28,213 research outputs found
Propagation of relativistic charged particles in ultracold atomic gases with Bose-Einstein condensates
We study theoretically some effects produced by a propagation of the charged
particles in dilute gases of alkali-metal atoms in the state with Bose-Einstein
condensates. The energy change of the high-speed (relativistic) particle that
corresponds to the Cherenkov effect in the condensate is investigated. We show
that in the studied cases the particle can both loose and receive the energy
from a gas. We find the necessary conditions for the particle acceleration in
the multi-component condensate. It is shown that the Cherenkov effect in
Bose-Einstein condensates can be used also for defining the spectral
characteristics of atoms.Comment: 6 pages, 3 figure
Squeezars: Tidally powered stars orbiting a massive black hole
We propose that there exists a class of transient sources, "squeezars", which
are stars caught in highly eccentric orbits around a massive (m<10^8 Mo) black
hole (MBH), whose atypically high luminosity (up to a significant fraction of
their Eddington luminosity) is powered by tidal interactions with the MBH.
Their existence follows from the presence of a mass sink, the MBH, in the
galactic center, which drives a flow of stars into nearly radial orbits to
replace those it has destroyed. We consider two limits for the stellar response
to tidal heating: surface heating with radiative cooling ("hot squeezars") and
bulk heating with adiabatic expansion ("cold squeezars"), and calculate the
evolution of the squeezar orbit, size, luminosity and effective temperature.
The squeezar formation rate is only ~0.05 that of tidal disruption flares, but
squeezar lifetimes are many orders of magnitude longer, and so future
observations of squeezars in nearby galaxies can probe the tidal process that
feeds MBHs and the effects of extreme tides on stars. The mean number of
squeezars orbiting the Galactic MBH is estimated at 0.1-1.Comment: ApJ Lett. accepted. 4 pp. 1 fi
Stability conditions and Fermi surface topologies in a superconductor
Candidate homogeneous, isotropic superfluid or superconducting states of
paired fermion species with different chemical potentials, can lead to
quasiparticle excitation energies that vanish at either zero, one, or two
spheres in momentum space. With no zeroes, we have a conventional BCS
superconductor. The other two cases, ``gapless'' superconductors, appear in
mean field theory for sufficiently large mismatches and/or sufficiently large
coupling strengths. Here we examine several stability criteria for those
candidate phases. Positivity of number susceptibility appears to provide the
most powerful constraint, and renders all the two-zero states that we have
examined mechanically unstable. Our results should apply directly to ultracold
fermionic atom systems.Comment: 18 pages, 7 figures; v2: some clarifications in Sec. IIC; references
added; version accepted for publication in Phys. Rev.
Cherenkov Radiation from Pairs and Its Effect on Induced Showers
We calculate the Cherenkov radiation from an pair at small
separations, as occurs shortly after a pair conversion. The radiation is
reduced (compared to that from two independent particles) when the pair
separation is smaller than the wavelength of the emitted light. We estimate the
reduction in light in large electromagnetic showers, and discuss the
implications for detectors that observe Cherenkov radiation from showers in the
Earth's atmosphere, as well as in oceans and Antarctic ice.Comment: Final version, with minor changes, to appear in PRD. 5 pages with 4
figure
Superfluid Motion of Light
Superfluidity, the ability of a fluid to move without dissipation, is one of
the most spectacular manifestations of the quantum nature of matter. We explore
here the possibility of superfluid motion of light. Controlling the speed of a
light packet with respect to a defect, we demonstrate the presence of
superfluidity and, above a critical velocity, its breakdown through the onset
of a dissipative phase. We describe a possible experimental realization based
on the transverse motion through an array of waveguides. These results open new
perspectives in transport optimization.Comment: 4 pages, 3 figure
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