2,624 research outputs found

### Dielectric Breakdown of a Mott Insulator

We study the nonequilibrium steady state of a Mott insulator coupled to a
thermostat and driven by a constant electric field, starting from weak fields,
until the dielectric breakdown, and beyond. We find that the conventional Zener
picture does not describe the steady-state physics. In particular, the current
at weak field is found to be controlled by the dissipation. Moreover, in
connection with the electric-field-driven dimensional crossover, we find that
the dielectric breakdown occurs when the field strength is on the order of the
Mott gap of the corresponding lower-dimensional system. We also report a
resonance and the meltdown of the quasiparticle peak when the field strength is
half of this Mott gap.Comment: 5 pages, 5 figures. v2: references adde

### Approximate programmable quantum processors

A quantum processor is a programmable quantum circuit in which both the data
and the program, which specifies the operation that is carried out on the data,
are quantum states. We study the situation in which we want to use such a
processor to approximate a set of unitary operators to a specified level of
precision. We measure how well an operation is performed by the process
fidelity between the desired operation and the operation produced by the
processor. We show how to find the program for a given processor that produces
the best approximation of a particular unitary operation. We also place bounds
on the dimension of the program space that is necessary to approximate a set of
unitary operators to a specified level of precision.Comment: 8 page

### Thermoelectric properties of AgGaTe$_2$ and related chalcopyrite structure materials

We present an analysis of the potential thermoelectric performance of p-type
AgGaTe$_{2}$, which has already shown a $ZT$ of 0.8 with partial optimization,
and observe that the same band structure features, such as a mixture of light
and heavy bands and isotropic transport, that lead to this good performance are
present in certain other ternary chalcopyrite structure semiconductors. We find
that optimal performance of AgGaTe$_2$ will be found for hole concentrations
between 4 $\times 10^{19}$ and 2 $\times 10^{20}$cm$^{-3}$ at 900 K, and 2
$\times 10^{19}$ and 10$^{20}$ cm$^{-3}$ at 700 K, and that certain other
chalcopyrite semiconductors might show good thermoelectric performance at
similar doping ranges and temperatures if not for higher lattice thermal
conductivity

### Negative effective mass transition and anomalous transport in power-law hopping bands

We study the stability of spinless Fermions with power law hopping $H_{ij}
\propto |i - j|^{-\alpha}$. It is shown that at precisely $\alpha_c =2$, the
dispersive inflection point coalesces with the band minimum and the charge
carriers exhibit a transition into negative effective mass regime, $m_\alpha^*
< 0$ characterized by retarded transport in the presence of an electric field.
Moreover, bands with $\alpha < 2$ must be accompanied by counter-carriers with
$m_\alpha^* > 0$, having a positive band curvature, thus stabilizing the system
in order to maintain equilibrium conditions and a proper electrical response.
We further examine the semi-classical transport and response properties,
finding an infrared divergent conductivity for 1/r hopping($\alpha =1$). The
analysis is generalized to regular lattices in dimensions $d$ = 1, 2, and 3.Comment: 6 pages. 2 figure

### Impurity states in graphene with intrinsic spin-orbit interaction

We consider the problem of electron energy states related to strongly
localized potential of a single impurity in graphene. Our model simulates the
effect of impurity atom substituting the atom of carbon, on the energy spectrum
of electrons near the Dirac point. We take into account the internal spin-orbit
interaction, which can modify the structure of electron bands at very small
neighborhood of the Dirac point, leading to the energy gap. This makes possible
the occurrence of additional impurity states in the vicinity of the gap.Comment: 10 pages, 5 figure

### Green-Function-Based Monte Carlo Method for Classical Fields Coupled to Fermions

Microscopic models of classical degrees of freedom coupled to non-interacting
fermions occur in many different contexts. Prominent examples from solid state
physics are descriptions of colossal magnetoresistance manganites and diluted
magnetic semiconductors, or auxiliary field methods for correlated electron
systems. Monte Carlo simulations are vital for an understanding of such
systems, but notorious for requiring the solution of the fermion problem with
each change in the classical field configuration. We present an efficient,
truncation-free O(N) method on the basis of Chebyshev expanded local Green
functions, which allows us to simulate systems of unprecedented size N.Comment: 4 pages, 3 figure

### Mott transition in the Hubbard model away from particle-hole symmetry

We solve the Dynamical Mean Field Theory equations for the Hubbard model away
from the particle-hole symmetric case using the Density Matrix Renormalization
Group method. We focus our study on the region of strong interactions and
finite doping where two solutions coexist. We obtain precise predictions for
the boundaries of the coexistence region. In addition, we demonstrate the
capabilities of this precise method by obtaining the frequency dependent
optical conductivity spectra.Comment: 4 pages, 4 figures; updated versio

### Dominant role of impurity scattering over crystalline anisotropy for magnetotransport properties in the quasi-1D Hollandite Ba1.2Rh8O16

Angular magnetotransport measurements have been performed to tackle the
origin of the magnetoresistance in the quasi-1D Hollandite Ba1.2Rh8O16. Three
samples of different impurities amount were measured. We observe that the low
temperature resistivity upturn is not due to a charge density wave transition,
and a dominant role of impurities scattering for low temperature transport
properties is instead demonstrated. The components of magnetoresistance were
separated by using the Kohler plot and the angular dependency of the resistance
under magnetic field. It shows the major contribution of an isotropic, likely
spin driven, negative magnetoresistance. Galvanomagnetic characteristics are
then consistent with a Kondo effect and appear to be essentially 3D at low
temperature.Comment: accepted for publication in PR

### Concurrence vs. purity: Influence of local channels on Bell states of two qubits

We analyze how a maximally entangled state of two-qubits (e.g., the singlet
$\psi_s$) is affected by action of local channels described by completely
positive maps \cE . We analyze the concurrence and the purity of states
\varrho_\cE=\cE\otimes\cI[\psi_s].Using the concurrence-{\it vs}-purity phase
diagram we characterize local channels \cE by their action on the singlet
state $\psi_s$. We specify a region of the concurrence-{\it vs.}-purity diagram
that is achievable from the singlet state via the action of unital channels. We
show that even most general (including non-unital) local channels acting just
on a single qubit of the original singlet state cannot generate the maximally
entangled mixed states (MEMS). We study in detail various time evolutions of
the original singlet state induced by local Markovian semigroups. We show that
the decoherence process is represented in the concurrence-{\it vs.}-purity
diagram by a line that forms the lower bound of the achievable region for
unital maps. On the other hand, the depolarization process is represented by a
line that forms the upper bound of the region of maps induced by unital maps.Comment: 9 pages, 6 figure

### Theory of cooling by flow through narrow pores

We consider the possibility of adding a stage to a dilution refrigerator to
provide additional cooling by ``filtering out'' hot atoms. Three methods are
considered: 1) Effusion, where holes having diameters larger than a mean-free
path allow atoms to pass through easily; 2) Particle waveguide-like motion
using very narrow channels that greatly restrict the quantum states of the
atoms in a channel. 3) Wall-limited diffusion through channels, in which the
wall scattering is disordered so that local density equilibrium is established
in a channel. We assume that channel dimension are smaller than the mean-free
path for atom-atom interactions. The particle waveguide and the wall-limited
diffusion methods using channels on order of the de Broglie wavelength give
cooling. Recent advances in nano-filters give this method some hope of being
practical.Comment: 10 pages, 3 figures. Corrected typos and made some minor wording
change

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