7,599 research outputs found
Multiple Lifshitz transitions driven by short-range antiferromagnetic correlations in the two-dimensional Kondo lattice model
With a mean field approach, the heavy Fermi liquid in the two-dimensional
Kondo lattice model is carefully considered in the presence of short-range
antiferromagnetic correlations. As the ratio of the local Heisenberg
superexchange coupling to the Kondo coupling increases, the Fermi surface
structure changes dramatically. From the analysis of the ground state energy
density, multiple Lifshitz type phase transitions occur at zero temperature.Comment: 4 pages, 3 figures, contribution to SCES201
Bose-Einstein condensation in an optical lattice: A perturbation approach
We derive closed analytical expressions for the order parameter
and for the chemical potential of a Bose-Einstein Condensate loaded into
a harmonically confined, one dimensional optical lattice, for sufficiently
weak, repulsive or attractive interaction, and not too strong laser
intensities. Our results are compared with exact numerical calculations in
order to map out the range of validity of the perturbative analytical approach.
We identify parameter values where the optical lattice compensates the
interaction-induced nonlinearity, such that the condensate ground state
coincides with a simple, single particle harmonic oscillator wave function
Alternative approach to computing transport coefficients: application to conductivity and Hall coefficient of hydrogenated amorphous silicon
We introduce a theoretical framework for computing transport coefficients for
complex materials. As a first example, we resolve long-standing inconsistencies
between experiment and theory pertaining to the conductivity and Hall mobility
for amorphous silicon and show that the Hall sign anomaly is a consequence of
localized states. Next, we compute the AC conductivity of amorphous
polyanaline. The formalism is applicable to complex materials involving defects
and band-tail states originating from static topological disorder and extended
states. The method may be readily integrated with current \textit{ab initio}
methods.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Monte Carlo modeling of spin injection through a Schottky barrier and spin transport in a semiconductor quantum well
We develop a Monte Carlo model to study injection of spin-polarized electrons
through a Schottky barrier from a ferromagnetic metal contact into a
non-magnetic low-dimensional semiconductor structure. Both mechanisms of
thermionic emission and tunneling injection are included in the model. Due to
the barrier shape, the injected electrons are non-thermalized. Spin dynamics in
the semiconductor heterostructure is controlled by the Rashba and Dresselhaus
spin-orbit interactions and described by a single electron spin density matrix
formalism. In addition to the linear term, the third order term in momentum for
the Dresselhaus interaction is included. Effect of the Schottky potential on
the spin dynamics in a 2 dimensional semiconductor device channel is studied.
It is found that the injected current can maintain substantial spin
polarization to a length scale in the order of 1 micrometer at room temperature
without external magnetic fields.Comment: 18 pages, 4 figures, J. Appl. Phys., accepted for publicatio
Formation of energy gap in higher dimensional spin-orbital liquids
A Schwinger boson mean field theory is developed for spin liquids in a
symmetric spin-orbital model in higher dimensions. Spin, orbital and coupled
spin-orbital operators are treated equally. We evaluate the dynamic correlation
functions and collective excitations spectra. As the collective excitations
have a finite energy gap, we conclude that the ground state is a spin-orbital
liquid with a two-fold degeneracy, which breaks the discrete spin-orbital
symmetry. Possible relevence of this spin liquid state to several realistic
systems, such as CaVV and NaSbTiO, are discussed.Comment: 4 pages with 1 figur
Density Matrices for a Chain of Oscillators
We consider chains with an optical phonon spectrum and study the reduced
density matrices which occur in density-matrix renormalization group (DMRG)
calculations. Both for one site and for half of the chain, these are found to
be exponentials of bosonic operators. Their spectra, which are correspondingly
exponential, are determined and discussed. The results for large systems are
obtained from the relation to a two-dimensional Gaussian model.Comment: 15 pages,8 figure
Strategies for Searching Video Content with Text Queries or Video Examples
The large number of user-generated videos uploaded on to the Internet
everyday has led to many commercial video search engines, which mainly rely on
text metadata for search. However, metadata is often lacking for user-generated
videos, thus these videos are unsearchable by current search engines.
Therefore, content-based video retrieval (CBVR) tackles this metadata-scarcity
problem by directly analyzing the visual and audio streams of each video. CBVR
encompasses multiple research topics, including low-level feature design,
feature fusion, semantic detector training and video search/reranking. We
present novel strategies in these topics to enhance CBVR in both accuracy and
speed under different query inputs, including pure textual queries and query by
video examples. Our proposed strategies have been incorporated into our
submission for the TRECVID 2014 Multimedia Event Detection evaluation, where
our system outperformed other submissions in both text queries and video
example queries, thus demonstrating the effectiveness of our proposed
approaches
Black Holes with a Generalized Gravitational Action
Microscopic black holes are sensitive to higher dimension operators in the
gravitational action. We compute the influence of these operators on the
Schwarzschild solution using perturbation theory. All (time reversal invariant)
operators of dimension six are included (dimension four operators don't alter
the Schwarzschild solution). Corrections to the relation between the Hawking
temperature and the black hole mass are found. The entropy is calculated using
the Gibbons-Hawking prescription for the Euclidean path integral and using
naive thermodynamic reasoning. These two methods agree, however, the entropy is
not equal to 1/4 the area of the horizon.Comment: plain tex(uses phyzzx.tex), 8 pages, CALT-68-185
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