6,401 research outputs found
Onset of classical behaviour after a phase transition
We analyze the onset of classical behaviour in a scalar field after a
continuous phase transition, in which the system-field, the long wavelength
order parameter of the model, interacts with an environment of its own
short-wavelength modes. We compute the decoherence time for the system-field
modes from the master equation and compare it with the other time scales of the
model. Within our approximations the decoherence time is in general the
smallest dynamical time scale. Demanding diagonalisation of the decoherence
functional produces identical results. The inclusion of other environmental
fields makes diagonalisation occur even earlier.Comment: Seven pages, no figures. Contributed talk to the Second International
Workshop DICE2004, Piombino, Italy. To be published in the Brazilian Journal
of Physic
Decoherence and Loss of Entanglement in Acoustic Black Holes
We studied the process of decoherence in acoustic black holes. We focused on
the ion trap model proposed by Horstmann et al. (Phys. Rev. Lett. 104, 250403
(2010)) but the formalism is general to any experimental implementation. For
that particular setup, we computed the decoherence time for the experimental
parameters that they proposed. We found that a quantum to classical transition
occurs during the measurement and we proposed improved parameters to avoid such
a feature. We also studied the entanglement between the Hawking-pair phonons
for an acoustic black hole while in contact with a reservoir, through the
quantum correlations, showing that they remain strongly correlated for small
enough times and temperatures.Comment: 5 pages, 2 figures, accepted in Phys. Rev. Let
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
Radiatively Induced Breaking of Conformal Symmetry in a Superpotential
Radiatively induced symmetry breaking is considered for a toy model with one
scalar and one fermion field unified in a superfield. It is shown that the
classical quartic self-interaction of the superfield possesses a quantum
infrared singularity. Application of the Coleman-Weinberg mechanism for
effective potential leads to the appearance of condensates and masses for both
scalar and fermion components. That induces a spontaneous breaking of the
initial classical symmetries: the supersymmetry and the conformal one. The
energy scales for the scalar and fermion condensates appear to be of the same
order, while the renormalization scale is many orders of magnitude higher. A
possibility to relate the considered toy model to conformal symmetry breaking
in the Standard Model is discussed.Comment: Improved final version with new references and misprints corrected, 9
pages , no 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
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