Production of q bar-q Pairs in Proton-Nucleus Collisions at High Energies

We calculate production of quark-antiquark pairs in high energy proton-nucleus collisions both in the quasi-classical approximation of McLerran-Venugopalan model and including quantum small-$x$ evolution. The resulting production cross section is explicitly expressed in terms of Glauber-Mueller multiple rescatterings in the classical case and in terms of dipole-nucleus scattering amplitude in the quantum evolution case. We generalize the result of one of us (K.T.) beyond the aligned jet configurations. We expand on the earlier results of Blaizot, Gelis and Venugopalan by deriving quark production cross section including quantum evolution corrections in rapidity intervals both between the quarks and the target and between the quarks and the projectile.Comment: 18 pages, 3 figures; typos corrected, discussion extende

Coherent spin dynamics in quantum wells in quantizing magnetic field

We investigate theoretically the coherent longitudinal and transversal spin relaxation of photoexcited electrons in quantum wells in quantized magnetic fields. We find the relaxation time for typical quantum well parameters between 100 and 1000 ps. For a realistic random potential the relaxation process depends on the electron energy and g-factor, demonstrating oscillations in the spin polarization accompanying the spin relaxation. The dependence of spin relaxation on applied field, and thus on the corresponding "magnetic" length, can be used to characterize the spatial scale of disorder in quantum wells.Comment: 13 pages, 4 figure

Anomalous ordering in inhomogeneously strained materials

We study a continuous quasi-two-dimensional order-disorder phase transition that occurs in a simple model of a material that is inhomogeneously strained due to the presence of dislocation lines. Performing Monte Carlo simulations of different system sizes and using finite size scaling, we measure critical exponents describing the transition of beta=0.18\pm0.02, gamma=1.0\pm0.1, and alpha=0.10\pm0.02. Comparable exponents have been reported in a variety of physical systems. These systems undergo a range of different types of phase transitions, including structural transitions, exciton percolation, and magnetic ordering. In particular, similar exponents have been found to describe the development of magnetic order at the onset of the pseudogap transition in high-temperature superconductors. Their common universal critical exponents suggest that the essential physics of the transition in all of these physical systems is the same as in our model. We argue that the nature of the transition in our model is related to surface transitions, although our model has no free surface.Comment: 5 pages, 3 figure

Four-Particle Anyon Exciton: Boson Approximation

A theory of anyon excitons consisting of a valence hole and three quasielectrons with electric charges $(-e/3)$ is presented. A full symmetry classification of the $k=0$ states is given, where $k$ is the exciton momentum. The energy levels of these states are expressed by quadratures of confluent hypergeometric functions. It is shown that the angular momentum $L$ of the exciton ground state depends on the distance between electron and hole confinement planes and takes the values $L=3n$, where $n$ is an integer. With increasing $k$ the electron density shows a spectacular splitting on bundles. At first a single anyon splits off of the two-anyon core, and finally all anyons become separated.Comment: Revtex 13 pages + 6 uuencoded postscript figure

Improved Theory of the Muonium Hyperfine Structure

Terms contributing to the hyperfine structure of the muonium ground state at the level of few tenths of kHz have been evaluated. The $\alpha^2 (Z\alpha)$ radiative correction has been calculated numerically to the precision of 0.02 kHz. Leading $\ln (Z\alpha )$ terms of order $\alpha^{4-n} (Z\alpha)^n , n=1,2,3,$ and some relativistic corrections have been evaluated analytically. The theoretical uncertainty is now reduced to 0.17 kHz. At present, however, it is not possible to test QED to this precision because of the 1.34 kHz uncertainty due to the muon mass.Comment: 11 pages + 2 figures (included), RevTeX 3.0, CLNS 94/127

Universal Quantum Computation through Control of Spin-Orbit Coupling

We propose a method for quantum computation which uses control of spin-orbit coupling in a linear array of single electron quantum dots. Quantum gates are carried out by pulsing the exchange interaction between neighboring electron spins, including the anisotropic corrections due to spin-orbit coupling. Control over these corrections, even if limited, is sufficient for universal quantum computation over qubits encoded into pairs of electron spins. The number of voltage pulses required to carry out either single qubit rotations or controlled-Not gates scales as the inverse of a dimensionless measure of the degree of control of spin-orbit coupling.Comment: 4 pages, 3 figures (minor revision, references added

Topological Change of the Fermi Surface in Low Density Rashba Gases: Application to Superconductivity

Strong spin-orbit coupling can have a profound effect on the electronic structure in a metal or semiconductor, particularly for low electron concentrations. We show how, for small values of the Fermi energy compared to the spin-orbit splitting of Rashba type, a topological change of the Fermi surface leads to an effective reduction of the dimensionality in the electronic density of states. We investigate its consequences on the onset of the superconducting instability. We show, by solving the Eliashberg equations for the critical temperature as a function of spin-orbit coupling and electron density, that the superconducting critical temperature is significantly tuned in this regime by the spin-orbit coupling. We suggest that materials with strong spin-orbit coupling are good candidates for enhanced superconductivity.Comment: 5 pages, 2 figures ep

"Phase Diagram" of the Spin Hall Effect

We obtain analytic formulas for the frequency-dependent spin-Hall conductivity of a two-dimensional electron gas (2DEG) in the presence of impurities, linear spin-orbit Rashba interaction, and external magnetic field perpendicular to the 2DEG. We show how different mechanisms (skew-scattering, side-jump, and spin precession) can be brought in or out of focus by changing controllable parameters such as frequency, magnetic field, and temperature. We find, in particular, that the d.c. spin Hall conductivity vanishes in the absence of a magnetic field, while a magnetic field restores the skew-scattering and side-jump contributions proportionally to the ratio of magnetic and Rashba fields.Comment: Some typos correcte