6,256 research outputs found
Reversal of particle-hole scattering-rate asymmetry in Anderson impurity model
We study the particle-hole asymmetry of the scattering rate in strongly
correlated electron systems by examining the cubic and
terms in the imaginary part of the self-energy of the Anderson impurity model.
We show that the sign is opposite in the weak-coupling and strong-coupling
limits, explaining the differences found in theoretical approaches taking the
respective limits as the starting points. The sign change in fact precisely
delineates the cross-over between the weak and strong correlation regimes of
the model. For weak interaction the sign reversal occurs for small values
of the doping , while for interaction of order , being the hybridization strength, the cross-over curve
rapidly shifts to the large-doping range. This curve based on the impurity
dynamics is genuinely different from other cross-over curves defined through
impurity thermodynamic and static properties.Comment: 4 pages, 5 figure
On the Capacity of the Finite Field Counterparts of Wireless Interference Networks
This work explores how degrees of freedom (DoF) results from wireless
networks can be translated into capacity results for their finite field
counterparts that arise in network coding applications. The main insight is
that scalar (SISO) finite field channels over are analogous
to n x n vector (MIMO) channels in the wireless setting, but with an important
distinction -- there is additional structure due to finite field arithmetic
which enforces commutativity of matrix multiplication and limits the channel
diversity to n, making these channels similar to diagonal channels in the
wireless setting. Within the limits imposed by the channel structure, the DoF
optimal precoding solutions for wireless networks can be translated into
capacity optimal solutions for their finite field counterparts. This is shown
through the study of the 2-user X channel and the 3-user interference channel.
Besides bringing the insights from wireless networks into network coding
applications, the study of finite field networks over also
touches upon important open problems in wireless networks (finite SNR, finite
diversity scenarios) through interesting parallels between p and SNR, and n and
diversity.Comment: Full version of paper accepted for presentation at ISIT 201
Quantum phase transition in capacitively coupled double quantum dots
We investigate two equivalent, capacitively coupled semiconducting quantum
dots, each coupled to its own lead, in a regime where there are two electrons
on the double dot. With increasing interdot coupling a rich range of behavior
is uncovered: first a crossover from spin- to charge-Kondo physics, via an
intermediate SU(4) state with entangled spin and charge degrees of freedom;
followed by a quantum phase transition of Kosterlitz-Thouless type to a
non-Fermi liquid `charge-ordered' phase with finite residual entropy and
anomalous transport properties. Physical arguments and numerical
renormalization group methods are employed to obtain a detailed understanding
of the problem.Comment: 4 pages, 3 figure
Reduced Complexity Filtering with Stochastic Dominance Bounds: A Convex Optimization Approach
This paper uses stochastic dominance principles to construct upper and lower
sample path bounds for Hidden Markov Model (HMM) filters. Given a HMM, by using
convex optimization methods for nuclear norm minimization with copositive
constraints, we construct low rank stochastic marices so that the optimal
filters using these matrices provably lower and upper bound (with respect to a
partially ordered set) the true filtered distribution at each time instant.
Since these matrices are low rank (say R), the computational cost of evaluating
the filtering bounds is O(XR) instead of O(X2). A Monte-Carlo importance
sampling filter is presented that exploits these upper and lower bounds to
estimate the optimal posterior. Finally, using the Dobrushin coefficient,
explicit bounds are given on the variational norm between the true posterior
and the upper and lower bounds
Musical chairs: a comment on the credit crisis.
Uncertainty –that is, a rise in unknown and immeasurable risk rather than the measurable risk that the financial sector specializes in managing– is at the heart of the recent liquidity crisis. The financial instruments and derivative structures underpinning the recent growth in credit markets are complex. Because of the rapid proliferation of these instruments, market participants cannot refer to a historical record to measure how these financial structures will behave during a time of stress. These two factors, complexity and lack of history, are the preconditions for rampant uncertainty. We explain how a rise in uncertainty can cause a liquidity crisis and discuss central bank policies in this context.
Spectral properties in the charge density wave phase of the half-filled Falicov-Kimball Model
We study the spectral properties of charge density wave (CDW) phase of the
half-filled spinless Falicov-Kimball model within the framework of the
Dynamical Mean Field Theory. We present detailed results for the spectral
function in the CDW phase as function of temperature and . We show how the
proximity of the non-fermi liquid phase affects the CDW phase, and show that
there is a region in the phase diagram where we get a CDW phase without a gap
in the spectral function. This is a radical deviation from the mean-field
prediction where the gap is proportional to the order parameter
Doping a correlated band insulator: A new route to half metallic behaviour
We demonstrate in a simple model the surprising result that turning on an
on-site Coulomb interaction U in a doped band insulator leads to the formation
of a half-metallic state. In the undoped system, we show that increasing U
leads to a first order transition between a paramagnetic, band insulator and an
antiferomagnetic Mott insulator at a finite value U_{AF}. Upon doping, the
system exhibits half metallic ferrimagnetism over a wide range of doping and
interaction strengths on either side of U_{AF}. Our results, based on dynamical
mean field theory, suggest a novel route to half-metallic behavior and provide
motivation for experiments on new materials for spintronics.Comment: 5 pages, 7 figure
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