1,658 research outputs found
Study of the topological Hall effect on simple models
Recently, a chirality-driven contribution to the anomalous Hall effect has
been found that is induced by the Berry phase and does not directly involve
spin-orbit coupling. In this paper, we will investigate this effect numerically
in a two-dimensional electron gas with a simple magnetic texture model. Both
the adiabatic and non-adiabatic regimes are studied, including the effect of
disorder. By studying the transition between both regimes the discussion about
the correct adiabaticity criterium in the diffusive limit is clarified.Comment: 8 pages, 7 figures, revtex
Transverse voltage in high-Tc superconductors in zero magnetic fields
Longitudinal and transverse voltages have been measured in zero external
magnetic fields. In close vicinity of the superconducting transition nonzero
transverse voltage has been observed while far away from Tc, both above and
below no such voltage has been detected. The value of the transverse
resistivity depends on the value of the transport current. Several models have
been discussed taking into account also the penetration of self field due to
the applied transport current. It seems that observed results can be explained
using the Kosterlitz-Thouless model as a result of an unpairing of
vortex-antivortex pairs created below Tkt due to fluctuations. At Tkt free
vortices and antivortices are created and can contribute to a dissipation of
energy. Their movement should also be responsible for the observed nonzero
transverse voltage.Comment: 3 pages in Latex, 3 figs.ep
Single-valley high-mobility (110) AlAs quantum wells with anisotropic mass
We studied a doping series of (110)-oriented AlAs quantum wells (QWs) and
observed transport evidence of single anisotropic-mass valley occupancy for the
electrons in a 150 \AA wide QW. Our calculations of strain and quantum
confinement for these samples predict single anisotropic-mass valley occupancy
for well widths greater than 53 \AA. Below this, double-valley occupation
is predicted such that the longitudinal mass axes are collinear. We observed
mobility anisotropy in the electronic transport along the crystallographic
directions in the ratio of 2.8, attributed to the mass anisotropy as well as
anisotropic scattering of the electrons in the X-valley of AlAs
Measuring carrier density in parallel conduction layers of quantum Hall systems
An experimental analysis for two parallel conducting layers determines the
full resistivity tensor of the parallel layer, at magnetic fields where the
other layer is in the quantum Hall regime. In heterostructures which exhibit
parallel conduction in the modulation-doped layer, this analysis quantitatively
determines the charge density in the doping layer and can be used to estimate
the mobility. To illustrate one application, experimental data show magnetic
freeze-out of parallel conduction in a modulation doped heterojunction. As
another example, the carrier density of a minimally populated second subband in
a two-subband quantum well is determined. A simple formula is derived that can
estimate the carrier density in a highly resistive parallel layer from a single
Hall measurement of the total system.Comment: 7 pages, 7 figure
X-ray photoemission characterization of La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} films
The Curie temperature and x-ray photoemission spectra of thin films of
La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} (LCSMO) have been studied as a function
of the Ca/Sr ratio. The films were grown by off-axis cosputtering from
individual targets of La_{0.67}Ca_{0.33}MnO_{3} (LCMO) and
La_{0.67}Sr_{0.33}MnO_{3} (LSMO) onto (100) oriented NdGaO_{3} substrates. The
films grow with a (100) orientation, with no other orientations observed by
x-ray diffraction. For the alloy mixtures, the Curie temperature, T_C, varies
slowly as the Ca/Sr is decreased, remaining 300 K, while for the LCMO
and LSMO films T_C is 260 and 330 K, respectively. The Mn-O valence structure
is composed of two dominant peaks, whose positions undergo a change as the Ca
fraction is decreased. The core lines behave as linear combinations of lines
from pure LCMO and LSMO.Comment: 3 pages, 5 eps figures. To be published in Journal of Applied Physics
(Proceedings of MMM'98
Different effects of Ni and Co substitution on the transport properties of BaFe2As2
We report resistivity and Hall effect results on Ba(Fe1-xNix)2As2 and compare
them with those in Ba(Fe1-xCox)2As2. The Hall number RH is negative for all x
values from 0.01 to 0.14, which indicates that electron carriers dominate the
transport both in the magnetic and paramagnetic regime. We analyse the data in
the framework of a two-band model. Without any assumption on the number of
carriers, we show that the electron resistivity can be estimated with good
accuracy in the low temperature paramagnetic range. Although the phase diagrams
of the two families are very similar with respect to the extra electrons added
in the system, we find that the transport properties differ in several aspects.
First, we evidence that the contribution of holes to the transport is more
important for Ni doping than for Co doping. Secondly, Ni behaves as a stronger
scatterer for the electrons, as the increase of the residual electron
resistivity rho/x is about four times larger for Ni than for Co in the most
doped samples.Comment: 8 pages, 8 figure
Sensitivity of resistive and Hall measurements to local inhomogeneities:Finite-field, intensity, and area corrections
Graduate Unemployment in the Face of Skill Shortages: A Labour Market Paradox
There is consensus among analysts that South Africa's unemployment is structural in the sense that the unemployed generally possess lower skills than what is required in the marketplace. In the context of increasing demand for skilled workers due to technical progress and the need to become more competitive globally, graduate unemployment would be expected to fall
Mesoscopic magnetoelectric effect in chaotic quantum dots
The magnitude of the inverse Faraday effect (IFE), a static magnetization due
to an ac electric field, can be strongly increased in a mesoscopic sample,
sensitive to time-reversal symmetry (TRS) breaking. Random rectification of ac
voltages leads to a magnetization flux, which can be detected by an asymmetry
of Hall resistances in a multi-terminal setup. In the absence of applied
magnetic field through a chaotic quantum dot the IFE scale, quadratic in
voltage, is found as an analytic function of the ac frequency, screening, and
coupling to the contacts and floating probes, and numerically it does not show
any effect of spin-orbit interaction. Our results qualitatively agree with a
recent experiment on TRS-breaking in a six-terminal Hall cross.Comment: 4+ pages, 2 figures; v2-published version, small change
Hole and Electron Contributions to the Transport Properties of Ba(Fe_(1-x)Ru_x)_2As_2 Single Crystals
We report a systematic study of structural and transport properties in single
crystals of Ba(Fe_(1-x)Ru_x)_2As_2 for x ranging from 0 to 0.5. The isovalent
substitution of Fe by Ru leads to an increase of the a parameter and a decrease
of the c parameter, resulting in a strong increase of the AsFeAs angle and a
decrease of the As height above the Fe planes. Upon Ru substitution, the
magnetic order is progressively suppressed and superconductivity emerges for x
> 0.15, with an optimal Tc ~ 20K at x = 0.35 and coexistence of magnetism and
superconductivity between these two Ru contents. Moreover, the Hall coefficient
RH which is always negative and decreases with temperature in BaFe2As2, is
found to increase here with decreasing T and even change sign for x > 0.15. For
x_Ru = 0.35, photo-emission studies have shown that the number of holes and
electrons are similar with n_e = n_h ~ 0.11, that is twice larger than found in
BaFe2As2 [1]. Using this estimate, we find that the transport properties of
Ba(Fe_0.65Ru_0.35)_2As_2 can be accounted for by the conventional multiband
description for a compensated semi-metal. In particular, our results show that
the mobility of holes is strongly enhanced upon Ru addition and overcomes that
of electrons at low temperature when x_Ru > 0.15.Comment: new version with minor correction
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