Spinning Holes in Semiconductors

The electron spin is emerging as a new powerful tool in the electronics and optics industries. Many proposed applications involve the creation of spin currents, which so far have proven to be difficult to produce in semiconductor environments. A new theoretical analysis shows this might be achieved using holes rather than electrons in semiconductors with significant spin-orbit coupling.Comment: 5 pages, 1 figure. A Perspective on "Dissipationless Quantum Spin Current at Room Temperature" by Shuichi Murakami, Naoto Nagaosa, and Shou-Cheng Zhang, Science 301, 1348 (2003

Electromagnetic Response of a Pinned Wigner Crystal

A microscopic model for analyzing the microwave absorption properties of a pinned, two-dimensional Wigner crystal in a strong perpendicular magnetic field is developed. The method focuses on excitations within the lowest Landau level, and corresponds to a quantum version of the harmonic approximation. For pure systems (no disorder), the method reproduces known results for the collective mode density of states of this system, and clearly identifies the origin of previously unexplained structure in this quantity. The application of the method to a simple diagonal disorder model uncovers a surprising result: a sharp (delta-function) response at zero temperature that is consistent with recent experiments. A simple spin lattice model is developed that reproduces the results of the quantum harmonic approximation, and shows that the sharp response is possible because the size scale $L_c$ of patches moving together in the lowest frequency collective mode is extremely large compared to the sample size for physically relevant parameters. This result is found to be a direct repercussion of the long-range nature of the Coulomb interaction. Finally, the model is used to analyze different disorder potentials that may pin the Wigner crystal, and it is argued that interface disorder is likely to represent the dominant pinning source for the system. A simple model of the interface is shown to reproduce some of the experimental trends for the magnetic field dependence of the pinning resonance.Comment: Version to appear in Phys. Rev. B. Improved interface pinning model. Many typos corrected. One figure deleted, one replace

Landau Level Mixing and Skyrmion Stability in Quantum Hall Ferromagnets

We present a Hartree-Fock study that incorporates the effects of Landau level mixing and screening due to filled levels into the computation of energies and states of quasiparticles in quantum Hall ferromagnets. We use it to construct a phase diagram for skyrmion stability as a function of magnetic field and Zeeman coupling strengths. We find that Landau level mixing tends to favor spin-polarized quasiparticles, while finite thickness corrections favor skyrmions. Our studies show that skyrmion stability in high Landau levels is very sensitive to the way in which electron-electron interactions are modified by finite thickness, and indicate that it is crucial to use models with realistic short distance behavior to get qualitatively correct results. We find that recent experimental evidence for skyrmions in higher Landau levels cannot be explained within our model

MagnetoResistance of graphene-based spin valves

We study the magnetoresistance of spin-valve devices using graphene as a non-magnetic material to connect ferromagnetic leads. As a preliminary step we first study the conductivity of a graphene strip connected to metallic contacts for a variety of lead parameters, and demonstrate that the resulting conductivity is rather insensitive to them. We then compute the conductivity of the spin-valve device in the parallel and antiparallel spin polarization configurations, and find that it depends only weakly on the relative spin orientations of the leads, so that the magnetoresistance $MR$ of the system is very small. The smallness of $MR$ is a consequence of the near independence of the graphene conductivity from the electronic details of the leads. Our results indicate that, although graphene has properties that make it attractive for spintronic devices, the performance of an graphene-based spin-valve is likely to be poor.Comment: 10 page

Coherence Network in the Quantum Hall Bilayer

Recent experiments on quantum Hall bilayers near total filling factor 1 have demonstrated that they support an ``imperfect'' two-dimensional superfluidity, in which there is nearly dissipationless transport at non-vanishing temperature observed both in counterflow resistance and interlayer tunneling. We argue that this behavior may be understood in terms of a {\it coherence network} induced in the bilayer by disorder, in which an incompressible, coherent state exists in narrow regions separating puddles of dense vortex-antivortex pairs. A renormalization group analysis shows that it is appropriate to describe the system as a vortex liquid. We demonstrate that the dynamics of the nodes of the network leads to a power law temperature dependence of the tunneling resistance, whereas thermally activated hops of vortices across the links control the counterflow resistance.Comment: Published versio

Correlation functions for the XYXY model in a Magnetic Field

Recent studies of the two-dimensional, classical $XY$ magnet in a magnetic field suggest that it has three distinct vortex phases: a linearly confined phase, a logarithmically confined phase, and a free vortex phase. In this work we study spin-spin correlation functions in this model by analytical analysis and numerical simulations to search for signatures of the various phases. In all three phases, the order parameter is nonzero and $<\cos(\th({\bf r}_1))\cos(\th({\bf r}_2))>$ remains nonzero for $r \equiv |{\bf r}_1-{\bf r}_2|\rightarrow \infty$, indicating the expected long range order. The correlation function for transverse fluctuations of the spins, $C(r)=$, falls exponentially in all three phases. A renormalization group analysis suggests that the logarithmically confined phase should have a spatially anisotropic correlation length. In addition, there is a generic anisotropy in the prefactor which is always present. We find that this prefactor anisotropy becomes rather strong in the presence of a magnetic field, masking the effects of any anisotropy in the correlation length in the simulations.Comment: 17 pages, 5 figure

Superfluidity without Symmetry-Breaking: The Time-Dependent Hartree-Fock Approximation for Bose-Condensed Systems

We develop a time-dependent Hartree-Fock approximation that is appropriate for Bose-condensed systems. Defining a {\it depletion Green's function} allows the construction of condensate and depletion particle densities from eigenstates of a single time-dependent Hamiltonian, guaranteeing that our approach is a conserving approximation. The poles of this Green's function yield the energies of number-changing excitations for which the condensate particle number is held fixed, which we show has a gapped spectrum in the superfluid state. The linearized time-dependent version of this has poles at the collective frequencies of the system, yielding the expected zero sound mode for a uniform infinite system. We show how the approximations may be expressed in a general linear response formalism.Comment: 2 figure. Submitted to PR

Elementary Electronic Excitations in Graphene Nanoribbons

We analyze the collective mode spectrum of graphene nanoribbons within the random phase approximation. In the undoped case, only metallic armchair nanoribbons support a propagating plasmon mode. Landau damping of this mode is shown to be suppressed through the chirality of the single particle wavefunctions. We argue that undoped zigzag nanoribbons should not support plasmon excitations because of a broad continuum of particle-hole excitations associated with surface states, into which collective modes may decay. Doped nanoribbons have properties similar to those of semiconductor nanowires, including a plasmon mode dispersing as $q\sqrt{-\ln qW}$ and a static dielectric response that is divergent at $q=2k_F$.Comment: 7 pages. 5 figures include

Vortices and Dissipation in a Bilayer Thin Film Superconductor

Vortex dynamics in a bilayer thin film superconductor are studied through a Josephson-coupled double layer XY model. A renormalization group analysis shows that there are three possible states associated with the relative phase of the layers: a free vortex phase, a logarithmically confined vortex-antivortex pair phase, and a linearly confined phase. The phases may be distinguished by measuring the resistance to counterflow current. For a geometry in which current is injected and removed from the two layers at the same edge by an ideal (dissipationless) lead, we argue that the three phases yield distinct behaviors: metallic conductivity in the free vortex phase, a power law I-V in the logarithmically confined phase, and true dissipationless superconductivity in the linearly confined phase. Numerical simulations of a resistively shunted Josephson junction model reveal size dependences for the resistance of this system that support these expectations.Comment: 17 pages, 2 figure

Gapped phase in AA stacked bilayer graphene

AA-stacked bilayer graphene supports Fermi circles in its bonding and antibonding bands which coincide exactly, leading to symmetry-breaking in the presence of electron-electron interactions. We analyze a continuum model of this system in the Hartree-Fock approximation, using a self-consistently screened interaction that accounts for the gap in the spectrum in the broken symmetry state. The order parameter in the groundstate is shown to be of the Ising type, involving transfer of charge between the layers in opposite directions for different sublattices. We analyze the Ising phase transition for the system, and argue that it continuously evolves into a Kosterlitz-Thouless transition in the limit of vanishing interlayer separation $d$. The transition temperature is shown to depend only on the effective spin stiffness of the system even for $d>0$, and an estimate its value suggests the transition temperature is of order a few degrees Kelvin