2,031 research outputs found
Electronic Structure of Pyrochlore Iridates: From Topological Dirac Metal to Mott Insulator
In 5d transition metal oxides such as the iridates, novel properties arise
from the interplay of electron correlations and spin-orbit interactions. We
investigate the electronic structure of the pyrochlore iridates, (such as
YIrO) using density functional theory, LDA+U method, and
effective low energy models. A remarkably rich phase diagram emerges on tuning
the correlation strength U. The Ir magnetic moment are always found to be
non-collinearly ordered. However, the ground state changes from a magnetic
metal at weak U, to a Mott insulator at large U. Most interestingly, the
intermediate U regime is found to be a Dirac semi-metal, with vanishing density
of states at the Fermi energy. It also exhibits topological properties -
manifested by special surface states in the form of Fermi arcs, that connect
the bulk Dirac points. This Dirac phase, a three dimensional analog of
graphene, is proposed as the ground state of YIrO and related
compounds. A narrow window of magnetic `axion' insulator, with axion parameter
, may also be present at intermediate U. An applied magnetic field
induces ferromagnetic order and a metallic ground state.Comment: 7pages + 2pages appendices. 7 figures; see also viewpoint article by
L. Balents, "Weyl Electrons Kiss", at Physics 4, 36 (2011
Electron-ion and ion-ion potentials for modeling warm-dense-matter: applications to laser-heated or shock-compressed Al and Si
The pair-interactions U_{ij}(r) determine the thermodynamics and linear
transport properties of matter via the pair-distribution functions (PDFs),
i.e., g_{ij}(r). Great simplicity is achieved if U_{ij}(r) could be directly
used to predict material properties via classical simulations, avoiding
many-body wavefunctions. Warm dense matter (WDM) is encountered in
quasi-equilibria where the electron temperature differs from the ion
temperature T_i, as in laser-heated or in shock-compressed matter. The electron
PDFs g_{ee}(r) as perturbed by the ions are used to evaluate fully non-local
exchange-correlation corrections to the free energy, using Hydrogen as an
example. Electron-ion potentials for ions with a bound core are discussed with
Al and Si as examples, for WDM with T_e \ne T_i, and valid for times shorter
than the electron-ion relaxation time. In some cases the potentials develop
attractive regions, and then become repulsive and `Yukawa-like' for higher
. These results clarify the origin of initial phonon-hardening and rapid
release. Pair-potentials for shock-heated WDM show that phonon hardening would
not occur in most such systems. Defining meaningful quasi-equilibrium static
transport coefficients consistent with the dynamic values is addressed. There
seems to be no meaningful `static conductivity' obtainable by extrapolating
experimental or theoretical \sigma(\omega, T_i, T_e) to \omega \to 0, unless
T_i \to T_e as well. Illustrative calculations of quasi-static resistivities
R(T_i,T_e) of laser-heated as well as shock-heated Aluminum and Silicon are
presented using our pseudopotentials, pair-potentials and classical integral
equations. The quasi-static resistivities display clear differences in their
temperature evolutions, but are not the strict \omega \to 0 limits of the
dynamic values.Comment: 12 pages, 6 figues, Latex file
Absence of low-temperature dependence of the decay of 7Be and 198Au in metallic hosts
The electron-capture (EC) decay rate of 7Be in metallic Cu host and the
beta-decay rate of 198Au in the host alloy Al-Au have been measured
simultaneously at several temperatures, ranging from 0.350 K to 293 K. No
difference of the half-life of 198Au between 12.5 K and 293 K is observed to a
precision of 0.1%. By utilizing the special characteristics of our
double-source assembly, possible geometrical effects that influence the
individual rates could be eliminated. The ratio of 7Be to 198Au activity thus
obtained also remains constant for this temperatures range to the experimental
precision of 0.15(0.16)%. The resulting null temperature dependence is
discussed in terms of the inadequacy of the often-used Debye-Huckel model for
such measurements.Comment: Four pages, three figures. Accepted for publication in Phys. Rev. C
(Rapd Communications
Self consistent theory of unipolar charge-carrier injection in metal/insulator/metal systems
A consistent device model to describe current-voltage characteristics of
metal/insulator/metal systems is developed. In this model the insulator and the
metal electrodes are described within the same theoretical framework by using
density of states distributions. This approach leads to differential equations
for the electric field which have to be solved in a self consistent manner by
considering the continuity of the electric displacement and the electrochemical
potential in the complete system. The model is capable of describing the
current-voltage characteristics of the metal/insulator/metal system in forward
and reverse bias for arbitrary values of the metal/ insulator injection
barriers. In the case of high injection barriers, approximations are provided
offering a tool for comparison with experiments. Numerical calculations are
performed exemplary using a simplified model of an organic semiconductor.Comment: 21 pages, 8 figure
Properties of the Nearly Free Electron Superconductor Ag5Pb2O6 Inferred from Fermi Surface Measurements
We measured the Fermi surface of the recently discovered superconductor
Ag5Pb2O6 via a de Haas-van Alphen rotation study. Two frequency branches were
observed and identified with the neck and belly orbits of a very simple, nearly
free electron Fermi surface. We use the observed Fermi surface geometry to
quantitatively deduce superconducting properties such as the in-plane and
out-of-plane penetration depths, the coherence length in the clean limit, and
the critical field; as well as normal state properties such as the specific
heat and the resistivity anisotropy.Comment: 2 pages, 1 figure, submitted to Physica C (M2S Proceedings
Glucose modulation of ATP-sensitive K-currents in wild-type, homozygous and heterozygous glucokinase knock-out mice
Netons: Vibrations of Complex Networks
We consider atoms interacting each other through the topological structure of
a complex network and investigate lattice vibrations of the system, the quanta
of which we call {\em netons} for convenience. The density of neton levels,
obtained numerically, reveals that unlike a local regular lattice, the system
develops a gap of a finite width, manifesting extreme rigidity of the network
structure at low energies. Two different network models, the small-world
network and the scale-free network, are compared: The characteristic structure
of the former is described by an additional peak in the level density whereas a
power-law tail is observed in the latter, indicating excitability of netons at
arbitrarily high energies. The gap width is also found to vanish in the
small-world network when the connection range .Comment: 9 pages, 6 figures, to appear in JP
Fractional photon-assisted tunneling in an optical superlattice: large contribution to particle transfer
Fractional photon-assisted tunneling is investigated both analytically and
numerically for few interacting ultra-cold atoms in the double-wells of an
optical superlattice. This can be realized experimentally by adding periodic
shaking to an existing experimental setup [Phys. Rev. Lett. 101, 090404
(2008)]. Photon-assisted tunneling is visible in the particle transfer between
the wells of the individual double wells. In order to understand the physics of
the photon-assisted tunneling, an effective model based on the rotating wave
approximation is introduced. The validity of this effective approach is tested
for wide parameter ranges which are accessible to experiments in double-well
lattices. The effective model goes well beyond previous perturbation theory
approaches and is useful to investigate in particular the fractional
photon-assisted tunneling resonances. Analytic results on the level of the
experimentally realizable two-particle quantum dynamics show very good
agreement with the numerical solution of the time-dependent Schr\"odinger
equation. Far from being a small effect, both the one-half-photon and the
one-third-photon resonance are shown to have large effects on the particle
transfer.Comment: 9 pages, 11 png-figure
Indirect coupling between spins in semiconductor quantum dots
The optically induced indirect exchange interaction between spins in two
quantum dots is investigated theoretically. We present a microscopic
formulation of the interaction between the localized spin and the itinerant
carriers including the effects of correlation, using a set of canonical
transformations. Correlation effects are found to be of comparable magnitude as
the direct exchange. We give quantitative results for realistic quantum dot
geometries and find the largest couplings for one dimensional systems.Comment: 4 pages, 3 figure
Application of the Lifshitz theory to poor conductors
The Lifshitz formula for the dispersive forces is generalized to the
materials, which cannot be described with the local dielectric response.
Principal nonlocality of poor conductors is related with the finite screening
length of the penetrating field and the collisional relaxation; at low
temperatures the role of collisions plays the Landau damping. The spatial
dispersion makes the theory self consistent. Our predictions are compared with
the recent experiment. It is demonstrated that at low temperatures the
Casimir-Lifshitz entropy disappears as in the case of degenerate plasma and
as for the nondegenerate one.Comment: Accepted for publication in PR
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