240 research outputs found
Electron-Ion Structure Factors and the General Accuracy of Linear Response
We show that electron-ion structure factors in fluid metallic systems can be
well understood from an application of linear response in the electron system,
combined with hard-sphere like correlation for the ionic component. In
particular, we predict that electron-ion structure factors fall into two
general classes, one for high () and one for low () valence
metals, and make suggestions for experiments to test these ideas. In addition,
we show how the general success of electronic linear response for most metallic
systems stems in part from an intrinsic interference between atomic and
electronic length scales which weakens the nonlinear response. The main
exception to this is metallic hydrogen.Comment: to appear in J. Non-Crystalline Solids, part of LAM-10 conference
proceedings. RevTex, 12 pages, 2 figure
Low-Temperature Hall Effect in Substituted Sr2RuO4
We report the results of a study of the Hall effect and magnetoresistance in
single crystals of Sr2RuO4 in which Sr^(2+) has been substituted by La^(3+)
(Sr(2-y)La(y)RuO(4)) or Ru^(4+) by Ti^(4+) (Sr(2)Ru(1-x)Ti(x)O(4)). For undoped
Sr2RuO4, the purity is so high that the strong-field Hall coefficient can be
measured for fields above 4 T. The conventional weak-field Hall coefficient as
a function of doping shows a sharp jump and sign change at y ~ 0.01 that is
unrelated to either a sharp change in Fermi-surface topography or a magnetic
instability. The implications of these results are discussed.Comment: 5 pages, 4 figure
Electronic coherence in metals: comparing weak localization and time-dependent conductance fluctuations
Quantum corrections to the conductivity allow experimental assessment of
electronic coherence in metals. We consider whether independent measurements of
different corrections are quantitatively consistent, particularly in systems
with spin-orbit or magnetic impurity scattering. We report weak localization
and time-dependent universal conductance fluctuation data in quasi-one- and
two-dimensional AuPd wires between 2 K and 20 K. The data inferred from both
methods are in excellent quantitative agreement, implying that precisely the
same coherence length is relevant to both corrections.Comment: 5 pages, 4 figures. Scheduled to appear in PRB 70, 041304 (2004
Detailed Examination of Transport Coefficients in Cubic-Plus-Quartic Oscillator Chains
We examine the thermal conductivity and bulk viscosity of a one-dimensional
(1D) chain of particles with cubic-plus-quartic interparticle potentials and no
on-site potentials. This system is equivalent to the FPU-alpha beta system in a
subset of its parameter space. We identify three distinct frequency regimes
which we call the hydrodynamic regime, the perturbative regime and the
collisionless regime. In the lowest frequency regime (the hydrodynamic regime)
heat is transported ballistically by long wavelength sound modes. The model
that we use to describe this behaviour predicts that as the frequency goes to
zero the frequency dependent bulk viscosity and the frequency dependent thermal
conductivity should diverge with the same power law dependence on frequency.
Thus, we can define the bulk Prandtl number as the ratio of the bulk viscosity
to the thermal conductivity (with suitable prefactors to render it
dimensionless). This dimensionless ratio should approach a constant value as
frequency goes to zero. We use mode-coupling theory to predict the zero
frequency limit. Values of the bulk Prandtl number from simulations are in
agreement with these predictions over a wide range of system parameters. In the
middle frequency regime, which we call the perturbative regime, heat is
transported by sound modes which are damped by four-phonon processes. We call
the highest frequency regime the collisionless regime since at these
frequencies the observing times are much shorter than the characteristic
relaxation times of phonons. The perturbative and collisionless regimes are
discussed in detail in the appendices.Comment: Latex with references in .bib file. 36 pages, 8 figures. Submitted to
J. Stat. Phys. on Sept. 2
Automated computation of materials properties
Materials informatics offers a promising pathway towards rational materials
design, replacing the current trial-and-error approach and accelerating the
development of new functional materials. Through the use of sophisticated data
analysis techniques, underlying property trends can be identified, facilitating
the formulation of new design rules. Such methods require large sets of
consistently generated, programmatically accessible materials data.
Computational materials design frameworks using standardized parameter sets are
the ideal tools for producing such data. This work reviews the state-of-the-art
in computational materials design, with a focus on these automated
frameworks. Features such as structural prototyping and
automated error correction that enable rapid generation of large datasets are
discussed, and the way in which integrated workflows can simplify the
calculation of complex properties, such as thermal conductivity and mechanical
stability, is demonstrated. The organization of large datasets composed of
calculations, and the tools that render them
programmatically accessible for use in statistical learning applications, are
also described. Finally, recent advances in leveraging existing data to predict
novel functional materials, such as entropy stabilized ceramics, bulk metallic
glasses, thermoelectrics, superalloys, and magnets, are surveyed.Comment: 25 pages, 7 figures, chapter in a boo
Realistic Tunneling States for the Magnetic Effects in Non-Metallic Real Glasses
The discovery of magnetic and compositional effects in the low temperature
properties of multi-component glasses has prompted the need to extend the
standard two-level systems (2LSs) tunneling model. A possible extension
\cite{Jug2004} assumes that a subset of tunneling quasi-particles is moving in
a three-welled potential (TWP) associated with the ubiquitous inhomogeneities
of the disordered atomic structure of the glass. We show that within an
alternative, cellular description of the intermediate-range atomic structure of
glasses the tunneling TWP can be fully justified. We then review how the
experimentally discovered magnetic effects can be explained within the approach
where only localized atomistic tunneling 2LSs and quasi-particles tunneling in
TWPs are allowed. We discuss the origin of the magnetic effects in the heat
capacity, dielectric constant (real and imaginary parts), polarization echo and
SQUID magnetization in several glassy systems. We conclude by commenting on a
strategy to reveal the mentioned tunneling states (2LSs and TWPs) by means of
atomistic computer simulations and discuss the microscopic nature of the
tunneling states in the context of the potential energy landscape of
glass-forming systems.Comment: 48 pages, 27 figures; mini-review for the Proceedings of the XIV
International Workshop on Complex Systems (Fai della Paganella, Trento, March
2015) (submitted to Phil.Mag.). arXiv admin note: text overlap with
arXiv:cond-mat/0210221 by other author
Fermionic Linear Optics Revisited
We provide an alternative view of the efficient classical simulatibility of
fermionic linear optics in terms of Slater determinants. We investigate the
generic effects of two-mode measurements on the Slater number of fermionic
states. We argue that most such measurements are not capable (in conjunction
with fermion linear optics) of an efficient exact implementation of universal
quantum computation. Our arguments do not apply to the two-mode parity
measurement, for which exact quantum computation becomes possible, see
quant-ph/0401066.Comment: 16 pages, submitted to the special issue of Foundation of Physics in
honor of Asher Peres' 70th birthda
Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys
We report the results of ab initio molecular dynamics simulations of liquid
Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600
K, just above the melting point of GaAs. The liquid is predicted to be metallic
at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity
maximum near x = 0.5, consistent with the Faber-Ziman expression. The
electronic density of states is finite at the Fermi energy for all
concentrations; there is, however, a significant pseudogap especially in the
As-rich samples. The Ga-rich density of states more closely resembles that of a
free-electron metal. The partial structure factors show only a weak indication
of chemical short-range order. There is also some residue of the covalent
bonding found in the solid, which shows up in the bond-angle distribution
functions of the liquid state. Finally, the atomic diffusion coefficients at
1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in
Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in
Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.
Electronic and Magnetic Properties of Febr2
Electronic and magnetic (e-m) properties of FeBr2 have been surprisingly well
described as originating from the Fe2+ ions and their fine electronic
structure. The fine electronic structure have been evaluated taking into
account the spin-orbit (s-o) coupling, crystal-field and inter-site
spin-dependent interactions. The required magnetic doublet ground state with an
excited singlet at D=2.8 meV results from the trigonal distortion. This effect
of the trigonal distortion and a large magnetic moment of iron, of 4.4 mB, can
be theoretically derived provided the s-o coupling is correctly taking into
account. The obtained good agreement with experimental data indicates on
extremaly strong correlations of the six 3d electrons in the Fe2+ ion yielding
their full localization and the insulating state. These calculations show that
for the meaningful analysis of e-m properties of FeBr2 the spin-orbit coupling
is essentially important and that the orbital moment (0.74 mB) is largely
unquenched (by the off-cubic trigonal distortion in the presence of the
spin-orbit coupling).Comment: 11 pages in RevTex, 5 figure
A Plaquette Basis for the Study of Heisenberg Ladders
We employ a plaquette basis-generated by coupling the four spins in a
lattice to a well-defined total angular momentum-for the study of
Heisenberg ladders with antiferromagnetic coupling. Matrix elements of the
Hamiltonian in this basis are evaluated using standard techniques in
angular-momentum (Racah) algebra. We show by exact diagonalization of small
( and ) systems that in excess of 90% of the ground-state
probability is contained in a very small number of basis states. These few
basis states can be used to define a severely truncated basis which we use to
approximate low-lying exact eigenstates. We show how, in this low-energy basis,
the isotropic spin-1/2 Heisenberg ladder can be mapped onto an anisotropic
spin-1 ladder for which the coupling along the rungs is much stronger than the
coupling between the rungs. The mapping thereby generates two distinct energy
scales which greatly facilitates understanding the dynamics of the original
spin-1/2 ladder. Moreover, we use these insights to define an effective
low-energy Hamiltonian in accordance to the newly developed COntractor
REnormalization group (CORE) method. We show how a simple range-2 CORE
approximation to the effective Hamiltonian to be used with our truncated basis
reproduces the low-energy spectrum of the exact theory at the \alt
1% level.Comment: 12 pages with two postscript figure
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