KLUEDO RSS FeedNeueste Dokumente / Latest documents
https://kluedo.ub.uni-kl.de/index/index/
Thu, 16 Nov 2000 00:00:00 +0100Thu, 16 Nov 2000 00:00:00 +0100Harmonic oscillator subject to parametric pulses: an amplitude (Milne) oscillator approach
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1149
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 phase-amplitude decomposition of the harmonic-oscillator solutions results in the so-called 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.K.-E. Thylwe; H. J. Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1149Thu, 16 Nov 2000 00:00:00 +0100Wannier-Stark states of a quantum particle in 2D lattices
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1172
A simple method of calculating the Wannier-Stark resonances in 2D lattices is suggested. Using this method we calculate the complex Wannier-Stark spectrum for a non-separable 2D potential realized in optical lattices and analyze its general structure. The dependence of the lifetime of Wannier-Stark states on the direction of the static field (relative to the crystallographic axis of the lattice) is briefly discussed.M. Glück; F. Keck; A. R. Kolovsky; H. J. Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1172Tue, 24 Oct 2000 00:00:00 +0200A quantum cable car for Wannier-Stark ladders
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1170
The paper studies the dynamics of transitions between the levels of a Wannier-Stark 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 Wannier-Stark states. It is shown that the periodic driving creates from a localized Wannier-Stark state an extended Bloch-like 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.M. Glück; A. R. Kolovsky; H.J. Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1170Sat, 30 Sep 2000 00:00:00 +0200Some Remarks on Complex Hamiltonian Systems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1206
The analyticity property of the one-dimensional 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 two-dimensional 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 auto-B"acklund transformation.R. S. Kaushal; H. J. Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1206Sat, 30 Sep 2000 00:00:00 +0200Bloch particle in presence of dc and ac fields: Statistics of the Wigner delay time
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1165
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.Markus Glück; Andrey R. Kolovsky; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1165Sat, 16 Sep 2000 00:00:00 +0200A truncated shift - operator technique for the calculation of resonances in Stark systems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/870
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 shift-operator in momentum space. Numerical results for space periodic and non-periodic systems illustrate the extreme simplicity of the method.Markus Glück; A. R. Kolovsky; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/870Sat, 02 Sep 2000 00:00:00 +0200Semiclassical analysis of tunneling splittings in periodically driven quantum systems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/872
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 non-harmonic oscillator.Hans Jürgen Korsch; Bruno Mirbach; Bernd Schellhaaßarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/872Sat, 02 Sep 2000 00:00:00 +0200Bloch particle in presence of dc and ac fields
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/879
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.Markus Glück; A. R. Kolovsky; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/879Sat, 02 Sep 2000 00:00:00 +0200Chaotic Wannier-Bloch resonance states
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/906
The Wannier-Bloch resonance states are metastable states of a quantum particle in a space-periodic potential plus a homogeneous field. Here we analyze the states of quantum particle in space- and time-periodic 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.+bMarkus Glück; A. R. Kolovsky; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/906Sat, 02 Sep 2000 00:00:00 +0200Stark Resonances in dc fields from short time propagation of the field-free Hamiltonian
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/907
A new method for calculating Stark resonances is presented and applied for illustration to the simple case of a one-particle, one-dimensional model Hamiltonian. The method is applicable for weak and strong dc fields. The only need, also for the case of many particles in multi-dimensional space, are either the short time evolution matrix elements or the eigenvalues and Fourier components of the eigenfunctions of the field-free Hamiltonian.Nimrod Moiseyev; Markus Glück; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/907Sat, 02 Sep 2000 00:00:00 +0200Lifetime statistics for a Bloch particle in ac and dc fields
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/908
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.Markus Glück; A. R. Kolovsky; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/908Sat, 02 Sep 2000 00:00:00 +0200A generalized entropy measuring quantum localization
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/865
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)] phase-space 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.Bruno Mirbach; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/865Wed, 30 Aug 2000 00:00:00 +0200Selective Quasienergies from Short Time Cross-Correlation Probability Amplitudes by the Filter-Diagonalization Method
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/866
The Filter-Diagonalization 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 cross-correlation 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 auto-correlation calculations, the cross-correlation time T (N) is shorter by the factor N , that is T (1) = N T (N).Markus Glück; Hans Jürgen Korsch; Nimrod Moiseyevarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/866Wed, 30 Aug 2000 00:00:00 +0200A quantum/classical entropy concept for measuring phase-space localization
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/867
We present an entropy concept measuring phase-space localization in dynamical systems based on time-averaged phase-space densities. This entropy has a direct classical counterpart; its local scaling with ln _h.Bruno Mirbach; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/867Wed, 30 Aug 2000 00:00:00 +0200Phase space entropies and global quantum phase space organisation: A two-dimensional anharmonic system
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/868
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 two-dimensional time independent systems. Numerical results for the Pullen Edmonds systems demonstrate the properties of the method for systems with mixed chaotic and regular dynamics.Hans Jürgen Korsch; Georg Groh; W. Schweizerarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/868Wed, 30 Aug 2000 00:00:00 +0200The electron as a selfinteracting lightlike pointcharge: Classification of lightlike curves in spacetime under the group of SO(1,3) motions
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/863
Heinz Krügerarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/863Mon, 03 Apr 2000 00:00:00 +0200Resonances from short time complex-scaled cross-correlation probability amplitudes by the Filter-Diagonalization Method
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/873
The Filter-Diagonalization 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 complex-scaled cross-correlation probability amplitudes from short time propagation enables the calculation of broad overlapping resonances, which can not be resolved from the amplitude of a single complex-scaled autocorrelation calculation.Hans Jürgen Korsch; Nimrod Moiseyev; Edvardas Narevicius; Daniel Neuheuserarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/873Mon, 03 Apr 2000 00:00:00 +0200Global and local dynamical invariants and quasienergy states of time-periodic Hamiltonians
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/874
A formalism is developed for calculating the quasienergy states and spectrum for time-periodic quantum systems when a time-periodic dynamical invariant operator with a nondegenerate spectrum is known. The method, which circumvents the integration of the Schr-odinger equation, is applied to an integrable class of systems, where the global invariant operator is constructed. Furthermore, a local integrable approximation for more general non-integrable systems is developed. Numerical results are presented for the doubleresonance model.Bruno Mirbach; Hans Jürgen Korsch; D.B. Monteolivaarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/874Mon, 03 Apr 2000 00:00:00 +0200The 'Ermakov-Lewis' invariants for Coupled Linear Oscillators
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/875
We consider N coupled linear oscillators with time-dependent 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 so-called Ermakov-Lewis 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.Karl-Erik Thylwe; Hans Jürgen Korscharticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/875Mon, 03 Apr 2000 00:00:00 +0200Quantum mechanical and quasi-classical dynamics of coupled quasiparticle-boson systems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/876
Hans Jürgen Korsch; R. Steib; J.L. Schoendorff; P. Reineckerarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/876Mon, 03 Apr 2000 00:00:00 +0200Quantum Chaos
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/877
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 time-dependent Schr-odinger 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 one-dimensional anharmonic oscillator is studied, both classically and quantum mechanically. In addition, spectral properties and chaotic tunneling are addressed.Hans Jürgen Korsch; Henning Wiescherarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/877Mon, 03 Apr 2000 00:00:00 +0200Wall energy and wall thickness of exchange-coupled rare-earth transition-metal triple layer stacks
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/922
The room-temperature wall energy sw 54.0310 23 J/m 2 of an exchange-coupled 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 in-plane easy axis! and compared the results with data obtained from Brillouin light-scattering 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 in-plane 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.D. Raasch; Christoph Mathieuarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/922Mon, 03 Apr 2000 00:00:00 +0200Ferromagnetism above room temperature in Mn-Si-C alloy films
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/923
Mn-Si-C alloy films are prepared by e-beam coevaporation onto a Si substrate held at 600 °C. Ferromagnetism is observed below T = (360 +/- 5) K with SQUID magnetometry and magneto-optical Kerr effect. This is the highest Curie temperature T yet observed for a Mn-based 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 well-defined composition independent of precipitations.M. Gajdzik; C. Sürgers; M. Kelemen; Burkard Hillebrands; H. von Löhneysenarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/923Mon, 03 Apr 2000 00:00:00 +0200Biquadratic interlayer coupling inlayered magnetic systems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/925
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.Serguei Demokritovarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/925Mon, 03 Apr 2000 00:00:00 +0200Brillouin light scattering investigations of structured permalloy films
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/927
The static and spin wave properties of regular square lattices of magnetic dots of 0.5-2 microm dot diameter and 1-4 microm periodicity patterned in permalloy films have been investigated by Brillouin light scattering. The samples have been structured using x-ray 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 in-plane magnetic anisotropy with the easy axis directed along the pattern diagonal is observed, indicating anisotropic coupling between the dots.Burkard Hillebrandsarticlehttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/927Mon, 03 Apr 2000 00:00:00 +0200