Modern Physics Letters B

Correspondence in quantum chaotic systems is lost in short time scales. Introducing some noise we study the spectrum of the resulting coarse-grained propagator of density matrices. Some different methods to compute the spectrum are reviewed. The relationship between the eigenvalues of the coarse-grained superoperator and the classical Ruelle–Pollicott resonances is remarked on. As a consequence, classical decay rates in quantum time-dependent quantities appear.

It is shown that the method of intermediate problems, which provides improvable convergent lower bounds for eigenvalues of linear half-bound Hermitian operators in Hilbert space, can be linked in a natural way to the stochastic variational method, thus resulting in an effective tool for analytical and numerical investigation of the energy spectrum and eigenstates of various few-body quantum systems, among them quantum dots with relatively large numbers of electrons.

In this paper we calculate the loss of fidelity due to quantum leakage for the Josephson charge qubit (JCQ). In this investigation the characteristic values and characteristic states of the Mathieu equation are used. It is shown that for present typical parameters of JCQ, E_J/E_ch ~ 0.02, the loss of the fidelity per elementary operation is about 10^-4 which satisfies DiVincenzo's low decoherence criterion. By appropriately improving the designs of the Josephson junction, namely, decreasing E_J/E_ch to ~ 0.01, the loss of fidelity per elementary operation can decrease even smaller to 10^-5. The first-order nonadiabatic correction is also obtained by using the approach.

We revisit the problem of quantum diffusion of a particle moving on a lattice with dynamical disorder. Decoherence, essential for the diffusive motion, is introduced via a set of Lindblad operators, known to guarantee per se the positivity, Hermiticity and the trace-class nature of the reduced density matrix, are derived and solved analytically for several transport quantities of interest. For the special Hermitian choice of the Lindblad operators projected onto the lattice sites, we recover several known results, obtained by others, e.g. through the stochastic Liouville equation using phenomenological damping terms for the off-diagonal density-matrix elements. An interesting result that we obtained is for the case of a 1D lattice with static potential bias and a time-harmonic modulation (ac drive) of its transition-matrix element, where the diffusion coefficient shows an oscillatory behavior as function of the drive amplitude and frequency — clearly, a Wannier–Stark ladder signature. The question of dissipation is also briefly discussed.

Existing proposals for the crystal structure of the high temperature superconducting Bi₂Sr₂Ca₂Cu₃O_7-x (Bi2223) phase have been reviewed. For the purpose of automated orientation determination in the scanning electron microscope using electron backscatter pattern (EBSP) analysis, a new crystal structure is proposed, where the unit cell is assumed to be tetragonal with P4/mmm point-group symmetry. Simulated EBSP images corresponding to the various unit cells exhibit significant differences, confirming the importance of choosing the correct unit-cell information. The suitability of various choices of the unit cell for automated orientation determination using EBSP analysis has been investigated using experimental data. The newly proposed unit cell is superior to the other unit cells for each of the criteria used to assess the indexing ability.

The magnetic ion Gd⁺³, having L = 0 and J ≠ 0, is unsplit by crystal fields and, unlike the other trivalent L ≠ 0 rare-earth ions (which are crystal-field split), is a pair-breaker in high-temperature superconductors. Consequently two-layer compounds with Gd (i.e., Gd_2-zCe_zCuO₄ and Ba₂GdRu_1-uCu_uO₆) do not superconduct, but their sister compounds without unsplit and pair-breaking Gd, do superconduct (e.g., Nd_2-zCe_zCuO₄, with crystal-field split Nd, and Sr₂YRu_1-uCu_uO₆, with L = 0 Y, both superconduct). The superconductivity clearly originates in the oxygen of the SrO or BaO layers, or in interstitial oxygen, not in the CuO₂ planes.