224,531 research outputs found
Hall resistance in the hopping regime, a "Hall Insulator"?
The Hall conductivity and resistivity of strongly localized electrons at low
temperatures and at small magnetic fields are obtained. It is found that the
results depend on whether the conductivity or the resistivity tensors are
averaged to obtain the macroscopic Hall resistivity. In the second case the
Hall resistivity always {\it diverges} exponentially as the temperature tends
to zero. But when the Hall resistivity is derived from the averaged
conductivity, the resulting temperature dependence is sensitive to the disorder
configuration. Then the Hall resistivity may approach a constant value as . This is the Hall insulating behavior. It is argued that for strictly dc
conditions, the transport quantity that should be averaged is the resistivity.Comment: Late
Introduction to tensorial resistivity probability tomography
The probability tomography approach developed for the scalar resistivity
method is here extended to the 2D tensorial apparent resistivity acquisition
mode. The rotational invariant derived from the trace of the apparent
resistivity tensor is considered, since it gives on the datum plane anomalies
confined above the buried objects. Firstly, a departure function is introduced
as the difference between the tensorial invariant measured over the real
structure and that computed for a reference uniform structure. Secondly, a
resistivity anomaly occurrence probability (RAOP) function is defined as a
normalised crosscorrelation involving the experimental departure function and a
scanning function derived analytically using the Frechet derivative of the
electric potential for the reference uniform structure. The RAOP function can
be calculated in each cell of a 3D grid filling the investigated volume, and
the resulting values can then be contoured in order to obtain the 3D
tomographic image. Each non-vanishing value of the RAOP function is interpreted
as the probability which a resistivity departure from the reference resistivity
obtain in a cell as responsible of the observed tensorial apparent resistivity
dataset on the datum plane. A synthetic case shows that the highest RAOP values
correctly indicate the position of the buried objects and a very high spacial
resolution can be obtained even for adjacent objects with opposite resistivity
contrasts with respect to the resistivity of the hosting matrix. Finally, an
experimental field case dedicated to an archaeological application of the
resistivity tensor method is presented as a proof of the high resolution power
of the probability tomography imaging, even when the data are collected in
noisy open field conditions.Comment: 8 pages, 7 figure
Radiation induced oscillations of the Hall resistivity in two-dimensional electron systems
We consider the effect of microwave radiation on the Hall resistivity in
two-dimension electron systems. It is shown that the photon-assisted impurity
scattering of electrons can result in oscillatory dependences of both
dissipative and Hall components of the conductivity and resistivity tensors on
the ratio of radiation frequency to cyclotron frequency. The Hall resistivity
can include a component induced by microwave radiation which is an even
function of the magnetic field. The phase of the dissipative resistivity
oscillations and the polarization dependence of their amplitude are compared
with those of the Hall resistivity oscillations. The developed model can
clarify the results of recent experimental observations of the radiation
induced Hall effect.Comment: 4 pages, 1 figur
Model for the magnetoresistance and Hall coefficient of inhomogeneous graphene
We show that when bulk graphene breaks into n-type and p-type puddles, the
in-plane resistivity becomes strongly field dependent in the presence of a
perpendicular magnetic field, even if homoge- neous graphene has a
field-independent resistivity. We calculate the longitudinal resistivity
\rho_{xx} and Hall resistivity \rho_{xy} as a function of field for this
system, using the effective-medium approximation. The conductivity tensors of
the individual puddles are calculated using a Boltzmann approach suit- able for
the band structure of graphene near the Dirac points. The resulting resistivity
agrees well with experiment, provided that the relaxation time is weakly
field-dependent. The calculated Hall resistivity has the sign of the majority
carrier and vanishes when there are equal number of n and p type puddles.Comment: 5 pages, 4 figure
Deviations from Matthiessen rule and resistivity saturation effects in Gd and Fe
According to earlier first-principles calculations, the spin-disorder
contribution to the resistivity of rare-earth metals in the paramagnetic state
is strongly underestimated if Matthiessen's rule is assumed to hold. To
understand this discrepancy, the resistivity of paramagnetic Fe and Gd is
evaluated by taking into account both spin and phonon disorder. Calculations
are performed using the supercell approach within the linear muffin-tin orbital
method. Phonon disorder is modeled by introducing random displacements of the
atomic nuclei, and the results are compared with the case of fictitious
Anderson disorder. In both cases the resistivity shows a nonlinear dependence
on the square of the disorder potential, which is interpreted as a resistivity
saturation effect. This effect is much stronger in Gd than in Fe. The
non-linearity makes the phonon and spin-disorder contributions to the
resistivity non-additive, and the standard procedure of extracting the
spin-disorder resistivity by extrapolation from high temperatures becomes
ambiguous. An "apparent" spin-disorder resistivity obtained through such
extrapolation is in much better agreement with experiment compared to the
results obtained by considering only spin disorder. By analyzing the spectral
function of the paramagnetic Gd in the presence of Anderson disorder, the
resistivity saturation is explained by the collapse of a large area of the
Fermi surface due to the disorder-induced mixing between the electronic and
hole sheets.Comment: 9 pages, 7 figure
Resistivity scaling model for metals with conduction band anisotropy
It is generally understood that the resistivity of metal thin films scales
with film thickness mainly due to grain boundary and boundary surface
scattering. Recently, several experiments and ab initio simulations have
demonstrated the impact of crystal orientation on resistivity scaling. The
crystal orientation cannot be captured by the commonly used resistivity scaling
models and a qualitative understanding of its impact is currently lacking. In
this work, we derive a resistivity scaling model that captures grain boundary
and boundary surface scattering as well as the anisotropy of the band
structure. The model is applied to Cu and Ru thin films, whose conduction bands
are (quasi-)isotropic and anisotropic respectively. After calibrating the
anisotropy with ab initio simulations, the resistivity scaling models are
compared to experimental resistivity data and a renormalization of the fitted
grain boundary reflection coefficient can be identified for textured Ru.Comment: 12 pages, 7 figure
Low-resistivity homogeneous elastomers
Mixture of polyurethane polyelectrolyte and soluble, conducting organic compound produces homogeneous elastomer which has resistivity several orders of magnitude less than polyelectrolyte alone. Elastomeric material has novel resistivity dependence on temperature, that is, resistivity changes dramatically over narrow temperature range in vicinity of glass transition temperature
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