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The Lagrangian fieldantifield formalism of Batalin and Vilkovisky (BV) is used to investigate the application of the collec tive coordinate method to soliton quantisation. In field theories with soliton solutions, the Gaussian fluctuation operator has zero modes due to the breakdown of global symmetries of the Lagrangian in the soliton solutions. It is shown how Noether identities and local symmetries of the Lagrangian arise when collective coordinates are introduced in order to avoid divergences related to these zero modes. This transformation to collective and fluctuation degrees of freedom is interpreted as a canonical transformation in the symplectic fieldantifield space which induces a timelocal gauge symmetry. Separating the corresponding Lagrangian path integral of the BV scheme in lowest order into harmonic quantum fluctuations and a free motion of the collective coordinate with the classical mass of the soliton, we show how the BV approach clarifies the relation between zero modes, collective coordinates, gauge invariance and the center ofmass motion of classical solutions in quantum fields. Finally, we apply the procedure to the reduced nonlinear O(3) oemodel.^L
Matterwave Optics of Darkstate Polaritons: Applications to Interferometry and Quantum Information
(2006)
The present work "Materwave Optics with Darkstate Polaritons: Applications to Interferometry and Quantum Information" deals in a broad sense with the subject of darkstates and in particular with the socalled darkstate polaritons introduced by M. Fleischhauer and M. D. Lukin. The darkstate polaritons can be regarded as a combined excitation of electromagnetic fields and spin/matterwaves. Within the framework of this thesis the special optical properties of the combined excitation are studied. On one hand a new procedure to spatially manipulate and to increase the excitation density of stored photons is described and on the other hand the properties are used to construct a new type of Sagnac Hybrid interferometer. The thesis is devided into four parts. In the introduction all notions necessary to understand the work are described, e.g.: electromagnetically induced transparency (EIT), darkstate polaritons and the Sagnac effect. The second chapter considers the method developed by A. Andre and M. D. Lukin to create stationary light pulses in specially dressed EITmedia. In a first step a set of field equations is derived and simplified by introducing a new set of normal modes. The absorption of one of the normal modes leads to the phenomenon of pulsematching for the other mode and thereby to a diffusive spreading of its field envelope. All these considerations are based on a homogeneous field setup of the EIT preparation laser. If this restriction is dismissed one finds that a drift motion is superimposed to the diffusive spreading. By choosing a special laser configuration the drift motion can be tailored such that an effective force is created that counteracts the spreading. Moreover, the force can not only be strong enough to compensate the diffusive spreading but also to exceed this dynamics and hence to compress the field envelope of the excitation. The compression can be discribed using a FokkerPlanck equation of the OrnsteinUhlenbeck type. The investigations show that the compression leads to an excitation of higherorder modes which decay very fast. In the last section of the chapter this exciation will be discussed in more detail and conditions will be given how the excitation of higherorder modes can be avoided or even suppressed. All results given in the chapter are supported by numerical simulatons. In the third chapter the matterwave optical properties of the darkstate polaritons will be studied. They will be used to construct a lightmatterwave hybrid Sagnac interferometer. First the principle setup of such an interferometer will be sketched and the relevant equations of motion of lightmatter interaction in a rotating frame will be derived. These form the basis of the following considerations of the darkstate polariton dynamics with and without the influence of external trapping potentials on the matterwave part of the polariton. It will be shown that a sensitivity enhancement compared to a passive laser gyroscope can be anticipated if the gaseous medium is initially in a superfluid quantum state in a ringtrap configuration. To achieve this enhancement a simultaneous coherence and momentum transfer is furthermore necessary. In the last part of the chapter the quantum sensitivity limit of the hybrid interferometer is derived using the oneparticle density matrix equations incorporating the motion of the particles. To this end the MaxwellBloch equations are considered perturbatively in the rotation rate of the noninertial frame of reference and the susceptibility of the considered 3level \(\Lambda\)type system is derived in arbitrary order of the probefield. This is done to determine the optimum operation point. With its help the anticipated quantum sensitivity of the lightmatterwave hybrid Sagnac interferometer is calculated at the shotnoise limit and the results are compared to stateoftheart laser and matterwave Sagnac interferometers. The last chapter of the thesis originates from a joint theoretical and experimental project with the AG Bergmann. This chapter does no longer consider the darkstate polaritons of the last two chapters but deals with the more general concept of dark states and in particular with the transient velocity selective dark states as introduced by E. Arimondo et al. In the experiment we could for the first time measure these states. The chapter starts with an introduction into the concept of velocity selective dark states as they occur in a \(\Lambda\)configuration. Then we introduce the transient velocity selective darkstates as they occur in an particular extension of the \(\Lambda\)system. For later use in the simulations the relevant equations of motion are derived in detail. The simulations are based on the solution of the generalized optical Bloch equations. Finally the experimental setup and procedure are explained and the theoretical and experimental results are compared.
Abstract: We calculate exact analytical expressions for O(alpha s) 3jet and O (alpha^2 s ) 4jet cross sections in polarized deep inelastic lepton nucleon scattering. Introducing an invariant jet definition scheme, we present differential distributions of 3 and 4jet cross sections in the basic kinematical variables x and W^2 as well as total jet cross sections and show their dependence on the chosen spindependent (polarized) parton distributions. Noticebly differences in the predictions are found for the two extreme choices, i.e. a large negative seaquark density or a large positive gluon density. Therefore, it may be possible to discriminate between different parametrizations of polarized parton densities, and hence between the different physical pictures of the proton spin underlying these parametrizations.
A new method is used to investigate the tunneling between two weaklylinked BoseEinstein con densates confined in doublewell potential traps. The nonlinear interaction between the atoms in each well contributes to a finite chemical potential, which, with consideration of periodic instantons, leads to a remarkably high tunneling frequency. This result can be used to interpret the newly found Macroscopic Quantum Self Trapping (MQST) effect. Also a new kind of firstorder crossover between different regions is predicted.
Abstract: We present experimental and theoretical results of a detailed study of laserinduced continuum structures (LICS) in the photoionization continuum of helium out of the metastable state 2s^1 S_0. The continuum dressing with a 1064 nm laser, couples the same region of the continuum to the 4s^1 S_0 state. The experimental data, presented for a range of intensities, show pronounced ionization suppression (by asmuch as 70% with respect to the farfromresonance value) as well as enhancement, in a BeutlerFano resonance profile. This ionization suppression is a clear indication of population trapping mediated by coupling to a contiuum. We present experimental results demonstrating the effect of pulse delay upon the LICS, and for the behavior of LICS for both weak and strong probe pulses. Simulations based upon numerical solution of the Schrödinger equation model the experimental results. The atomic parameters (Rabi frequencies and Stark shifts) are calculated using a simple modelpotential method for the computation of the needed wavefunctions. The simulations of the LICS profiles are in excellent agreement with experiment. We also present an analytic formulation of pulsed LICS. We show that in the case of a probe pulse shorter than the dressing one the LICS profile is the convolution of the power spectra of the probe pulse with the usual Fano profile of stationary LICS. We discuss some consequences of deviation from steadystate theory.
Phase velocities of surface acoustic waves in several boron nitride films were investigated by Brillouin light scattering. In the case of films with predominantly hexagonal crystal structure, grown under conditions close to the nucleation threshold of cubic BN, four independent elastic constants have been determined from the dispersion of the Rayleigh and the first Sezawa mode. The large elastic anisotropy of up to c11/c33 = 0.1 is attributed to a pronounced texture with the caxes of the crystallites parallel to the film plane. In the case of cubic BN films the dispersion of the Rayleigh wave provides evidence for the existence of a more compliant layer at the substratefilm interface. The observed broadening of the Rayleigh mode is identified to be caused by the film morphology.
Hexagonal BN films have been deposited by rfmagnetron sputtering with simultaneous ion plating. The elastic properties of the films grown on silicon substrates under identical coating conditions have been determined by Brillouin light scattering from thermally excited surface phonons. Four of the five independent elastic constants of the deposited material are found to be c11 = 65 GPa, c13 = 7 GPa, c33 = 92 GPa and c44 = 53 GPa exhibiting an elastic anisotropy c11/c33 of 0.7. The Young's modulus determined with load indentation is distinctly larger than the corresponding value taken from Brillouin light scattering. This discrepancy is attributed to the specific morphology of the material with nanocrystallites embedded in an amorphous matrix.
The magnetic anisotropy of Co/Cu~001! films has been investigated by the magnetooptical Kerr effect, both in the pseudomorphic growth regime and above the critical thickness where strain relaxation sets in. A clear correlation between the onset of strain relaxation as measured by means of reflection highenergy electron diffraction and changes of the magnetic anisotropy has been found.
Functional Metallic Microcomponents via LiquidPhase Multiphoton Direct Laser Writing: A Review
(2019)
We present an overview of functional metallic microstructures fabricated via direct laser writing out of the liquid phase. Metallic microstructures often are key components in diverse applications such as, e.g., microelectromechanical systems (MEMS). Since the metallic component’s functionality mostly depends on other components, a technology that enables onchip fabrication of these metal structures is highly desirable. Direct laser writing via multiphoton absorption is such a fabrication method. In the past, it has mostly been used to fabricate multidimensional polymeric structures. However, during the last few years different groups have put effort into the development of novel photosensitive materials that enable fabrication of metallic—especially gold and silver—microstructures. The results of these efforts are summarized in this review and show that direct laser fabrication of metallic microstructures has reached the level of applicability.
Small concentrations of alloying elements can modify the
α
α

γ
γ
phase transition temperature
T
c
Tc
of Fe. We study this effect using an atomistic model based on a set of manybody interaction potentials for iron and several alloying elements. Freeenergy calculations based on perturbation theory allow us to determine the change in
T
c
Tc
introduced by the alloying element. The resulting changes are in semiquantitative agreement with experiment. The effect is traced back to the shape of the pair potential describing the interaction between the Fe and the alloying atom
We report on generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the lowtemperaturegrown GaAs (LTGaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals.
We consider N coupled linear oscillators with timedependent coecients. An exact complex amplitude  real phase decomposition of the oscillatory motion is constructed. This decomposition is further used to derive N exact constants of motion which generalise the socalled ErmakovLewis invariant of a single oscillator. In the Floquet problem of periodic oscillator coecients we discuss the existence of periodic complex amplitude functions in terms of existing Floquet solutions.
A harmonic oscillator subject to a parametric pulse is examined. The aim of the paper is to present a new theory for analysing transitions due to parametric pulses. The new theoretical notions which are introduced relate the pulse parameters in a direct way with the transition matrix elements. The harmonic oscillator transitions are expressed in terms of asymptotic properties of a companion oscillator, the Milne (amplitude) oscillator. A traditional phaseamplitude decomposition of the harmonicoscillator solutions results in the socalled Milne's equation for the amplitude, and the phase is determined by an exact relation to the amplitude. This approach is extended in the present analysis with new relevant concepts and parameters for pulse dynamics of classical and quantal systems. The amplitude oscillator has a particularly nice numerical behavior. In the case of strong pulses it does not possess any of the fast oscillations induced by the pulse on the original harmonic oscillator. Furthermore, the new dynamical parameters introduced in this approach relate closely to relevant characteristics of the pulse. The relevance to quantum mechanical problems such as reflection and transmission from a localized well and mechanical problems of controlling vibrations is illustrated.
The particle flux produced by an obliquely incident Nd Qswitched pulse (20 ns) on a Ta target has been analysed with regard to its angular distribution resolved for both its neutral and ion components. The laser intensity has been varied in the range between about 10^10  10^11 W cm2, which is appropriate for many lowirradiance applications. It is observed that, at all emission angles and for the whole range of laser intensities, the number of neutral species clearly dominates the composition of the particles. At 1.3 x 10^10 W cm2 the total number of emitted particles is 4 x 10^14, scaling as E_L^¾ with the laser energy. While for relatively low laser energies the angular distribution shows the usual smooth cosbehaviour, an additional strong directive emission cone, superimposed upon the cosdistribution, develops if the laser energy is enhanced. Both the strength and the width strongly depend on the laser intensity. While at lower intensities a fit by a cos^n function with n ~ 10 seems appropriate, n increases to 26 at an intensity of 10^11 W cm2 . It can be assumed that secondary energy transfer processes that are not yet fully understood are responsible for this anomalous emission.
We present results of anisotropy and exchangecoupling studies of asymmetric Co/Cr/Fe trilayers and superlattices grown by molecular beam epitaxy on Cr~001!/Mg~001! buffers and substrates. The magnetic properties have been investigated using both the longitudinal magnetooptical Kerr effect and ferromagnetic resonance. The hysteresis data obtained from the trilayer system were fit to a theoretical model which contains both bilinear and biquadratic coupling. The effective inplane anisotropy was found to be of fourfold symmetry with the same easyaxis orientation for both the Fe and Co layers. An analysis of the easyaxis hysteresis loops indicates longperiod oscillatory coupling and also suggests a short periodic coupling. We show that weakly antiferromagnetically coupled asymmetric films might serve as potential candidates for improved spinvalve systems.
We present detailed studies of the enhanced coercivity of exchangebias bilayer Fe/MnPd, both experimentally and theoretically. We have demonstrated that the existence of large higherorder anisotropies due to exchange coupling between different Fe and MnPd layers can account for the large increase of coercivity in Fe/MnPd system. The linear dependence of coercivity on inverse Fe thickness are well explained by a phenomenological model by introducing higherorder anisotropy terms into the total free energy of the system.
Initiated by a task in tunable microoptics, but not limited to this application, a microfluidic droplet array in an upright standing module with 3 × 3 subcells and droplet actuation via electrowetting is presented. Each subcell is filled with a single (of course transparent) water droplet, serving as a movable iris, surrounded by opaque blackened decane. Each subcell measures 1 × 1 mm ² and incorporates 2 × 2 quadratically arranged positions for the droplet. All 3 × 3 droplets are actuated synchronously by electrowetting on dielectric (EWOD). The droplet speed is up to 12 mm/s at 130 V (Vrms) with response times of about 40 ms. Minimum operating voltage is 30 V. Horizontal and vertical movement of the droplets is demonstrated. Furthermore, a minor modification of the subcells allows us to exploit the flattening of each droplet. Hence, the opaque decane fluid sample can cover each water droplet and render each subcell opaque, resulting in switchable irises of constant opening diameter. The concept does not require any mechanically moving parts or external pumps.
Although for photon Bose–Einstein condensates the main mechanism of the observed photon–photon interaction has already been identified to be of a thermooptic nature, its influence on the condensate dynamics is still unknown. Here a meanfield description of this effect is derived, which consists of an opendissipative Schrödinger equation for the condensate wave function coupled to a diffusion equation for the temperature of the dye solution. With this system at hand, the lowestlying collective modes of a harmonically trapped photon Bose–Einstein condensate are calculated analytically via a linear stability analysis. As a result, the collective frequencies and, thus, the strength of the effective photon–photon interaction turn out to strongly depend on the thermal diffusion in the cavity mirrors. In particular, a breakdown of the Kohn theorem is predicted, i.e. the frequency of the centreofmass oscillation is reduced due to the thermooptic photon–photon interaction.
High frequency switching of single domain, uniaxial magnetic particles is discussed in terms of transition rates controlled by a small transverse bias field. It is shown that fast switching times can be achieved using bias fields an order of magnitude smaller than the effective anisotropy field. Analytical expressions for the switching time are derived in special cases and general configurations of practical interest are examined using numerical simulations.
A pure YangMills theory extended by addition of a quartic term is considered in order to study the transition from the quantum tunneling regime to that of classical, i.e. thermal, behaviour. The periodic field confiurations are found, which interpolate between the vacuum and sphaleron field configurations. It is shown by explicit calculation that only smooth second order transitions occur for all permissible values of the parameter A introduced with the quartic term. The theory is one of the rare cases which canbe handled analytically.
Microsystem technology has been a fast evolving field over the last few years. Its ability to handle volumes in the submicroliter range makes it very interesting for potential application in fields such as biology, medicine and pharmaceutical research. However, the use of microfabricated devices for the analysis of liquid biological samples still has to prove its applicability for many particular demands of basic research. This is particularly true for samples consisting of complex protein mixtures. The presented study therefore aimed at evaluating if a commonly used glasscoating technique from the field of microfluidic technology can be used to fabricate an analysis system for molecular biology. It was ultimately motivated by the demand to develop a technique that allows the analysis of biological samples at the singlecell level. Gene expression at the transcription level is initiated and regulated by DNAbinding proteins. To fully understand these regulatory processes, it is necessary to monitor the interaction of specific transcription factors with other elements  proteins as well as DNA sites  in living cells. One wellestablished method to perform such analysis is the Chromatin Immunoprecipitation (CHIP) assay. To map proteinDNA interactions, living cells are treated with formaldehyde in vivo to crosslink DNAbinding proteins to their resident sites. The chromatin is then broken into small fragments, and specific antibodies against the protein of interest are used to immunopurify the chromatin fragments to which those factors are bound. After purification, the associated DNA can be detected and analyzed using Polymerase Chain Reaction (PCR). Current CHIP technology is limited as it needs a relatively large number of cells while there is increasing interest in monitoring DNAprotein interactions in very few, if not single cells. Most notably this is the case in research on early organism development (embryogenesis). To investigate if microsystem technology can be used to analyze DNAprotein complexes from samples containing chromatin from only few cells, a new setup for fluid transport in glass capillaries of 75 µm inner diameter has been developed, forming an array of microcolumns for parallel affinity chromatography. The inner capillary walls were antibodycoated using a silanebased protocol. The remaining surface was made chemically inert by saturating free binding sites with suitable biomolecules. Variations of this protocol have been tested. Furthermore, the sensitivity of the PCR method to detect immunoprecipitated proteinDNA complexes was improved, resulting in the reliable detection of about 100 DNA fragments from chromatin. The aim of the study was to successively decrease the amount of analyzed chromatin in order to investigate the lower limits of this technology in regard to sensitivity and specificity of detection. The Drosophila GAGA transcription factor was used as an established model system. The protein has already been analyzed in several largescale CHIP experiments and antibodies of excellent specificity are available. The results of the study revealed that this approach is not easily applicable to "realworld" biological samples in regard to volume reduction and specificity. Particularly, material that nonspecifically adsorbed to capillary surfaces outweighed the specific antibodyantigen interaction, the system was designed for. It became clear that complex biological structures, such as chromatinprotein compositions, are not as easily accessible by techniques based on chemically modified glass surfaces as prepurified samples. In the case of the investigated system, it became evident that there is a need for more research that goes beyond the scope of this work. It is necessary to develop novel coatings and materials to prevent nonspecific adsorption. In addition to improving existing techniques, fundamentally new concepts, such as microstructures in biocompatible polymers or liquid transport on hydrophobic stripes on planar substrates to minimize surface contact, may also help to advance the miniaturization of biological experiments.
Abstract: We utilize the generation of large atomic coherence to enhance the resonant nonlinear magnetooptic effect by several orders of magnitude, thereby eliminating power broadening and improving the fundamental signaltonoise ratio. A proofofprinciple experiment is carried out in a dense vapor of Rb atoms. Detailed numerical calculations are in good agreement with the experimental results. Applications such as optical magnetometry or the search for violations of parity and time reversal symmetry are feasible.
We present a detailed analysis of a scalar conformal fourpoint function obtained from AdS/CFT correspondence. We study the scalar exchange graphs in AdS and discuss their analytic properties. Using methods of conformal partial wave analysis, we present a general procedure to study conformal fourpoint functions in terms of exchanges of scalar and tensor fields. The logarithmic terms in the fourpoint functions are connected to the anomalous dimensions of the exchanged fields. Comparison of the results from AdS graphs with the conformal partial wave analysis, suggests a possible general form for the operator product expansion of scalar fields in the boundary CFT.
We discuss the analytic properties of AdS scalar exchange graphs in the crossed channel. We show that the possible nonanalytic terms drop out by virtue of nontrivial properties of generalized hypergeometric functions. The absence of nonanalytic terms is a necessary condition for the existence of an operator product expansion for CFT amplitudes obtained from AdS/CFT correspondence.
The conversion efficiency of laser energy into kinetic ion energy in a laserproduced plasma has been investigated for two quite different targets: graphite and tantalum. The laser energy (intensity) varied from several mJ to 200 mJ (1O^9 to 7 x 10^10 W cm2) which is appropriate to many applications of a laser produced ion source. The conversion efficiency as a function of the laser energy was directly determined by differential measurements of the charge, kinetic energy and angular emission distribution of the plasma ions in absolute units. Whilst for the Ta target a nearly constant efficiency of about 30% was observed, the graphite result shows an unexpectedly strong enhancement of the transfer efficiency of up to 80% in the laser intensity range around 1.5 x l0^10 W cm2. It is assumed that the results are related to the difference in the surface roughness of the targets.
Ion energy spectra of a laserproduced Ta plasma have been investigated as a function of the flight distance from the focus. The laser (Nd:YAG, 20 ns, 210 mJ) is incident obliquely (45°) and focused to an intensity of about 10^11 W cm2. The changes in the ion distributions have been analysed for the Ta+ to Ta6+ ions in an expansion range 64  220 cm. With increasing distance from the target, a weak but monotonic decrease is observed for the total number of ions, which is essentially due to the decrease in the number of the more highly charged species. For the Ta+ and Ta2+ ions the net changes approximately cancel. A more sophisticated picture of the recombination dynamics is obtained, however, if the changes within individual groups of ions expanding with different velocities are compared. Here, in the same spectrum, both increasing and decreasing ion numbers can be observed. This can be interpreted as direct evidence of recombination and its dependence on temperature, density and charge.
Epitaxial growth of metastable Pd(001) at high deposition temperatures up to a critical thickness of 6 monolayers on bccFe(001) is reported, the critical thickness being depending dramatically on the deposition temperature. For larger thicknesses the Pd film undergoes a roughening transition with strain relaxation by forming a top polycrystalline layer. These results allow to make a correlation between previously reported unusual magnetic properties of Fe/Pd double layers and the crystallographic structure of the Pd overlayer.
We report on the exchange bias effect as a function of the inplane direction of the applied field in twofold symmetric, epitaxial Ni80Fe20/Fe50Mn50 bilayers grown on Cu(110) single crystal substrates. An enhancement of the exchange bias field, Heb, up to a factor of two is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the exchange bias field as a function of the inplane angle of the applied field, the unidirectional, uniaxial and fourfold anisotropy contributions are determined with high precision. The symmetry direction of the unidirectional anisotropy switches with increasing NiFe thickness from [110] to [001].
We report on the exchange bias effect as a function of the inplane direction of the applied field in twofold symmetric, epitaxial Ni 80 Fe 20 /Fe 50 Mn 50 bilayers grown on Cu~110! singlecrystal substrates. An enhancement of the exchange bias field, H eb , up to a factor of 2 is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the exchange bias field as a function of the inplane angle of the applied field, the unidirectional, uniaxial and fourfold anisotropy contributions are determined with high precision. The symmetry direction of the unidirectional anisotropy switches with increasing NiFe thickness from [110] to [001].
Based on the Lindblad master equation approach we obtain a detailed microscopic model of photons in a dyefilled cavity, which features condensation of light. To this end we generalise a recent nonequilibrium approach of Kirton and Keeling such that the dyemediated contribution to the photonphoton interaction in the light condensate is accessible due to an interplay of coherent and dissipative dynamics. We describe the steadystate properties of the system by analysing the resulting equations of motion of both photonic and matter degrees of freedom. In particular, we discuss the existence of two limiting cases for steady states: photon BoseEinstein condensate and laserlike. In the former case, we determine the corresponding dimensionless photonphoton interaction strength by relying on realistic experimental data and find a good agreement with previous theoretical estimates. Furthermore, we investigate how the dimensionless interaction strength depends on the respective system parameters.
Wall energy and wall thickness of exchangecoupled rareearth transitionmetal triple layer stacks
(1999)
The roomtemperature wall energy sw 54.0310 23 J/m 2 of an exchangecoupled Tb 19.6 Fe 74.7 Co 5.7 /Dy 28.5 Fe 43.2 Co 28.3 double layer stack can be reduced by introducing a soft magnetic intermediate layer in between both layers exhibiting a significantly smaller anisotropy compared to Tb+ FeCo and Dy+ FeCo. sw will decrease linearly with increasing intermediate layer thickness, d IL , until the wall is completely located within the intermediate layer for d IL d w , where d w denotes the wall thickness. Thus, d w can be obtained from the plot sw versus d IL .We determined sw and d w on Gd+ FeCo intermediate layers with different anisotropy behavior ~perpendicular and inplane easy axis! and compared the results with data obtained from Brillouin lightscattering measurements, where exchange stiffness, A, and uniaxial anisotropy, K u , could be determined. With the knowledge of A and K u , wall energy and thickness were calculated and showed an excellent agreement with the magnetic measurements. A ten times smaller perpendicular anisotropy of Gd 28.1 Fe 71.9 in comparison to Tb+ FeCo and Dy+ FeCo resulted in a much smaller sw 51.1310 23 J/m 2 and d w 524 nm at 300 K. A Gd 34.1 Fe 61.4 Co 4.5 with inplane anisotropy at room temperature showed a further reduced sw 50.3310 23 J/m 2 and d w 517 nm. The smaller wall energy was a result of a different wall structure compared to perpendicular layers.
thesis deals with the investigation of the dynamics of optically excited (hot) electrons in thin and ultrathin layers. The main interests concern about the time behaviour of the dissipation of energy and momentum of the excited electrons. The relevant relaxation times occur in the femtosecond time region. The twophoton photoemission is known to be an adequate tool in order to analyse such dynamical processes in realtime. This work expands the knowledge in the fields of electron relaxation in ultrathin silver layers on different substrates, as well as in adsorbate states in a bandgap of a semiconductor. It contributes facts to the comprehension of spin transport through an interface between a metal and a semiconductor. The primary goal was to prove the predicted theory by reducing the observed crystal in at least one direction. One expects a change of the electron relaxation behaviour while altering the crystal’s shape from a 3d bulk to a 2d (ultrathin) layer. This is due to the fact that below a determined layer thickness, the electron gas transfers to a twodimensional one. This behaviour could be proven in this work. In an about 3nm thin silver layer on graphite, the hot electrons show a jump to longer relaxation time all over the whole accessible energy range. It is the first time that the temporal evolution of the relaxation of excited electrons could be observed during the transition from a 3d to a 2d system. In order to reduce or even eliminate the influence coming from the substrate, the system of silver on the semiconductor GaAs, which has a bandgap of 1.5eV at the Gammapoint, was investigated. The observations of the relaxation behaviour of hot electron in different ultrathin silver layers on this semiconductor could show, that at metalinsulatorjunctions, plasmons in the silver and in the interface, as well as cascading electrons from higher lying energies, have a huge influence to the dissipation of momentum and energy. This comes mainly from the band bending of the semiconductor, and from the electrons, which are excited in GaAs. The limitation of the silver layer on GaAs in one direction led to the expected generation of quantum well states (QWS) in the bandgap. Those adsorbate states have quantised energy and momentum values, which are directly connected to the layer thickness and the standing electron wave therein. With the experiments of this work, published values could not only be completed and proved, but it could also be determined the time evolution of such a QWS. It came out that this QWS might only be filled by electrons, which are moving from the lower edge of the conduction band of the semiconductor to the silver and suffer cascading steps there. By means of the system silver on GaAs, and of the known fact that an excitation of electrons in GaAs with circularly polarised light of the energy 1.5eV does produce spin polarised electrons in the conduction band, it became possible to bring a contribution to the hot topic of spin injection. The main target of spin injection is the transfer of spin polarised electrons out of a ferromagnet into a semiconductor, in order to develop spin dependent switches and memories. It could be demonstrated here that spin polarised electrons from GaAs can move through the interface into silver, could be photoemitted from there and their spin was still being detectable. As a third investigation system, ultrathin silver layers were deposited on the insulator MgO, which has a bandgap of 7.8eV. Also in this system, one could recognize a change in the relaxation time while reducing the dimension of the silver layer from thick to ultrathin. Additionally, it came out an extreme large relaxation time at a layer thickness of 0.6 – 1.2nm. This time is an order of magnitude longer than at thick films, and this is a consequence of two factors: first, the reduction of the phase space due to the confined electron gas in the zdirection, and second, the slowlier thermalisation of the electron gas due to less accessible scattering partners.
Abstract: We describe quantumfieldtheoretical (QFT) techniques for mapping quantum problems onto cnumber stochastic problems. This approach yields results which are identical to phasespace techniques [C.W. Gardiner, Quantum Noise (1991)] when the latter result in a FokkerPlanck equation for a corresponding pseudoprobability distribution. If phasespace techniques do not result in a FokkerPlanck equation and hence fail to produce a stochastic representation, the QFT techniques nevertheless yield stochastic di erence equations in discretised time.
Abstract: We aim to establish a link between pathintegral formulations of quantum and classical field theories via diagram expansions. This link should result in an independent constructive characterisation of the measure in Feynman path integrals in terms of a stochastic differential equation (SDE) and also in the possibility of applying methods of quantum field theory to classical stochastic problems. As a first step we derive in the present paper a formal solution to an arbitrary cnumber SDE in a form which coincides with that of Wick's theorem for interacting bosonic quantum fields. We show that the choice of stochastic calculus in the SDE may be regarded as a result of regularisation, which in turn removes ultraviolet divergences from the corresponding diagram series.
We show that the solution to an arbitrary cnumber stochastic differential equation (SDE) can be represented as a diagram series. Both the diagram rules and the properties of the graphical elements reflect causality properties of the SDE and this series is therefore called a causal diagram series. We also discuss the converse problem, i.e. how to construct an SDE of which a formal solution is a given causal diagram series. This then allows for a nonperturbative summation of the diagram series by solving this SDE, numerically or analytically.
Thermal Properties of Interacting Bose Fields and ImaginaryTime Stochastic Differential Equations
(1998)
Abstract: Matsubara Green's functions for interacting bosons are expressed as classical statistical averages corresponding to a linear imaginarytime stochastic differential equation. This makes direct numerical simulations applicable to the study of equilibrium quantum properties of bosons in the nonperturbative regime. To verify our results we discuss an oscillator with quartic anharmonicity as a prototype model for an interacting Bose gas. An analytic expression for the characteristic function in a thermal state is derived and a Higgstype phase transition discussed, which occurs when the oscillator frequency becomes negative.
Abstract: Standard methods of nonlinear dynamics are used to investigate the stability of particles, branes and Dbranes of abelian BornInfeld theory. In particular the equation of small fluctuations about the Dbrane is derived and converted into a modified Mathieu equation and  complementing earlier lowenergy investigations in the case of the dilatonaxion system  studied in the highenergy domain. Explicit expressions are derived for the Smatrix and absorption and reflection amplitudes of the scalar fluctuation in the presence of the Dbrane. The results confirm physical expectations and numerical studies of others. With the derivation and use of the (hitherto practically unknown) high energy expansion of the Floquet exponent our considerations also close a gap in earlier treatments of the Mathieu equation.
Abstract: A BornInfeld theory describing a D2brane coupled to a 4form RR field strength is considered, and the general solutions of the static and Euclidean time equations are derived and discussed. The period of the bounce solutions is shown to allow a consideration of tunneling and quantumclassical transitions in the sphaleron region. The order of such transitions, depending on the strength of the RR field strength, is determined. A criterion is then derived to confirm these findings.
Abstract: Following our earlier investigations we examine the quantumclassical winding number transition in the AbelianHiggs system. It is demonstrated that the winding number transition in this system is of the smooth second order type in the full range of parameter space. Comparison of the action of classical vortices with that of the sphaleron supports our finding.
Abstract: Winding number transitions from quantum to classical behavior are studied in the case of the 1+1 dimensional MottolaWipf model with the space coordinate on a circle for exploring the possibility of obtaining transitions of second order. The model is also studied as a prototype theory which demonstrates the procedure of such investigations. In the model at hand we find that even on a circle the transitions remain those of first order.
Abstract: The functional relation between interquark potential and interquark distance is explicitly derived by considering the NambuGoto action in the AdS5 X S 5 background. It is also shown that a similar relation holds in a general background. The implications of this relation for confinement are briefly discussed.
Abstract: The transition from the instantondominated quantum regime to the sphalerondominated classical regime is studied in the d = 2 abelianHiggs model when the spatial coordinate is compactified to S1. Contrary to the noncompactified case, this model allows both sharp firstorder and smooth secondorder transitions depending on the size of the circle. This finding may make the model a useful toy model for the analysis of baryon number violating processes.
Abstract: The calculation of absorption cross sections for minimal scalars in supergravity backgrounds is an important aspect of the investigation of AdS/CFT correspondence and requires a matching of appropriate wave functions. The low energy case has attracted particular attention. In the following the dependence of the cross section on the matching point is investigated. It is shown that the low energy limit is independent of the matching point and hence exhibits universality. In the high energy limit the independence is not maintained, but the result is believed to possess the correct energy dependence.
The greybody factors in BTZ black holes are evaluated from 2D CFT in the spirit of AdS3/CFT correspondence. The initial state of black holes in the usual calculation of greybody factors by effective CFT is described as Poincar'e vacuum state in 2D CFT. The normalization factor which cannot be fixed in the effective CFT without appealing to string theory is shown to be determined by the normalized bulktoboundary Green function. The relation among the greybody factors in different dimensional black holes is exhibited. Two kinds of (h; _h) = (1; 1) operators which couple with the boundary value of massless scalar field are discussed.
Combining ultracold atomic gases with the peculiar properties of Rydberg excited atoms gained a lot of theoretical and experimental attention in recent years. Embedded in the ultracold gas, an interaction between the Rydberg atom and the surrounding ground state atoms arises through the scattering of the Rydberg electron from an intruding perturber atom. This peculiar interaction gives rise to a plenitude of previously unobserved effects. Within the framework of the present thesis, this interaction is studied in detail for Rydberg \(P\)states in rubidium.
Due to their long lifetime, atoms in Rydberg states are subject to scattering with the surrounding ground state atoms in the ultracold cloud. By measuring their lifetime as a function of the ground state atom flux, we are able to obtain the total inelastic scattering cross section as well as the partial cross section for associative ionisation. The fact that the latter is three orders of magnitude larger than the size of the formed molecular
ion indicates the presence of an efficient mass transport mechanism that is mediated by the Rydberg–ground state interaction. The immense acceleration of the collisional process shows a close analogy to a catalytic process. The increase of the scattering cross section renders associative ionisation an important process that has to be considered for experiments in dense ultracold systems.
The interaction of the Rydberg atom with a ground state perturber gives rise to a highly oscillatory potential that supports molecular bound states. These socalled ultralongrange Rydberg molecules are studied with high resolution timeofflight spectroscopy, where we are able to determine the binding energies and lifetimes of the molecular states between the two fine structure split \(25P\)states. Inside an electric field, we observe a broadening of the
molecular lines that indicates the presence of a permanent electric dipole moment, induced by the mixing with high angular momentum states. Due to the mixing of the ground state atom’s hyperfine states by the molecular interaction, we are able to observe a spinflip of the perturber upon creation of a Rydberg molecule. Furthermore, an incidental neardegeneracy in the underlying level scheme of the \(25P\)state gives rise to highly entangled states between the Rydberg fine structure state and the perturber’s hyperfine structure. These mechanisms can be used to manipulate the quantum state of a remote particle over distances that exceed by far the typical contact interaction range.
Apart from the ultralongrange Rydberg molecules that predominantly consist of only one low angular momentum state, a class of Rydberg molecules is predicted to exist that strongly mixes the high angular momentum states of the degenerate hydrogenic manifolds. These states, the socalled trilobite and butterfly Rydberg molecules, show very peculiar properties that cannot be observed for conventional molecules. Here we present the first experimental observation of butterfly Rydberg molecules. In addition to an extensive spectroscopy that reveals the binding energy, we are also able to observe the rotational structure of these exotic molecules. The arising pendular states inside an electric field allow us, in comparison to the model of a dipolar rotor, to extract the precise bond
length and dipole moment of the molecule. With the information obtained in the present study, it is possible to photoassociate butterfly molecules with a selectable bond length, vibrational state, rotational state, and orientation inside an electric field.
By shedding light on various previously unrevealed aspects, the experiments presented in this thesis significantly deepen our knowledge on the Rydberg–ground state interaction and the peculiar effects arising from it. The obtained spectroscopic information on Rydberg molecules and the changed reaction dynamics for molecular ion creation will surely provide valuable data for quantum chemical simulations and provide necessary data to plan future experiments. Beyond that, our study reveals that the hyperfine interaction in Rydberg molecules and the peculiar properties of butterfly states provide very promising new ways to alter the short and longrange interactions in ultracold manybody systems. In this sense the investigated Rydberg–ground state interaction not only lies right at
the interface between quantum chemistry, quantum manybody systems, and Rydberg physics, but also creates many new and fascinating possibilities by combining these fields.
Starting from the Hamiltonian operator of the noncompensated twosublattice model of a small antiferromagnetic particle, we derive the e effective Lagrangian of a biaxial antiferromagnetic particle in an external magnetic field with the help of spincoherentstate path integrals. Two unequal levelshifts induced by tunneling through two types of barriers are obtained using the instanton method. The energy spectrum is found from Bloch theory regarding the periodic potential as a superlattice. The external magnetic field indeed removes Kramers' degeneracy, however a new quenching of the energy splitting depending on the applied magnetic field is observed for both integer and halfinteger spins due to the quantum interference between transitions through two types of barriers.
Abstact. The tunnel splitting in biaxial antiferromagnetic particles is studied with a magnetic field applied along the hard anisotropy axis. We observe the oscillation of tunnel splitting as a function of the magnetic field due to the quantum phase interference of two tunneling paths of opposite windings. The oscillation is similar to the recent experimental result with Fe8 molecular clusters.
We consider a (2 + 1)dimensional mechanical system with the Lagrangian linear in the torsion of a lightlike curve. We give Hamiltonian formulation of this system and show that its mass and spin spectra are defined by onedimensional nonrelativistic mechanics with a cubic potential. Consequently, this system possesses the properties typical of resonancelike particles.
Adsorption and Diffusion of Cisplatin Molecules in Nanoporous Materials: A Molecular Dynamics Study
(2019)
Using molecular dynamics simulations, the adsorption and diffusion of cisplatin drug molecules in nanopores is investigated for several inorganic materials. Three different materials are studied with widelyvarying properties: metallic gold, covalent silicon, and silica. We found a strong influence of both the van der Waals and the electrostatic interaction on the adsorption behavior on the pore walls, which in turn influence the diffusion coefficients. While van der Waals forces generally lead to a reduction of the diffusion coefficient, the fluctuations in the electrostatic energy induced by orientation changes of the cisplatin molecule were found to help desorb the molecule from the wall.
2D quantum dilaton gravitational Hamiltonian, boundary terms and new definition for total energy
(1995)
The ADM and Bondi mass for the RST model have been first discussed from Hawking and Horowitz's argument. Since there is a nonlocal term in the RST model, the RST lagrangian has to be localized so that Hawking and Horowitz's proposal can be carried out. Expressing the localized RST action in terms of the ADM formulation, the RST Hamiltonian can be derived, meanwhile keeping track of all boundary terms. Then the total boundary terms can be taken as the total energy for the RST model. Our result shows that the previous expression for the ADM and Bondi mass actually needs to be modified at quantum level, but at classical level, our mass formula can be reduced to that given by Bilal and Kogan [5] and de Alwis [6]. It has been found that there is a new contribution to the ADM and Bondi mass from the RST boundary due to the existence of the hidden dynamical field. The ADM and Bondi mass with and without the RST boundary for the static and dynamical solutions have been discussed respectively in detail, and some new properties have been found. The thunderpop of the RST model has also been encountered in our new Bondi mass formula.
Skyrme Sphalerons of an O(3)oe Model and the Calculation of Transition Rates at Finite Temperature
(1997)
The reduced O(3)oe model with an O(3) ! O(2) symmetry breaking potential is considered with an additional Skyrmionic term, i. e. a totally antisymmetric quartic term in the field derivatives. This Skyrme term does not affect the classical static equations of motion which, however, allow an unstable sphaleron solution. Quantum fluctuations around the static classical solution are considered for the determination of the rate of thermally induced transitions between topologically distinct vacua mediated by the sphaleron. The main technical effect of the Skyrme term is to produce an extra measure factor in one of the fluctuation path integrals which is therefore evaluated using a measuremodified FourierMatsubara decomposition (this being one of the few cases permitting this explicit calculation). The resulting transition rate is valid in a temperature region different from that of the original Skyrmeless model, and the crossover from transitions dominated by thermal fluctuations to those dominated by tunneling at the lower limit of this range depends on the strength of the Skyrme coupling.
The pureSkyrme limit of a scalebreaking Skyrmed O(3) sigma model in 1+1 dimensions is employed to study the effect of the Skyrme term on the semiclassical analysis of a field theory with instantons. The instantons of this model are selfdual and can be evaluated explicitly. They are also localised to an absolute scale, and their fluctuation action can be reduced to a scalar subsystem. This permits the explicit calculation of the fluctuation determinant and the shift in vacuum energy due to instantons. The model also illustrates the semiclassical quantisation of a Skyrmed field theory.
Starting from the coherent state representation of the evolution operator with the help of the pathintegral, we derive a formula for the lowlying levels E = ffl0 Gamma 24ffl cos(s + ,)ss of a quantum spin system. The quenching of macroscopic quantum coherence is understood as the vanishing of cos(s + ,)ss in disagreement with the suppression of tunneling (i.e. 4ffl = 0) as claimed in the literature. A new configuration called the macroscopic Fermiparticle is suggested by the character of its wave function. The tunne ling rate ( 24fflss ) does not vanish, not for integer spin s nor for a halfinteger value of s, and is calculated explicitly (for the position dependent mass) up to the oneloop approximation.
Quantum tunneling between degenerate ground states through the central barrier of a potential is extended to excited states with the instanton method. This extension is achieved with the help of an LSZ reduction technique as in field theory and may be of importance in the study of macroscopic quantum phenomena in magnetic systems.
In this paper we present a renormalizability proof for spontaneously broken SU (2) gauge theory. It is based on Flow Equations, i.e. on the Wilson renormalization group adapted to perturbation theory. The power counting part of the proof, which is conceptually and technically simple, follows the same lines as that for any other renormalizable theory. The main difficulty stems from the fact that the regularization violates gauge invariance. We prove that there exists a class of renormalization conditions such that the renormalized Green functions satisfy the SlavnovTaylor identities of SU (2) YangMills theory on which the gauge invariance of the renormalized theory is based.
Abstract: In this paper we present a renormalizability proof for spontaneously broken SU (2) gauge theory. It is based on Flow Equations, i.e. on the Wilson renormalization group adapted to perturbation theory. The power counting part of the proof, which is conceptually and technically simple, follows the same lines as that for any other renormalizable theory. The main difficulty stems from the fact that the regularization violates gauge invariance. We prove that there exists a class of renormalization conditions such that the renormalized Green functions satisfy the SlavnovTaylor identities of SU (2) YangMills theory on which the gauge invariance of the renormalized theory is based.
Recently renewed interest in solitons has arisen in connection with exceptional statistics occuring in lowdimensional quantum field theory. The nonperturbative approach to quantum solitons [1, 2, 3, 4, 5], based on the notion of a disorder variable [6, 7], does not make use of the wellknown semiclassical quantisation procedure around classical soliton solutions [8]. In a recent article [9] the author introduced multicomponent scalar field models, treated nonperturbatively on a Euclidean spacetime lattice. The exponentially decaying disorder correlation functions are connected with soliton fields showing nonAbelian braid group statistics. It is the aim of this note to present the corresponding classical soliton solutions, which do not seem to have appeared in the literature.
FeNi/FeMn exchange bias samples with a large exchange bias field at room temperature have been prepared on a Cu buffer layer. Upon irradiation with He ions, both the exchange bias field and the coercive field are modified. For low ion doses the exchange bias field is enhanced by nearly a factor of 2. Above a threshold dose of 0.3olsi 10 15 ions/cm 2 , the exchange bias field decreases continuously as the ion dose increases. The observed modifications are explained in terms of defect creation acting as pinning sites for domain walls and atomic intermixing.
A new method for calculating Stark resonances is presented and applied for illustration to the simple case of a oneparticle, onedimensional model Hamiltonian. The method is applicable for weak and strong dc fields. The only need, also for the case of many particles in multidimensional space, are either the short time evolution matrix elements or the eigenvalues and Fourier components of the eigenfunctions of the fieldfree Hamiltonian.
A formalism is developed for calculating the quasienergy states and spectrum for timeperiodic quantum systems when a timeperiodic dynamical invariant operator with a nondegenerate spectrum is known. The method, which circumvents the integration of the Schrodinger equation, is applied to an integrable class of systems, where the global invariant operator is constructed. Furthermore, a local integrable approximation for more general nonintegrable systems is developed. Numerical results are presented for the doubleresonance model.
We present an entropy concept measuring quantum localization in dynamical systems based on time averaged probability densities. The suggested entropy concept is a generalization of a recently introduced [PRL 75, 326 (1995)] phasespace entropy to any representation chosen according to the system and the physical question under consideration. In this paper we inspect the main characteristics of the entropy and the relation to other measures of localization. In particular the classical correspondence is discussed and the statistical properties are evaluated within the framework of random vector theory. In this way we show that the suggested entropy is a suitable method to detect quantum localization phenomena in dynamical systems.
Abstract: The duality symmetries of various chiral boson actions are investigated using D = 2 and D = 6 spacetime dimensions as examples. These actions involve the Siegel, FloreaniniJackiw, Srivastava and PastiSorokinTonin formulations. We discover that the Siegel, FloreaniniJackiw and PastiSorokinTonin actions have selfduality with respect to a common antidualization of chiral boson fields in D = 2 and D = 6 dimensions, respectively, while the Srivastava action is selfdual with respect to a generalized dualization of chiral boson fields. Moreover, the action of the FloreaniniJackiw chiral bosons interacting with gauge fields in D = 2 dimensions also has selfduality but with respect to a generalized antidualization of chiral boson fields.
Abstract: The selfduality of chiral pforms was originally investigated by Pasti, Sorokin and Tonin in a manifestly Lorentz covariant action with nonpolynomial auxiliary fields. The investigation was then extended to other chiral pform actions. In this paper we point out that the selfduality appears in a wider context of theoretical models that relate to chiral pforms. We demonstrate this by considering the interacting model of Floreanini Jackiw chiral bosons and gauge fields, the generalized chiral Schwinger model (GCSM) and the latter's gauge invariant formulation, and discover that the selfduality of the GCSM corresponds to the vector and axial vector current duality.
Influence of the Crystal Surface on the Austenitic and Martensitic Phase Transition in Pure Iron
(2018)
Using classical molecular dynamics simulations, we studied the influence that free
surfaces exert on the austenitic and martensitic phase transition in iron. For several singleindexed
surfaces—such as (100)bcc and (110)bcc as well as (100)fcc and (110)fcc surfaces—appropriate
pathways exist that allow for the transformation of the surface structure. These are the Bain,
Mao, Pitsch, and Kurdjumov–Sachs pathways, respectively. Tilted surfaces follow the pathway
of the neighboring singleindexed plane. The austenitic transformation temperature follows the
dependence of the specific surface energy of the native bcc phase; here, the new phase nucleates at
the surface. In contrast, the martensitic transformation temperature steadily decreases when tilting
the surface from the (100)fcc to the (110)fcc orientation. This dependence is caused by the strong
outofplane deformation that (110)fcc facets experience under the transformation; here, the new
phase also nucleates in the bulk rather than at the surface.
Indentation into a metastable austenite may induce the phase transformation to the bcc phase. We study this process using
atomistic simulation. At temperatures low compared to the equilibrium transformation temperature, the indentation triggers the
transformation of the entire crystallite: after starting the transformation, it rapidly proceeds throughout the simulation crystallite.
The microstructure of the transformed sample is characterized by twinned grains. At higher temperatures, around the equilibrium
transformation temperature, the crystal transforms only locally, in the vicinity of the indent pit. In addition, the indenter
produces dislocation plasticity in the remaining austenite. At intermediate temperatures, the crystal continuously transforms
throughout the indentation process.
The scales of white beetles strongly scatter light within a thin disordered network of
chitin filaments. There is no comparable artificial material achieving such a high scat
tering strength within a thin layer of low refractive index material. Several analyses
investigated the scattering but could not explain the underlying concept. Here a model
system is described, which has the same optical properties as the white beetles’ scales
in the visible wavelength range. With some modification, it also explains the behavior
of the structures in the near infrared range. The comparison of the original structure and
the model system is done by finitedifference timedomain calculations. The calcula
tions show excellent agreement with the beetles’ scales with respect to the reflectance,
the timeofflight, and the intensity distribution in the farfield.
The present dissertation contains the theoretical studies performed on the topic of a high energy deposition in matter. The work focuses on electronic excitation and relaxation processes on ultrafast timescales. Energy deposition by means of intense ultrashort (femtosecond) laser pulses or by means of swift heavy ions irradiation have a certain similarities: the final observable material modifications result from a number of processes on different timescales. First, the electronic excitation by photoabsorption or by ion impact takes place on subfemtosecond timescales. Then these excited electrons propagate and redistribute their energy interacting among themselves and exciting secondary generations of electrons. This typically takes place on femtosecond timescales. On the order of tens to hundreds femtoseconds the excited electrons are usually thermalized. The energy exchange with the lattice atoms lasts up to tens of picoseconds. The lattice temperature can reach melting point; then the material cools down and recrystalizes, forming the final modified nanostructures, which are observed experimentally. The processes on each previous step form the initial conditions for the following step. Thus, to describe the final phase transition and formation of nanostructures, one has to start from the very beginning and follow through all the steps.
The present work focuses on the early stages of the energy dissipation after its deposition, taking place in the electronic subsystems of excited materials. Different models applicable for different excitation mechanisms will be presented: in the thesis I will start from the description of high energy excitation (electron energies of \(\sim\) keV), then I shall focus on excitations to intermediate energies of electrons (\(\sim\) 100 eV), and finally coming down to a few eV electron excitations (visible light). The results will be compared with experimental observations.
For the high energy material excitation assumed to be caused by irradiation with swift heavy ions, the classical Asymptotical Trajectory MonteCarlo (ATMC) is applied to describe the excitation of electrons by the impact of the projectile, the initial kinetics of electrons, secondary electron creation and Augerredistribution of holes. I first simulate the early stage (first tens of fs) of kinetics of the electronic subsystem (in silica target, SiO\(_2\)) in tracks of ions decelerated in the electronic stopping regime. It will be shown that the well pronounced front of excitation in the electronic and ionic subsystems is formed due to the propagation of electrons, which cannot be described by models based on diffusion mechanisms (e.g. parabolic equations of heat diffusion). On later timescales, the thermalization time of electrons can be estimated as a time when the particle and the energy propagation turns from the ballistic to the diffusive one. As soon as the electrons are thermalized, one can apply the Two Temperature Model. It will be demonstrated how to combine the MC output with the two temperature model. The results of this combination demonstrate that secondary ionizations play a very important role for the track formation process, leading to energy stored in the hole subsystem. This energy storage causes a significant delay of heating and prolongs the timescales of lattice modifications up to tens of picoseconds.
For intermediate energies of excitation (XUVVUV laser pulse excitation of materials) I applied the MonteCarlo simulation, modified where necessary and extended in order to take into account the electronic band structure and Pauli's principle for electrons within the conduction band. I apply the new method for semiconductors and for metals on examples of solid silicon and aluminum, respectively.
It will be demonstrated that for the case of semiconductors the final kinetic energy of free electrons is much less than the total energy provided by the laser pulse, due to the energy spent to overcome ionization potentials. It was found that the final total number of electrons excited by a single photon is significantly less than \(\hbar \omega / E_{gap}\). The concept of an 'effective energy gap' is introduced for collective electronic excitation, which can be applied to estimate the free electron density after highintensity VUV laser pulse irradiation.
For metals, experimentally observed spectra of emitted photons from irradiated aluminum can be explained well with our results. At the characteristic time of a photon emission due to radiative decay of \(L\)shell hole (\(t < 60\) fs), the distribution function of the electrons is not yet fully thermalized. This distribution consists of two main branches: low energy distribution as a distorted Fermidistribution, and a long high energy tail. Therefore, the experimentally observed spectra demonstrate two different branches of results: the one observed with \(L\)shell radiation emission reflects the low energy distribution, the Bremsstrahlung spectra reflects high energy (nonthermalized) tail. The comparison with experiments demonstrated a good agreement of the calculated spectra with the experimentally observed ones.
For the irradiation of semiconductor with low energy photons (visible light), a statistical model named the "extended multiple rate equation" is proposed. Based on the earlier developed multiple rate equation, the model additionally includes the interaction of electrons with the phononic subsystem of the lattice and allows for the direct determination of the conditions for crystal damage. Our model effectively describes the dynamics of the electronic subsystem, dynamical changes in the optical properties, and lattice heating, and the results are in very good agreement with experimental measurements on the transient reflectivity and the fluence damage threshold of silicon irradiated with a femtosecond laser pulse.
We report on the observation of quantized surface spin waves in periodic arrays of magnetic Ni81Fe19 wires by means of Brillouin light scattering spectroscopy. At small wavevectors (q_1 = 0  0.9*100000 cm^1 ) several discrete, dispersionless modes with a frequency splitting of up to 0.9 GHz were observed for the wavevector oriented perpendicular to the wires. From the frequencies of the modes and the wavevector interval, where each mode is observed, the modes are identified as dipoleexchange surface spin wave modes of the film with quantized wavevector values determined by the boundary conditions at the lateral edges of the wires. With increasing wavevector the separation of the modes becomes smaller, and the frequencies of the discrete modes converge to the dispersion of the dipoleexchange surface mode of a continuous film.
We report on Brillouin light scattering investigations of the elastic properties in Co/Ni superlattices which exhibit localized electronic eigenstates near the Fermi level causing an oscillation of the resistivity as a function of the superlattice periodicity A. No oscillations of the Rayleigh and Sezawa mode as a function of A could be observed within an error margin of + 2% indicating that the localized electronic states do not contribute to the elastic constants.
Brillouin light scattering investigations of exchange biased (110)oriented NiFe/FeMn bilayers
(1997)
All contributing magnetic anisotropies in (110)oriented exchange biased Ni 80 Fe 20 /Fe 50 Mn 50 double layers prepared by molecular beam epitaxy on Cu(110) single crystals have been determined by means of Brillouin light scattering. Upon covering the Ni 80 Fe 20 films by Fe 50 Mn 50 , a unidirectional anisotropy contribution appears, which is consistent with the measured exchange bias field. The uniaxial and fourfold inplane anisotropy contributions are largely modified by an amount, which scales with the Ni 80 Fe 20 thickness, indicating an interface effect. The strong uniaxial anisotropy contribution shows an inplane switching of the easy axis from [110] to [001] with increasing Ni 80 Fe 20 layer thickness. The large mode width of the spin wave excitations, which exceeds the linewidth of uncovered Ni 80 Fe 20 films by a factor of more than six, indicates large spatial variations of the exchange coupling constant. (C) 1998 American Institute of Physics.
Static magnetic and spin wave properties of square lattices of permalloy micron dots with thicknesses of 500 Å and 1000 Å and with varying dot separations have been investigated. A magnetic fourfold anisotropy was found for the lattice with dot diameters of 1 micrometer and a dot separation of 0.1 micrometer. The anisotropy is attributed to an anisotropic dipoledipole interaction between magnetically unsaturated parts of the dots. The anisotropy strength (order of 100000 erg/cm^3 ) decreases with increasing inplane applied magnetic field.
It is shown, that recently constructed PST Lagrangians for chiral supergravities follow directly from earlier KavalovMkrtchyan Lagrangians by an Ansatz for the ' tensor by expressing this in terms of the PST scalar. The susy algebra which included earlier ffsymmetry in the commutator of supersymmetry transformations, is now shown to include both PST symmetries, which arise from the single ffsymmetry term. The Lagrangian for the 5brane is not described by this correspondence, and probably can be obtained from more general Lagrangians, posessing ffsymmetry.
The lightcone Hamiltonian approach is applied to the super D2 brane, and the equivalent areapreserving and U(1) gaugeinvariant effective Lagrangian, which is quadratic in the U(1) gauge field, is derived. The latter is recognised to be that of the three dimensional U(1) gauge theory, interacting with matter supermultiplets, in a special external induced supergravity metric and the gravitino field, depending on matter fields. The duality between this theory and 11d supermembrane theory is demonstrated in the lightcone gauge.
Abstract: It has recently been shown that the equation of motion of a massless scalar field in the background of some specific p branes can be reduced to a modified Mathieu equation. In the following the absorption rate of the scalar by a D3 brane in ten dimensions is calculated in terms of modified Mathieu functions of the first kind, using standard Mathieu coefficients. The relation of the latter to Dougall coefficients (used by others) is investigated. The Smatrix obtained in terms of modified Mathieu functions of the first kind is easily evaluated if known rapidly convergent low energy expansions of these in terms of products of Bessel functions are used. Leading order terms, including the interesting logarithmic contributions, can be obtained analytically.
Abstract: The behavior of the divergent part of the bulk AdS/CFT effective action is considered with respect to the special finite diffeomorphism transformations acting on the boundary as a Weyl transformation of the boundary metric. The resulting 1cocycle of the Weyl group is in full agreement with the 1cocycle of the Weyl group obtained from the cohomological consideration of the effective action of the corresponding CFT.
Double Scaling Limits, Airy Functions and Multicritical Behaviour in O(N) Vektor Sigma Models
(1995)
O(N) vector sigma models possessing catastrophes in their action are studied. Coupling the limit N  > infinity with an appropriate scaling behaviour of the coupling constants, the partition function develops a singular factor. This is a generalized Airy function in the case of spacetime dimension zero and the partition function of a scalar field theory for positive spacetime dimension.
For the case of the singleO(N)vector linear sigma models the critical behaviour following from any A_k singularity in the action is worked out in the double scaling limit N>infinity, f_r > f_r^c, 2 <= r <= k. After an exact elimination of Gaussian degrees of freedom, the critical objects such as coupling constants, indices and susceptibility matrix are derived for all A_k and spacetime dimensions 0 <= D <= 4. There appear exceptional spacetime dimensions where the degree k of the singularity A_k is more strongly constrained than by the renormalizability requirement.
Introduction: Recent developments in quantum communication and computing [13] stimulated an intensive search for physical systems that can be used for coherent processing of quantum information. It is generally believed that quantum entanglement of distinguishable quantum bits (qubits) is at the heart of quantum information processing. Significant efforts have been directed towards the design of elementary logic gates, which perform certain unitary processes on pairs of qubits. These gates must be capable of generating specific, in general entangled, superpositions of the two qubits and thus require a strong qubitqubit interaction. Using a sequence of single and twobit operations, an arbitrary quantum computation can be performed [2]. Over the past few years many systems have been identified for potential implementations of logic gates and several interesting experiments have been performed. Proposals for strong qubitqubit interaction involve e.g. the vibrational coupling of cooled trapped ions [4], near dipoledipole or spinspin interactions such as in nuclear magnetic resonance [5], collisional interactions of confined cooled atoms [6] or radiative interactions between atoms in cavity QED [7]. The possibility of simple preparation and measurement of qubit states as well as their relative insensitivity to a thermal environment makes the latter schemes particularly interesting for quantum information processing. Most theoretical proposals on cavityQED systems focus on fundamental systems involving a small number of atoms and few photons. These systems are sufficiently simple to allow for a firstprinciple description. Their experimental implementation is however quite challenging. For example, extremely highQ microcavities are needed to preserve coherence during all atomphoton interactions. Furthermore, single atoms have to be confined inside the cavities for a sufficiently long time. This requires developments of novel cooling and trapping techniques, which is in itself a fascinating direction of current research. Despite these technical obstacles, a remarkable progress has been made in this area: quantum processors consisting of several coupled qubits now appear to be feasible.
Abstract: The effect of intracavity Electromagnetically Induced Transparency on the properties of optical resonators and active laser devices is discussed theoretically. A pronounced frequency pulling and cavity linewidth narrowing are predicted. The effect can be used to substantially reduce classical and quantum phase noise of the beatnote of optical oscillators. Fundamental limits of this stabilization mechanism are discussed as well as its potential application to highresolution spectroscopy.
Abstract: We describe a technique for manipulating quantum information stored in collective states of mesoscopic ensembles. Quantum processing is accomplished by optical excitation into states with strong dipoledipole interactions. The resulting "dipole blockade" can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as nonclassical photonic states and for scalable quantum logic gates. An example involving a cold Rydberg gas is analyzed.
Abstract: We predict the possibility of sharp, highcontrast resonances in the optical response of a broad class of systems, wherein interference effects are generated by coherent perturbation or interaction of dark states. The properties of these resonances can be manipulated to design a desired atomic response.
Abstract: We describe a general technique that allows for an ideal transfer of quantum correlations between light fields and metastable states of matter. The technique is based on trapping quantum states of photons in coherently driven atomic media, in which the group velocity is adiabatically reduced to zero. We discuss possible applications such as quantum state memories, generation of squeezed atomic states, preparation of entangled atomic ensembles and quantum information processing.
Abstract: We investigate the quantum properties of fields generated by resonantly enhanced wave mixing based on atomic coherence in Raman systems. We show that such a process can be used for generation of pairs of Stokes and antiStokes fields with nearly perfect quantum correlations, yielding almost complete (i.e. 100%) squeezing without the use of a cavity. We discuss the extension of the wave mixing interactions into the domain of a few interacting light quanta.
It is shown that nonvacuum pseudoparticles can account for quantum tunneling and metastability. In particular the saddle point nature of the pseudoparticles is demonstrated, and the evaluation of pathintegrals in their neighbourhood. Finally the relation between instantons and bounces is used to derive a result conjectured by Bogomolny and Fateyev.
Significance of zero modes in pathintegral quantization of solitonic theories with BRST invariance
(1996)
The significance of zero modes in the pathintegral quantization of some solitonic models is investigated. In particular a Skyrmelike theory with topological vortices in (1 + 2) dimensions is studied, and with a BRST invariant gauge fixing a well defined transition amplitude is obtained in the one loop approximation. We also present an alternative method which does not necessitate evoking the timedependence in the functional integral, but is equivalent to the original one in dealing with the quantization in the background of the static classical solution of the nonlinear field equations. The considerations given here are particularly useful in  but also limited to the oneloop approximation.
A new look at the RST model
(1996)
The RST model is augmented by the addition of a scalar field and a boundary term so that it is wellposed and local. Expressing the RST action in terms of the ADM formulation, the constraint structure can be analysed completely. It is shown that from the view point of local field theories, there exists a hidden dynamical field 1 in the RST model. Thanks to the presence of this hidden dynamical field, we can reconstruct the closed algebra of the constraints which guarantee the general invariance of the RST action. The resulting stress tensors TSigma Sigma are recovered to be true tensor quantities. Especially, the part of the stress tensors for the hidden dynamical field 1 gives the precise expression for tSigma . At the quantum level, the cancellation condition for the total central charge is reexamined. Finally, with the help of the hidden dynamical field 1, the fact that the semiclassical static soluti on of the RST model has two independent parameters (P,M), whereas for the classical CGHS model there is only one, can be explained.
A new approach with BRST invariance is suggested to cure the degeneracy problem of ill defined path integrals in the path integral calculation of quantum mechanical tunneling effects in which the problem arises due to the occurrence of zero modes. The FaddeevPopov procedure is avoided and the integral over the zero mode is transformed in a systematic way into a well defined integral over instanton positions. No special procedure has to be adopted as in the FaddeevPopov method in calculating the Jacobian of the transformation. The quantum mechanical tunneling for the SineGordon potential is used as a test of the method and the width of the lowest energy band is obtained in exact agreement with that of WKB calculations.
Abstract: It is shown that nonvacuum pseudoparticles can account forquantum tunneling and metastability. In particular the saddlepoint nature of the pseudoparticles is demonstrated, and the evaluation of pathintegrals in their neighbourhood. Finally the relation between instantons and bounces is used to derive a result conjectured by Bogomolny andFateyev.
A formula suitable for a quantitative evaluation of the tunneling effect in a ferromagnetic particle is derived with the help of the instanton method. The tunneling between nth degenerate states of neighboring wells is dominated by a periodic pseudoparticle configuration. The lowlying levelsplitting previously obtained with the LSZ method in field theory in which the tunneling is viewed as the transition of n bosons induced by the usual(vacuum) instanton is recovered.The observation made with our new result is that the tunneling effect increases at excited states. The results should be useful in analyzing results of experimental tests of macroscopic quantum coherence in ferromagnetic particles.