26 research outputs found
Violation of Quasineutrality in Semiconductor Transport: The Dember Effect
Exact solution of the linearized equations for steady-state transport in
semiconductors yields two modes that vary exponentially in space, one involving
screening (without entropy production) and one involving diffusion and
recombination (with entropy production). Neither mode is quasineutral. For
constant surface photoexcitation with generation of electrons and holes, the
steady-state response is a linear combination of these modes, subject to global
electroneutrality. The resultant charge separation produces a voltage
difference across the sample (the Dember effect)
Supersolid He Likely Has Nearly Isotropic Superflow
We extend previous calculations of the zero temperature superfluid fraction
(SFF) {\it vs} localization, from the fcc lattice to the experimentally
realized (for solid He) hcp and bcc lattices. The superfluid velocity is
assumed to be a one-body function, and dependent only on the local density,
taken to be a sum over sites of gaussians of width . Localization is
defined as , with the nearest-neighbor distance. As expected, for
fcc and bcc lattices the superfluid density tensor is proportional to the unit
tensor. To numerical accuracy of three-places (but no more), the hcp superfluid
density tensor is proportional to the unit tensor. This implies that a larger
spread in data on , if measured on pure crystals, is unlikely to be due to
crystal orientation. In addition, to three decimal places (but no more) the
curves of {\it vs} are the same for both the hcp and fcc
cases. An expected value for the localization gives an in reasonable
agreement with experiment. The bcc lattice has a similar curve of {\it
vs} , but is generally smaller because the lattice is more dilute.Comment: 9 pages, 1 figure, 3 table
Hysteresis of Finite Arrays of Magnetic nano Dots
Hysteresis curves for finite arrays of ferromagnetic nano dots
subject to the dipole-dipole interaction are investigated for . Spin
arrangements up to N=6 are presented, which indicate the onset of bulk-like
behavior associated with odd (N=5) and even (N=6) systems. The effect of field
misalignment on the hysteresis loops is also studied for . The area
of the hysteresis loop is studied as a function of . We find that
approximately scales as for odd and as
for even.Comment: 6 pages, 8 figures, Submitted to PR
Andreev-Lifshitz Hydrodynamics Applied to an Ordinary Solid under Pressure
We have applied the Andreev-Lifshitz hydrodynamic theory of supersolids to an
ordinary solid. This theory includes an internal pressure , distinct from
the applied pressure and the stress tensor . Under uniform
static , we have . For , Maxwell relations imply that . The theory also permits
vacancy diffusion but treats vacancies as conserved. It gives three sets of
propagating elastic modes; it also gives two diffusive modes, one largely of
entropy density and one largely of vacancy density (or, more generally, defect
density). For the vacancy diffusion mode (or, equivalently, the lattice
diffusion mode) the vacancies behave like a fluid within the solid, with the
deviations of internal pressure associated with density changes nearly
canceling the deviations of stress associated with strain. We briefly consider
pressurization experiments in solid He at low temperatures in light of this
lattice diffusion mode, which for small has diffusion constant . The general principles of the theory -- that both volume and
strain should be included as thermodynamic variables, with the result that both
and appear -- should apply to all solids under pressure,
especially near the solid-liquid transition. The lattice diffusion mode
provides an additional degree of freedom that may permit surfaces with
different surface treatments to generate different responses in the bulk.Comment: 10 pages. Accepted by Physical Review
Generation Efficiencies for Propagating Modes in a Supersolid
Using Andreev and Lifshitz's supersolid hydrodynamics, we obtain the
propagating longitudinal modes at non-zero applied pressure (necessary
for solid 4He), and their generation efficiencies by heaters and transducers.
For small , a solid develops an internal pressure . This
theory has stress contributions both from the lattice and an internal pressure
. Because both types of stress are included, the normal mode analysis
differs from previous works. Not surprisingly, transducers are significantly
more efficient at producing elastic waves and heaters are significantly more
efficient at producing fourth sound waves. We take the system to be isotropic,
which should apply to systems that are glassy or consist of many crystallites;
the results should also apply, at least qualitatively, to single-crystal hcp
4He.Comment: 10 pages. Accepted by Physical Review
Thermal Equilibration and Thermally-Induced Spin Currents in a Thin-Film Ferromagnet on a Substrate
Recent spin-Seebeck experiments on thin ferromagnetic films apply a
temperature difference along the length and measure a
(transverse) voltage difference along the width . The
connection between these effects is complex, involving: (1) thermal
equilibration between sample and substrate; (2) spin currents along the height
(or thickness) ; and (3) the measured voltage difference. The present work
studies in detail the first of these steps, and outlines the other two steps.
Thermal equilibration processes between the magnons and phonons in the sample,
as well as between the sample and the substrate leads to two surface modes,
with surface lengths , to provide for thermal equilibration.
Increasing the coupling between the two modes increases the longer mode length
and decreases the shorter mode length. The applied thermal gradient along
leads to a thermal gradient along that varies as ,
which can in turn produce fluxes of the carriers of up- and down- spins along
, and gradients of their associated \textit{magnetoelectrochemical
potentials} , which vary as
. By the inverse spin Hall effect, this spin current along
can produce a transverse (along ) voltage difference , which
also varies as .Comment: 14 pages, 7 figures, 1 tabl
Andreev-Lifshitz Supersolid Hydrodynamics Including the Diffusive Mode
We have re-examined the Andreev-Lifshitz theory of supersolids. This theory
implicitly neglects uniform bulk processes that change the vacancy number, and
assumes an internal pressure in addition to lattice stress .
Each of and takes up a part of an external, or applied,
pressure (necessary for solid 4He). The theory gives four pairs of
propagating elastic modes, of which one pair corresponds to a fourth-sound
mode, and a single diffusive mode, which has not been analyzed previously. The
diffusive mode has three distinct velocities, with the superfluid velocity much
larger than the normal fluid velocity, which in turn is much larger than the
lattice velocity. The mode structure depends on the relative values of certain
kinetic coefficients and thermodynamic derivatives. We consider pressurization
experiments in solid 4He at low temperatures in light of this diffusion mode
and a previous analysis of modes in a normal solid with no superfluid
component.Comment: 8 pages. Accepted by Physical Review
Dissipation due to pure spin-current generated by spin pumping
Based on spin-dependent transport theory and thermodynamics, we develop a
generalized theory of the Joule heating in the presence of a spin current.
Along with the conventional Joule heating consisting of an electric current and
electrochemical potential, it is found that the spin current and spin
accumulation give an additional dissipation because the spin-dependent
scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a
change of entropy. The theory is applied to investigate the dissipation due to
pure spin-current generated by spin pumping across a
ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from
an interface because the spin pumping is a transfer of both the spin angular
momentum and the energy from the ferromagnet to conduction electrons near the
interface. It is found that the dissipation is proportional to the enhancement
of the Gilbert damping constant by spin pumping.Comment: 12 pages, 4 figure
Macroscopic Magnetic Dynamics
Ferromagnetic metals and spin-polarized He are spin 1/2 systems with
the same macroscopic symmetry, and thus should have macroscopic magnetic
dynamics with the same structure. Using Onsager's irreversible thermodynamics,
we develop a theory for these systems that contains two relaxation times (one
for the magnetization and the other for the spin current
), a magnetic compressibility, and a mean-field parameter.
Currently spintronics data on metallic ferromagnets are analyzed using a
complex decay length from a theory employing a diffusion constant, a lifetime,
and a mean-field parameter. The present theory leads to a complex decay length
with the same structure. On neglecting decay of , the present theory
applies to liquids and gases. For macroscopic equations the particle statistics
is not relevant, so the theory also applies to bosons. The theory predicts a
longitudinal spin wave whose velocity we estimate for liquid He and for
paramagnetic metals; but such a wave should also occur for ferromagnets and for
gases.Comment: 6 page