7,346 research outputs found
Towards first-principles understanding of the metal-insulator transition in fluid alkali metals
By treating the electron-ion interaction as perturbation in the
first-principles Hamiltonian, we have calculated the density response functions
of a fluid alkali metal to find an interesting charge instability due to
anomalous electronic density fluctuations occurring at some finite wave vector
{\bi Q} in a dilute fluid phase above the liquid-gas critical point. Since
|{\bi Q}| is smaller than the diameter of the Fermi surface, this instability
necessarily impedes the electric conduction, implying its close relevance to
the metal-insulator transition in fluid alkali metals.Comment: 11 pages, 5 figure
Wigner-Mott scaling of transport near the two-dimensional metal-insulator transition
Electron-electron scattering usually dominates the transport in strongly
correlated materials. It typically leads to pronounced resistivity maxima in
the incoherent regime around the coherence temperature , reflecting the
tendency of carriers to undergo Mott localization following the demise of the
Fermi liquid. This behavior is best pronounced in the vicinity of
interaction-driven (Mott-like) metal-insulator transitions, where the
decreases, while the resistivity maximum increases. Here we show
that, in this regime, the entire family of resistivity curves displays a
characteristic scaling behavior while
the and assume a powerlaw dependence on the
quasi-particle effective mass . Remarkably, precisely such trends are
found from an appropriate scaling analysis of experimental data obtained from
diluted two-dimensional electron gases in zero magnetic fields. Our analysis
provides strong evidence that inelastic electron-electron scattering -- and not
disorder effects -- dominates finite temperature transport in these systems,
validating the Wigner-Mott picture of the two-dimensional metal-insulator
transition.Comment: 7 page
Magneto-structural transformations via a solid-state nudged elastic band method: Application to iron under pressure
We extend the solid-state nudged elastic band method to handle a
non-conserved order parameter - in particular, magnetization, that couples to
volume and leads to many observed effects in magnetic systems. We apply this
formalism to the well-studied magneto-volume collapse during the
pressure-induced transformation in iron - from ferromagnetic body-centered
cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a
bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state
enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and
coherent stress occurs at the transition state, which has a form of a cusp on
the potential-energy surface (yet all the atomic and cell degrees of freedom
are continuous); the calculated pressure jump of 25 GPa is related to the
observed 25 GPa spread in measured coexistence pressures arising from
martensitic and coherency stresses in samples. Our results agree with
experiments, but necessarily differ from those arising from drag and restricted
parametrization methods having improperly constrained or uncontrolled degrees
of freedom.Comment: 7 pages, 7 figure
Seebeck effect in the conducting LaAlO_{3}/SrTiO_{3} interface
The observation of metallic behavior at the interface between insulating
oxides has triggered worldwide efforts to shed light on the physics of these
systems and clarify some still open issues, among which the dimensional
character of the conducting system. In order to address this issue, we measure
electrical transport (Seebeck effect, Hall effect and conductivity) in
LaAlO_{3}/SrTiO_{3} interfaces and, for comparison, in a doped SrTiO_{3} bulk
single crystal. In these experiments, the carrier concentration is tuned, using
the field effect in a back gate geometry. The combined analysis of all
experimental data at 77 K indicates that the thickness of the conducting layer
is ~7 nm and that the Seebeck effect data are well described by a
two-dimensional (2D) density of states. We find that the back gate voltage is
effective in varying not only the charge density, but also the thickness of the
conducting layer, which is found to change by a factor of ~2, using an electric
field between -4 and +4MV/m at 77K. No enhancement of the Seebeck effect due to
the electronic confinement and no evidence for two-dimensional quantization
steps are observed at the interfaces.Comment: 15 pages, 5 figure
Analysis of broadband microwave conductivity and permittivity measurements of semiconducting materials
We perform broadband phase sensitive measurements of the reflection
coefficient from 45 MHz up to 20 GHz employing a vector network analyzer with a
2.4 mm coaxial sensor which is terminated by the sample under test. While the
material parameters (conductivity and permittivity) can be easily extracted
from the obtained impedance data if the sample is metallic, no direct solution
is possible if the material under investigation is an insulator. Focusing on
doped semiconductors with largely varying conductivity, here we present a
closed calibration and evaluation procedure for frequencies up to 5 GHz, based
on the rigorous solution for the electromagnetic field distribution inside the
sample combined with the variational principle; basically no limiting
assumptions are necessary. A simple static model based on the electric current
distribution proves to yield the same frequency dependence of the complex
conductivity up to 1 GHz. After a critical discussion we apply the developed
method to the hopping transport in Si:P at temperature down to 1 K.Comment: 9 pages, 10 figures, accepted for publication in the Journal of
Applied Physic
Low-energy excitations of the one-dimensional half-filled SU(4) Hubbard model with an attractive on-site interaction: Density-matrix renormalization-group calculations and perturbation theory
We investigate low-energy excitations of the one-dimensional half-filled
SU(4) Hubbard model with an attractive on-site interaction U < 0 using the
density matrix renormalization group method as well as a perturbation theory.
We find that the ground state is a charge density wave state with a long range
order. The ground state is completely incompressible since all the excitations
are gapful. The charge gap which is the same as the four-particle excitation
gap is a non-monotonic function of U, while the spin gap and others increase
with increasing |U| and have linear asymptotic behaviors.Comment: 4 pages, 3 figures, submitte
First order Mott transition at zero temperature in two dimensions: Variational plaquette study
The nature of the metal-insulator Mott transition at zero temperature has
been discussed for a number of years. Whether it occurs through a quantum
critical point or through a first order transition is expected to profoundly
influence the nature of the finite temperature phase diagram. In this paper, we
study the zero temperature Mott transition in the two-dimensional Hubbard model
on the square lattice with the variational cluster approximation. This takes
into account the influence of antiferromagnetic short-range correlations. By
contrast to single-site dynamical mean-field theory, the transition turns out
to be first order even at zero temperature.Comment: 6 pages, 5 figures, version 2 with additional results for 8 bath
site
Epitaxial Growth of LaSrFeO thin films by laser ablation
We report on the synthesis of high quality LaSrFeO (LSFO)
thin films using the pulsed laser deposition technique on both SrTiO (STO)
and LaAlO (LAO) substrates (100)-oriented. From X-Ray diffraction (XRD)
studies, we find that the films have an out-of-plane lattice parameter around
0.3865nm, almost independent of the substrate (i.e. the nature of the strains).
The transport properties reveal that, while LSFO films deposited on STO exhibit
an anomaly in the resistivity vs temperature at 180K (corresponding to the
charge-ordered transition and associated with a transition from a paramagnetic
to an antiferromagnetic state), the films grown on LAO display a very small
magnetoresistance behavior and present an hysteresis around 270K under the
application of a 4T magnetic field. The changes in transport properties between
both substrates are discussed and compared with the corresponding single
crystals.Comment: 9 pages, 4 figure
Coulomb corrections to the extrinsic spin-Hall effect of a two-dimensional electron gas
We develop the microscopic theory of the extrinsic spin Hall conductivity of
a two-dimensional electron gas, including skew-scattering, side-jump, and
Coulomb interaction effects. We find that while the spin-Hall conductivity
connected with the side-jump is independent of the strength of
electron-electron interactions, the skew-scattering term is reduced by the
spin-Coulomb drag, so the total spin current and the total spin-Hall
conductivity are reduced for typical experimental mobilities. Further, we
predict that in paramagnetic systems the spin-Coulomb drag reduces the spin
accumulations in two different ways: (i) directly through the reduction of the
skew-scattering contribution (ii) indirectly through the reduction of the spin
diffusion length. Explicit expressions for the various contributions to the
spin Hall conductivity are obtained using an exactly solvable model of the
skew-scattering.Comment: The Coulomb corrections to the spin-Hall conductivity and spin
accumulations to first order in strength of spin-orbit coupling and
electron-electron interactions are include
Strain induced pressure effect in pulsed laser deposited thin films of the strongly correlated oxide V2O3
V2O3 thin films about 10 nm thick were grown on Al2O3 (0001) by pulsed laser
deposition. The XRD analysis is in agreement with R-3c space group. Some of
them exhibit the metal / insulator transition characteristic of V2O3 bulk
material and others samples exhibit a metallic behavior. For the latter, the
XPS analysis indicates an oxidation state of +III for vanadium. There is no
metal / insulator transition around 150 K in this sample and a strongly
correlated Fermi liquid rho = AT2 behavior of the resistivity at low
temperature is observed, with a value of A of 1.2 10-4 ohm cm, 3 times larger
than the bulk value at 25 kbar
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