Static and dynamic properties of crystalline phases of two-dimensional electrons in a strong magnetic field

We study the cohesive energy and elastic properties as well as normal modes of the Wigner and bubble crystals of the two-dimensional electron system (2DES) in higher Landau levels. Using a simple Hartree-Fock approach, we show that the shear moduli ($c_{66}$'s) of these electronic crystals show a non-monotonic behavior as a function of the partial filling factor $\nu^*$ at any given Landau level, with $c_{66}$ increasing for small values of $\nu^*$, before reaching a maximum at some intermediate filling factor $\nu^*_m$, and monotonically decreasing for $\nu^*>\nu^*_m$. We also go beyond previous treatments, and study how the phase diagram and elastic properties of electron solids are changed by the effects of screening by electrons in lower Landau levels, and by a finite thickness of the experimental sample. The implications of these results on microwave resonance experiments are briefly discussed.Comment: Discussion updated - 16 pages, 10 figures; version accepted for publication in Phys. Rev.

Anisotropic states of two-dimensional electrons in high magnetic fields

We study the collective states formed by two-dimensional electrons in Landau levels of index $n\ge 2$ near half-filling. By numerically solving the self-consistent Hartree-Fock (HF) equations for a set of oblique two-dimensional lattices, we find that the stripe state is an anisotropic Wigner crystal (AWC), and determine its precise structure for varying values of the filling factor. Calculating the elastic energy, we find that the shear modulus of the AWC is small but finite (nonzero) within the HF approximation. This implies, in particular, that the long-wavelength magnetophonon mode in the stripe state vanishes like $q^{3/2}$ as in an ordinary Wigner crystal, and not like $q^{5/2}$ as was found in previous studies where the energy of shear deformations was neglected.Comment: minor corrections; 5 pages, 4 figures; version to be published in Physical Review Letter

Control of Risk Factors Among People With Diagnosed Diabetes, by Lower Extremity Disease Status

Introduction We examined the control of modifiable risk factors among a national sample of diabetic people with and without lower extremity disease (LED). Methods The sample from the 1999-2004 National Health and Nutrition Examination Survey consisted of 948 adults aged 40 years or older with diagnosed diabetes and who had been assessed for LED. LED was defined as peripheral arterial disease (ankle-brachial index <0.9), peripheral neuropathy (≥1 insensate area), or presence of foot ulcer. Good control of modifiable risk factors, based on American Diabetes Association recommendations, included being a nonsmoker and having the following measurements: hemoglobin A1c (HbA1c) less than 7%, systolic blood pressure less than or equal to 130 mm Hg, diastolic blood pressure less than or equal to 80 mm Hg, high-density lipoprotein (HDL) cholesterol greater than 50 mg/dL, and body mass index (BMI) between 18.5 kg/m2 and 24.9 kg/m2. Results Diabetic people with LED were less likely than were people without LED to have recommended levels of HbA1c (39.3% vs 53.5%) and HDL cholesterol (29.7% vs 41.1%), but there were no differences in systolic or diastolic blood pressure, BMI classification, or smoking status between people with and without LED. Control of some risk factors differed among population subgroups. Notably, among diabetic people with LED, non-Hispanic blacks were more likely to have improper control of HbA1c (adjusted odds ratio [AOR] = 2.0; 95% confidence interval [CI], 1.1-3.9), systolic blood pressure (AOR = 1.9; 95% CI, 1.1-3.2), and diastolic blood pressure (AOR = 2.6; 95% CI, 1.1-5.8), compared with non-Hispanic whites. Conclusion Control of 2 of 6 modifiable risk factors was worse in diabetic adults with LED compared with diabetic adults without LED. Among diabetic people with LED, non-Hispanic blacks had worse control of 3 of 6 risk factors compared with non-Hispanic whites

Double sign reversal of the vortex Hall effect in YBa2Cu3O7-delta thin films in the strong pinning limit of low magnetic fields

Measurements of the Hall effect and the resistivity in twinned YBa2Cu3O7-delta thin films in magnetic fields B oriented parallel to the crystallographic c-axis and to the twin boundaries reveal a double sign reversal of the Hall coefficient for B below 1 T. In high transport current densities, or with B tilted off the twin boundaries by 5 degrees, the second sign reversal vanishes. The power-law scaling of the Hall conductivity to the longitudinal conductivity in the mixed state is strongly modified in the regime of the second sign reversal. Our observations are interpreted as strong, disorder-type dependent vortex pinning and confirm that the Hall conductivity in high temperature superconductors is not independent of pinning.Comment: 4 pages, 4 figure

Velocity Selection for Propagating Fronts in Superconductors

Using the time-dependent Ginzburg-Landau equations we study the propagation of planar fronts in superconductors, which would appear after a quench to zero applied magnetic field. Our numerical solutions show that the fronts propagate at a unique speed which is controlled by the amount of magnetic flux trapped in the front. For small flux the speed can be determined from the linear marginal stability hypothesis, while for large flux the speed may be calculated using matched asymptotic expansions. At a special point the order parameter and vector potential are dual, leading to an exact solution which is used as the starting point for a perturbative analysis.Comment: 4 pages, 2 figures; submitted to Phys. Rev. Letter

NPHC and MGC Sororities and Fraternities as Spaces of Activism within Predominantly White Institutions

This study explored how Students of Color within National Pan-Hellenic Council and Multicultural Greek Council sororities and fraternities engaged in activism and in what ways this involvement connected to their membership. Using a qualitative critical narrative approach, we examined the journeys of ten participants. Findings unpack ways participants engaged in activism and resistance aimed at educating individuals and increasing awareness of societal injustices, addressing inequities through service, and inciting disruption and cultivating institutional and societal level change

Bound states of edge dislocations: The quantum dipole problem in two dimensions

We investigate bound state solutions of the 2D Schr\"odinger equation with a dipole potential originating from the elastic effects of a single edge dislocation. The knowledge of these states could be useful for understanding a wide variety of physical systems, including superfluid behavior along dislocations in solid $^4$He. We present a review of the results obtained by previous workers together with an improved variational estimate of the ground state energy. We then numerically solve the eigenvalue problem and calculate the energy spectrum. In our dimensionless units, we find a ground state energy of -0.139, which is lower than any previous estimate. We also make successful contact with the behavior of the energy spectrum as derived from semiclassical considerations.Comment: 6 pages, 3 figures, submitted to PR

Longitudinal Current Dissipation in Bose-glass Superconductors

A scaling theory of vortex motion in Bose glass superconductors with currents parallel to the common direction of the magnetic field and columnar defects is presented. Above the Bose-glass transition the longitudinal DC resistivity $\rho_{||}(T)\sim (T-T_{BG})^{\nu' z'}$ vanishes much faster than the corresponding transverse resistivity $\rho_{\perp}(T)\sim (T-T_{BG})^{\nu' (z'-2)}$, thus {\it reversing} the usual anisotropy of electrical transport in the normal state of layered superconductors. In the presence of a current $\bf J$ at an angle $\theta_J$ with the common field and columnar defect axis, the electric field angle $\theta_E$ approaches $\pi/2$ as $T\rightarrow T_{BG}^+$. Scaling also predicts the behavior of penetration depths for the AC currents as $T\rightarrow T_{BG}^-$, and implies a {\it jump discontinuity} at $T_{BG}$ in the superfluid density describing transport parallel to the columns.Comment: 5 pages, revte

Transmit TACAN Bearing Information with a Circular Array

Using TACAN and array fundamentals, we derive an architecture for transmitting TACAN bearing information from a circular array with time-varying weights. We evaluate performance for a simulated example array of Vivaldi elements