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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.

Wannier-Stark states for semiconductor superlattices in strong static fields, where the interband Landau-Zener tunneling cannot be neglected, are rigorously calculated. The lifetime of these metastable states was found to show multiscale oscillations as a function of the static field, which is explained by an interaction with above-barrier resonances. An equation, expressing the absorption spectrum of semiconductor superlattices in terms of the resonance Wannier-Stark states is obtained and used to calculate the absorption spectrum in the region of high static fields.

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.

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.

An extremely simple and convenient method is presented for computing eigenvalues in quantum mechanics by representing position and momentum operators in a simple matrix form. The simplicity and success of the method is illustrated by numerical results concerning eigenvalues of bound systems and resonances for hermitian and non-hermitian Hamiltonians as well as driven quantum systems.

Chaotic Billiards
(2000)

The frictionless motion of a particle on a plane billiard table The frictionless motion of a particle on a plane billiard table bounded by a closed curve provides a very simple example of a conservative classical system with non-trivial, chaotic dynamics. The limiting cases of strictly regular ("integrable") and strictly irregular ("ergodic") systems can be illustrated, as well as the typical case which shows an intricate mixture of regular and irregular behavior. Irregular orbits are characterized by an extremely sensitivity with respect to the initial conditions. Such billiard systems are exemplarily suited for educational purposes as models for simple systems with complicated dynamics as well as for far-reaching fundamental investigations.

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.