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 ($Z>3$) and one for low ($Z\leq2$) 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 $\textit{ab-initio}$ 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 $\textit{ab-initio}$ 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 $2\times2$ 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 ($2\times4$ and $2\times6$) 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 $2\times6$ theory at the \alt 1% level.Comment: 12 pages with two postscript figure