991 research outputs found
Phonon assisted tunneling in Josephson junctions
The expression for additional subgap current in the presence of
electron-phonon interaction is derived. We show that the phonon assisted
tunneling leads to appearance of peaks on current-voltage characteristics at
the Josephson frequencies corresponding to the Raman-active phonons. The
relation of the obtained results to experimental observations are discussed.Comment: 8 pages, submitted to PR
Ab initio calculations of the physical properties of transition metal carbides and nitrides and possible routes to high-Tc
Ab initio linear-response calculations are reported of the phonon spectra and
the electron-phonon interaction for several transition metal carbides and
nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and
superconducting properties are calculated in detail at various pressures and
the normal-pressure results are found to well agree with the experiment.
Factors accounting for the relatively low critical temperatures Tc in
transition metal compounds with light elements are considered and the possible
ways of increasing Tc are discussed.Comment: 19 pages, 7 figure
Critical temperature and giant isotope effect in presence of paramagnons
We reconsider the long-standing problem of the effect of spin fluctuations on
the critical temperature and isotope effect in a phonon-mediated
superconductor. Although the general physics of the interplay between phonons
and paramagnons had been rather well understood, the existing approximate
formulas fail to describe the correct behavior of for general phonon
and paramagnon spectra. Using a controllable approximation, we derive an
analytical formula for which agrees well with exact numerical solutions
of the Eliashberg equations for a broad range of parameters. Based on both
numerical and analytical results, we predict a strong enhancement of the
isotope effect when the frequencies of spin fluctuation and phonons are of the
same order. This effect may have important consequences for near-magnetic
superconductors such as MgCNiComment: 5 pages, 2 figure
Signatures of Quantum Chaos and fermionization in the incoherent transport of bosonic carriers in the Bose-Hubbard chain
We analyse the stationary current of Bose particles across the Bose-Hubbard
chain connected to a battery, focusing on the effect of inter-particle
interactions. It is shown that the current magnitude drastically decreases as
the strength of inter-particle interactions exceeds the critical value which
marks the transition to quantum chaos in the Bose-Hubbard Hamiltonian. We found
that this transition is well reflected in the non-equilibrium many-body density
matrix of the system. Namely, the level-spacing distribution for eigenvalues of
the density matrix changes from Poisson to Wigner-Dyson distributions. With the
further increase of the interaction strength, the Wigner-Dyson spectrum
statistics changes back to the Poisson statistics which now marks
fermionization of the bosonic particles. With respect to the stationary
current, this leads to the counter-intuitive dependence of the current
magnitude on the particle number.Comment: 5 pages, 5 figure
Elastic properties of cubic crystals: Every's versus Blackman's diagram
Blackman's diagram of two dimensionless ratios of elastic constants is
frequently used to correlate elastic properties of cubic crystals with
interatomic bondings. Every's diagram of a different set of two dimensionless
variables was used by us for classification of various properties of such
crystals. We compare these two ways of characterization of elastic properties
of cubic materials and consider the description of various groups of materials,
e.g. simple metals, oxides, and alkali halides. With exception of intermediate
valent compounds, the correlation coefficients for Every's diagrams of various
groups of materials are greater than for Blackaman's diagrams, revealing the
existence of a linear relationship between two dimensionless Every's variables.
Alignment of elements and compounds along lines of constant Poisson's ratio
, ( arbitrary perpendicular to ) is
observed. Division of the stability region in Blackman's diagram into region of
complete auxetics, auxetics and non-auxetics is introduced. Correlations of a
scaling and an acoustic anisotropy parameter are considered.Comment: 8 pages, 9 figures, presented on The Ninth International School on
Theoretical Physics "Symmetry and Structural Properties of Condensed Matter",
5 - 12 September 2007, Myczkowce, Polan
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