Thermal equation of state and stability of (Mg_(0.06)Fe_(0.94))O

We present the pressure-volume-temperature (P-V-T) equation of state of polycrystalline (Mg_(0.06)Fe_(0.94))O (Mw94) determined from laser-heated x-ray diffraction experiments up to 122 GPa and 2100 K, conditions approaching those of the deep mantle. We conducted two sets of experiments, one with an in situ Fe metal oxygen fugacity buffer and one without such a buffer. The internal pressure markers used in these experiments were B2-NaCl and hcp-Fe in the buffered experiment and B2-NaCl in the unbuffered experiment. In the sampled P-T range of the high temperature part of this study, only the B1 structure of Mw94 was observed, indicating that the addition of Mg to FeO stabilizes the B1 phase with respect to the B8 phase at these conditions. Both datasets were fit to a Birch-Murnaghan and Mie-Grüneisen-Debye thermal equation of state using a new open-source fitting routine, also presented here. Analysis of these data sets using the same internal pressure marker shows that the P–V–T data of Mw94 obtained in the unbuffered experiment are well explained by the equation of state parameters determined from the buffered data set. We have also compared the thermal equation of state of Mw94 with that of wüstite and conclude that Mw94 has measurably distinct thermoelastic properties compared with those of wüstite. We use the results obtained in the buffered experiment to determine the density and bulk sound velocity of Mw94 at the base of the mantle and compare these values to geophysical observations of ultralow-velocity zones

Negative Impurity Magnetic Susceptibility and Heat Capacity in a Kondo Model with Narrow Peaks in the Local Density of Electron States

Temperature dependencies of the impurity magnetic susceptibility, entropy, and heat capacity have been obtained by the method of numerical renormalization group and exact diagonalization for the Kondo model with peaks in the electron density of states near the Fermi energy (in particular, with logarithmic Van Hove singularities). It is shown that these quantities can be {\it negative}. A new effect has been predicted (which, in principle, can be observed experimentally), namely, the decrease in the magnetic susceptibility and heat capacity of a nonmagnetic sample upon the addition of magnetic impurities into it

Anomalous behaviors of the charge and spin degrees of freedom in the CuO double chains of PrBa2_2Cu4_4O8_8

The density-matrix renormalization-group method is used to study the electronic states of a two-chain Hubbard model for CuO double chains of PrBa$_2$Cu$_4$O$_8$. We show that the model at quarter filling has the charge ordered phases with stripe-type and in-line--type patterns in the parameter space, and in-between, there appears a wide region of vanishing charge gap; the latter phase is characteristic of either Tomonaga-Luttinger liquid or a metallic state with a spin gap. We argue that the low-energy electronic state of the CuO double chains of PrBa$_2$Cu$_4$O$_8$ should be in the metallic state with a possibly small spin gap.Comment: REVTEX 4, 10 pages, 9 figures; submitted to PR

Nonperturbative Scaling Theory of Free Magnetic Moment Phases in Disordered Metals

The crossover between a free magnetic moment phase and a Kondo phase in low dimensional disordered metals with dilute magnetic impurities is studied. We perform a finite size scaling analysis of the distribution of the Kondo temperature as obtained from a numerical renormalization group calculation of the local magnetic susceptibility and from the solution of the self-consistent Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic moments when the exchange coupling falls below a disorder-dependent critical value $J_{\rm c}$. Our numerical results show that between the free moment phase due to Anderson localization and the Kondo screened phase there is a phase where free moments occur due to the appearance of random local pseudogaps at the Fermi energy whose width and power scale with the elastic scattering rate $1/\tau$.Comment: 4 pages, 6 figure

Spin-flop transition in uniaxial antiferromagnets: magnetic phases, reorientation effects, multidomain states

The classical spin-flop is the field-driven first-order reorientation transition in easy-axis antiferromagnets. A comprehensive phenomenological theory of easy-axis antiferromagnets displaying spin-flops is developed. It is shown how the hierarchy of magnetic coupling strengths in these antiferromagnets causes a strongly pronounced two-scale character in their magnetic phase structure. In contrast to the major part of the magnetic phase diagram, these antiferromagnets near the spin-flop region are described by an effective model akin to uniaxial ferromagnets. For a consistent theoretical description both higher-order anisotropy contributions and dipolar stray-fields have to be taken into account near the spin-flop. In particular, thermodynamically stable multidomain states exist in the spin-flop region, owing to the phase coexistence at this first-order transition. For this region, equilibrium spin-configurations and parameters of the multidomain states are derived as functions of the external magnetic field. The components of the magnetic susceptibility tensor are calculated for homogeneous and multidomain states in the vicinity of the spin-flop. The remarkable anomalies in these measurable quantities provide an efficient method to investigate magnetic states and to determine materials parameters in bulk and confined antiferromagnets, as well as in nanoscale synthetic antiferromagnets. The method is demonstrated for experimental data on the magnetic properties near the spin-flop region in the orthorhombic layered antiferromagnet (C_2H_5NH_3)_2CuCl_4.Comment: (15 pages, 12 figures; 2nd version: improved notation and figures, correction of various typos

The Kondo effect in the presence of Van Hove singularities: A numerical renormalization group study

A numerical renormalization group (NRG) investigation of the one-centre $t-t'$ Kondo problem is performed for the square lattice with account of logarithmic Van Hove singularities (VHS) in the electron density of states. The magnetic susceptibility, entropy and specific heat are calculated. The temperature dependences of the thermodynamic properties in the presence of VHS turn out to be non-trivial. For finite $t'$ inverse logarithm of the corresponding Kondo temperature $T_K$ demonstrates a crossover from the square-root to standard linear dependence on the $s-d$ exchange coupling. The low-temperature behavior of magnetic susceptibility and linear specific heat are investigated, and the Wilson ratio is obtained. For $t' -> 0$ the Fermi-liquid behavior is broken.Comment: 13 pages, 10 figure

Coherence in the Quasi-Particle 'Scattering' by the Vortex Lattice in Pure Type-II Superconductors

The effect of quasi-particle (QP) 'scattering' by the vortex lattice on the de-Haas van-Alphen oscillations in a pure type-II superconductor is investigated within mean field,asymptotic perturbation theory. Using a 2D electron gas model it is shown that, due to a strict phase coherence in the many-particle correlation functions, the 'scattering' effect in the asymptotic limit ($\sqrt{E_F/\hbar\omega_c}\gg 1$) is much weaker than what is predicted by the random vortex lattice model proposed by Maki and Stephen, which destroys this coherence . The coherent many particle configuration is a collinear array of many particle coordinates, localized within a spatial region with size of the order of the magnetic length. The amplitude of the magnetization oscillations is sharply damped just below $$ because of strong $180^{\circ}$ out of phase magnetic oscillations in the superconducting condensation energy ,which tend to cancel the normal electron oscillations. Within the ideal 2D model used it is found, however, that because of the relative smallness of the quartic and higher order terms in the expansion, the oscillations amplitude at lower fields does not really damp to zero, but only reverses sign and remains virtually undamped well below $H_{c2}$. This conclusion may be changed if disorder in the vortex lattice, or vortex lines motion will be taken into account. The reduced QP 'scattering' effect may be responsible for the apparent crossover from a strong damping of the dHvA oscillations just below $H_{c2}$ to a weaker damping at lower fields observed experimentally in several 3D superconductors.Comment: 26 pages, Revtex no Figure