81 research outputs found
On amplitude oscillation of vibrations of strongly anisotropic high-temperature superconductors of BiPbSrCaCuO system
Effect of oscillations of the vibration amplitude of cylindrical sample
suspended by a thin elastic thread and vibrating in a transverse magnetic field
and containing 2D quasi-two-dimensional vortices (pancakes), was observed in
the strongly anisotropic high- superconductor of
system.Comment: 8 pages, 7 figure
Density Waves in a Transverse Electric Field
In a quasi-one-dimensional conductor with an open Fermi surface, a Charge or
a Spin Density Wave phase can be destroyed by an electric field perpendicular
to the direction of high conductivity. This mechanism, due to the breakdown of
electron-hole symmetry, is very similar to the orbital destruction of
superconductivity by a magnetic field, due to time-reversal symmetry.Comment: 3 pages, Latex, 2 figures, to appear in Phys. Rev. B Rapid Com
Exponential martingales and changes of measure for counting processes
We give sufficient criteria for the Dol\'eans-Dade exponential of a
stochastic integral with respect to a counting process local martingale to be a
true martingale. The criteria are adapted particularly to the case of counting
processes and are sufficiently weak to be useful and verifiable, as we
illustrate by several examples. In particular, the criteria allow for the
construction of for example nonexplosive Hawkes processes as well as counting
processes with stochastic intensities depending on diffusion processes
Disorder and thermally driven vortex-lattice melting in La{2-x}Sr{x}CuO{4} crystals
Magnetization measurements in La{2-x}Sr{x}CuO{4} crystals indicate vortex
order-disorder transition manifested by a sharp kink in the second
magnetization peak. The transition field exhibits unique temperature
dependence, namely a strong decrease with temperature in the entire measured
range. This behavior rules out the conventional interpretation of a
disorder-driven transition into an entangled vortex solid phase. It is shown
that the transition in La{2-x}Sr{x}CuO{4} is driven by both thermally- and
disorder-induced fluctuations, resulting in a pinned liquid state. We conclude
that vortex solid-liquid, solid-solid and solid to pinned-liquid transitions
are different manifestations of the same thermodynamic order-disorder
transition, distinguished by the relative contributions of thermal and
disorder-induced fluctuations.Comment: To be published in phys. Rev. B Rapid Com
Casimir Effect on the Worldline
We develop a method to compute the Casimir effect for arbitrary geometries.
The method is based on the string-inspired worldline approach to quantum field
theory and its numerical realization with Monte-Carlo techniques. Concentrating
on Casimir forces between rigid bodies induced by a fluctuating scalar field,
we test our method with the parallel-plate configuration. For the
experimentally relevant sphere-plate configuration, we study curvature effects
quantitatively and perform a comparison with the ``proximity force
approximation'', which is the standard approximation technique. Sizable
curvature effects are found for a distance-to-curvature-radius ratio of a/R >~
0.02. Our method is embedded in renormalizable quantum field theory with a
controlled treatment of the UV divergencies. As a technical by-product, we
develop various efficient algorithms for generating closed-loop ensembles with
Gaussian distribution.Comment: 27 pages, 10 figures, Sect. 2.1 more self-contained, improved data
for Fig. 6, minor corrections, new Refs, version to be published in JHE
Thermal Casimir effect in ideal metal rectangular boxes
The thermal Casimir effect in ideal metal rectangular boxes is considered
using the method of zeta functional regularization. The renormalization
procedure is suggested which provides the finite expression for the Casimir
free energy in any restricted quantization volume. This expression satisfies
the classical limit at high temperature and leads to zero thermal Casimir force
for systems with infinite characteristic dimensions. In the case of two
parallel ideal metal planes the results, as derived previously using thermal
quantum field theory in Matsubara formulation and other methods, are reproduced
starting from the obtained expression. It is shown that for rectangular boxes
the temperature-dependent contribution to the electromagnetic Casimir force can
be both positive and negative depending on side lengths. The numerical
computations of the scalar and electromagnetic Casimir free energy and force
are performed for cubesComment: 10 pages, 4 figures, to appear in Europ. Phys. J.
Tunneling Violates Special Relativity
Experiments with evanescent modes and tunneling particles have shown that i)
their signal velocity may be faster than light, ii) they are described by
virtual particles, iii) they are nonlocal and act at a distance, iv)
experimental tunneling data of phonons, photons, and electrons display a
universal scattering time at the tunneling barrier front, and v) the properties
of evanescent, i.e. tunneling modes is not compatible with the special theory
of relativity
Long-Range Forces of QCD
We consider the scattering of two color dipoles (e.g., heavy quarkonium
states) at low energy - a QCD analog of Van der Waals interaction. Even though
the couplings of the dipoles to the gluon field can be described in
perturbation theory, which leads to the potential proportional to
(N_c^2-1)/R^{7}, at large distances R the interaction becomes totally
non-perturbative. Low-energy QCD theorems are used to evaluate the leading
long-distance contribution \sim (N_f^2-1)/(11N_c - 2N_f)^2 R^{-5/2} exp(-2 \mu
R) (\mu is the Goldstone boson mass), which is shown to arise from the
correlated two-boson exchange. The sum rule which relates the overall strength
of the interaction to the energy density of QCD vacuum is derived.
Surprisingly, we find that when the size of the dipoles shrinks to zero (the
heavy quark limit in the case of quarkonia), the non-perturbative part of the
interaction vanishes more slowly than the perturbative part as a consequence of
scale anomaly. As an application, we evaluate elastic \pi J/\psi and \pi J/\psi
\to \pi \psi' cross sections.Comment: 16pages, 9 eps figures; discussion extended, 2 new references added,
to appear in Phys.Rev.
Network models for localisation problems belonging to the chiral symmetry classes
We consider localisation problems belonging to the chiral symmetry classes,
in which sublattice symmetry is responsible for singular behaviour at a band
centre. We formulate models which have the relevant symmetries and which are
generalisations of the network model introduced previously in the context of
the integer quantum Hall plateau transition. We show that the generalisations
required can be re-expressed as corresponding to the introduction of absorption
and amplification into either the original network model, or the variants of it
that represent disordered superconductors. In addition, we demonstrate that by
imposing appropriate constraints on disorder, a lattice version of the Dirac
equation with a random vector potential can be obtained, as well as new types
of critical behaviour. These models represent a convenient starting point for
analytic discussions and computational studies, and we investigate in detail a
two-dimensional example without time-reversal invariance. It exhibits both
localised and critical phases, and band-centre singularities in the critical
phase approach more closely in small systems the expected asymptotic form than
in other known realisations of the symmetry class.Comment: 14 pages, 15 figures, Submitted to Physical Review
The first dozen years of the history of ITEP Theoretical Physics Laboratory
The theoretical investigations at ITEP in the years 1945-1958 are reviewed.
There are exposed the most important theoretical results, obtained in the
following branches of physics: 1) the theory of nuclear reactors on thermal
neutrons; 2) the hydrogen bomb project ("Tube" in USSR and "Classical Super" in
USA); 3) radiation theory; ~4) low temperature physics; 5) quantum
electrodynamics and quantum field theories; 6) parity violation in weak
interactions, the theory of -decay and other weak processes; 7) strong
interaction and nuclear physics. To the review are added the English
translations of few papers, originally published in Russian, but unknown (or
almost unknown) to Western readers.Comment: 55 pages, 5 fig
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