427 research outputs found
Instanton approach to the Langevin motion of a particle in a random potential
We develop an instanton approach to the non-equilibrium dynamics in
one-dimensional random environments. The long time behavior is controlled by
rare fluctuations of the disorder potential and, accordingly, by the tail of
the distribution function for the time a particle needs to propagate along the
system (the delay time). The proposed method allows us to find the tail of the
delay time distribution function and delay time moments, providing thus an
exact description of the long-time dynamics. We analyze arbitrary environments
covering different types of glassy dynamics: dynamics in a short-range random
field, creep, and Sinai's motion.Comment: 4 pages, 1 figur
Superconductivity, phase fluctuations and the c-axis conductivity of bilayer high temperature superconductors
We present a theory of the interplane conductivity of bilayer high
temperature superconductors, focusing on the effect of quantal and thermal
fluctuations on the oscillator strengths of the superfluid stiffness and the
bilayer plasmon. We find that the opening of the superconducting gap and
establishment of superconducting phase coherence each lead to redistribution of
spectral weight over wide energy scales. The factor-of-two relation between the
superfluid stiffness and the change below in the oscillator strength of
the absorptive part of the conductivity previously derived for single-layer
systems, is found to be substantially modified in bilayer systems.Comment: 11 pages, 14 figure
Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope
Van der Waals materials exhibit intriguing structural, electronic, and photonic properties. Electron energy loss spectroscopy within scanning transmission electron microscopy allows for nanoscale mapping of such properties. However, its detection is typically limited to energy losses in the eV range-too large for probing low-energy excitations such as phonons or mid-infrared plasmons. Here, we adapt a conventional instrument to probe energy loss down to 100 meV, and map phononic states in hexagonal boron nitride, a representative van der Waals material. The boron nitride spectra depend on the flake thickness and on the distance of the electron beam to the flake edges. To explain these observations, we developed a classical response theory that describes the interaction of fast electrons with (anisotropic) van der Waals slabs, revealing that the electron energy loss is dominated by excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, our work is of fundamental importance for interpreting future low-energy loss spectra of van der Waals materials.We acknowledge financial support from the European Commission under the Graphene Flagship (GrapheneCore1, grant no. 696656), the ERC starting grant SPINTROS (grant no. 257654), and the Spanish Ministry of Economy and Competitiveness (National plans MAT2014-53432-C5-4-R, MAT2015-65159-R, MAT2015-65525-R, and FIS2016-80174-P). A.K. also thanks for the Czechoslovak Microscopic Society/FEI scholarship.Peer Reviewe
Conductance renormalization and conductivity of a multi-subband Tomonaga-Luttinger model
We studied the conductance renormalization and conductivity of multi-subband
Tomonaga-Luttinger models with inter-subband interactions. We found that, as in
single-band systems, the conductance of a multi-subband system with an
arbitrary number of subbands is not renormalized due to interaction between
electrons. We derived a formula for the conductivity in multi-subband models.
We applied it to a simplified case and found that inter-subband interaction
enhances the conductivity, which is contrary to the intra-subband repulsive
interaction, and that the conductivity is further enhanced for a larger number
of subbands.Comment: 12 pages, no figures. to be published in Physical Review B as a brief
repor
Superconducting zero temperature phase transition in two dimensions and in the magnetic field
We derive the Ginzburg-Landau-Wilson theory for the superconducting phase
transition in two dimensions and in the magnetic field. Without disorder the
theory describes a fluctuation induced first-order quantum phase transition
into the Abrikosov lattice. We propose a phenomenological criterion for
determining the transition field and discuss the qualitative effects of
disorder. Comparison with recent experiments on MoGe films is discussed.Comment: 7 pages, 2 figure
Precision calculation of magnetization and specific heat of vortex liquids and solids in type II superconductors
A new systematic calculation of magnetization and specific heat contributions
of vortex liquids and solids (not very close to the melting line) is presented.
We develop an optimized perturbation theory for the Ginzburg - Landau
description of thermal fluctuations effects in the vortex liquids. The
expansion is convergent in contrast to the conventional high temperature
expansion which is asymptotic. In the solid phase we calculate first two orders
which are already quite accurate. The results are in good agreement with
existing Monte Carlo simulations and experiments. Limitations of various
nonperturbative and phenomenological approaches are noted. In particular we
show that there is no exact intersection point of the magnetization curves both
in 2D and 3D.Comment: 4 pages, 3 figure
Granular Electronic Systems
A granular metal is an array of metallic nano-particles imbedded into an
insulating matrix. Tuning the intergranular coupling strength a granular system
can be transformed into either a good metal or an insulator and, in case of
superconducting particles, experience superconductor-insulator transition. The
ease of adjusting electronic properties of granular metals makes them most
suitable for fundamental studies of disordered solids and assures them a
fundamental role for nanotechnological applications. This Review discusses
recent important theoretical advances in the study of granular metals,
emphasizing on the interplay of disorder, quantum effects, fluctuations and
effects of confinement in formation of electronic transport and thermodynamic
properties of granular materials.Comment: 51 pages, 23 figures, submitted to Reviews of Modern Physic
From Luttinger to Fermi liquids in organic conductors
This chapter reviews the effects of interactions in quasi-one dimensional
systems, such as the Bechgaard and Fabre salts, and in particular the Luttinger
liquid physics. It discusses in details how transport measurements both d.c.
and a.c. allow to probe such a physics. It also examine the dimensional
crossover and deconfinement transition occurring between the one dimensional
case and the higher dimensional one resulting from the hopping of electrons
between chains in the quasi-one dimensional structure.Comment: To be published In the book "The Physics of Organic Conductors and
Superconductors", Springer, 2007, ed. A. Lebe
Graphite and Hexagonal Boron-Nitride Possess the Same Interlayer Distance. Why?
Graphite and hexagonal boron nitride (h-BN) are two prominent members of the
family of layered materials possessing a hexagonal lattice. While graphite has
non-polar homo-nuclear C-C intra-layer bonds, h-BN presents highly polar B-N
bonds resulting in different optimal stacking modes of the two materials in
bulk form. Furthermore, the static polarizabilities of the constituent atoms
considerably differ from each other suggesting large differences in the
dispersive component of the interlayer bonding. Despite these major differences
both materials present practically identical interlayer distances. To
understand this finding, a comparative study of the nature of the interlayer
bonding in both materials is presented. A full lattice sum of the interactions
between the partially charged atomic centers in h-BN results in vanishingly
small monopolar electrostatic contributions to the interlayer binding energy.
Higher order electrostatic multipoles, exchange, and short-range correlation
contributions are found to be very similar in both materials and to almost
completely cancel out by the Pauli repulsions at physically relevant interlayer
distances resulting in a marginal effective contribution to the interlayer
binding. Further analysis of the dispersive energy term reveals that despite
the large differences in the individual atomic polarizabilities the
hetero-atomic B-N C6 coefficient is very similar to the homo-atomic C-C
coefficient in the hexagonal bulk form resulting in very similar dispersive
contribution to the interlayer binding. The overall binding energy curves of
both materials are thus very similar predicting practically the same interlayer
distance and very similar binding energies.Comment: 18 pages, 5 figures, 2 table
Energy-efficient installations of multi-zone multi-frequency induction heating of steel billets for large deformation
This article discusses the basic construction principles of the device for multi-zone induction heating. The mathematical model of a cascade inverter is described. Various modes of heating cylindrical billets are considered depending on the ratio of powers in the heating zones and current frequencies in the inductors. The indicators of heating uniformity over the cross section and energy efficiency are studied. Based on the data obtained, a conclusion is drawn about the optimal choice of the number of heating zones, frequencies of inductors and heating modes. © Published under licence by IOP Publishing Ltd
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