9 research outputs found
Non-perturbative electron dynamics in crossed fields
Intense AC electric fields on semiconductor structures have been studied in
photon-assisted tunneling experiments with magnetic field applied either
parallel (B_par) or perpendicular (B_per) to the interfaces. We examine here
the electron dynamics in a double quantum well when intense AC electric fields
F, and tilted magnetic fields are applied simultaneously. The problem is
treated non-perturbatively by a time-dependent Hamiltonian in the effective
mass approximation, and using a Floquet-Fourier formalism. For B_par=0, the
quasi-energy spectra show two types of crossings: those related to different
Landau levels, and those associated to dynamic localization (DL), where the
electron is confined to one of the wells, despite the non-negligible tunneling
between wells. B_par couples parallel and in-plane motions producing
anti-crossings in the spectrum. However, since our approach is
non-perturbative, we are able to explore the entire frequency range. For high
frequencies, we reproduce the well known results of perfect DL given by zeroes
of a Bessel function. We find also that the system exhibits DL at the same
values of the field F, even as B_par non-zero, suggesting a hidden dynamical
symmetry in the system which we identify with different parity operations. The
return times for the electron at various values of field exhibit interesting
and complex behavior which is also studied in detail. We find that smaller
frequencies shifts the DL points to lower field F, and more importantly, yields
poorer localization by the field. We analyze the explicit time evolution of the
system, monitoring the elapsed time to return to a given well for each Landau
level, and find non-monotonic behavior for decreasing frequencies.Comment: REVTEX4 + 11 eps figs, submitted to Phys. Rev.
Conventionalvs. Gauge-invariant quantum transition probabilities in simple systems. Morse oscillator
Global and local dynamical invariants and quasienergy states of time-periodic Hamiltonians
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