8,032 research outputs found
Stimulated Raman backscattering of laser radiation in deep plasma channels
Stimulated Raman backscattering (RBS) of intense laser radiation confined by
a single-mode plasma channel with a radial variation of plasma frequency
greater than a homogeneous-plasma RBS bandwidth is characterized by a strong
transverse localization of resonantly-driven electron plasma waves (EPW). The
EPW localization reduces the peak growth rate of RBS and increases the
amplification bandwidth. The continuum of non-bound modes of backscattered
radiation shrinks the transverse field profile in a channel and increases the
RBS growth rate. Solution of the initial-value problem shows that an
electromagnetic pulse amplified by the RBS in the single-mode deep plasma
channel has a group velocity higher than in the case of homogeneous-plasma
Raman amplification. Implications to the design of an RBS pulse compressor in a
plasma channel are discussed.Comment: 11 pages, 3 figures; submitted to Physics of Plasma
Supersolidity and phase diagram of softcore bosons in a triangular lattice
We study the softcore extended Bose Hubbard model in a two-dimensional
triangular lattice by using the quantum Monte Carlo methods. The ground state
phase diagram of the system exhibits a very fruitful structure. Except the Mott
insulating state, four kinds of solid states with respect to the commensurate
filling factors and are identified. Two of them (CDW II
and CDW III) are newly predicted. In incommensurate fillings, superfluid,
spuersolid as well as phase separation states are detected . As in the case for
the hardcore bosons, a supersolid phase exists in while it is
unstable towards the phase separation in . However, this instability
is refrained in due to the softening of the bosons and then a
supersolid phase survives.Comment: 4 pages, 5 figure
Crossed Andreev reflection in diffusive contacts
Crossed Andreev reflection in multiterminal structures in the diffusive
regime is addressed within the quasiclassical Keldysh-Usadel formalism. The
elastic cotunneling and crossed Andreev reflection of quasiparticles give
nonlocal currents and voltages (depending on the actual biasing of the devices)
by virtue of the induced proximity effect in the normal metal electrodes. The
magnitude of the nonlocal processes is found to scale with the square of the
barrier transparency and to decay exponentially with interface spacing.
Nonlocal cotunneling and crossed Andreev conductances are found to contribute
equally to the nonlocal current, which is of relevance to the use of normal
metal-superconducting heterostructures as sources of entanglement
What makes a crystal supersolid ?
For nearly half a century the supersolid phase of matter has remained
mysterious, not only eluding experimental observation, but also generating a
great deal of controversy among theorists. Recent discovery of what is
interpreted as a non-classical moment of inertia at low temperature in solid
He-4 has elicited much excitement as a possible first observation of a
supersolid phase. In the two years following the discovery, however, more
puzzles than answers have been provided to the fundamental issue of whether the
supersolid phase exists, in helium or any other naturally occurring condensed
matter system. Presently, there is no established theoretical framework to
understand the body of experimental data on He-4. Different microscopic
mechanisms that have been suggested to underlie superfluidity in a perfect
quantum crystal do not seem viable for \he4, for which a wealth of experimental
and theoretical evidence points to an insulating crystalline ground state. This
perspective addresses some of the outstanding problems with the interpretation
of recent experimental observations of the apparent superfluid response in He-4
(seen now by several groups) and discusses various scenarios alternative to the
homogeneous supersolid phase, such as superfluidity induced by extended defects
of the crystalline structure which include grain boundaries, dislocations,
anisotropic stresses, etc. Can a metastable superfluid "glassy" phase exist,
and can it be relevant to some of the experimental observations ? One of the
most interesting and unsolved fundamental questions is what interatomic
potentials, given the freedom to design one, can support an ideal supersolid
phase in continuous space, and can they be found in Nature.Comment: Perspective to appear in Advances in Physics, 25 pages, 7 figure
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