6,939 research outputs found
Cold Quark Matter, Quadratic Corrections and Gauge/String Duality
We make an estimate of the quadratic correction in the pressure of cold quark
matter using gauge/string duality.Comment: 7 pages; v.2: reference added; v.3: reference and comments added,
version to appear in PRD; v4. final version to appear in PRD; v.5: key
reference adde
Massive Fields and the 2D String
The first massive level of closed bosonic string theory is studied.
Free-field equations are derived by imposing Weyl invariance on the world
sheet. A two-parameter solution to the equation of motion and constraints is
found in two dimensions with a flat linear-dilaton background. One-to-one
tachyon scattering is studied in this background. The results support Dhar,
Mandal and Wadia's proposal that 2D critical string theory corresponds to the
c=1 matrix model in which both sides of the Fermi sea are excited.Comment: 17 pages, Latex. V2: One ref added, minor rephrasing of the first
paragraph in Sec.3.1, typos in (56) and (57) correcte
Excitation gap of a graphene channel with superconducting boundaries
We calculate the density of states of electron-hole excitations in a
superconductor/normal-metal/superconductor (SNS) junction in graphene, in the
long-junction regime that the superconducting gap is much larger than the
Thouless energy. If the normal region is undoped, the excitation spectrum
consists of neutral modes that propagate along the boundaries - transporting
energy but no charge. These ``Andreev modes'' are a coherent superposition of
electron states from the conduction band and hole states from the valence band,
coupled by specular Andreev reflection at the superconductor. The lowest
Andreev mode has an excitation gap, which depends on the superconducting phase
difference across the SNS graphene channel. At high doping the excitation gap
vanishes and the usual gapless density of states of Andreev levels is
recovered. We use our results to calculate the superconducting phase dependence
of the thermal conductance of the graphene channel.Comment: 8 pages, 10 figure
Frobenius-Perron Resonances for Maps with a Mixed Phase Space
Resonances of the time evolution (Frobenius-Perron) operator P for phase
space densities have recently been shown to play a key role for the
interrelations of classical, semiclassical and quantum dynamics. Efficient
methods to determine resonances are thus in demand, in particular for
Hamiltonian systems displaying a mix of chaotic and regular behavior. We
present a powerful method based on truncating P to a finite matrix which not
only allows to identify resonances but also the associated phase space
structures. It is demonstrated to work well for a prototypical dynamical
system.Comment: 5 pages, 2 figures, 2nd version as published (minor changes
Polaritons and Pairing Phenomena in Bose--Hubbard Mixtures
Motivated by recent experiments on cold atomic gases in ultra high finesse
optical cavities, we consider the problem of a two-band Bose--Hubbard model
coupled to quantum light. Photoexcitation promotes carriers between the bands
and we study the non-trivial interplay between Mott insulating behavior and
superfluidity. The model displays a global U(1) X U(1) symmetry which supports
the coexistence of Mott insulating and superfluid phases, and yields a rich
phase diagram with multicritical points. This symmetry property is shared by
several other problems of current experimental interest, including
two-component Bose gases in optical lattices, and the bosonic BEC-BCS crossover
problem for atom-molecule mixtures induced by a Feshbach resonance. We
corroborate our findings by numerical simulations.Comment: 4 pages, 3 figure
Fluctuations of the Josephson current and electron-electron interactions in superconducting weak links
We derive a microscopic effective action for superconducting contacts with
arbitrary transmission distribution of conducting channels. Provided
fluctuations of the Josephson phase remain sufficiently small our formalism
allows to fully describe fluctuation and interaction effects in such systems.
As compared to the well studied tunneling limit our analysis yields a number of
qualitatively new features which occur due to the presence of subgap Andreev
bound states in the system. We investigate the equilibrium supercurrent noise
and evaluate the electron-electron interaction correction to the Josephson
current across superconducting contacts. At T=0 this correction is found to
vanish for fully transparent contacts indicating the absence of Coulomb effects
in this limit.Comment: 12 pages, 4 figure
Testing the time dependence of the fundamental constants in the spectra of multicharged ions
A new method for measuring a possible time dependence of the fine-structure
constant () is proposed. The method is based on the level-crossing in
two-electron highly-charged ions facilitating resonance laser measurements of
the distance between the levels at the point of crossing. This provides an
enhancement factor of about in Helium-like Europium and thus reduces
the requirements for the relative accuracy of resonance laser measurements at
about .Comment: 11 page
Atom-molecule coexistence and collective dynamics near a Feshbach resonance of cold fermions
Degenerate Fermi gas interacting with molecules near Feshbach resonance is
unstable with respect to formation of a mixed state in which atoms and
molecules coexist as a coherent superposition. Theory of this state is
developed using a mapping to the Dicke model, treating molecular field in the
single mode approximation. The results are accurate in the strong coupling
regime relevant for current experimental efforts. The exact solution of the
Dicke model is exploited to study stability, phase diagram, and nonadiabatic
dynamics of molecular field in the mixed state.Comment: 5 pages, 2 figure
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