Low-Temperature Spin Diffusion in a Spin-Polarized Fermi Gas

We present a finite temperature calculation of the transverse spin-diffusion coefficient, $D_\bot$, in a dilute degenerate Fermi gas in the presence of a small external magnetic field, $H$. While the longitudinal diffusion coefficient displays the conventional low-temperature Fermi-liquid behavior, $D_\parallel \propto T^{-2}$, the corresponding results for $D_\bot$ show three separate regimes: (a) $D_\bot \sim H^{-2}$ for $T \ll H$; (b) $D_\bot \sim T^{-2}$, $D_\bot /D_\parallel \neq 1$ for $T \gg H$ and large spin-rotation parameter $\xi \gg 1$, and (c) $D_\bot = D_\parallel \propto T^{-2}$ for $T \gg H$ and $\xi \ll 1$. Our results are qualitatively consistent with the available experimental data in weakly spin-polarized $^3{\rm He}$ and $^3{\rm He} - ^4{\rm He}$ mixtures.Comment: 13 pages, REVTEX, 3 figures available upon request, RU-94-4

Leptonic Photons and Nucleosynthesis

Should $U(1)$ long-range forces be associated to electron, muon and/or tau quantum number then their ''fine structure constants" are seen to be bound by nucleosynthesis data to be less than about $1.7 \times 10^{-11}$. For $\tau$ and $\mu$ this is the best upper limit up to dateComment: 4 pages, LaTeX. Full postscript file available via anonymous ftp at ftp://ftp.ifae.es/preprint/ft/uabft402.p

Exciton effective mass enhancement in coupled quantum wells in electric and magnetic fields

We present a calculation of exciton states in semiconductor coupled quantum wells (CQWs) in the presence of electric and magnetic fields applied perpendicular to the QW plane. The exciton Schr\"odinger equation is solved in real space in three dimensions to obtain the Landau levels of both direct and indirect excitons. Calculation of the exciton energy levels and oscillator strengths enables mapping of the electric and magnetic field dependence of the exciton absorption spectrum. For the ground state of the system, we evaluate the Bohr radius, optical lifetime, binding energy and dipole moment. The exciton mass renormalization due to the magnetic field is calculated using a perturbative approach. We predict a non-monotonous dependence of the exciton ground state effective mass on magnetic field. Such a trend is explained in a classical picture, in terms of the ground state tending from an indirect to a direct exciton with increasing magnetic field.Comment: 20 pages, 7 figure