4,570 research outputs found
Hund and pair-hopping signature in transport properties of degenerate nanoscale devices
We investigate the signature of a complete Coulomb interaction in transport
properties of double-orbital nanoscale devices. We analyze the specific effects
of Hund exchange and pair hopping terms, calculating in particular stability
diagrams. It turns out that a crude model, with partial Coulomb interaction,
may lead to a misinterpretation of experiments. In addition, it is shown that
spectral weight transfers induced by gate and bias voltages strongly influence
charge current. The low temperature regime is also investigated, displaying
inelastic cotunneling associated with the exchange term, as well as Kondo
conductance enhancement.Comment: 5 pages, 4 figure
Conditions for requiring nonlinear thermoelectric transport theory in nanodevices
In this paper, we examine the conditions under which the nonlinear transport
theory is inescapable, when a correlated quantum dot is symmetrically coupled
to two leads submitted to temperature and voltage biases. By detailed numerical
comparisons between nonlinear and linear currents, we show that the claimed
nonlinear behavior in a temperature gradient for the electric current is not so
genuine, and the linear theory made at the operating temperature is unexpectedly robust. This is demonstrated for the single
impurity Anderson model, in different regimes: resonant tunneling, Coulomb
blockade and Kondo regimes
Fluctuations, correlations and the sign problem in QCD
We study the distribution of the phase angle and the magnitude of the fermion
determinant as well as its correlation with the chiral condensate and the
baryon number for QCD at non-zero quark chemical potential. Results are derived
to one-loop order in Chiral Perturbation Theory (ChPT), as well as by
analytical and numerical calculations in QCD in one Euclidean dimension. We
find a qualitative change of the distribution of the phase of the fermion
determinant when the quark mass enters the spectrum of the Dirac operator: it
changes from a periodicized Gaussian distribution to a periodicized Lorentzian
distribution. We also explore the possibility that some observables remain
weakly correlated with the phase of the fermion determinant even though the
sign problem is severe. We discuss the practical implications of our findings
on lattice simulations of QCD at non-zero baryon chemical potential.Comment: Presented at the XXVII International Symposium on Lattice Field
Theory, July 26-31, 2009, Peking University, Beijing, China, 7 page
QCD in One Dimension at Nonzero Chemical Potential
Using an integration formula recently derived by Conrey, Farmer and
Zirnbauer, we calculate the expectation value of the phase factor of the
fermion determinant for the staggered lattice QCD action in one dimension. We
show that the chemical potential can be absorbed into the quark masses; the
theory is in the same chiral symmetry class as QCD in three dimensions at zero
chemical potential. In the limit of a large number of colors and fixed number
of lattice points, chiral symmetry is broken spontaneously, and our results are
in agreement with expressions based on a chiral Lagrangian. In this limit, the
eigenvalues of the Dirac operator are correlated according to random matrix
theory for QCD in three dimensions. The discontinuity of the chiral condensate
is due to an alternative to the Banks-Casher formula recently discovered for
QCD in four dimensions at nonzero chemical potential. The effect of temperature
on the average phase factor is discussed in a schematic random matrix model.Comment: Latex, 23 pages and 5 figures; Added two references and corrected
several typo
Geometric phases under the presence of a composite environment
We compute the geometric phase for a spin-1/2 particle under the presence of
a composite environment, composed of an external bath (modeled by an infinite
set of harmonic oscillators) and another spin-1/2 particle. We consider both
cases: an initial entanglement between the spin-1/2 particles and an initial
product state in order to see if the initial entanglement has an enhancement
effect on the geometric phase of one of the spins. We follow the nonunitary
evolution of the reduced density matrix and evaluate the geometric phase for a
single two-level system. We also show that the initial entanglement enhances
the sturdiness of the geometric phase under the presence of an external
composite environment.Comment: 10 pages, 12 figures. Version to appear in Phys. Rev.
BCS-BEC crossover of neutron pairs in symmetric and asymmetric nuclear matter
We propose new types of density dependent contact pairing interaction which
reproduce the pairing gaps in symmetric and neutron matter obtained by a
microscopic treatment based on the nucleon-nucleon interaction. These
interactions are able to simulate the pairing gaps of either the bare
interaction or the interaction screened by the medium polarization effects. It
is shown that the medium polarization effects cannot be cast into the density
power law function usually introduced together with the contact interaction and
require the introduction of another isoscalar term. The BCS-BEC crossover of
neutrons pairs in symmetric and symmetric nuclear matter is studied by using
these contact interactions. It is shown that the bare and screened pairing
interactions lead to different features of the BCS-BEC crossover in symmetric
nuclear matter. For the screened pairing interaction, a two-neutron BEC state
is formed in symmetric matter at fm (neutron density
). Contrary the bare interaction does not form the
BEC state at any neutron density
One,Two,Zero: Scales of Strong Interactions
We discuss our results on QCD with a number of fundamental fermions ranging
from zero to sixteen. These theories exhibit a wide array of fascinating
phenomena which have been under close scrutiny, especially in recent years,
first and foremost is the approach to conformality. To keep this review
focused, we have chosen scale generation, or lack thereof as a guiding theme,
however the discussion will be set in the general framework of the analysis of
the phases and phase transitions of strong interactions at zero and nonzero
temperature.Comment: 15 pages, prepared for IJMPA Special Issue 'Recent Nonperturbative
Developments in QCD-like Theories
On the particle spectrum and the conformal window
We study the SU(3) gauge theory with twelve flavours of fermions in the
fundamental representation as a prototype of non-Abelian gauge theories inside
the conformal window. Guided by the pattern of underlying symmetries, chiral
and conformal, we analyze the two-point functions theoretically and on the
lattice, and determine the finite size scaling and the infinite volume fermion
mass dependence of the would-be hadron masses. We show that the spectrum in the
Coulomb phase of the system can be described in the context of a universal
scaling analysis and we provide the nonperturbative determination of the
fermion mass anomalous dimension gamma*=0.235(46) at the infrared fixed point.
We comment on the agreement with the four-loop perturbative prediction for this
quantity and we provide a unified description of all existing lattice results
for the spectrum of this system, them being in the Coulomb phase or the
asymptotically free phase. Our results corroborate the view that the fixed
point we are studying is not associated to a physical singularity along the
bare coupling line and estimates of physical observables can be attempted on
either side of the fixed point. Finally, we observe the restoration of the U(1)
axial symmetry in the two-point functions.Comment: 40 pages, 22 figure
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