Magneto-optical evidence of the percolation nature of the metal-insulator transition in the 2D electron system

We compare the results of the transport and time-resolved magneto-luminescence measurements in disordered 2D electron systems in GaAs-AlGaAs heterostructures in the extreme quantum limit, in particular, in the vicinity of the metal-insulator transition (MIT). At filling factors $\nu <1$, the optical signal has two components: the single-rate exponentially decaying part attributed to a uniform liquid and a power-law long-living tail specific to a microscopically inhomogeneous state of electrons. We interprete this result as a separation of the 2D electron system into a liquid and localized phases, especially because the MIT occurs strikingly close to those filling factors where the liquid occupies ${1\over 2}$ of the sample area (the percollation threshold condition in two-component media).Comment: 5 pages RevTex + 4 fig., to appear in PRB, Rapid Com

A preliminary comparison of cranial and external morphological variability of Southern Crested Newt (Triturus karelinii) from the Crimea and the Caucasus

The Triturus karelinii group of crested newts comprises the eastern, central and western lineages that differ by mitochondrial DNA. The eastern lineage largely retains the ancestral cranium shape and is distributed throughout the Crimean Peninsula, the Caucasus Isthmus, and the Caspian Sea southern shore in Iran. In this report the comparison of the Crimean and Trascaucasian T. karelinii cranium shape, differences in the dimensional characteristics variability and body colouring patterns is presented. The samples of T. dobrogicus from the Danube River Delta have been employed for outgroup comparison. Data of the differences between several populations within the eastern lineage suggest that it is not uniform and T. karelinii from the Crimean Peninsula is considered to be somewhat different.Група Triturus karelinii з числа гребінчастих тритонів включає східну, центральну і західну гілку, які відрізняються за мітохондріальною ДНК. Східна гілка значною мірою зберігає анцестральну форму черепної коробки і поширюється всім Кримським півостровом, Кавказьким перешийком і південним узбережжям Каспійського моря в Ірані. Проведено порівняння кримської і закавказької черепної форми Т. karelinii, вказано на відмінності мінливості розмірних характеристик і забарвлення тіла. Екземпляри Т. dobrogicus з дельти Дунаю були використані для порівняння. Дані про відмінності між декількома популяціями в межах східної гілки припускають, що вони не є однорідним, і Т. karelinii з Кримського півострова дещо відрізняється.Группа Triturus karelinii гребенчатых тритонов включает восточную, центральную и западную ветвь, которые отличаются митохондриальной ДНК. Восточная ветвь в значительной мере сохраняет анцестральную форму черепной коробки и распространена по всему Крымскому полуострову, Кавказскому перешейку и южному побережью Каспийского моря в Иране. Проведено сравнение крымской и закавказской черепной формы Т. karelinii, указано на отличия изменчивости размерных характеристик и окраски тела. Экземпляры Т. dobrogicus из дельты Дуная были использованы для сравнения. Данные об отличиях между несколькими популяциями в пределах восточной ветви допускают, что они неоднородны, и Т. karelinii с Крымского полуострова несколько отличаются

Effect of in-plane magnetic field on the photoluminescence spectrum of modulation-doped quantum wells and heterojunctions

The photoluminescence (PL) spectrum of modulation-doped GaAs/AlGaAs quantum wells (MDQW) and heterojunctions (HJ) is studied under a magnetic field ($B_{\|}$) applied parallel to the two-dimensional electron gas (2DEG) layer. The effect of $B_{\|}$ strongly depends on the electron-hole separation ($d_{eh}$), and we revealed remarkable $B_{\|}$-induced modifications of the PL spectra in both types of heterostructures. A model considering the direct optical transitions between the conduction and valence subband that are shifted in k-space under $B_{\|}$, accounts qualitatively for the observed spectral modifications. In the HJs, the PL intensity of the bulk excitons is strongly reduced relatively to that of the 2DEG with increasing $B_{\|}$. This means that the distance between the photoholes and the 2DEG decreases with increased $B_{\|}$, and that free holes are responsible for the hole-2DEG PL.Comment: 6pages, 5figure

Electrically-Controlled Nuclear Spin Polarization and Relaxation by Quantum-Hall states

We investigate interactions between electrons and nuclear spins by using the resistance (Rxx) peak which develops near filling factor n = 2/3 as a probe. By temporarily tuning n to a different value, ntemp, with a gate, the Rxx peak is shown to relax quickly on both sides of ntemp = 1. This is due to enhanced nuclear spin relaxation by Skyrmions, and demonstrates the dominant role of nuclear spin in the transport anomaly near n = 2/3. We also observe an additional enhancement in the nuclear spin relaxation around n = 1/2 and 3/2, which suggests a Fermi sea of partially-polarized composite fermions.Comment: 6 pages, 3 figure

Harmonic Solid Theory of Photoluminescence in the High Field Two-Dimensional Wigner Crystal

Motivated by recent experiments on radiative recombination of two-dimensional electrons in acceptor doped GaAs-AlGaAs heterojunctions as well as the success of a harmonic solid model in describing tunneling between two-dimensional electron systems, we calculate within the harmonic approximation and the time dependent perturbation theory the line shape of the photoluminescence spectrum corresponding to the recombination of an electron with a hole bound to an acceptor atom. The recombination process is modeled as a sudden perturbation of the Hamiltonian for the in-plane degrees of freedom of the electron. We include in the perturbation, in addition to changes in the equilibrium positions of electrons, changes in the curvatures of the harmonically approximated potential. The computed spectra have line shapes similar to that seen in a recent experiment. The spectral width, however, is roughly a factor of 3 smaller than that seen in experiment if one assumes a perfect Wigner crystal for the initial state state of the system, whereas a simple random disorder model yields a width a factor of 3 too large. We speculate on the possible mechanisms that may lead to better quantitative agreement with experiment.Comment: 22 pages, RevTex, 8 figures. Submitted to the Physical Review

Exchange interaction effects in inter-Landau level Auger scattering in a two-dimensional electron gas

We consider the influence of spin effects on the inter-Landau level electron-electron scattering rate in a two-dimensional electron gas. Due to the exchange spin splitting, the Landau levels are not equidistant. This leads to the suppresion of Auger processes and a nonlinear dependence of the lifetime on the concentration of the excited electrons even at very low excitation levels.Comment: 10 pages, 3 figure

Temperature dependence of spin polarizations at higher Landau Levels

We report our results on temperature dependence of spin polarizations at $\nu=1$ in the lowest as well as in the next higher Landau level that compare well with recent experimental results. At $\nu=3$, except having a much smaller magnitude the behavior of spin polarization is not much influenced by higher Landau levels. In sharp contrast, for filling factor $\nu=\frac83$ we predict that unlike the case of $\nu=\frac23$ the system remains fully spin polarized even at vanishingly small Zeeman energies.Comment: 4 pages, REVTEX, and 3 .ps files, To be published in Physical Review Letter

Goldstone Mode Relaxation in a Quantum Hall Ferromagnet due to Hyperfine Interaction with Nuclei

Spin relaxation in quantum Hall ferromagnet regimes is studied. As the initial non-equilibrium state, a coherent deviation of the spin system from the ${\vec B}$ direction is considered and the breakdown of this Goldstone-mode state due to hyperfine coupling to nuclei is analyzed. The relaxation occurring non-exponentially with time is studied in terms of annihilation processes in the "Goldstone condensate" formed by "zero spin excitons". The relaxation rate is calculated analytically even if the initial deviation is not small. This relaxation channel competes with the relaxation mechanisms due to spin-orbit coupling, and at strong magnetic fields it becomes dominating.Comment: 8 page