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 Fachbereich Physik (59) (remove)
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.
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.
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.
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.
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.
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.
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.
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.
Quantum Chaos
(1999)
The study of dynamical quantum systems, which are classically chaotic, and the search for quantum manifestations of classical chaos, require large scale numerical computations. Special numerical techniques developed and applied in such studies are discussed: The numerical solution of the timedependent Schrodinger equation, the construction of quantum phase space densities, quantum dynamics in phase space, the use of phase space entropies for characterizing localization phenomena, etc. As an illustration, the dynamics of a driven onedimensional anharmonic oscillator is studied, both classically and quantum mechanically. In addition, spectral properties and chaotic tunneling are addressed.
The quasienergy spectrum of a periodically driven quantum system is constructed from classical dynamics by means of the semiclassical initial value representation using coherent states. For the first time, this method is applied to explicitly time dependent systems. For an anharmonic oscillator system with mixed chaotic and regular classical dynamics, the entire quantum spectrum (both regular and chaotic states) is reproduced semiclassically with surprising accuracy. In particular, the method is capable to account for the very small tunneling splittings.
The FilterDiagonalization Method is used to ,nd the broad and even overlapping resonances of a 1D Hamiltonian used before as a test model for new resonance theories and computational methods. It is found that the use of several complexscaled crosscorrelation probability amplitudes from short time propagation enables the calculation of broad overlapping resonances, which can not be resolved from the amplitude of a single complexscaled autocorrelation calculation.
For periodically driven systems, quantum tunneling between classical resonant stability islands in phase space separated by invariant KAM curves or chaotic regions manifests itself by oscillatory motion of wave packets centered on such an island, by multiplet splittings of the quasienergy spectrum, and by phase space localisation of the quasienergy states on symmetry related ,ux tubes. Qualitatively di,erent types of classical resonant island formation  due to discrete symmetries of the system  and their quantum implications are analysed by a (uniform) semiclassical theory. The results are illustrated by a numerical study of a driven nonharmonic oscillator.
The global dynamical properties of a quantum system can be conveniently visualized in phase space by means of a quantum phase space entropy in analogy to a Poincare section in classical dynamics for twodimensional time independent systems. Numerical results for the Pullen Edmonds systems demonstrate the properties of the method for systems with mixed chaotic and regular dynamics.
Relating mathematical concepts to graphical representations is a challenging task for students. In this paper, we introduce two visual strategies to qualitatively interpret the divergence of graphical vector field representations. One strategy is based on the graphical interpretation of partial derivatives, while the other is based on the flux concept. We test the effectiveness of both strategies in an instructionbased eyetracking study with N = 41 physics majors. We found that students’ performance improved when both strategies were introduced (74% correct) instead of only one strategy (64% correct), and students performed best when they were free to choose between the two strategies (88% correct). This finding supports the idea of introducing multiple representations of a physical concept to foster student understanding.Relevant eyetracking measures demonstrate that both strategies imply different visual processing of the vector field plots, therefore reflecting conceptual differences between the strategies. Advanced analysis methods further reveal significant differences in eye movements between the best and worst performing students. For instance, the best students performed predominantly horizontal and vertical saccades, indicating correct interpretation of partial derivatives. They also focused on smaller regions when they balanced positive and negative flux. This mixed method research leads to new insights into student visual processing of vector field representations, highlights the advantages and limitations of eyetracking methodologies in this context, and discusses implications for teaching and for future research. The introduction of saccadic direction analysis expands traditional methods, and shows the potential to discover new insights into student understanding and learning difficulties.
The coordination of multiple external representations is important for learning, but yet a difficult task for students, requiring instructional support. The subject in this study covers a typical relation in physics between abstract mathematical equations (definitions of divergence and curl) and a visual representation (vector field plot). To support the connection across both representations, two instructions with written explanations, equations, and visual representations (differing only in the presence of visual cues) were designed and their impact on students’ performance was tested. We captured students’ eye movements while they processed the written instruction and solved subsequent coordination tasks. The results show that students instructed with visual cues (VC students) performed better, responded with higher confidence, experienced less mental effort, and rated the instructional quality better than students instructed without cues. Advanced eyetracking data analysis methods reveal that cognitive integration processes appear in both groups at the same point in time but they are significantly more pronounced for VC students, reflecting a greater attempt to construct a coherent mental representation during the learning process. Furthermore, visual cues increase the fixation count and total fixation duration on relevant information. During problem solving, the saccadic eye movement pattern of VC students is similar to experts in this domain. The outcomes imply that visual cues can be beneficial in coordination tasks, even for students with high domain knowledge. The study strongly confirms an important multimedia design principle in instruction, that is, that highlighting conceptually relevant information shifts attention to relevant information and thus promotes learning and problem solving. Even more, visual cues can positively influence students’ perception of course materials.
The analyticity property of the onedimensional complex Hamiltonian system H(x,p)=H_1(x_1,x_2,p_1,p_2)+iH_2(x_1,x_2,p_1,p_2) with p=p_1+ix_2, x=x_1+ip_2 is exploited to obtain a new class of the corresponding twodimensional integrable Hamiltonian systems where H_1 acts as a new Hamiltonian and H_2 is a second integral of motion. Also a possible connection between H_1 and H_2 is sought in terms of an autoB"acklund transformation.
We report the design, fabrication and experimental investigation of a spectrally wideband terahertz spatial light modulator (THzSLM) based on an array of 768 actuatable mirrors with each having a length of 220 μm and a width of 100 μm. A mirror length of several hundred micrometers is required to reduce diffraction from individual mirrors at terahertz frequencies and to increase the pixeltopixel modulation contrast of the THzSLM. By means of spatially selective actuation, we used the mirror array as reconfigurable grating to spatially modulate terahertz waves in a frequency range from 0.97 THz to 2.28 THz. Over the entire frequency band, the modulation contrast was higher than 50% with a peak modulation contrast of 87% at 1.38 THz. For spatial light modulation, almost arbitrary spatial pixel sizes can be realized by grouping of mirrors that are collectively switched as a pixel. For fabrication of the actuatable mirrors, we exploited the intrinsic residual stress in chromecopperchrome multilayers that forces the mirrors into an upstanding position at an inclination angle of 35°. By applying a bias voltage of 37 V, the mirrors were pulled down to the substrate. By hysteretic switching, we were able to spatially modulate terahertz radiation at arbitrary pixel modulation patterns.
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. The spin wave frequencies can be well described taking into account the demagnetization factor of each single dot. 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.
The static and spin wave properties of regular square lattices of magnetic dots of 0.52 microm dot diameter and 14 microm periodicity patterned in permalloy films have been investigated by Brillouin light scattering. The samples have been structured using xray lithography and ion beam etching. The Brillouin light scattering spectra reveal both surface and bulk spin wave modes. The spin wave frequencies can be well described taking into account the demagnetization factor of each single dot. For the samples with smallest dot separation of 0.1 microm a fourfold inplane magnetic anisotropy with the easy axis directed along the pattern diagonal is observed, indicating anisotropic coupling between the dots.
A computer control for a Sandercocktype multipath tandem FabryPerot interferometer is described, which offers many advantages over conventionally used analog control: The range of stability is increased due to active control of the laser light intensity and the mirror dither amplitude. The alignment is fully automated enabling start of a measurement within a minute after start of align, including optionally finding the optimum focus on the sample. The software control enables a programmable series of measurements with control of, e.g., the position and rotation of the sample, the angle of light incidence, the sample temperature, or the strength and direction of an applied magnetic field. Builtin fitting routines allow for a precise determination of frequency positions of excitation peaks combined with increased frequency accuracy due to a correction of a residual nonlinearity of the mirror stage drive.
We investigate the temperature dependence of the magnetization reversal process and of spinwaves in epitaxially grown (001)oriented [Fem/Aun]30 multilayers (m = 1, 2; n = 1 6). Both polar magnetooptic Kerrr effect and Brillouin light scattering measurements reveal that all investigated multilayers, apart from the [Fe2/Au1]30sample, are magnetized perpendicular to the film plane. The outofplane anisotropy constants are obtained. At high temperature, the magnetization curves are well described by an alternating stripe domain structure with free mobile domain walls, and at low temperature by a thermal activation model for the domain wall motion.
Static and dynamic properties of patterned magnetic permalloy films are investigated. In square lattices of circular shaped permalloy dots an anisotropic coupling mechanism has been found, which is identified as being due to intrinsically unsaturated parts of the dots caused by spatial variations of demagnetizing field. In arrays of magnetic wires a quantization of the surface spin wave mode in several dispersionless modes is observed and quantitatively described. For large wavevectors the frequency separation between the modes becomes smaller and the frequencies converge to the dispersion of the dipoleexchange surface mode of a continuous film.
Passive graduated filters with fixed absorption profile are currently used in image recording to avoid overexposure. However, a whole set of filters with prescribed gradients is required to cope with changing illumination conditions. Furthermore, they demand mechanical adjustment during operation. To overcome these deficiencies we present a microfabricated active electrochromic graduated filter which combines multiple functionalities: The overall absorbance, the position of medium transmission as well as the magnitude of its gradient can be tuned continuously by electrical means. Live image control is possible using low operation voltages in the range of ±2 V to reach a high change in optical density ΔOD of 1.01 (400 nm to 780 nm) with a coloration and bleaching time 1.3 s and 0.2 s, respectively. Owing to their low volume and power consumption they are suitable for widespread applications like in smartphones, surveillance cameras or microscopes.
The dispersions of dipolar (DamonEshbach modes) and exchange dominated spin waves are calculated for inplane magnetized thin and ultrathin cubic films with (111) crystal orientation and the results are compared with those obtained for the other principal planes. The properties of these magnetic excitations are examined from the point of view of Brillouin light scattering experiments. Attention is paid to study the spinwave frequency variation as a function of the magnetization direction in the film plane for different film thicknesses. Interface anisotropies and the bulk magnetocrystalline anisotropy are considered in the calculation. A quantitative comparison between an analytical expression obtained in the limit of small film thickness and wave vector and the full numerical calculation is given.
The paper studies the effect of a weak periodic driving on metastable WannierStark states. The decay rate of the ground WannierStark states as a continuous function of the driving frequency is calculated numerically. The theoretical results are compared with experimental data of Wilkinson et at. [Phys.Rev.Lett.76, 4512 (1996)] obtained for cold sodium atoms in an accelerated optical lattice.
We study the transitions between the ground and excited Wannier states induced by a weak ac field. Because the upper Wannier states are several order of magnitude less stable than the ground states, these transitions decrease the global stability of the system characterized by the rate of probability leakage or decay rate. Using nonhermitian resonant perturbation theory we obtain an analytical expression for this induced decay rate. The analytical results are compared with exact numerical calculations of the system decay rate.
The FilterDiagonalization Method is applied to time periodic Hamiltonians and used to find selectively the regular and chaotic quasienergies of a driven 2D rotor. The use of N crosscorrelation probability amplitudes enables a selective calculation of the quasienergies from short time propagation to the time T (N). Compared to the propagation time T (1) which is required for resolving the quasienergy spectrum with the same accuracy from autocorrelation calculations, the crosscorrelation time T (N) is shorter by the factor N , that is T (1) = N T (N).
The paper studies quantum states of a Bloch particle in presence of external ac and dc fields. Provided the period of the ac field and the Bloch period are commensurate, an effective scattering matrix is introduced, the complex poles of which are the system quasienergy spectrum. The statistics of the resonance width and the Wigner delay time shows a close relation of the problem to random matrix theory of chaotic scattering.
The statistics of the resonance widths and the behavior of the survival probability is studied in a particular model of quantum chaotic scattering (a particle in a periodic potential subject to static and timeperiodic forces) introduced earlier in Ref. [5,6]. The coarsegrained distribution of the resonance widths is shown to be in good agreement with the prediction of Random Matrix Theory (RMT). The behavior of the survival probability shows, however, some deviation from RMT.
The quasienergy spectrum of a Bloch electron affected by dcac fields is known to have a fractal structure as function of the socalled electric matching ratio, which is the ratio of the ac field frequency and the Bloch frequency. This paper studies a manifestation of the fractal nature of the spectrum in the system "atom in a standing laser wave", which is a quantum optical realization of a Bloch electron. It is shown that for an appropriate choice of the system parameters the atomic survival probability (a quantity measured in laboratory experiments) also develops a fractal structure as a function of the electric matching ratio. Numerical simulations under classically chaotic scattering conditions show good agreement with theoretical predictions based on random matrix theory.
A novel method is presented which allows a fast computation of complex energy resonance states in Stark systems, i.e. systems in a homogeneous field. The technique is based on the truncation of a shiftoperator in momentum space. Numerical results for space periodic and nonperiodic systems illustrate the extreme simplicity of the method.
The paper studies metastable states of a Bloch electron in the presence of external ac and dc fields. Provided resonance condition between period of the driving frequency and the Bloch period, the complex quasienergies are numerically calculated for two qualitatively different regimes (quasiregular and chaotic) of the system dynamics. For the chaotic regime an effect of quantum stabilization, which suppresses the classical decay mechanism, is found. This effect is demonstrated to be a kind of quantum interference phenomenon sensitive to the resonance condition.
The WannierBloch resonance states are metastable states of a quantum particle in a spaceperiodic potential plus a homogeneous field. Here we analyze the states of quantum particle in space and timeperiodic potential. In this case the dynamics of the classical counterpart of the quantum system is either quasiregular or chaotic depending on the driving frequency. It is shown that both the quasiregular and the chaotic motion can also support quantum resonances. The relevance of the obtained result to the problem a of crystal electron under simultaneous influence of d.c. and a.c. electric fields is briefly discussed. PACS: 73.20Dx, 73.40Gk, 05.45.+b
We study the statistics of the Wigner delay time and resonance width for a Bloch particle in ac and dc fields in the regime of quantum chaos. It is shown that after appropriate rescaling the distributions of these quantities have universal character predicted by the random matrix theory of chaotic scattering.
The paper studies the dynamics of transitions between the levels of a WannierStark ladder induced by a resonant periodic driving. The analysis of the problem is done in terms of resonance quasienergy states, which take into account the metastable character of the WannierStark states. It is shown that the periodic driving creates from a localized WannierStark state an extended Blochlike state with a spatial length varying in time as ~ t^1/2. Such a state can find applications in the field of atomic optics because it generates a coherent pulsed atomic beam.
A simple method of calculating the WannierStark resonances in 2D lattices is suggested. Using this method we calculate the complex WannierStark spectrum for a nonseparable 2D potential realized in optical lattices and analyze its general structure. The dependence of the lifetime of WannierStark states on the direction of the static field (relative to the crystallographic axis of the lattice) is briefly discussed.
MnSiC alloy films are prepared by ebeam coevaporation onto a Si substrate held at 600 °C. Ferromagnetism is observed below T = (360 +/ 5) K with SQUID magnetometry and magnetooptical Kerr effect. This is the highest Curie temperature T yet observed for a Mnbased alloy. Although the composition determined by Auger depth profiling varies appreciably for different films, their T is the same indicating that ferromagnetism is caused by an alloy of welldefined composition independent of precipitations.
III/V semiconductor quantum dots (QD) are in the focus of optoelectronics research for about 25 years now. Most of the work
has been done on InAs QD on GaAs substrate. But, e.g., Ga(As)Sb (antimonide) QD on GaAs substrate/buffer have also gained
attention for the last 12 years.There is a scientific dispute on whether there is a wetting layer before antimonide QD formation, as
commonly expected for StranskyKrastanov growth, or not. Usually ex situ photoluminescence (PL) and atomic force microscope
(AFM) measurements are performed to resolve similar issues. In this contribution, we show that reflectance anisotropy/difference
spectroscopy (RAS/RDS) can be used for the same purpose as an in situ, realtime monitoring technique. It can be employed not
only to identify QD growth via a distinct RAS spectrum, but also to get information on the existence of a wetting layer and its
thickness. The data suggest that for antimonide QD growth the wetting layer has a thickness of 1 ML (one monolayer) only.
Previously in this journal we have reported on fundamental transversemode selection (TMS#0) of broad area semiconductor lasers
(BALs) with integrated twiceretracted 4f setup and filmwaveguide lens as the Fouriertransform element. Now we choose and
report on a simpler approach for BALTMS#0, i.e., the use of a stable confocal longitudinal BAL resonator of length L with a
transverse constriction.The absolute value of the radius R of curvature of both mirrorfacets convex in one dimension (1D) is R = L
= 2f with focal length f.The round trip length 2L = 4f againmakes up for a Fourieroptical 4f setup and the constriction resulting
in a resonatorinternal beam waist stands for a Fourieroptical lowpass spatial frequency filter. Good TMS#0 is achieved, as long
as the constriction is tight enough, but filamentation is not completely suppressed.
1. Introduction
Broad area (semiconductor diode) lasers (BALs) are intended
to emit high optical output powers (where “high” is relative
and depending on the material system). As compared to
conventional narrow stripe lasers, the higher power is distributed
over a larger transverse crosssection, thus avoiding
catastrophic optical mirror damage (COMD). Typical BALs
have emitter widths of around 100 ????m.
Thedrawback is the distribution of the high output power
over a large number of transverse modes (in cases without
countermeasures) limiting the portion of the light power in
the fundamental transverse mode (mode #0), which ought to
be maximized for the sake of good light focusability.
Thus techniques have to be used to support, prefer, or
select the fundamental transverse mode (transverse mode
selection TMS#0) by suppression of higher order modes
already upon buildup of the laser oscillation.
In many cases reported in the literature, either a BAL
facet, the
A measurement technique, i.e. reflectance anisotropy/difference spectroscopy (RAS/RDS), which had originally been developed for insitu
epitaxial growth control, is employed here for insitu realtime etchdepth control during reactive ion etching (RIE) of cubic crystalline III/V
semiconductor samples. Temporal optical FabryPerot oscillations of the genuine RAS signal (or of the average reflectivity) during etching due
to the ever shrinking layer thicknesses are used to monitor the current etch depth. This way the achievable insitu etchdepth resolution has
been around 15 nm. To improve etchdepth control even further, i.e. down to below 5 nm, we now use the optical equivalent of a mechanical
vernier scale– by employing FabryPerot oscillations at two different wavelengths or photon energies of the RAS measurement light – 5%
apart, which gives a vernier scale resolution of 5%. For the AlGaAs(Sb) material system a 5 nm resolution is an improvement by a factor of 3
and amounts to a precision in insitu etchdepth control of around 8 lattice constants.
An unusual interlayer coupling, recently discovered in layered magnetic systems, is analysed from the experimental and theoretical points of view. This coupling favours the 90° orientation of the magnetization of the adjacent magnetic films. It can be phenomenologically described by a term in the energy expression, which is biquadratic with respect to the magnetizations of the two films. The main experimental findings, as well as the theoretical models, explaining the phenomenon are discussed.
Low damping magnetic properties and perpendicular magnetic anisotropy in the Heusler alloy Fe1.5CoGe
(2019)
We present a study of the dynamic magnetic properties of TiNbuffered epitaxial thin films of the Heusler alloy Fe1.5CoGe. Thickness series annealed at different temperatures are prepared and the magnetic damping is measured, a lowest value of α = 2.18 × 10−3 is obtained. The perpendicular magnetic anisotropy properties in Fe1.5CoGe/MgO are also characterized. The evolution of the interfacial perpendicular anisotropy constant K⊥S with the annealing temperature is shown and compared with the widely used CoFeB/MgO interface. A large volume contribution to the perpendicular anisotropy of (4.3 ± 0.5) × 105 J/m3 is also found, in contrast with vanishing bulk contribution in common Co and Febased Heusler alloys.
The first observation of selffocusing of dipolar spin waves in garnet film media is reported. In particular, we show that the quasistationary diffraction of a finiteaperture spin wave beam in a focusing medium leads to the concentration of the wave power in one focal point rather than along a certain line (channel). The obtained results demonstrate the wide applicability of nonlinear spin wave media to study nonlinear wave phenomena using an advanced combined microwaveBrillouin light scattering technique for a twodimensional mapping of the spin wave amplitudes.
Using molecular dynamics simulation, we study nanoindentation in large samples of Cu–Zr glass at various temperatures between zero and the glass transition temperature. We find that besides the elastic modulus, the yielding point also strongly (by around 50%) decreases with increasing temperature; this behavior is in qualitative agreement with predictions of the cooperative shear model. Sheartransformation zones (STZs) show up in increasing sizes at low temperatures, leading to shearband activity. Cluster analysis of the STZs exhibits a powerlaw behavior in the statistics of STZ sizes. We find strong plastic activity also during the unloading phase; it shows up both in the deactivation of previous plastic zones and the appearance of new zones, leading to the observation of popouts. The statistics of STZs occurring during unloading show that they operate in a similar nature as the STZs found during loading. For both cases, loading and unloading, we find the statistics of STZs to be related to directed percolation. Material hardness shows a weak strainrate dependence, confirming previously reported experimental findings; the number of popins is reduced at slower indentation rate. Analysis of the dependence of our simulation results on the quench rate applied during preparation of the glass shows only a minor effect on the properties of STZs.