Hall Drag Research Articles

Observation of an odd longitudinal magnetoresistance in YBa 2Cu 3O 7− δ single crystals with unidirected twins

Unidirected twin planes are the cause of anisotropic pinning of vortices in YBa 2Cu 3O 7− δ (below YBaCuO) single crystals. In contrast to the case of only isotropic pinning, two new effects may appear due to the pinning anisotropy: guiding of vortices along twin planes and anisotropy of the Hall resistivity. The former leads to the appearance of the even (with respect to the reversal of a magnetic field direction) transverse resistivity and influences the Hall angle, while the last causes new additional odd contribution to the longitudinal resistivity. The paper presents magnetoresistivity measurements in the vortex liquid phase of YBaCuO single crystals with different angles between unidirected twins and current direction in the magnetic fields up to 12 T. Field inversion was used for separation of the even and odd components of the measured transverse and longitudinal voltages in order to select new contributions to the resistivity in the case of anisotropic pinning. The results are consistent with the model of anisotropic pinning where the anisotropy of the Hall drag coefficient is caused by twins.

Anisotropic pinning and the mixed-state galvanothermomagnetic properties of superconductors—a phenomenological approach

In the presence of isotropic and anisotropic pinning the vortex dynamics is discussed in terms of phenomenologically introduced, nonlinear viscosities. The formulas for linear galvanothermomagnetic effects are derived and analyzed under the condition at which the transport current or temperature gradient is directed at arbitrary angle with respect to the unidirected twins, which cause the anisotropic pinning. It is shown that two new effects which appear due to the anisotropic pinning, namely (with respect to the reversal of the magnetic field direction) even transverse and odd longitudinal voltage, have a distinct origin. The first is due to the guided vortex motion, while the second appears only when anisotropic (in contrast with isotropic) pinning changes the Hall drag coefficient. We also show that the last effect might be masked in the experiment by a large, odd contribution, which has the same angular dependence and which appears due to the Ettingshausen effect. In order to clarify the problem of influence of the twins on the Hall drag coefficient, we discuss the possibility of separating these two contributions in the experiment.

Can Hall drag be observed in Coulomb coupled quantum wells in a magnetic field?

We study the transresistivity \U0001d6d221 (or equivalently, the drag rate) of two Coulomb-coupled quantum wells in the presence of a perpendicular magnetic field, using semi-classical transport theory. Elementary arguments seem to preclude any possibility of observation of "Hall drag" (i.e., a nonzero off-diagonal component in \U0001d6d221). We show that these arguments are specious, and in fact Hall drag can be observed at sufficiently high temperatures when the intralayer transport time τ has significant energy-dependence around the Fermi energy εF. The ratio of the Hall to longitudinal transresistivities goes as T2Bs, where T is the temperature, B is the magnetic field, and s = [δτ/δε](εF).

Modified geostrophy in the thermosphere

A number of modeling studies, as well as analyses of satellite data, have shown that the neutral winds in the F region are characterized by an approximate balance between the pressure gradient, the Coriolis and curvature acceleration, and viscous and ion drag in the direction perpendicular to the electric field momentum source. The ion drag in the F region is associated with the Pedersen conductivity terms and is often called the Pedersen drag. The dynamics associated with such a balanced flow are reasonably well understood. In the lower thermosphere, the balance of forces becomes more complicated and leads to interesting new flow configurations as the effect of the Hall drag becomes increasingly important. Specifically, the Hall drag coefficient has the effect of decreasing the magnitude of the effective Coriolis parameter, since the Coriolis parameter always appears in combination with the Hall drag term. We examine the balance of forces in the lower E region across the range of heights where the Hall drag terms dominate at the lower altitudes and the Pedersen drag terms dominate at the higher altitudes. The balanced flow solutions predict distinctly different flows in the regions in which the plasma accelerates the neutrals and in the regions in which the plasma induces a drag. The analogs of the thermal wind equations are also presented. The results show that the flow in the lower E region is enhanced and will be in a direction that is antiparallel to the F region plasma flow direction and that the largest shears should occur on the bottomside of the wind maximum. Large convergent/divergent flows are expected on the equatorward side of the auroral oval, and the maximum convergence/divergence is expected near a height of ∼110–120 km.

Coulomb drag in normal metals and superconductors: Diagrammatic approach.

The diagrammatic linear response formalism for the Coulomb drag in two--layer systems is developed. This technique can be used to treat both elastic disorder and intralayer interaction effects. In the absence of intralayer electron--electron correlations we reproduce earlier results, obtained using the kinetic equation and the memory--function formalism. In addition we calculate weak--localization corrections to the drag coefficient and the Hall drag coefficient in a perpendicular magnetic field. As an example of the intralayer interaction effects we consider a situation where one (or both) layers are close to (but above) the superconducting transition temperature. Fluctuation corrections, analogous to the Aslamazov--Larkin corrections, to the drag coefficient are calculated. Although the fluctuation corrections do not enhance the drag coefficient for normal--superconductor systems, a dramatic enhancement is found for superconductor--superconductor structures.

E Region neutral winds in the postmidnight diffuse aurora during the atmospheric response in Aurora 1 Rocket Campaign

Measured E region neutral winds from the Atmospheric Response in Aurora (ARIA 1) rocket campaign are compared with winds predicted by a high‐resolution nonhydrostatic dynamical thermosphere model. The ARIA 1 rockets were launched into the postmidnight diffuse aurora during the recovery phase of a substorm. Simulations have shown that electrodynamical coupling between the auroral ionosphere and the thermosphere was expected to be strong during active diffuse auroral conditions (Walterscheid and Lyons, 1989). This is the first time that simulations using the time history of detailed specifications of the magnitude and latitudinal variation of the auroral forcing based on measurements have been compared to simultaneous wind measurements. Model inputs included electron densities derived from ground‐based airglow measurements, precipitating electron fluxes measured by the rocket, electron densities measured on the rocket, electric fields derived from magnetometer and satellite ion drift measurements, and large‐scale background winds from a thermospheric general circulation model. Our model predicted a strong jet of eastward winds at E region heights. A comparison between model predicted and observed winds showed modest agreement. Above 135 km the model predicted zonal winds with the correct sense, the correct profile shape, and the correct altitude of the peak wind. However, it overpredicted the magnitude of the eastward winds by more than a factor or 2. For the meridional winds the model predicted the general sense of the winds but was unable to predict the structure or strength of the winds seen in the observations. Uncertainties in the magnitude and latitudinal structure of the electric field and in the magnitude of the background winds are the most likely sources of error contributing to the differences between model and observed winds. Between 110 and 135 km the agreement between the model and observations was poor because of a large unmodeled jetlike feature in the observed winds (140 m s−1). Agreement between the present simulation and the earlier simulations of Walterscheid and Lyons (1989) is favorable, although the winds in the present simulation are considerably weaker for particle precipitation of similar characteristic energy and flux. The reasons for the difference were the smaller latitudinal extent of the model diffuse aurora and the weaker electric fields in our simulation. We have shown that the enhanced electron densities and electric fields associated with the postmidnight diffuse aurora provide the potential for a rapid acceleration of the zonal winds as shown by Walterscheid and Lyons (1989). However, the modeled response to the large‐scale electric field is too great. This suggests that the assimilated mapping of ionospheric electrodynamics (AMIE) electric field is also too large. The actual electric field is most likely reduced locally in regions of enhanced ionization and conductivity within the diffuse aurora. In addition, we have shown that the “exotic” jetlike wind feature between 110 and 135 km is not aurorally forced. However, it may be the result of an enhancement of the Hall drag relative to the Coriolis force that modifies the geostrophic balance with the large‐scale pressure gradient.

Additive quasiparticle and vortex Hall conductivities inLa2−xSrxCuO4and untwinnedYBa2Cu3O6.93

The flux-flow Hall conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathit{xy}}$ in untwinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.93}$ (YBCO) is determined from Hall and resistivity measurements with the current applied along the a and the b axes. We show that sign reversal results from competition between a positive quasiparticle current (\ensuremath{\sim}H) and a negative vortex-motion term (\ensuremath{\sim}1/H). The Hall drag coefficient in YBCO is measured to be \ensuremath{\alpha}(T)=-4.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$(1-T/${\mathit{T}}_{\mathit{c}}$) N s/${\mathrm{m}}^{2}$ (75T93 K). In single-crystal ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$, a similar decomposition into quasiparticle and vortex currents (both positive) is possible even though ${\mathrm{\ensuremath{\sigma}}}_{\mathit{xy}}$ shows no sign reversal.

Hall conductivity in Tl 2Ba 2CaCu 2O 8 thin films

We have studied the Hall and longitudinal conductivities in c-axis oriented Tl 2Ba 2CaCu 2O 8 thin films. While the longitudinal conductivity displays thermally activated behavior increasing exponentially in the mixed state, the Hall conductivity tends to saturate with decreasing temperature. The data provide evidence that the Hall conductivity in the mixed state is independent of pinning strength and allow the determination of the bare Hall drag coefficient.

A two‐dimensional, high‐resolution, nested‐grid model of the thermosphere: 2. Response of the thermosphere to narrow and broad electrodynamic features

Using a two‐dimensional, nested‐grid model of the thermosphere, with high‐latitude resolution, the dynamic, energy, and composition response to a number of electrodynamic features has been considered. The forcing is characterized by a Gaussian latitude distribution of the convective electric field across the dawn or dusk auroral ovals with scale sizes ranging from 1° to 5° latitude full width at half maximum, together with an enhancement of E region ion density characteristic of the diffuse aurora. An extreme case for long‐lived features of the 100 mV m−1 electric field is considered, together with a more realistic magnitude of 50 mV m−1, coexisting with a 3×1011 m−3 ion density enhancement at low altitudes. The dynamic response is shown to be critically dependent on the magnitude of the winds across the source channel, either self‐induced or imposed from external sources. Self‐induced cross winds result from the action of the Coriolis force and, in addition, in the lower thermosphere, by the action of Hall drag. Crosswinds are shown to create plumes of momentum downwind and inhibit the peak neutral wind response that would otherwise exist. The combination of increased self‐induced cross wind and increasing width of the source creates a surprisingly similar peak in the zonal neutral wind response at high altitudes. The crosswinds induced by the Hall drag imply that a greater lower thermosphere zonal neutral wind is created by the 50 mV m−1 electric field compared with the 100 mV m−1 source. During the recovery a lower thermosphere jet is shown to persist for many hours with a slight preference for the smaller‐scale features. A stronger response is predicted in the dusk sector due to an inertial balance that exists, which implies that small‐scale features existing in the dusk sector are more likely to have an observable neutral response. The temperature perturbation is shown to be dependent on a self‐consistent treatment of composition and to increase in magnitude as the width of the source increases.