1,793 research outputs found
Orbital Polarization in Itinerant Magnets
We propose a parameter-free scheme of calculation of the orbital polarization
(OP) in metals, which starts with the strong-coupling limit for the screened
Coulomb interactions in the random-phase approximation (RPA). For itinerant
magnets, RPA can be further improved by restoring the spin polarization of the
local-spin-density approximation (LSDA) through the local-field corrections.
The OP is then computed in the static GW approach, which systematically
improves the orbital magnetization and the magnetic anisotropy energies in
transition-metal and actinide compounds.Comment: 5 pages, 4 figure
Localized states due to expulsion of resonant impurity levels from the continuum in bilayer graphene
Anderson impurity problem is considered for a graphene bilayer subject to a
gap-opening bias. In-gap localized states are produced even when the impurity
level overlaps with the continuum of band electrons. The effect depends
strongly on the polarity of the applied bias as long as hybridization with the
impurity occurs within a single layer. For an impurity level inside the
conduction band a positive bias creates the new localized in-gap state. A
negative bias does not produce the same result and leads to a simple broadening
of the impurity level. The implications for transport are discussed including a
possibility of gate-controlled Kondo effect.Comment: 5 pages, 2 figure
Low-Temperature Decoherence of Qubit Coupled to Background Charges
We have found an exact expression for the decoherence rate of a Josephson
charge qubit coupled to fluctuating background charges. At low temperatures
the decoherence rate is linear in while at high temperatures it
saturates in agreement with a known classical solution which, however, reached
at surprisingly high . In contrast to the classical picture, impurity states
spread in a wide interval of energies () may essentially contribute to
.Comment: Both figures are changed to illustrate a more generic case of
impurity states spread in wide interval of energies. Some changes have been
made to the abstract and the introductio
Suppression of electron relaxation and dephasing rates in quantum dots caused by external magnetic fields
An external magnetic field has been applied in laterally coupled dots (QDs)
and we have studied the QD properties related to charge decoherence. The
significance of the applied magnetic field to the suppression of
electron-phonon relaxation and dephasing rates has been explored. The coupled
QDs have been studied by varing the magnetic field and the interdot distance as
other system parameters. Our numerical results show that the electron
scattering rates are strongly dependent on the applied external magnetic field
and the details of the double QD configuration.Comment: 13 pages, 6 figure
Mott Insulator to Superfluid transition in Bose-Bose mixtures in a two-dimensional lattice
We perform a numeric study (Worm algorithm Monte Carlo simulations) of
ultracold two-component bosons in two-dimensional optical lattices. We study
how the Mott insulator to superfluid transition is affected by the presence of
a second superfluid bosonic species. We find that, at fixed interspecies
interaction, the upper and lower boundaries of the Mott lobe are differently
modified. The lower boundary is strongly renormalized even for relatively low
filling factor of the second component and moderate (interspecies) interaction.
The upper boundary, instead, is affected only for large enough filling of the
second component. Whereas boundaries are renormalized we find evidence of
polaron-like excitations. Our results are of interest for current experimental
setups.Comment: 4 pages, 3 figures, accepted as PRA Rapid Communicatio
Effects of electron coupling to intra- and inter-molecular vibrational modes on the transport properties of single crystal organic semiconductors
Electron coupling to intra- and inter-molecular vibrational modes is
investigated in models appropriate to single crystal organic semiconductors,
such as oligoacenes. Focus is on spectral and transport properties of these
systems beyond perturbative approaches. The interplay between different
couplings strongly affects the temperature band renormalization that is the
result of a subtle equilibrium between opposite tendencies: band narrowing due
to interaction with local modes, band widening due to electron coupling to non
local modes. The model provides an accurate description of the mobility as
function of temperature: indeed, it has the correct order of magnitude, at low
temperatures, it scales as a power-law with the exponent
larger than unity, and, at high temperatures, shows an hopping behavior with a
small activation energy.Comment: 3 Figures, Submitte
One Dimensional Gas of Bosons with Feshbach Resonant Interactions
We present a study of a gas of bosons confined in one dimension with Feshbach
resonant interactions, at zero temperature. Unlike the gas of one dimensional
bosons with non-resonant interactions, which is known to be equivalent to a
system of interacting spinless fermions and can be described using the
Luttinger liquid formalism, the resonant gas possesses novel features.
Depending on its parameters, the gas can be in one of three possible regimes.
In the simplest of those, it can still be described by the Luttinger liquid
theory, but its Fermi momentum cannot be larger than a certain cutoff momentum
dependent on the details of the interactions. In the other two regimes, it is
equivalent to a Luttinger liquid at low density only. At higher densities its
excitation spectrum develops a minimum, similar to the roton minimum in helium,
at momenta where the excitations are in resonance with the Fermi sea. As the
density of the gas is increased further, the minimum dips below the Fermi
energy, thus making the ground state unstable. At this point the standard
ground state gets replaced by a more complicated one, where not only the states
with momentum below the Fermi points, but also the ones with momentum close to
that minimum, get filled, and the excitation spectrum develops several
branches. We are unable so far to study this new regime in detail due to the
lack of the appropriate formalism.Comment: 20 pages, 18 figure
Multilayer Thermionic Refrigerator and Generator
A new method of refrigeration is proposed. Cooling is obtained by thermionic
emission of electrons over periodic barriers in a multilayer geometry. These
could be either Schottky barriers between metals and semiconductors or else
barriers in a semiconductor superlattice. The same device is an efficient power
generator. A complete theory is provided.Comment: 17 pages with 5 postscript figures, submitted to J. Appl. Phy
Orthogonality catastrophe and Kondo effect in graphene
Anderson's orthogonality catastrophe in graphene, at energies close to the
Dirac point, is analyzed. It is shown that, in clean systems, the orthogonality
catastrophe is suppressed, due to the vanishing density of states at the Dirac
point. In the presence of preexisting localized states at the Dirac energy, the
orthogonality catastrophe shows similar features to those found in normal
metals with a finite density of states at the Fermi level. The implications for
the Kondo effect induced by magnetic impurities, and for the Fermi edge
singularities in tunneling processes are also discussed.Comment: 7 pages, 7 figure
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