98 research outputs found
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.
Transport of magnetoexcitons in single and coupled quantum wells
The transport relaxation time and the mean free path of
magnetoexcitons in single and coupled quantum wells are calculated ( is the
magnetic momentum of the magnetoexciton). We present the results for
magnetoexciton scattering in a random field due to (i) quantum well width
fluctuations, (ii) composite fluctuations and (iii) ionized impurities. The
time depends nonmonotonously on in the case (ii) and in the cases
(i), (iii) for smaller than some critical value ( is the interwell
separation, is the magnetic length). For the
transport relaxation time increases monotonously with . The magnetoexciton
mean free path has a maximum at in the cases (i), (iii).
It decreases with increasing . The mean free path calculated for the case
(ii) may have two maxima. One of them disappears with the variation of the
random fields parameters. The maximum of increases with for
types (i,iii) of scattering processes and decreases in the case (ii).Comment: 13 pages, 8 figures in EPS format; Physica Scripta (in print
Monte-Carlo Simulations of the Dynamical Behavior of the Coulomb Glass
We study the dynamical behavior of disordered many-particle systems with
long-range Coulomb interactions by means of damage-spreading simulations. In
this type of Monte-Carlo simulations one investigates the time evolution of the
damage, i.e. the difference of the occupation numbers of two systems, subjected
to the same thermal noise. We analyze the dependence of the damage on
temperature and disorder strength. For zero disorder the spreading transition
coincides with the equilibrium phase transition, whereas for finite disorder,
we find evidence for a dynamical phase transition well below the transition
temperature of the pure system.Comment: 10 pages RevTeX, 8 Postscript figure
Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions
Molecular dynamics simulations are performed to study the lithium jumps in
LiPO3 glass. In particular, we calculate higher-order correlation functions
that probe the positions of single lithium ions at several times. Three-time
correlation functions show that the non-exponential relaxation of the lithium
ions results from both correlated back-and-forth jumps and the existence of
dynamical heterogeneities, i.e., the presence of a broad distribution of jump
rates. A quantitative analysis yields that the contribution of the dynamical
heterogeneities to the non-exponential depopulation of the lithium sites
increases upon cooling. Further, correlated back-and-forth jumps between
neighboring sites are observed for the fast ions of the distribution, but not
for the slow ions and, hence, the back-jump probability depends on the
dynamical state. Four-time correlation functions indicate that an exchange
between fast and slow ions takes place on the timescale of the jumps
themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites
featuring fast and slow lithium dynamics, respectively, are intimately mixed.
In addition, a backward correlation beyond the first neighbor shell for highly
mobile ions and the presence of long-range dynamical heterogeneities suggest
that fast ion migration occurs along preferential pathways in the glassy
matrix. In the melt, we find no evidence for correlated back-and-forth motions
and dynamical heterogeneities on the length scale of the next-neighbor
distance.Comment: 12 pages, 13 figure
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
On the unipolarity of charge transport in methanofullerene diodes
Fullerenes are electron transporting organic semiconductors with a wide range of applications. In particular, methanofullereneshave been the preferred choice for solution-processed solar cells and photodiodes. The wide applicability of fullerenes as both ‘ntype’transport materials and electron acceptors is clear. However, what is still a matter of debate is whether the fullerenes can alsosupport efficient transport of holes, particularly in diode geometries. In this letter, we utilize a number of recently developedexperimental methods for selective electron and hole mobility measurements. We show for the two most widely used solutionprocessable fullerenes, PC70- and-PC60BM, that whilst both exhibit electron mobilities as high as 10−3 cm2/Vs, their hole mobilities
Single-particle excitations under coexisting electron correlation and disorder: a numerical study of the Anderson-Hubbard model
Interplay of electron correlation and randomness is studied by using the
Anderson-Hubbard model within the Hartree-Fock approximation. Under the
coexistence of short-range interaction and diagonal disorder, we obtain the
ground-state phase diagram in three dimensions, which includes an
antiferromagnetic insulator, an antiferromagnetic metal, a paramagnetic
insulator (Anderson-localized insulator) and a paramagnetic metal. Although
only the short-range interaction is present in this model, we find
unconventional soft gaps in the insulating phases irrespective of electron
filling, spatial dimensions and long-range order, where the single-particle
density of states (DOS) vanishes with a power-law scaling in one dimension (1D)
or even faster in two dimensions (2D) and three dimensions (3D) toward the
Fermi energy. We call it soft Hubbard gap. Moreover, exact-diagonalization
results in 1D support the formation of the soft Hubbard gap beyond the
mean-field level. The formation of the soft Hubbard gap cannot be attributed to
a conventional theory by Efros and Shklovskii (ES) owing the emergence of soft
gaps to the long-range Coulomb interaction. Indeed, based on a picture of
multivalley energy landscape, we propose a phenomenological scaling theory,
which predicts a scaling of the DOS in perfect agreement with the numerical
results. We further discuss a correction of the scaling of the DOS by the
long-range part of the Coulomb interaction, which modifies the scaling of Efros
and Shklovskii. Furthermore, explicit formulae for the temperature dependence
of the DC resistivity via variable-range hopping under the influence of the
soft gaps are derived. Finally, we compare the present theory with experimental
results of SrRu_{1-x}Ti_xO_3.Comment: 22 pages, 19 figure
Temperature-dependent optical properties of InAs GaAs quantum dots: Independent carrier versus exciton relaxation
Beneficiation on a semi-industrial scale of satkin magnesites by the flotation technique with recycled water
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