5,148 research outputs found
Anderson-Hubbard model with box disorder: Statistical dynamical mean-field theory investigation
Strongly correlated electrons with box disorder in high-dimensional lattices
are investigated. We apply the statistical dynamical mean-field theory, which
treats local correlations non-perturbatively. The incorporation of a finite
lattice connectivity allows for the detection of disorder-induced localization
via the probability distribution function of the local density of states. We
obtain a complete paramagnetic ground state phase diagram and find
correlation-induced as well as disorder-induced metal-insulator transitions.
Our results qualitatively confirm predictions obtained by typical medium
theory. Moreover, we find that the probability distribution function of the
local density of states in the metallic phase strongly deviates from a
log-normal distribution as found for the non-interacting case.Comment: 13 pages, 15 figures, published versio
Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)
Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and
dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15).
Static dielectric constants are determined from the low-T limiting behavior of
the permittivity. The estimated polarizability for bound holes ~ 10^{-22}
cm^{-3} implies a radius comparable to the interatomic spacing, consistent with
the small polaron picture established from prior transport studies near room
temperature and above on nearby compositions. Relaxation peaks in the
dielectric loss associated with charge-carrier hopping yield activation
energies in good agreement with low-T hopping energies determined from
variable-range hopping fits of the dc resistivity. The doping dependence of
these energies suggests that the orthorhombic, canted antiferromagnetic ground
state tends toward an insulator-metal transition that is not realized due to
the formation of the ferromagnetic insulating state near Mn(4+) concentration ~
0.13.Comment: PRB in press, 5 pages, 6 figure
Uso de habitats e movimentos migratórios por Pseudis platensis (ANURA: HYLIDAE) no Pantanal de Nhecolândia.
Orbital selective insulator-metal transition in V2O3 under external pressure
We present a detailed account of the physics of Vanadium sesquioxide (), a benchmark system for studying correlation induced metal-insulator
transition(s). Based on a detailed perusal of a wide range of experimental
data, we stress the importance of multi-orbital Coulomb interactions in concert
with first-principles LDA bandstructure for a consistent understanding of the
PI-PM MIT under pressure. Using LDA+DMFT, we show how the MIT is of the orbital
selective type, driven by large changes in dynamical spectral weight in
response to small changes in trigonal field splitting under pressure. Very good
quantitative agreement with () the switch of orbital occupation and ()
S=1 at each site across the MIT, and () carrier effective mass in
the PM phase, is obtained. Finally, using the LDA+DMFT solution, we have
estimated screening induced renormalisation of the local, multi-orbital Coulomb
interactions. Computation of the one-particle spectral function using these
screened values is shown to be in excellent quantitative agreement with very
recent experimental (PES and XAS) results. These findings provide strong
support for an orbital-selective Mott transition in paramagnetic .Comment: 12 pages, 7 figure
Skew scattering due to intrinsic spin-orbit coupling in a two-dimensional electron gas
We present the generalization of the two-dimensional quantum scattering
formalism to systems with Rashba spin-orbit coupling. Using symmetry
considerations, we show that the differential scattering cross section depends
on the spin state of the incident electron, and skew scattering may arise even
for central spin-independent scattering potentials. The skew scattering effect
is demonstrated by exact results of a simple hard wall impurity model. The
magnitude of the effect for short-range impurities is estimated using the first
Born approximation. The exact formalism we present can serve as a foundation
for further theoretical investigations.Comment: 4 pages, 3 figur
Analytical calculation of the Green's function and Drude weight for a correlated fermion-boson system
In classical Drude theory the conductivity is determined by the mass of the
propagating particles and the mean free path between two scattering events. For
a quantum particle this simple picture of diffusive transport loses relevance
if strong correlations dominate the particle motion. We study a situation where
the propagation of a fermionic particle is possible only through creation and
annihilation of local bosonic excitations. This correlated quantum transport
process is outside the Drude picture, since one cannot distinguish between free
propagation and intermittent scattering. The characterization of transport is
possible using the Drude weight obtained from the f-sum rule, although its
interpretation in terms of free mass and mean free path breaks down. For the
situation studied we calculate the Green's function and Drude weight using a
Green's functions expansion technique, and discuss their physical meaning.Comment: final version, minor correction
Stabilization of charge ordering in La_(1/3)Sr_(2/3)FeO_(3-d) by magnetic exchange
The magnetic exchange energies in charge ordered La_(1/3)Sr_(2/3)FeO_(3-d)
(LSFO) and its parent compound LaFeO_(3) (LFO) have been determined by
inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic
exchange between Fe3+ - Fe5+ pairs (J_F) and antiferromagnetic exchange between
Fe3+ - Fe3+ pairs (J_AF) fulfills the criterion for charge ordering driven by
magnetic interactions (|J_F/J_AF| > 1). The 30% reduction of J_AF as compared
to LFO indicates that doped holes are delocalized, and charge ordering occurs
without a dominant influence from Coulomb interactions.Comment: 18 pages, 4 color figure
Effect of Strain Relaxation on Magnetotransport properties of epitaxial La_0.7Ca_0.3MnO_3 films
In this paper, we have studied the effect of strain relaxation on
magneto-transport properties of La_0.7Ca_0.3MnO_3 epitaxial films (200 nm
thick), which were deposited by pulsed laser deposition technique under
identical conditions. All the films are epitaxial and have cubic unit cell. The
amount of strain relaxation has been varied by taking three different single
crystal substrates of SrTiO_3, LaAlO_3 and MgO. It has been found that for
thicker films the strain gets relaxed and produces variable amount of disorder
depending on the strength of strain relaxation. The magnitude of lattice
relaxation has been found to be 0.384, 3.057 and 6.411 percent for film
deposited on SrTiO_3, LaAlO_3 and MgO respectively. The films on LaAlO_3 and
SrTiO_3 show higher T_{IM} of 243 K and 217 K respectively as compared to
T_{IM} of 191 K for the film on MgO. Similarly T_C of the films on SrTiO_3 and
LaAlO_3 is sharper and has value of 245 K and 220 K respectively whereas the TC
of the film on MgO is 175 K. Higher degree of relaxation creates more defects
and hence TIM (T_C) of the film on MgO is significantly lower than of SrTiO_3
and LaAlO_3. We have adopted a different approach to correlate the effect of
strain relaxation on magneto-transport properties of LCMO films by evaluating
the resistivity variation through Mott's VRH model. The variable presence of
disorder in these thick films due to lattice relaxation which have been
analyzed through Mott's VRH model provides a strong additional evidence that
the strength of lattice relaxation produces disorder dominantly by increase in
density of defects such as stacking faults, dislocations, etc. which affect the
magneto-transport properties of thick epitaxial La_0.7Ca_0.3MnO_3 films
Search for Ferromagnetism in doped semiconductors in the absence of transition metal ions
In contrast to semiconductors doped with transition metal magnetic elements,
which become ferromagnetic at temperatures below ~ 100K, semiconductors doped
with non-magnetic ions (e.g. silicon doped with phosphorous) have not shown
evidence of ferromagnetism down to millikelvin temperatures. This is despite
the fact that for low densities the system is expected to be well modeled by
the Hubbard model, which is predicted to have a ferromagnetic ground state at
T=0 on 2- or 3-dimensional bipartite lattices in the limit of strong
correlation near half-filling. We examine the impurity band formed by
hydrogenic centers in semiconductors at low densities, and show that it is
described by a generalized Hubbard model which has, in addition to strong
electron-electron interaction and disorder, an intrinsic electron-hole
asymmetry. With the help of mean field methods as well as exact diagonalization
of clusters around half filling, we can establish the existence of a
ferromagnetic ground state, at least on the nanoscale, which is more robust
than that found in the standard Hubbard model. This ferromagnetism is most
clearly seen in a regime inaccessible to bulk systems, but attainable in
quantum dots and 2D heterostructures. We present extensive numerical results
for small systems that demonstrate the occurrence of high-spin ground states in
both periodic and positionally disordered 2D systems. We consider how
properties of real doped semiconductors, such as positional disorder and
electron-hole asymmetry, affect the ground state spin of small 2D systems. We
also discuss the relationship between this work and diluted magnetic
semiconductors, such as Ga_(1-x)Mn_(x)As, which though disordered, show
ferromagnetism at relatively high temperatures.Comment: 47 page
Large temperature dependence of the Casimir force at the metal-insulator transition
The dependence of the Casimir force on material properties is important for
both future applications and to gain further insight on its fundamental
aspects. Here we derive a general theory of the Casimir force for
low-conducting compounds, or poor metals. For distances in the micrometer
range, a large variety of such materials is described by universal equations
containing a few parameters: the effective plasma frequency, dissipation rate
of the free carriers, and electric permittivity in the infrared range. This
theory can also describe inhomogeneous composite materials containing small
regions with different conductivity. The Casimir force for mechanical systems
involving samples made with compounds that have a metal-insulator transition
shows an abrupt large temperature dependence of the Casimir force within the
transition region, where metallic and dielectric phases coexist.Comment: 23 pages, 9 figure
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