3,101 research outputs found

### The yoga of commutators

In the present paper we discuss some recent versions of localisation methods
for calculations in the groups of points of algebraic-like and classical-like
groups. Namely, we describe relative localisation, universal localisation, and
enhanced versions of localisation-completion. Apart from the general strategic
description of these methods, we state some typical technical results of the
conjugation calculus and the commutator calculus. Also, we state several recent
results obtained therewith, such as relative standard commutator formulae,
bounded width of commutators, with respect to the elementary generators, and
nilpotent filtrations of congruence subgroups. Overall, this shows that
localisation methods can be much more efficient, than expected

### Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS

Electron correlation effects are studied in ZrSiS using a combination of
first-principles and model approaches. We show that basic electronic properties
of ZrSiS can be described within a two-dimensional lattice model of two nested
square lattices. High degree of electron-hole symmetry characteristic for ZrSiS
is one of the key features of this model. Having determined model parameters
from first-principles calculations, we then explicitly take electron-electron
interactions into account and show that at moderately low temperatures ZrSiS
exhibits excitonic instability, leading to the formation of a pseudogap in the
electronic spectrum. The results can be understood in terms of
Coulomb-interaction-assisted pairing of electrons and holes reminiscent to that
of an excitonic insulator. Our finding allows us to provide a physical
interpretation to the unusual mass enhancement of charge carriers in ZrSiS
recently observed experimentally.Comment: 6 pages, 4 figures. Final versio

### Langmuir wave linear evolution in inhomogeneous nonstationary anisotropic plasma

Equations describing the linear evolution of a non-dissipative Langmuir wave
in inhomogeneous nonstationary anisotropic plasma without magnetic field are
derived in the geometrical optics approximation. A continuity equation is
obtained for the wave action density, and the conditions for the action
conservation are formulated. In homogeneous plasma, the wave field E
universally scales with the electron density N as E ~ N^{3/4}, whereas the
wavevector evolution varies depending on the wave geometry

### Electron-phonon relaxation in periodic granular films

We study the electron-phonon relaxation in the model of a granular metal
film, where the grains are formed by regularly arranged potential barriers of
arbitrary transparency. The relaxation rate of Debye acoustic phonons is
calculated taking into account two mechanisms of electron-phonon scattering:
the standard Frohlich interaction of the lattice deformation with the electron
density and the interaction mediated by the displacement of grain boundaries
dragged by the lattice vibration. At lowest temperatures, the electron-phonon
cooling power follows the power-law temperature dependence typical for clean
systems, but with the prefactor growing as the transparency of the grain
boundaries decreases.Comment: 8 pages, 4 figure

### Renormalized theory of the ion cyclotron turbulence in magnetic field--aligned plasma shear flow

The analytical treatment of nonlinear evolution of the shear-flow-modified
current driven ion cyclotron instability and shear-flow-driven ion cyclotron
kinetic instabilities of magnetic field--aligned plasma shear flow is
presented. Analysis is performed on the base of the nonlinear dispersion
equation, which accounts for a new combined effect of plasma turbulence and
shear flow. It consists in turbulent scattering of ions across the shear flow
with their convection by shear flow and results in enhanced nonlinear
broadening of ion cyclotron resonances. This effect is found to lead to the
saturation of ion cyclotron instabilities as well as to the development of
nonlinear shear flow driven ion cyclotron instability. 52.35.RaComment: 21 page

### Vacuum energy induced by an impenetrable flux tube of finite radius

We consider the effect of the magnetic field background in the form of a tube
of the finite transverse size on the vacuum of the quantized charged massive
scalar field which is subject to the Dirichlet boundary condition at the edge
of the tube. The vacuum energy is induced, being periodic in the value of the
magnetic flux enclosed in the tube. Our previous study in J. Phys. A: Vol.43,
175401 (2010) is extended to the case of smaller radius of the tube and larger
distances from it. The dependence of the vacuum energy density on the distance
from the tube and on the coupling to the space-time curvature scalar is
comprehensively analyzed.Comment: 11 pages, 8 figures, journal version, abstract extended. arXiv admin
note: substantial text overlap with arXiv:0911.287

### Evidence for two-gap nodeless superconductivity in SmFeAsO$_{0.8}$F$_{0.2}$ from point-contact Andreev-reflection spectroscopy

Point-contact Andreev-reflection measurements were performed in
SmFeAsO_{0.8}F_{0.2} polycrystals with T_c \simeq 53 K. The experimental
conductance curves reproducibly exhibit peaks around \pm 6 mV and shoulders at
V \sim 16-20 mV, indicating the presence of two nodeless superconducting gaps.
While the single-band Blonder-Tinkham-Klapwijk fit can only reproduce a small
central portion of the conductance curve, the two-gap one accounts remarkably
well for the shape of the whole experimental dI/dV. The fits of the normalized
curves give Delta_1(0) = 6.15 \pm 0.45 meV and Delta_2(0) = 18 \pm 3 meV. Both
gaps close at the same temperature and follow a BCS-like behavior.Comment: 4 pages, 4 eps color figure

### Self-consistent Dual Boson approach to single-particle and collective excitations in correlated systems

We propose an efficient dual boson scheme, which extends the DMFT paradigm to
collective excitations in correlated systems. The theory is fully
self-consistent both on the one- and on the two-particle level, thus describing
the formation of collective modes as well as the renormalization of electronic
and bosonic spectra on equal footing. The method employs an effective impurity
model comprising both fermionic and bosonic hybridization functions. Only
single- and two-electron Green's functions of the reference problem enter the
theory, due to the optimal choice of the self-consistency condition for the
effective bosonic bath. We show that the theory is naturally described by a
dual Luttinger-Ward functional and obeys the relevant conservation laws.Comment: 17 pages, 12 figure

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