75 research outputs found
Condensation of N interacting bosons: Hybrid approach to condensate fluctuations
We present a new method of calculating the distribution function and
fluctuations for a Bose-Einstein condensate (BEC) of N interacting atoms. The
present formulation combines our previous master equation and canonical
ensemble quasiparticle techniques. It is applicable both for ideal and
interacting Bogoliubov BEC and yields remarkable accuracy at all temperatures.
For the interacting gas of 200 bosons in a box we plot the temperature
dependence of the first four central moments of the condensate particle number
and compare the results with the ideal gas. For the interacting mesoscopic BEC,
as with the ideal gas, we find a smooth transition for the condensate particle
number as we pass through the critical temperature.Comment: 6 pages, 4 figures, to appear in Phys. Rev. Let
The effect of an external magnetic field on the maximum current of SNINS junctions near the critical temperature
The behaviour of superconducting junctions of SNINS-type (where S is
superconductor, N is normal metal, I is insulator) with anharmonic
current-phase relations in the external magnetic field near the critical
temperature was investigated. The dependence of the maximum current on the
value of the magnetic flux in a wide range of electron transmission coefficient
values was considered. Also, it was investigated how the presence of a normal
layer of an arbitrary thickness in the scale of coherence length and in the
presence of impurities in superconducting regions affect the sensitivity of the
maximum current to the magnetic field magnitude.Comment: 7 pages, 4 figure
Oscillating axion bubbles as alternative to supermassive black holes at galactic centers
Recent observations of near-infrared and X-ray flares from Sagittarius A*,
which is believed to be a supermassive black hole at the Galactic center, show
that the source exhibits about 20-minute periodic variability. Here we provide
arguments based on a quantitative analysis that supermassive objects at
galactic centers may be bubbles of dark matter axions rather than black holes.
An oscillating axion bubble can explain periodic variability of Sagittarius A*
and yields the axion mass about 0.6 meV which fits in the open axion mass
window. The bubble scenario with no other free parameters explains lack of
supermassive "black holes" with mass M<10^6 M_{Sun}. Low-mass bubbles decay
fast and as a result are very rare. We also found that the mass of an axion
bubble can not exceed 1.5\times 10^9 M_{Sun}, in agreement with the upper limit
on the supermassive "black hole" mass obtained from observations. Our finding,
if confirmed, suggests that Einstein general relativity is invalid for strong
gravity and the gravitational field for the bubble effectively becomes
repulsive at large potential. Imaging a shadow of the "black hole" at the
Galactic center with VLBI in the next decade can distinguish between the black
hole and the oscillating axion bubble scenarios. In the case of axion bubble, a
steady shadow will not be observed. Instead, the shadow will appear and
disappear periodically with a period of about 20 min.Comment: 10 pages, 4 figures, added derivation of the exponential metric based
on superposition principl
Unusual condensates in quark and atomic systems
In these lectures we discuss condensates which are formed in quark matter
when it is squeezed and in a gas of fermionic atoms when it is cooled. The
behavior of these two seemingly very different systems reveals striking
similarities. In particular, in both systems the Bose-Einstein condensate to
Bardeen--Cooper-Schrieffer (BEC-BCS) crossover takes place.Comment: Lectures delivered at 8th Moscow school of Physics (33rd ITEP Winter
School of Physics
Correspondence between Andreev reflection and Klein tunneling in bipolar graphene
Andreev reflection at a superconductor and Klein tunneling through an n-p
junction in graphene are two processes that couple electrons to holes -- the
former through the superconducting pair potential Delta and the latter through
the electrostatic potential U. We derive that the energy spectra in the two
systems are identical, at low energies E<<Delta and for an antisymmetric
potential profile U(-x,y)=-U(x,y). This correspondence implies that bipolar
junctions in graphene may have zero density of states at the Fermi level and
carry a current in equilibrium, analogously to superconducting Josephson
junctions. It also implies that nonelectronic systems with the same band
structure as graphene, such as honeycomb-lattice photonic crystals, can exhibit
pseudo-superconducting behavior.Comment: 7 pages, 7 figures; much expanded version, with a revised title, test
of the analytics by computer simulation, temperature dependence of the
persistent current, and an appendix with details of the calculatio
BCS-Bose Crossover in Color Superconductivity
It is shown that the onset of the color superconducting phase occurs in the
BCS-BE crossover region.Comment: 5 pages, LaTeX, references adde
Radiation of Neutron Stars Produced by Superfluid Core
We find that neutron star interior is transparent for collisionless electron
sound, the same way as it is transparent for neutrinos. In the presence of
magnetic field the electron sound is coupled with electromagnetic radiation and
form the fast magnetosonic wave. We find that electron sound is generated by
superfluid vortices in the stellar core. Thermally excited helical vortex waves
produce fast magnetosonic waves in the stellar crust which propagate toward the
surface and transform into outgoing electromagnetic radiation. The vortex
radiation has the spectral index -0.45 and can explain nonthermal radiation of
middle-aged pulsars observed in the infrared, optical and hard X-ray bands. The
radiation is produced in the stellar interior which allows direct determination
of the core temperature. Comparing the theory with available spectra
observations we find that the core temperature of the Vela pulsar is T=8*10^8K,
while the core temperature of PSR B0656+14 and Geminga exceeds 2*10^8K. This is
the first measurement of the temperature of a neutron star core. The
temperature estimate rules out equation of states incorporating Bose
condensations of pions or kaons and quark matter in these objects. Based on the
temperature estimate and cooling models we determine the critical temperature
of triplet neutron superfluidity in the Vela core Tc=(7.5\pm 1.5)*10^9K which
agrees well with recent data on behavior of nucleon interactions at high
energies. Another finding is that in the middle aged neutron stars the vortex
radiation, rather then thermal conductivity, is the main mechanism of heat
transfer from the stellar core to the surface. Electron sound opens a
perspective of direct spectroscopic study of superdense matter in the neutron
star interiors.Comment: 43 pages, 7 figures, to appear in Astrophysical Journa
Giant Josephson current through a single bound state in a superconducting tunnel junction
We study the microscopic structure of the Josephson current in a single-mode
tunnel junction with a wide quasiclassical tunnel barrier. In such a junction
each Andreev bound state carries a current of magnitude proportional to the
{\em amplitude} of the normal electron transmission through the junction.
Tremendous enhancement of the bound state current is caused by the resonance
coupling of superconducting bound states at both superconductor-insulator
interfaces of the junction. The possibility of experimental observation of the
single bound state current is discussed.Comment: 11 pages, [aps,preprint]{revtex
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