1,254 research outputs found
Validity of the Gor'kov expansion near the upper critical field in type II superconductors
We have examined the validity of the Gor'kov expansion in the strength of the
order parameter of type II superconductors near the upper critical field.
Although the degeneracy of the electron levels in a magnetic field gives non-
perturbative terms in the solution to the Bogoliubov-de Gennes equations we
find, contrary to recent claims, that these non-perturbative terms cancel in
the expression for the thermodynamic potential, and that the traditional
Gor'kov theory is correct sufficiently close to Hc2 at finite temperature. We
have derived conditions for the validity of the Gor'kov theory which
essentially state, that the change in the quasiparticle energies as compared to
the normal state energies cannot be too large compared to the temperature.Comment: 5 pages, 3 figures. One reference adde
Decay of polarons and molecules in a strongly polarized Fermi gas
The ground state of an impurity immersed in a Fermi sea changes from a
polaron to a molecule as the interaction strength is increased.
We show here that the coupling between these two states is strongly
suppressed due to a combination of phase space effects and Fermi statistics,
and that it vanishes much faster than the energy difference between the two
states, thereby confirming the first order nature of the polaron-molecule
transition. In the regime where each state is metastable, we find quasiparticle
lifetimes which are much longer than what is expected for a usual Fermi liquid.
Our analysis indicates that the decay rates are sufficiently slow to be
experimentally observable.Comment: Version accepted in PRL. Added discussion of three-body losses to
deeply bound molecular state
Spin Excitations in a Fermi Gas of Atoms
We have experimentally investigated a spin excitation in a quantum degenerate
Fermi gas of atoms. In the hydrodynamic regime the damping time of the
collective excitation is used to probe the quantum behavior of the gas. At
temperatures below the Fermi temperature we measure up to a factor of 2
reduction in the excitation damping time. In addition we observe a strong
excitation energy dependence for this quantum statistical effect.Comment: 4 pages, 3 figure
The Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817
The historic detection of gravitational waves from a binary neutron star
merger (GW170817) and its electromagnetic counterpart led to the first accurate
(sub-arcsecond) localization of a gravitational-wave event. The transient was
found to be 10" from the nucleus of the S0 galaxy NGC 4993. We report
here the luminosity distance to this galaxy using two independent methods. (1)
Based on our MUSE/VLT measurement of the heliocentric redshift () we infer the systemic recession velocity of the
NGC 4993 group of galaxies in the cosmic microwave background (CMB) frame to be
km s. Using constrained cosmological
simulations we estimate the line-of-sight peculiar velocity to be km s, resulting in a cosmic velocity of km s () and a
distance of Mpc assuming a local Hubble constant of
km s Mpc. (2) Using Hubble Space Telescope
measurements of the effective radius (15.5" 1.5") and contained intensity
and MUSE/VLT measurements of the velocity dispersion, we place NGC 4993 on the
Fundamental Plane (FP) of E and S0 galaxies. Comparing to a frame of 10
clusters containing 226 galaxies, this yields a distance estimate of Mpc. The combined redshift and FP distance is Mpc. This 'electromagnetic' distance estimate is consistent
with the independent measurement of the distance to GW170817 as obtained from
the gravitational-wave signal ( Mpc) and
confirms that GW170817 occurred in NGC 4993.Comment: 9 pages, 5 figure
A dipolar self-induced bosonic Josephson junction
We propose a new scheme for observing Josephson oscillations and macroscopic
quantum self-trapping phenomena in a toroidally confined Bose-Einstein
condensate: a dipolar self-induced Josephson junction. Polarizing the atoms
perpendicularly to the trap symmetry axis, an effective ring-shaped,
double-well potential is achieved which is induced by the dipolar interaction.
By numerically solving the three-dimensional time-dependent Gross-Pitaevskii
equation we show that coherent tunneling phenomena such as Josephson
oscillations and quantum self-trapping can take place. The dynamics in the
self-induced junction can be qualitatively described by a two-mode model taking
into account both s-wave and dipolar interactions.Comment: Major changes. Accepted for publication in EP
Cross-Disorder Genome-Wide Analyses Suggest a Complex Genetic Relationship Between Tourette\u27s Syndrome and OCD
OBJECTIVE: Obsessive-compulsive disorder (OCD) and Tourette\u27s syndrome are highly heritable neurodevelopmental disorders that are thought to share genetic risk factors. However, the identification of definitive susceptibility genes for these etiologically complex disorders remains elusive. The authors report a combined genome-wide association study (GWAS) of Tourette\u27s syndrome and OCD. METHOD: The authors conducted a GWAS in 2,723 cases (1,310 with OCD, 834 with Tourette\u27s syndrome, 579 with OCD plus Tourette\u27s syndrome/chronic tics), 5,667 ancestry-matched controls, and 290 OCD parent-child trios. GWAS summary statistics were examined for enrichment of functional variants associated with gene expression levels in brain regions. Polygenic score analyses were conducted to investigate the genetic architecture within and across the two disorders. RESULTS: Although no individual single-nucleotide polymorphisms (SNPs) achieved genome-wide significance, the GWAS signals were enriched for SNPs strongly associated with variations in brain gene expression levels (expression quantitative loci, or eQTLs), suggesting the presence of true functional variants that contribute to risk of these disorders. Polygenic score analyses identified a significant polygenic component for OCD (p=2x10(-4)), predicting 3.2% of the phenotypic variance in an independent data set. In contrast, Tourette\u27s syndrome had a smaller, nonsignificant polygenic component, predicting only 0.6% of the phenotypic variance (p=0.06). No significant polygenic signal was detected across the two disorders, although the sample is likely underpowered to detect a modest shared signal. Furthermore, the OCD polygenic signal was significantly attenuated when cases with both OCD and co-occurring Tourette\u27s syndrome/chronic tics were included in the analysis (p=0.01). CONCLUSIONS: Previous work has shown that Tourette\u27s syndrome and OCD have some degree of shared genetic variation. However, the data from this study suggest that there are also distinct components to the genetic architectures of these two disorders. Furthermore, OCD with co-occurring Tourette\u27s syndrome/chronic tics may have different underlying genetic susceptibility compared with OCD alone
Metastability in spin polarised Fermi gases and quasiparticle decays
We investigate the metastability associated with the first order transition from normal to superfluid phases in the phase diagram of two-component polarised Fermi gases.We begin by detailing the dominant decay processes of single quasiparticles.Having determined the momentum thresholds of each process and calculated their rates, we apply this understanding to a Fermi sea of polarons by linking its metastability to the stability of individual polarons, and predicting a region of metastability for the normal partially polarised phase. In the limit of a single impurity, this region extends from the interaction strength at which a polarised phase of molecules becomes the groundstate, to the one at which the single quasiparticle groundstate changes character from polaronic to molecular. Our argument in terms of a Fermi sea of polarons naturally suggests their use as an experimental probe. We propose experiments to observe the threshold of the predicted region of metastability, the interaction strength at which the quasiparticle groundstate changes character, and the decay rate of polarons
Ginzburg-Landau-Gor'kov Theory of Magnetic oscillations in a type-II 2-dimensional Superconductor
We investigate de Haas-van Alphen (dHvA) oscillations in the mixed state of a
type-II two-dimensional superconductor within a self-consistent Gor'kov
perturbation scheme. Assuming that the order parameter forms a vortex lattice
we can calculate the expansion coefficients exactly to any order. We have
tested the results of the perturbation theory to fourth and eight order against
an exact numerical solution of the corresponding Bogoliubov-de Gennes
equations. The perturbation theory is found to describe the onset of
superconductivity well close to the transition point . Contrary to
earlier calculations by other authors we do not find that the perturbative
scheme predicts any maximum of the dHvA-oscillations below . Instead we
obtain a substantial damping of the magnetic oscillations in the mixed state as
compared to the normal state. We have examined the effect of an oscillatory
chemical potential due to particle conservation and the effect of a finite
Zeeman splitting. Furthermore we have investigated the recently debated issue
of a possibility of a sign change of the fundamental harmonic of the magnetic
oscillations. Our theory is compared with experiment and we have found good
agreement.Comment: 39 pages, 8 figures. This is a replacement of supr-con/9608004.
Several sections changed or added, including a section on the effect of spin
and the effect of a conserved number of particles. To be published in Phys.
Rev.
Exact particle and kinetic energy densities for one-dimensional confined gases of non-interacting fermions
We propose a new method for the evaluation of the particle density and
kinetic pressure profiles in inhomogeneous one-dimensional systems of
non-interacting fermions, and apply it to harmonically confined systems of up
to N=1000 fermions. The method invokes a Green's function operator in
coordinate space, which is handled by techniques originally developed for the
calculation of the density of single-particle states from Green's functions in
the energy domain. In contrast to the Thomas-Fermi (local density)
approximation, the exact profiles under harmonic confinement show negative
local pressure in the tails and a prominent shell structure which may become
accessible to observation in magnetically trapped gases of fermionic alkali
atoms.Comment: 8 pages, 3 figures, accepted for publication in Phys. Rev. Let
Evaporative Cooling of a Two-Component Degenerate Fermi Gas
We derive a quantum theory of evaporative cooling for a degenerate Fermi gas
with two constituents and show that the optimum cooling trajectory is
influenced significantly by the quantum statistics of the particles. The
cooling efficiency is reduced at low temperatures due to Pauli blocking of
available final states in each binary collision event. We compare the
theoretical optimum trajectory with experimental data on cooling a quantum
degenerate cloud of potassium-40, and show that temperatures as low as 0.3
times the Fermi temperature can now be achieved.Comment: 6 pages, 4 figure
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