Quasi-Ballistic Electron Transport in Random Superlattices

We theoretically study electron transport in disordered, quantum-well based, semiconductor superlattices with structural short-range correlations. Our system consists of equal width square barriers and quantum wells with two different thicknesses. The two kinds of quantum wells are randomly distributed along the growth direction. Structural correlations are introduced by adding the constraint that one of the wells always appears in pairs. We show that such correlated disordered superlattices exhibit a strong enhancement of their dc conductance as compared to usual random ones, giving rise to quasi-ballistic electron transport. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder. We specifically describe the way superlattices should be built and experiments should be carried out for that purpose.Comment: REVTeX 3.0, 7 pages, 4 figures on request from FD-A ([email protected]). Submitted to Physical Review B. Preprint MA/UC3M/12/199

Absence of localization and large dc conductance in random superlattices with correlated disorder

We study how the influence of structural correlations in disordered systems manifests itself in experimentally measurable magnitudes, focusing on dc conductance of semiconductor superlattices with general potential profiles. We show that the existence of bands of extended states in these structures gives rise to very noticeable peaks in the finite temperature dc conductance as the chemical potential is moved through the bands or as the temperature is increased from zero. On the basis of these results we discuss how dc conductance measurements can provide information on the location and width of the bands of extended states. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder.Comment: REVTeX 3.0, 14 pages, 11 figures available on request from ED ([email protected]). Submitted to Phys Rev B. MA/UC3M/06/9

Excitation decay in one-dimensional disordered systems with paired traps

Incoherent transport of excitations in one-dimensional disordered lattices with pairs of traps placed at random is studied by numerically solving the corresponding master equation. Results are compared to the case of lattices with the same concentration of unpaired traps, and it is found that pairing of traps causes a slowdown of the decay rate of both the mean square displacement and the survival probability of excitations. We suggest that this result is due to the presence of larger trap-free segments in the lattices with paired disorder, which implies that pairing of traps causes less disruption on the dynamics of excitations. In the conclusion we discuss the implications of our work, placing it in a more general context.Comment: REVTeX 3.0, 10 pages, 7 figures available on request from FD-A ([email protected]), Universidad Carlos III preprint MA/UC3M/08/9

Dynamic transition in Landau-Zener-St\"uckelberg interferometry of dissipative systems: the case of the flux qubit

We study Landau-Zener-Stuckelberg (LZS) interferometry in multilevel systems coupled to an Ohmic quantum bath. We consider the case of superconducting flux qubits driven by a dc+ac magnetic fields, but our results can apply to other similar systems. We find a dynamic transition manifested by a symmetry change in the structure of the LZS interference pattern, plotted as a function of ac amplitude and dc detuning. The dynamic transition is from a LZS pattern with nearly symmetric multiphoton resonances to antisymmetric multiphoton resonances at long times (above the relaxation time). We also show that the presence of a resonant mode in the quantum bath can impede the dynamic transition when the resonant frequency is of the order of the qubit gap. Our results are obtained by a numerical calculation of the finite time and the asymptotic stationary population of the qubit states, using the Floquet-Markov approach to solve a realistic model of the flux qubit considering up to 10 energy levels.Comment: One new figure added. Final version to be published in PR

Incoherent Exciton Trapping in Self-Similar Aperiodic Lattices

Incoherent exciton dynamics in one-dimensional perfect lattices with traps at sites arranged according to aperiodic deterministic sequences is studied. We focus our attention on Thue-Morse and Fibonacci systems as canonical examples of self-similar aperiodic systems. Excitons progressively extend over the lattice on increasing time and, in this sense, they act as a probe of the particular arrangements of traps in each system considered. The analysis of the characteristic features of their time decay indicates that exciton dynamics in self-similar aperiodic arrangements of traps is quite close to that observed in periodic ones, but differs significatively from that corresponding to random lattices. We also report on characteristic features of exciton motion suggesting that Fibonacci and Thue-Morse orderings might be clearly observed by appropriate experimental measurements. In the conclusions we comment on the implications of our work on the way towards a unified theory of the orderings of matter.Comment: REVTeX 3.0, 10 pages, 2 figures on request from FD-A ([email protected]). Submitted to Phys Rev B. MA/UC3M/11/9