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Search Results: 1 - 10 of 117776 matches for " T. Schanz "
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Signature of Chaotic Diffusion in Band Spectra
T. Dittrich,B. Mehlig,H. Schanz,U. Smilansky
Physics , 1997, DOI: 10.1103/PhysRevE.57.359
Abstract: We investigate the two-point correlations in the band spectra of spatially periodic systems that exhibit chaotic diffusion in the classical limit. By including level pairs pertaining to non-identical quasimomenta, we define form factors with the winding number as a spatial argument. For times smaller than the Heisenberg time, they are related to the full space-time dependence of the classical diffusion propagator. They approach constant asymptotes via a regime, reflecting quantal ballistic motion, where they decay by a factor proportional to the number of unit cells. We derive a universal scaling function for the long-time behaviour. Our results are substantiated by a numerical study of the kicked rotor on a torus and a quasi-one-dimensional billiard chain.
Universal spectral properties of spatially periodic quantum systems with chaotic classical dynamics
T. Dittrich,B. Mehlig,H. Schanz,U. Smilansky
Physics , 1997, DOI: 10.1016/S0960-0779(97)00016-7
Abstract: We consider a quasi one-dimensional chain of N chaotic scattering elements with periodic boundary conditions. The classical dynamics of this system is dominated by diffusion. The quantum theory, on the other hand, depends crucially on whether the chain is disordered or invariant under lattice translations. In the disordered case, the spectrum is dominated by Anderson localization whereas in the periodic case, the spectrum is arranged in bands. We investigate the special features in the spectral statistics for a periodic chain. For finite N, we define spectral form factors involving correlations both for identical and non-identical Bloch numbers. The short-time regime is treated within the semiclassical approximation, where the spectral form factor can be expressed in terms of a coarse-grained classical propagator which obeys a diffusion equation with periodic boundary conditions. In the long-time regime, the form factor decays algebraically towards an asymptotic constant. In the limit $N\to\infty$, we derive a universal scaling function for the form factor. The theory is supported by numerical results for quasi one-dimensional periodic chains of coupled Sinai billiards.
Reaction matrix for Dirichlet billiards with attached waveguides
Holger Schanz
Physics , 2002, DOI: 10.1016/S1386-9477(03)00147-4
Abstract: The reaction matrix of a cavity with attached waveguides connects scattering properties to properties of a corresponding closed billiard for which the waveguides are cut off by straight walls. On the one hand this matrix is directly related to the S-matrix, on the other hand it can be expressed by a spectral sum over all eigenfunctions of the closed system. However, in the physically relevant situation where these eigenfunctions vanish on the impenetrable boundaries of the closed billiard, the spectral sum for the reaction matrix, as it was used before, fails to converge and does not reliably reproduce the scattering properties. We derive here a convergent representation of the reaction matrix in terms of eigenmodes satisfying Dirichlet boundary conditions and demonstrate its validity in the rectangular and the Sinai billiards.
Phase-space correlations of chaotic eigenstates
Holger Schanz
Physics , 2004, DOI: 10.1103/PhysRevLett.94.134101
Abstract: It is shown that the Husimi representations of chaotic eigenstates are strongly correlated along classical trajectories. These correlations extend across the whole system size and, unlike the corresponding eigenfunction correlations in configuration space, they persist in the semiclassical limit. A quantitative theory is developed on the basis of Gaussian wavepacket dynamics and random-matrix arguments. The role of symmetries is discussed for the example of time-reversal invariance.
Spectral correlations in systems undergoing a transition from periodicity to disorder
T. Dittrich,B. Mehlig,H. Schanz,Uzy Smilansky,Peter Pollner,Gabor Vattay
Physics , 1998, DOI: 10.1103/PhysRevE.59.6541
Abstract: We study the spectral statistics for extended yet finite quasi 1-d systems which undergo a transition from periodicity to disorder. In particular we compute the spectral two-point form factor, and the resulting expression depends on the degree of disorder. It interpolates smoothly between the two extreme limits -- the approach to Poissonian statistics in the (weakly) disordered case, and the universal expressions derived for the periodic case. The theoretical results agree very well with the spectral statistics obtained numerically for chains of chaotic billiards and graphs.
Does race really matter? Career goals, perceptions of criminal justice practitioners and competence among criminal justice undergraduates
Youngyol Y. Schanz
Journal of Arts and Humanities , 2013,
Abstract: This study examines whether race or ethnicity affects student decision-making pertaining to career goals and choices. The career goals and choices of undergraduates in criminal justice (CJ) were surveyed in early spring of 2006. The research also investigates students’ perceptions of their own perceived competence as future CJ practitioners. Data were collected from CJ undergraduates from an urban university in the upper Midwestern part of the U.S. The results indicate that racial or ethnic minority students have significantly different career goals and choices than those of non-minority students. Some future research implications and policy implications are discussed.
Quantization of Sinai's Billiard - A Scattering Approach
Holger Schanz,Uzy Smilansky
Physics , 1995, DOI: 10.1016/0960-0779(94)E0066-X
Abstract: We obtained the spectrum of the Sinai billiard as the zeroes of a secular equation, which is based on the scattering matrix of a related scattering problem. We show that this quantization method provides an efficient numerical scheme, and its implementation for the present case gives a few thousands of levels without encountering any serious difficulty. We use the numerical data to check some approximations which are essential for the derivation of a semiclassical quantization method based also on this scattering approach.
Excitonic - vibronic coupled dimers: A dynamic approach
Bernd Esser,Holger Schanz
Physics , 1995, DOI: 10.1007/BF01313855
Abstract: The dynamical properties of exciton transfer coupled to polarization vibrations in a two site system are investigated in detail. A fixed point analysis of the full system of Bloch - oscillator equations representing the coupled excitonic - vibronic flow is performed. For overcritical polarization a bifurcation converting the stable bonding ground state to a hyperbolic unstable state which is basic to the dynamical properties of the model is obtained. The phase space of the system is generally of a mixed type: Above bifurcation chaos develops starting from the region of the hyperbolic state and spreading with increasing energy over the Bloch sphere leaving only islands of regular dynamics. The behaviour of the polarization oscillator accordingly changes from regular to chaotic.
Nonadiabatic couplings and incipience of quantum chaos
Holger Schanz,Bernd Esser
Physics , 1996, DOI: 10.1007/s002570050212
Abstract: The quantization of the electronic two site system interacting with a vibration is considered by using as the integrable reference system the decoupled oscillators resulting from the adiabatic approximation. A specific Bloch projection method is applied which demonstrates how besides some regular regions in the fine structure of the spectrum and the associated eigenvectors irregularities appear when passing from the low to the high coupling case. At the same time even for strong coupling some of the regular structure of the spectrum rooted in the adiabatic potentials is kept intact justifying the classification of this situation as incipience of quantum chaos.
Spectral Statistics for Quantum Graphs: Periodic Orbits and Combinatorics
Holger Schanz,Uzy Smilansky
Physics , 1999, DOI: 10.1080/13642810010000635
Abstract: We consider the Schroedinger operator on graphs and study the spectral statistics of a unitary operator which represents the quantum evolution, or a quantum map on the graph. This operator is the quantum analogue of the classical evolution operator of the corresponding classical dynamics on the same graph. We derive a trace formula, which expresses the spectral density of the quantum operator in terms of periodic orbits on the graph, and show that one can reduce the computation of the two-point spectral correlation function to a well defined combinatorial problem. We illustrate this approach by considering an ensemble of simple graphs. We prove by a direct computation that the two-point correlation function coincides with the CUE expression for 2x2 matrices. We derive the same result using the periodic orbit approach in its combinatorial guise. This involves the use of advanced combinatorial techniques which we explain.
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