Low-temperature spin relaxation in n-type GaAs

Low-temperature electron spin relaxation is studied by the optical orientation method in bulk n-GaAs with donor concentrations from 10^14 cm^{-3} to 5x10^17 cm^{-3}. A peculiarity related to the metal-to-insulator transition (MIT) is observed in the dependence of the spin lifetime on doping near n_D = 2x10^16 cm^{-3}. In the metallic phase, spin relaxation is governed by the Dyakonov-Perel mechanism, while in the insulator phase it is due to anisotropic exchange interaction and hyperfine interactio

Chaos and the Quantum Phase Transition in the Dicke Model

We investigate the quantum chaotic properties of the Dicke Hamiltonian; a quantum-optical model which describes a single-mode bosonic field interacting with an ensemble of $N$ two-level atoms. This model exhibits a zero-temperature quantum phase transition in the N \go \infty limit, which we describe exactly in an effective Hamiltonian approach. We then numerically investigate the system at finite $N$ and, by analysing the level statistics, we demonstrate that the system undergoes a transition from quasi-integrability to quantum chaotic, and that this transition is caused by the precursors of the quantum phase-transition. Our considerations of the wavefunction indicate that this is connected with a delocalisation of the system and the emergence of macroscopic coherence. We also derive a semi-classical Dicke model, which exhibits analogues of all the important features of the quantum model, such as the phase transition and the concurrent onset of chaos.Comment: 51 pages, 15 figures, late

Retinal nerve fibre layer thickness analysis in X-linked retinoschisis using Fourier-domain

PURPOSE: To evaluate the presence of retinal nerve fibre layer (RNFL) defects in patients with X-linked retinoschisis (XLRS) using high-speed, high-resolution, Fourier domain OCT (FD-OCT). METHODS: Twenty-four patients with XLRS seen by the authors were enrolled in the study. All patients underwent a complete eye examination. FD-OCT was performed using Optovue technology. A quadrant of the RNFL was considered to be thinned if at least two of the four segments in the quadrant were reduced in thickness. RESULTS: The average age of the 24 patients in the study was 28.8+/-14.7 years. Thinning of the RNFL in one quadrant was seen in 10 patients (41.7%), and thinning in two or more quadrants was seen in 8 patients (33.3%). Thinning in the inferior quadrant was most commonly seen and was observed in 12 patients (50%), followed by the temporal quadrant in 8 patients (33.3%), nasal quadrant in 4 patients (16.7%), and the superior quadrant in 4 patients (16.7%). CONCLUSIONS: Among our 24 patients with XLRS, 15 patients (62.5%) showed a thinning of the RNFL in one or more quadrants in at least one eye and 9 patients (37.5%) in both eyes. High-speed, high-resolution FD-OCT may be useful to determine the presence of possible changes in RNFL thickness in patients with XLRS. Reductions in RNFL thickness in such patients could be relevant in their selection for future therapeutic trials

Investigation of Oxidation Kinetics in Nonstoichiometric Ni-Zn Ferrites

The distinctive features of kinetics and oxidation mechanisms in the ferrites Ni2+(0.5862)Zn2+(0.0891)Co2+(0.019)Fe2+(0.2057)Fe3+(2)O2-(4) are investigated by methods of thermogravimetry, differential thermal analysis, calorimetry, radiography etc. The new data indicative of the influence of the morphological defect structure peculiarities on the oxidation reaction of nonstoichiometric nickel-zinc ferrites are obtained