Equation of State and Viscosities from a Gravity Dual of the Gluon Plasma

Employing new precision data of the equation of state of the SU(3) Yang-Mills theory (gluon plasma) the dilaton potential of a gravity-dual model is adjusted in the temperature range $(1 - 10) T_c$ within a bottom-up approach. The ratio of bulk viscosity to shear viscosity follows then as $\zeta/\eta \approx \pi \Delta v_s^2$ for $\Delta v_s^2 < 0.2$ and achieves a maximum value of $0.94$ at $\Delta v_s^2 \approx 0.3$, where $\Delta v_s^2 \equiv 1/3 - v_s^2$ is the non-conformality measure and $v_s^2$ is the velocity of sound squared, while the ratio of shear viscosity to entropy density is known as $(4 \pi)^{-1}$ for the considered set-up with Hilbert action on the gravity side.Comment: 19 pages, 13 figures. Version published in PL

Magnetic excitations in charge- ordered NaV2O5

An investigation of the spin excitation spectrum of charge ordered (CO) NaV2O5 is presented. We discuss several different exchange models which may be relevant for this compound, namely in- line and zig-zag chain models with weak as well as strong inter- chain coupling and also a ladder model and a CO/MV (mixed valent) model. We put special emphasis on the importance of large additional exchange across the diagonals of V- ladders and the presence of exchange anisotropies on the excitation spectrum. It is shown that the observed splitting of transverse dispersion branches may both be interpreted as anisotropy effect as well as acoustic- optic mode splitting in the weakly coupled chain models. In addition we calculate the field dependence of excitation modes in these models. Furthermore we show that for strong inter- chain coupling, as suggested by recent LDA+U results, an additional high energy optical excitation appears and the spin gap is determined by anisotropies. The most promising CO/MV model predicts a spin wave dispersion perpendicular to the chains which agrees very well with recent results obtained by inelastic neutron scattering.Comment: Revtex, 22 pages, 7 postscript figure

Cross-over versus first-order phase transition in holographic gravity-single-dilaton models of QCD thermodynamics

A dilaton potential is adjusted to recently confirmed lattice QCD thermodynamics data in the temperature range $(0.7 \ldots 3.5) T_c$ where $T_c = 155 \text{MeV}$ is the pseudo-critical temperature. The employed holographic model is based on a gravity--single-field dilaton dual. We discuss conditions for enforcing (for the pure gluon plasma) or avoiding (for the QCD quark-gluon plasma) a first-order phase transition, but still keeping a softest point (minimum of sound velocity).Comment: 11 pages, 9 figure

Fermi surface and heavy masses for UPd2_2Al3_3

We calculate the Fermi surface and the anisotropic heavy masses of UPd2Al3 by keeping two of the 5f electrons as localized. Good agreement with experiments is found. The theory contains essentially no adjustable parameter except for a small shift of the position of the Fermi energy of the order of a few meV. A discussion is given why localization of two f electrons is justified.Comment: 4 pages, 2 figure

Dynamic spin susceptibility of paramagnetic spinel LiV2O4

In an attempt to explain inelastic neutron scattering data for LiV2O4 the dynamical spin susceptibility \chi(Q,w) at zero temperature is calculated. Starting point is a weak coupling approach based on the LDA bandstructure for that material. It is supplemented by a RPA treatment of local on-site interactions and contains an adjustable parameter. Due to the geometrically frustrated lattice structure the magnetic response is strongly enhanced in the vicinity of a nearly spherical surface in Q-space. We compare these results with recent low-temperature neutron scattering data. The measured spin relaxation rate \Gamma is used to estimate the spin fluctuation contribution to the specific heat.Comment: 26 pages, 6 figures, submitted to PR

Magnetic properties of doped GdI2

Motivated by the recent experimental studies on layered ferromagnetic metallic system GdI2 and its doped variant GdI2Hx we develop a model to understand their ground state magnetic phase diagram. Based on first principle electronic structure calculations we write down a phenomenological model and solve it under certain approximations to obtain the ground state energy. In the process we work out the phase diagram of the correlated double exchange model on a triangular lattice for the specific band structure at hand.Comment: 13 pages, 5 figures, corrected typo