7,818 research outputs found
Lang-Firsov approaches to polaron physics: From variational methods to unbiased quantum Monte Carlo simulations
We review variational and quantum Monte Carlo approaches based on (extended)
Lang-Firsov transformations of the Hamiltonian. Derivations for one, two and
many electrons are given, and results for the Holstein polaron, the
Holstein-Hubbard bipolaron, and the spinless Holstein model at finite carrier
densities are presented.Comment: 40 pages, 15 figures, to be published in Polarons in Advanced
Materials, Ed. A. S. Alexandrov (Canopus/Springer Publishing, Bristol)
(2007); V2: typo in authors list correcte
Entropy Production and Equilibrium Conditions of General-Covariant Spin Systems
In generalizing the special-relativistic one-component version of Eckart's
continuum thermodynamics to general-relativistic space-times with Riemannian or
post-Riemannian geometry, we consider the entropy production and other
themodynamical quantities such as the entropy flux and the Gibbs fundamental
equation. We discuss equilibrium conditions in gravitational theories which are
based on such geometries. In particular, thermodynamic implications of the
non-symmetry of the energy-momentum tensor and the related spin balance
equations are investigated, also for the special case of General Relativity.Comment: General-covariant spin systems are carefully discussed in the
framework of non-equlibrium thermodynamics starting out with an already
published entropy identit
Anderson Orthogonality and the Numerical Renormalization Group
Anderson Orthogonality (AO) refers to the fact that the ground states of two
Fermi seas that experience different local scattering potentials, say |G_I> and
|G_F>, become orthogonal in the thermodynamic limit of large particle number N,
in that || ~ N^(- Delta_AO^2 /2) for N->infinity. We show that the
numerical renormalization group (NRG) offers a simple and precise way to
calculate the exponent Delta_AO: the overlap, calculated as function of Wilson
chain length k, decays exponentially, ~ exp(-k alpha), and Delta_AO can be
extracted directly from the exponent alpha. The results for Delta_AO so
obtained are consistent (with relative errors typically smaller than 1%) with
two other related quantities that compare how ground state properties change
upon switching from |G_I> to |G_F>: the difference in scattering phase shifts
at the Fermi energy, and the displaced charge flowing in from infinity. We
illustrate this for several nontrivial interacting models, including systems
that exhibit population switching.Comment: 10 pages, 7 figure
Prediction of high-Tc conventional superconductivity in the ternary lithium borohydride system
We investigate the superconducting ternary lithium borohydride phase diagram at pressures of 0 and 200 GPa using methods for evolutionary crystal structure prediction and linear-response calculations for the electron-phonon coupling. Our calculations show that the ground state phase at ambient pressure, LiBH4, stays in the Pnma space group and remains a wide band-gap insulator at all pressures investigated. Other phases along the 1:1:x Li:B:H line are also insulating. However, a full search of the ternary phase diagram at 200 GPa revealed a metallic Li2BH6
phase, which is thermodynamically stable down to 100 GPa. This superhydride phase, crystallizing in a Fm¯3m space group, is characterized by sixfold hydrogen-coordinated boron atoms occupying the fcc sites of the unit cell. Due to strong hydrogen-boron bonding this phase displays a critical temperature of ∼100K between 100 and 200 GPa. Our investigations confirm that ternary compounds used in hydrogen-storage applications should exhibit high-Tc conventional superconductivity in diamond anvil cell experiments, and suggest a viable route to optimize the superconducting behavior of high-pressure hydrides, exploiting metallic covalent bonds
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