arXiv subjects

A. L. Efros

Publications and source records attributed to A. L. Efros.

At least 19 recordsLinked to original sources

arXiv2011

High volumetric capacitance near insulator-metal percolation transition

A new type of a capacitor with a very high volumetric capacitance is proposed. It is based upon the known phenomenon of a sharp increase of the dielectric constant of the metal-insulator composite in the vicinity of the percolation threshold, but still on the insulator side. The optimization suggests that the metallic particles should be of nanoscale and that the distance between planar electrodes should be somewhat larger than the correlation length of the percolation theory and 10 to 20 times larger than the size of the particles while the area of the electrodes might be unlimited. The random electric field in the capacitors is found to be larger than the average field corresponding to the potential difference of electrodes. This random field is potentially responsible for dielectric breakdown. The estimated breakdown voltage of the new capacitor shows that the stored energy density might be significantly larger than that of electrolytic capacitors while the volumetric capacitances might be comparable. The charging and discharging times should be significantly smaller than corresponding times of batteries and even electrolytic capacitors.

cond-mat.mes-hall↗

arXiv2011

Coulomb gap in the one-particle density of states in three-dimensional systems with localized electrons

The one-particle density of states (1P-DOS) in a system with localized electron states vanishes at the Fermi level due to the Coulomb interaction between electrons. Derivation of the Coulomb gap uses stability criteria of the ground state. The simplest criterion is based on the excitonic interaction of an electron and a hole and leads to a quadratic 1P-DOS in the three-dimensional (3D) case. In 3D, higher stability criteria, including two or more electrons, were predicted to exponentially deplete the 1P-DOS at energies close enough to the Fermi level. In this paper we show that there is a range of intermediate energies where this depletion is strongly compensated by the excitonic interaction between single-particle excitations, so that the crossover from quadratic to exponential behavior of the 1P-DOS is retarded. This is one of the reasons why such exponential depletion was never seen in computer simulations.

cond-mat.dis-nn↗

arXiv2009

Anomalous low temperature ambipolar diffusion and Einstein relation

Regular Einstein relation, connecting the coefficient of ambipolar diffusion and the Dember field with mobilities, is generalized for the case of interacting electron-hole plasma. The calculations are presented for a non-degenerate plasma injected by light in semiconductors of silicon and germanium type. The Debye-Huckel correlation and the Wigner-Seitz exchange terms are considered. The corrections to the mobilities of carriers due to difference between average and acting electric fields within the electron-hole plasma is taken into account. The deviation of the generalized relation from the regular Einstein relation is pronounced at low temperatures and can explain anomaly of the coefficient of ambipolar diffusion, recently discovered experimentally.

cond-mat.str-el↗

arXiv2008

Negative density of states: screening, Einstein relation, and negative diffusion

In strongly interacting electron systems with low density and at low temperature the thermodynamic density of states is negative. It creates difficulties with understanding of the Einstein relation between conductivity and diffusion coefficient. Using the expression for electrochemical potential that takes into account the long range part of the Coulomb interaction it is shown that at negative density of states Einstein relation gives a negative sign of the diffusion coefficient D, but under this condition there is no thermodynamic limitation on the sign of D. It happens because the unipolar relaxation of inhomogeneous electron density is not described by the diffusion equation. The relaxation goes much faster due to electric forces caused by electron density and by neutralizing background. Diffusion coefficient is irrelevant in this case and it is not necessarily positive because process of diffusion does not contribute to the positive production of entropy. In the case of bipolar diffusion negative D results in a global absolute instability that leads to formation of neutral excitons. Graphene is considered as an example of a system, where the density relaxation is expected to be due to electric force rather than diffusion. It may also have a negative density of states.

cond-mat.str-el↗

arXiv2006

The problem of a perfect lens made of a slab with negative refraction

The problem of the principal existence of the perfect lens and superlensing is discussed. We have demonstrated that in the case of the virtual focus the idea of perfect lens based upon amplification of evanescent waves as proposed by Pendry is perfectly right unlike the case of the real foci. We think that some experimental results claiming superlensing can be explained in terms of the proposed theory for the case when the virtual focus is inside the lens but very close to the rear face of it.

cond-mat.mtrl-sci↗

arXiv2005

Image of Veselago lens based upon two-dimensional photonic crystal with triangular lattice

The construction of the multi-focal Veselago lens predicted earlier is proposed on the basis of a uniaxial photonic crystal consisting of cylindrical air holes in silicon that make a triangular lattice in a plane perpendicular to the axis of the crystal. The object and image are in air. The period of the crystal should be $0.44μ{\rm m}$ to work at the wavelength $1.5μ{\rm m}$. The lens does not provide superlensing but the half-width of the image is $0.5λ$. The lens is shown to have wave guiding properties depending on the substrate material.

cond-mat.mtrl-sci↗

arXiv2005

Far-field image of Veselago lens

It is shown that perfect imaging of a point source both in near- and far-field regions contradicts electrodynamics although ``superlensing'' is impossible only in the far-field region. These general statements are illustrated by detailed study of evanescent wave propagation in a photonic crystal that is known to be a left-handed medium. An analytical expression for the intensity distribution near the far-field focus is obtained. This distribution contains some novel features and establishes a new ``diffraction limit'' for flat lenses. It is generalized for multiple sources located at different points. The distribution is in very good agreement with computer simulation.

cond-mat.mtrl-sci↗

arXiv2005

Instability of human societies as a result of conformity

We introduce a new model that mimics the strong and sudden effects induced by conformity in tightly interacting human societies. Such effects range from mere crowd phenomena to dramatic political turmoil. The model is a modified version of the Ising Hamiltonian. We have studied the properties of this Hamiltonian using both a Metropolis simulation and analytical derivations. Our study shows that increasing the value of the conformity parameter, results in a first order phase transition. As a result a majority of people begin honestly to support the idea that may contradict the moral principles of a normal human beings though each individual would support the moral principle without tight interaction with the society. Thus, above some critical level of conformity our society occurs to be instable with respect to ideas that might be doubtful. Our model includes, in a simplified way, human diversity with respect to loyalty to the moral principles.

physics.soc-ph↗

arXiv2005

Evanescent waves in photonic crystals and image of Veselago lens

It is shown that negative electric permittivity and magnetic permeability recently discovered in a photonic crystal in the vicinity of the Gamma-point are properties of propagating modes only. The evanescent modes rather decay than increase in the bulk of the crystal though they may be amplified by surface waves. If surface support such waves, the evanescent waves may improve the image of a thin Veselago lens. It is shown that a ``perfect lens'' contradicts to the wave optics and a criterion of ``superlensing'' is formulated.

cond-mat.mtrl-sci↗

arXiv2004

Maximum thermal conductivity of aligned single wall carbon nanotubes

I estimate maximum thermal conductivity $κ$ of a perfectly aligned bundle of single wall carbon nanotubes. Each row of aligned nanotubes has a discrete structure. It consists of segments of nanotubes with length $L$. The spacing between the segments block the phonon path through the row. Only the scattering due to the finite length of the segments is taken into account. The result is that the 'effective'' mean free path is of the order of $L/7$. For 1 micron tubes (10,10) we get maximum value of $κ\approx 300$W/m K at room temperature. This result is in a reasonable agreement with the experiment by Hone {\it et al.} assuming that in their samples $L\approx 1μ{\rm m}$

cond-mat.mtrl-sci↗

arXiv2003

Dielectric photonic crystal as medium with negative electric permittivity and magnetic permeability

We show that a two-dimensional photonic crystal (PC) made from a non-magnetic dielectric is a left-handed material in the sense defined by Veselago. Namely, it has negative values of both the electric permittivity $ε$ and the magnetic permeability $μ$ in some frequency range. This follows from a recently proven general theorem. The negative values of $ε$ and $μ$ are found by a numerical simulation. Using these values we demonstrate the Veselago lens, a unique optical device predicted by Veselago. An approximate analytical theory is proposed to calculate the values of $ε$ and $μ$ from the PC band structure. It gives the results that are close to those obtained by the numerical simulation. The theory explains how a non-zero magnetization arises in a non-magnetic PC.

cond-mat↗

arXiv2003

Long time relaxation of interacting electrons in the regime of hopping conduction

Using numerical simulations we studied the long time relaxation of the hopping conductivity. Even though no modern computation is able to simulate the behavior of a large size system over minutes or hours so as to observe the relaxation, still we have been able to show that the long time relaxation and aging effect observed in experiments can be explained in terms of slow transitions between different pseudoground states. This was achieved by showing that different pseudoground states may have different conductivities and that the dispersion of conductivities is in agreement with the experimental data. We considered two different two-dimensional models with electron-electron interaction: the lattice model and the random site model, corresponding to ``strong'' and ``weak'' effective disorder. For the lattice model, effectively strong disorder, we have shown that the universality of the Coulomb gap, which is responsible for the universal Efros-Shklovskii law for the conductivity, suppresses the long time relaxation of conductivity since the universality strongly decreases the dispersion of conductivities of the pseudoground states.

cond-mat.dis-nn↗

arXiv2002

New type of lenses based upon left-handed materials

New type of lenses which are a slab of a left-handed material embedded into a regular material is proposed. These two materials should have equal refractive indices. Lenses with a focal length larger than the width of the slab can be constructed. These lenses should be easier to make than the well known Veselago lens, because the materials of the Veselago lens should obey an additional matching condition. Lenses of new type have multiple foci and might be useful for the 3D imaging.

cond-mat↗

arXiv2002

Sign of refractive index and group velocity in left-handed media

We argue that the widely spread opinion that the left-handed media (LHM) are characterized by a negative refractive index $n_-$ is misleading. Since n does not enter into Maxwell's equations and boundary conditions, any medium may be described by both positive n and negative $n_-=-n$. Two thermodynamic inequalities are presented, that make a difference between the LHM and the regular media (RM). The first one reads that the group velocity is positive in the RM and negative in the LHM. The second one is that the product ${\rm Re}(n) {\rm Im}(n)$ is positive in the RM and negative in the LHM. Both inequalities are invariant with respect to the change $n \to n_-$. However, to use $n_-$ one should change some traditional electrodynamics definitions.

cond-mat↗

arXiv2002

Diffraction theory and focusing of light by left-handed materials

A diffraction theory in a system consisting of left-handed and right-handed materials is proposed. The theory is based upon the Huygens's principle and the Kirchhoff's integral and it is valid if the wavelength is smaller than any relevant length of the system. The theory is applied to the calculation of the smearing of the foci of the Veselago lens due to the finite wavelength. We show that the Veselago lens is a unique optical instrument for the 3D imaging, but it is not a ``superlens'' as it has been claimed recently.

cond-mat↗

arXiv2002

Propagation of waves in metallic photonic crystals at low frequencies and some theoretical aspects of left-handed materials

An analytical theory of low frequency electromagnetic waves in metallic photonic crystals with a small volume fraction of a metal is presented. The evidence of the existence of such waves has been found recently via experiments and computations. We have obtained an exact dispersion equation for omega (k) and studied the cutoff frequency omega_0 = omega(0) as a function of parameters of the photonic crystal. An analytical expression for the permittivity epsilon is calculated. It is shown, that if the crystal is embedded into a medium with negative mu, it has no propagating modes at any frequency. Thus, such a compound system is not a left-handed material (LHM). The recent experimental results on the LHM are discussed.

cond-mat↗

arXiv2002

Electrodynamics of metallic photonic crystals and problem of left-handed materials

An exact analytical theory of the electromagnetic waves in metallic photonic crystals with a small volume fraction of a metal is presented. It is shown that there are waves with a very low cutoff frequency $ω_0$ and that the permittivity $ε$ is negative below $ω_0$. We show that if the crystal is embedded into a medium with negative $μ$, it has no propagating modes at any frequency. Thus, such a compound system is not a left handed material (LHM). The recent experimental results on the LHM are discussed.

cond-mat↗

arXiv2002

Diffraction in left-handed materials and theory of Veselago lens

A theory of diffraction in the system consisting of the left-handed and the right-handed materials is proposed. The theory is based upon the Huygens's principle and the Kirchhoff's integral and it is valid if the wavelength is smaller than any relevant length of the system. The theory is applied to the calculation of the smearing of the foci of the Veselago lens due to the finite wavelength. We show that the Veselago lens is a unique optical instrument for the 3D imaging, but it is not a ``superlens'' as it has been claimed recently.

cond-mat↗