106 research outputs found
Density of States and Conductivity of Granular Metal or Array of Quantum Dots
The conductivity of a granular metal or an array of quantum dots usually has
the temperature dependence associated with variable range hopping within the
soft Coulomb gap of density of states. This is difficult to explain because
neutral dots have a hard charging gap at the Fermi level. We show that
uncontrolled or intentional doping of the insulator around dots by donors leads
to random charging of dots and finite bare density of states at the Fermi
level. Then Coulomb interactions between electrons of distant dots results in
the a soft Coulomb gap. We show that in a sparse array of dots the bare density
of states oscillates as a function of concentration of donors and causes
periodic changes in the temperature dependence of conductivity. In a dense
array of dots the bare density of states is totally smeared if there are
several donors per dot in the insulator.Comment: 13 pages, 15 figures. Some misprints are fixed. Some figures are
dropped. Some small changes are given to improve the organizatio
Superfluid-insulator transition and BCS-BEC crossover in dirty ultracold Fermi gas
Superfluid-insulator transition in an ultracold Fermi gas in the external
disorder potential of the amplitude is studied as a function of the
concentration of the gas and magnetic field in the presence of the
Feshbach resonance. We find the zero temperature phase diagrams in the plane
() at a given and in the plane at a given . Our
results for BEC side of the diagram are also valid for the superfluid-insulator
transition in a Bose gas.Comment: Reference added, typos correcte
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.Comment: 6 pages, 1 figur
Slow relaxation of conductance of amorphous hopping insulators
We discuss memory effects in the conductance of hopping insulators due to
slow rearrangements of structural defects leading to formation of polarons
close to the electron hopping states. An abrupt change in the gate voltage and
corresponding shift of the chemical potential change populations of the hopping
sites, which then slowly relax due to rearrangements of structural defects. As
a result, the density of hopping states becomes time dependent on a scale
relevant to rearrangement of the structural defects leading to the excess time
dependent conductivity.Comment: 6 pages, 1 figur
Statistics of the Charging Spectrum of a Two-Dimensional Coulomb Glass Island
The fluctuations of capacitance of a two-dimensional island are studied in
the regime of low electron concentration and strong disorder, when electrons
can be considered classical particles. The universal capacitance distribution
is found, with the dispersion being of the order of the average. This
distribution is shown to be closely related to the shape of the Coulomb gap in
the one-electron density of states of the island. Behavior of the the
capacitance fluctuations near the metal - insulator transition is discussed.Comment: 4 pages, LaTex, 4 Postscript figures are included Discussion of the
situation with screening by metallic gate is adde
Levy statistics and anomalous transport in quantum-dot arrays
A novel model of transport is proposed to explain power law current
transients and memory phenomena observed in partially ordered arrays of
semiconducting nanocrystals. The model describes electron transport by a
stationary Levy process of transmission events and thereby requires no time
dependence of system properties. The waiting time distribution with a
characteristic long tail gives rise to a nonstationary response in the presence
of a voltage pulse. We report on noise measurements that agree well with the
predicted non-Poissonian fluctuations in current, and discuss possible
mechanisms leading to this behavior.Comment: 7 pages, 2 figure
On dispersive energy transport and relaxation in the hopping regime
A new method for investigating relaxation phenomena for charge carriers
hopping between localized tail states has been developed. It allows us to
consider both charge and energy {\it dispersive} transport. The method is based
on the idea of quasi-elasticity: the typical energy loss during a hop is much
less than all other characteristic energies. We have investigated two models
with different density of states energy dependencies with our method. In
general, we have found that the motion of a packet in energy space is affected
by two competing tendencies. First, there is a packet broadening, i.e. the
dispersive energy transport. Second, there is a narrowing of the packet, if the
density of states is depleting with decreasing energy. It is the interplay of
these two tendencies that determines the overall evolution. If the density of
states is constant, only broadening exists. In this case a packet in energy
space evolves into Gaussian one, moving with constant drift velocity and mean
square deviation increasing linearly in time. If the density of states depletes
exponentially with decreasing energy, the motion of the packet tremendously
slows down with time. For large times the mean square deviation of the packet
becomes constant, so that the motion of the packet is ``soliton-like''.Comment: 26 pages, RevTeX, 10 EPS figures, submitted to Phys. Rev.
Coulomb gap in a model with finite charge transfer energy
The Coulomb gap in a donor-acceptor model with finite charge transfer energy
describing the electronic system on the dielectric side of the
metal-insulator transition is investigated by means of computer simulations on
two- and three-dimensional finite samples with a random distribution of equal
amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry
of the model presented with respect to the exchange of donors and acceptors are
derived. In the immediate neighborhood of the Fermi energy the the
density of one-electron excitations is determined solely by
finite size effects and further away from is described by
an asymmetric power law with a non-universal exponent, depending on the
parameter .Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Electronic correlation effects and the Coulomb gap at finite temperature
We have investigated the effect of the long-range Coulomb interaction on the
one-particle excitation spectrum of n-type Germanium, using tunneling
spectroscopy on mechanically controllable break junctions. The tunnel
conductance was measured as a function of energy and temperature. At low
temperatures, the spectra reveal a minimum at zero bias voltage due to the
Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by
thermal excitations. This behavior is reflected in the temperature dependence
of the variable-range hopping resitivity measured on the same samples: Up to a
few degrees Kelvin the Efros-Shkovskii ln law is obeyed,
whereas at higher temperatures deviations from this law are observed,
indicating a cross-over to Mott's ln law. The mechanism of
this cross-over is different from that considered previously in the literature.Comment: 3 pages, 3 figure
Hopping conduction in strong electric fields: Negative differential conductivity
Effects of strong electric fields on hopping conductivity are studied
theoretically. Monte-Carlo computer simulations show that the analytical theory
of Nguyen and Shklovskii [Solid State Commun. 38, 99 (1981)] provides an
accurate description of hopping transport in the limit of very high electric
fields and low concentrations of charge carriers as compared to the
concentration of localization sites and also at the relative concentration of
carriers equal to 0.5. At intermediate concentrations of carriers between 0.1
and 0.5 computer simulations evidence essential deviations from the results of
the existing analytical theories.
The theory of Nguyen and Shklovskii also predicts a negative differential
hopping conductivity at high electric fields. Our numerical calculations
confirm this prediction qualitatively. However the field dependence of the
drift velocity of charge carriers obtained numerically differs essentially from
the one predicted so far. Analytical theory is further developed so that its
agreement with numerical results is essentially improved.Comment: 12 pages, 12 figures. References added, figures improve
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