4,974 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
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
Constraining crystalline color superconducting quark matter with gravitational-wave data
We estimate the maximum equatorial ellipticity sustainable by compact stars
composed of crystalline color-superconducting quark matter. For the
theoretically allowed range of the gap parameter , the maximum
ellipticity could be as large as , which is about 4 orders of
magnitude larger than the tightest upper limit obtained by the recent science
runs of the LIGO and GEO600 gravitational wave detectors based on the data from
78 radio pulsars. We point out that the current gravitational-wave strain upper
limit already has some implications for the gap parameter. In particular, the
upper limit for the Crab pulsar implies that is less than O(20) MeV
for a range of quark chemical potential accessible in compact stars, assuming
that the pulsar has a mass , radius 10 km, breaking strain
, and that it has the maximum quadrupole deformation it can sustain
without fracturing.Comment: Minor changes to match the published versio
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
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
Universal Decoherence in Solids
Symmetry implications for the decoherence of quantum oscillations of a
two-state system in a solid are studied. When the oscillation frequency is
small compared to the Debye frequency, the universal lower bound on the
decoherence due to the atomic environment is derived in terms of the
macroscopic parameters of the solid, with no unknown interaction constants.Comment: 4 pages, no figure
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
Theory of magnetic deflagration
Theory of magnetic deflagration (avalanches) in crystals of molecular magnets
has been developed. The phenomenon resembles the burning of a chemical
substance, with the Zeeman energy playing the role of the chemical energy.
Non-destructive reversible character of magnetic deflagration, as well as the
possibility to continuously tune the flammability of the crystal by changing
the magnetic field, makes molecular magnets an attractive toy system for a
detailed study of the burning process. Besides simplicity, new features, as
compared to the chemical burning, include possibility of quantum decay of
metastable spin states and strong temperature dependence of the heat capacity
and thermal conductivity. We obtain analytical and numerical solutions for
criteria of the ignition of magnetic deflagration, and compute the ignition
rate and the speed of the developed deflagration front.Comment: 17 Pages, 17 Figure caption
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