Theory of Electron Spin Relaxation in n-Doped Quantum Wells

Recent experiments have demonstrated long spin lifetimes in uniformly n-doped quantum wells. The spin dynamics of exciton, localized, and conduction spins are important for understanding these systems. We explain experimental behavior by invoking spin exchange between all spin species. By doing so we explain quantitatively and qualitatively the striking and unusual temperature dependence in (110)-GaAs quantum wells. We discuss possible future experiments to resolve the pertinent localized spin relaxation mechanisms. In addition, our analysis allows us to propose possible experimental scenarios that will optimize spin relaxation times in GaAs and CdTe quantum wells.Comment: Small corrections made. Accepted to Phys. Rev. B. 8 pages, 5 figure

Broken rotation symmetry in the fractional quantum Hall system

We demonstrate that the two-dimensonal electron system in a strong perpendicular magnetic field has stable states which break rotational but not translational symmetry. The Laughlin fluid becomes unstable to these states in quantum wells whose thickness exceeds a critical value which depends on the electron density. The order parameter at 1/3 reduced density resembles that of a nematic liquid crystal, in that a residual two-fold rotation axis is present in the low symmetry phase. At filling factors 1/5 and 1/7, there are states with four- and six-fold axes, as well. We discuss the experimental detection of these phases.Comment: 8 pages, LaTex 3.1, figures attache

Theory of Neutron Diffraction from the Vortex Lattice in UPt3

Neutron scattering experiments have recently been performed in the superconducting state of UPt3 to determine the structure of the vortex lattice. The data show anomalous field dependence of the aspect ratio of the unit cell in the B phase. There is apparently also a change in the effective coherence length on the transition from the B to the C phases. Such observations are not consistent with conventional superconductvity. A theory of these results is constructed based on a picture of two-component superconductivity for UPt3. In this way, these unusual observations can be understood. There is a possible discrepancy between theory and experiment in the detailed field dependence of the aspect ratio.Comment: 11 pages; uses REVTEX, APS and PRABIB styles; 2 Postscript figure files include

Phase diagram of UPt3_3 in the E1gE_{1g} model

The phase diagram of the unconventional superconductor UPt$_3$ is explained under the long-standing hypothesis that the pair wavefunction belongs to the $E_{1g}$ representation of the point group. The main objection to this theory has been that it disagrees with the experimental phase diagram when a field is applied along the c-axis. By a careful analysis of the free energy this objection is shown to be incorrect. This singlet theory also explains the unusual anisotropy in the upper critical field curves, often thought to indicate a triplet pair function.Comment: 11 pages, Revtex, 2 figures (uuencoded, gzip'ed Postscript

c-axis Josephson Tunneling in Twinned YBCO Crystals

Josephson tunneling between YBCO and Pb with the current flowing along the c-axis of the YBCO is persumed to come from an s-wave component of the superconductivity of the YBCO. Experiments on multi-twin samples are not entirely consistent with this hypothesis. The sign change of the s-wave order parameter across the N_T twin boundaries should give cancelations, resulting in a small $(\sqrt{N})$ tunneling current. The actual current is larger than this. We present a theory of this unexpectedly large current based upon a surface effect: disorder-induced supression of the d-wave component at the (001) surface leads to s-wave coherence across the twin boundaries and a non-random tunneling current. We solve the case of an ordered array of d+s and d-s twins, and estimate that the twin size at which s-wave surface coherence occurs is consistent with typical sizes observed in experiments. In this picture, there is a phase difference of $\pi/2$ between different surfaces of the material. We propose a corner junction experiment to test this picture.Comment: 5 pages, 4 eps figure

Electronic inhomogeneity at magnetic domain walls in strongly-correlated systems

We show that nano-scale variations of the order parameter in strongly-correlated systems can induce local spatial regions such as domain walls that exhibit electronic properties representative of a different, but nearby, part of the phase diagram. This is done by means of a Landau-Ginzburg analysis of a metallic ferromagnetic system near an antiferromagnetic phase boundary. The strong spin gradients at a wall between domains of different spin orientation drive the formation of a new type of domain wall, where the central core is an insulating antiferromagnet, and connects two metallic ferromagnetic domains. We calculate the charge transport properties of this wall, and find that its resistance is large enough to account for recent experimental results in colossal magnetoresistance materials. The technological implications of this finding for switchable magnetic media are discussed.Comment: Version submitted to Physical Review Letters, except for minor revisions to reference

Magnetism of Superconducting UPt3

The phase diagram of superconducting $U\!Pt_{3}$ in pressure-temperature plane, together with the neutron scattering data is studied within a two component superconducting order parameter scenario. In order to give a qualitative explanation to the experimental data a set of two linearly independent antiferromagnetic moments which emerge appropriately at the temperature \mbox{$T_{N}\sim 10\cdot T_{c}$} and \mbox{$T_{m}\sim T_{c}$} and couple to superconductivity is proposed. Several constraints on the fourth order coefficients in the Ginzburg-Landau free energy are obtained.Comment: 17 pages, figures available on request to [email protected]