19,280 research outputs found
Quantum phase transitions and Berezinskii-Kosterlitz-Thouless temperature in a two-dimensional spin-orbit-coupled Fermi gas
We study the effect of spin-orbit coupling on both the zero-temperature and
non-zero temperature behavior of a two-dimensional (2D) Fermi gas. We include a
generic combination of Rashba and Dresselhaus terms into the system
Hamiltonian, which allows us to study both the experimentally relevant
equal-Rashba-Dresselhaus (ERD) limit and the Rashba-only (RO) limit. At zero
temperature, we derive the phase diagram as a function of the two-body binding
energy and Zeeman field. In the ERD case, this phase diagram reveals several
topologically distinct uniform superfluid phases, classified according to the
nodal structure of the quasiparticle excitation energies. Furthermore, we use a
momentum dependent SU(2)-rotation to transform the system into a generalized
helicity basis, revealing that spin-orbit coupling induces a triplet pairing
component of the order parameter. At non-zero temperature, we study the
Berezinskii-Kosterlitz-Thouless (BKT) phase transition by including phase
fluctuations of the order parameter up to second order. We show that the
superfluid density becomes anisotropic due to the presence of spin-orbit
coupling (except in the RO case). This leads both to elliptic vortices and
antivortices, and to anisotropic sound velocities. The latter prove to be
sensitive to quantum phase transitions between topologically distinct phases.
We show further that at a fixed non-zero Zeeman field, the BKT critical
temperature is increased by the presence of ERD spin-orbit coupling.
Subsequently, we demonstrate that the Clogston limit becomes infinite:
remains non-zero at all finite values of the Zeeman field. We
conclude by extending the quantum phase transition lines to non-zero
temperature, using the nodal structure of the quasiparticle spectrum, thus
connecting the BKT critical temperature with the zero-temperature results.Comment: 17 pages, 7 figure
Effects of spin-orbit coupling on the Berezinskii-Kosterlitz-Thouless transition and the vortex-antivortex structure in two-dimensional Fermi gases
We investigate the Berezinskii-Kosterlitz-Thouless (BKT) transition in a
two-dimensional (2D) Fermi gas with spin-orbit coupling (SOC), as a function of
the two-body binding energy and a perpendicular Zeeman field. By including a
generic form of the SOC, as a function of Rashba and Dresselhaus terms, we
study the evolution between the experimentally relevant equal
Rashba-Dresselhaus (ERD) case and the Rashba-only (RO) case. We show that in
the ERD case, at fixed non-zero Zeeman field, the BKT transition temperature
is increased by the effect of SOC for all values of the binding
energy. We also find a significant increase in the value of the Clogston limit
compared to the case without SOC. Furthermore, we demonstrate that the
superfluid density tensor becomes anisotropic (except in the RO case), leading
to an anisotropic phase-fluctuation action that describes elliptic vortices and
antivortices, which become circular in the RO limit. This deformation
constitutes an important experimental signature for superfluidity in a 2D Fermi
gas with ERD SOC. Finally, we show that the anisotropic sound velocities
exhibit anomalies at low temperatures, in the vicinity of quantum phase
transitions between topologically distinct uniform superfluid phases.Comment: 5 pages, 3 figure
Noisy One-Way Quantum Computations: The Role of Correlations
A scheme to evaluate computation fidelities within the one-way model is
developed and explored to understand the role of correlations in the quality of
noisy quantum computations. The formalism is promptly applied to many
computation instances, and unveils that a higher amount of entanglement in the
noisy resource state does not necessarily imply a better computation.Comment: 10 pages, 6 figures, extension of a previous versio
Remarks on Charged Vortices in the Maxwell-Chern-Simons Model
We study vortex-like configuration in Maxwell-Chern-Simons Electrodynamics.
Attention is paid to the similarity it shares with the Nielsen-Olesen solutions
at large distances. A magnetic symmetry between a point-like and an
azimuthal-like current in this framework is also pointed out. Furthermore, we
address the issue of a neutral and spinless particle interacting with a charged
vortex, and obtain that the Aharonov-Casher-type phase depends upon mass and
distance parameters.Comment: New refs. added. Version accepted for publication in Phys. Lett.
Causal Structure and Birefringence in Nonlinear Electrodynamics
We investigate the causal structure of general nonlinear electrodynamics and
determine which Lagrangians generate an effective metric conformal to
Minkowski. We also proof that there is only one analytic nonlinear
electrodynamics presenting no birefringence.Comment: 11 pages, no figure
Schwinger's Principle and Gauge Fixing in the Free Electromagnetic Field
A manifestly covariant treatment of the free quantum eletromagnetic field, in
a linear covariant gauge, is implemented employing the Schwinger's Variational
Principle and the B-field formalism. It is also discussed the abelian Proca's
model as an example of a system without constraints.Comment: 8 pages. Format PTPtex. No figur
Fundraising and vote distribution: a non-equilibrium statistical approach
The number of votes correlates strongly with the money spent in a campaign,
but the relation between the two is not straightforward. Among other factors,
the output of a ballot depends on the number of candidates, voters, and
available resources. Here, we develop a conceptual framework based on Shannon
entropy maximization and Superstatistics to establish a relation between the
distributions of money spent by candidates and their votes. By establishing
such a relation, we provide a tool to predict the outcome of a ballot and to
alert for possible misconduct either in the report of fundraising and spending
of campaigns or on vote counting. As an example, we consider real data from a
proportional election with candidates, where a detailed data
verification is virtually impossible, and show that the number of potential
misconducting candidates to audit can be reduced to only nine
Genetic gain in an improvement program of irrigated rice in Minas Gerais.
An evaluation of the genetic improvement programme of irrigated rice of Minas Gerais (Brazil) estimated the genetic gain obtained in the 90s. Grain yield data of the advanced comparative trials of cultivars and lines of continuously flooded rice, conducted from 1990-91 to 2000-01, were used. The estimate of the genetic gain was obtained by the methodology of the adjusted means proposed by Breseghello (1998). The mean annual genetic gain in the 90s was 42.45+or-17.89 kg ha-1 (0.7% per year). The improvement programme proved auspicious for the development of lines that outmatched the controls. The mean of the cultivars released in the 90s did however not outstrip the mean of the elite lines, which were the genotypes with the highest means in this study and will be further evaluated in the ongoing programme
Filaments in Galactic Winds Driven by Young Stellar Clusters
The starburst galaxy M82 shows a system of H-emitting filaments which
extend to each side of the galactic disk. We model these filaments as the
result of the interaction between the winds from a distribution of Super
Stellar Clusters (SSCs). We first derive the condition necessary for producing
a radiative interaction between the cluster winds (a condition which is met by
the SSC distribution of M82). We then compute 3D simulations for SSC wind
distributions which satisfy the condition for a radiative interaction, and also
for distributions which do not satisfy this condition. We find that the highly
radiative models, that result from the interaction of high metallicity cluster
winds, produce a structure of H emitting filaments, which qualitatively
agrees with the observations of the M82, while the non-radiative SSC wind
interaction models do not produce filamentary structures. Therefore, our
criterion for radiative interactions (which depends on the mass loss rate and
the terminal velocity of the SSC winds, and the mean separation between SSCs)
can be used to predict whether or not an observed galaxy should have associated
H emitting filaments.Comment: 10 pages, 6 Figures. ApJ Accepted, August 7, 200
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