5,874 research outputs found
Magnetoelectric Response of the Time-Reversal Invariant Helical Metal
We derive compact analytical expressions for the coupled spin-charge
susceptibility of a clean helical metal at the surface of a three dimensional
topological insulator (TI). These expressions lead to unconventional
non-collinear RKKY interactions between two impurity magnetic moments placed on
the surface of a TI, and predict the generation of electric currents by
time-dependent magnetic moments. We determine the influence of gate and bias
voltages on the interlayer exchange coupling between two single-domain
ferromagnetic monolayers deposited on top of a TI.Comment: 4 pages, 2 figures; submitted to Phys. Rev. B R
Superfluid Helium On-Orbit Transfer (SHOOT) operatons
The in-flight tests and the operational sequences of the Superfluid Helium On-Orbit Transfer (SHOOT) experiment are outlined. These tests include the transfer of superfluid helium at a variety of rates, the transfer into cold and warm receivers, the operation of an extravehicular activity coupling, and tests of a liquid acquisition device. A variety of different types of instrumentation will be required for these tests. These include pressure sensors and liquid flow meters that must operate in liquid helium, accurate thermometry, two types of quantity gauges, and liquid-vapor sensors
Dirac electrons in a Kronig-Penney potential: dispersion relation and transmission periodic in the strength of the barriers
The transmission T and conductance G through one or multiple one-dimensional,
delta-function barriers of two-dimensional fermions with a linear energy
spectrum are studied. T and G are periodic functions of the strength P of the
delta-function barrier V(x,y) / hbar v_F = P delta(x). The dispersion relation
of a Kronig-Penney (KP) model of a superlattice is also a periodic function of
P and causes collimation of an incident electron beam for P = 2 pi n and n
integer. For a KP superlattice with alternating sign of the height of the
barriers the Dirac point becomes a Dirac line for P = (n + 1/2) pi.Comment: 5 pages, 6 figure
The importance of electron-electron interactions in the RKKY coupling in graphene
We show that the carrier-mediated exchange interaction, the so-called RKKY
coupling, between two magnetic impurity moments in graphene is significantly
modified in the presence of electron-electron interactions. Using the
mean-field approximation of the Hubbard- model we show that the
-oscillations present in the bulk for
non-interacting electrons disappear and the power-law decay becomes more long
ranged with increasing electron interactions. In zigzag graphene nanoribbons
the effects are even larger with any finite rendering the long-distance
RKKY coupling distance independent. Comparing our mean-field results with
first-principles results we also extract a surprisingly large value of
indicating that graphene is very close to an antiferromagnetic instability.Comment: 4 pages, 3 figure
Unidimensional model of the ad-atom diffusion on a substrate submitted to a standing acoustic wave I. Derivation of the ad-atom motion equation
The effect of a standing acoustic wave on the diffusion of an ad-atom on a
crystalline surface is theoretically studied. We used an unidimensional space
model to study the ad-atom+substrate system. The dynamic equation of the
ad-atom, a Generalized Langevin equation, is analytically derived from the full
Hamiltonian of the ad-atom+substrate system submitted to the acoustic wave. A
detailed analysis of each term of this equation, as well as of their
properties, is presented. Special attention is devoted to the expression of the
effective force induced by the wave on the ad-atom. It has essentially the same
spatial and time dependences as its parent standing acoustic wave
Influence of Zeeman splitting and thermally excited polaron states on magneto-electrical and magneto-thermal properties of magnetoresistive polycrystalline manganite La_{0.8}Sr_{0.2}MnO_3
Some possible connection between spin and charge degrees of freedom in
magneto-resistive manganites is investigated through a thorough experimental
study of the magnetic (AC susceptibility and DC magnetization) and transport
(resistivity and thermal conductivity) properties. Measurements are reported in
the case of well characterized polycrystalline La_{0.8}Sr_{0.2}MnO_3 samples.
The experimental results suggest rather strong field-induced polarization
effects in our material, clearly indicating the presence of ordered FM regions
inside the semiconducting phase. Using an analytical expression which fits the
spontaneous DC magnetization, the temperature and magnetic field dependences of
both electrical resistivity and thermal conductivity data are found to be well
reproduced through a universal scenario based on two mechanisms: (i) a
magnetization dependent spin polaron hopping influenced by a Zeeman splitting
effect, and (ii) properly defined thermally excited polaron states which have
to be taken into account in order to correctly describe the behavior of the
less conducting region. Using the experimentally found values of the magnetic
and electron localization temperatures, we obtain L=0.5nm and m_p=3.2m_e for
estimates of the localization length (size of the spin polaron) and effective
polaron mass, respectively.Comment: Accepted for publication in Journal of Applied Physic
Electron-Phonon Interaction in Embedded Semiconductor Nanostructures
The modification of acoustic phonons in semiconductor nanostructures embedded
in a host crystal is investigated including corrections due to strain within
continuum elasticity theory. Effective elastic constants are calculated
employing {\em ab initio} density functional theory. For a spherical InAs
quantum dot embedded in GaAs barrier material, the electron-phonon coupling is
calculated. Its strength is shown to be suppressed compared to the assumption
of bulk phonons
Comment on ``Analytical and numerical verification of the Nernst heat theorem for metals''
Recently, H{\o}ye, Brevik, Ellingsen and Aarseth (quant-ph/0703174) claimed
that the use of the Drude dielectric function leads to zero Casimir entropy at
zero temperature in accordance with Nernst's theorem. We demonstrate that their
proof is not applicable to metals with perfect crystal lattices having no
impurities. Thus there is no any contradiction with previous results in the
literature proving that the Drude dielectric function violates the Nernst
theorem for the Casimir entropy in the case of perfect crystal lattices. We
also indicate mistakes in the coefficients of their asymptotic expressions for
metals with impurities.Comment: 6 page
Effect of disorder studied with ferromagnetic resonance for arrays of tangentially magnetized sub-micron Permalloy discs fabricated by nanosphere lithography
Tangentially magnetized trigonal arrays of sub-micron Permalloy discs are
characterized with ferromagnetic resonance to determine the possible
contributions to frequency and linewidth from array disorder. Each array is
fabricated by a water-surface self-assembly lithographic technique, and
consists of a large trigonal array of 700 nm diameter magnetic discs. Each
array is characterized by a different degree of ordering. Two modes are present
in the ferromagnetic resonance spectra: a large amplitude, `fundamental' mode
and a lower amplitude mode at higher field. Angular dependence of the resonance
field in a very well ordered array is found to be negligible for both modes.
The relationship between resonance frequency and applied magnetic field is
found to be uncorrelated with array disorder. Linewidth is found to increase
with increasing array disorder
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