Nuclear magnetic susceptibility of metals with magnetic impurities

We consider the contribution of magnetic impurities to the nuclear magnetic susceptibility $\chi$ and to the specific heat $C$ of a metal. The impurity contribution to the magnetic susceptibility has a $1/T^2$ behaviour, and the impurity contribution to the specific heat has a $1/T$ behaviour, both in an extended region of temperatures $T$. In the case of a dirty metal the RKKY interaction of nuclear spins and impurity spins is suppressed for low temperatures and the main contribution to $C$ and $\chi$ is given by their dipole-dipole interaction.Comment: 9 pages, 4 figures, REVTE

Excitations in Spin Chains and Specific-Heat Anomalies in Yb(4)As(3)

An explanation is given for the observed magnetic-field dependence of the low-temperature specific heat coefficient of Yb(4)As(3). It is based on a recently developed model for that material which can explain the observed heavy-fermion behaviour. According to it the Yb(3+)-ions are positioned in a net of parallel chains with an effective spin coupling of the order of J = 25 K. The magnetic-field dependence can be understood by including a weak magnetic coupling J' between adjacent chains. The data require a ratio J'/J of about 10^{-4}. In that case the experimental results can be reproduced very well by the theory.Comment: 5 pages, 5 PostScript-figures, needs LaTeX2e and the graphics-packag

Orbital Localization and Delocalization Effects in the U 5f^2 Configuration: Impurity Problem

Anderson models, based on quantum chemical studies of the molecule of U(C_8H_8)_2, are applied to investigate the problem of an U impurity in a metal. The special point here is that the U 5f-orbitals are divided into two subsets: an almost completely localized set and a considerably delocalized one. Due to the crystal field, both localized and delocalized U 5f-orbitals affect the low-energy physics. A numerical renormalization group study shows that every fixed point is characterized by a residual local spin and a phase shift. The latter changes between 0 and \pi/2, which indicates the competition between two different fixed points. Such a competition between the different local spins at the fixed points reflects itself in the impurity magnetic susceptibility at high temperatures. These different features cannot be obtained if the special characters of U 5f-orbitals are neglected.Comment: 4 pages, REVTeX, email to [email protected]

Fast computation of the Kohn-Sham susceptibility of large systems

For hybrid systems, such as molecules grafted onto solid surfaces, the calculation of linear response in time dependent density functional theory is slowed down by the need to calculate, in N^4 operations, the susceptibility of N non interacting Kohn-Sham reference electrons. We show how this susceptibility can be calculated N times faster within finite precision. By itself or in combination with previous methods, this should facilitate the calculation of TDDFT response and optical spectra of hybrid systems.Comment: submitted 25/1/200

Fermiology of Cuprates from First Principles: From Small Pockets to the Luttinger Fermi surface

Fermiology, the shape and size of the Fermi surface, underpins the low-temperature physical properties of a metal. Recent investigations of the Fermi surface of high-Tc superconductors, however, show a most unusual behavior: upon addition of carriers, ``Fermi'' pockets appear around nodal (hole doping) and antinodal (electron doping) regions of the Brillouin zone in the ``pseudogap'' state. With progressive doping, p, these evolve into well-defined Fermi surfaces around optimal doping (p_opt), with no pseudogap. Correspondingly, various physical responses, including d-wave superconductivity, evolve from highly anomalous, up to p_opt, to more conventional beyond. Describing this evolution holds the key to understanding high-temperature superconductivity. Here, we present ab initio quantum chemical results for cuprates, providing a quantitative description of the evolution of the Fermi surface with doping. Our results constitute an ab initio justification for several, hitherto proposed semiphenomenological theories, offering an unified basis for understanding of various, unusual physical responses of doped cuprates

Eliashberg theory of superconductivity and inelastic rare-earth impurity scattering in filled skutterudite La1−x_{1-x}Prx_{x}Os4_{4}Sb12_{12}

We study the influence of inelastic rare-earth impurity scattering on electron-phonon mediated superconductivity and mass renormalization in (La$_{1-x}$Pr$_{x}$)Os$_{4}$Sb$_{12}$ compounds. Solving the strong coupling Eliashberg equations we find that the dominant quadrupolar component of the inelastic scattering on Pr impurities yields an enhancement of the superconducting transition temperature T$_c$ in LaOs$_{4}$Sb$_{12}$ and increases monotonically as a function of Pr concentration. The calculated results are in good agreement with the experimentally observed T$_c (x)$ dependence. Our analysis suggests that phonons and quadrupolar excitations cause the attractive electron interaction which results in the formation of Cooper pairs and singlet superconductivity in PrOs$_{4}$Sb$_{12}$.Comment: 5 pages,4 figures, revised title suggested by editor, original fig.4 and fig.5 combined together, discussion added before conclusio

Interaction of a Magnetic Impurity with Strongly Correlated Conduction Electrons

We consider a magnetic impurity which interacts by hybridization with a system of strongly correlated conduction electrons. The latter are described by a Hubbard Hamiltonian. By means of a canconical transformation the charge degrees of freedom of the magnetic impurity are eliminated. The resulting effective Hamiltonian $H_{\rm eff}$ is investigated and various limiting cases are considered. If the Hubbard interaction $U$ between the conduction electrons is neglected $H_{\rm eff}$ reduces to a form obtained by the Schrieffer-Wolff transformation, which is essentially the Kondo Hamiltonian. If $U$ is large and the correlations are strong $H_{\rm eff}$ is changed. One modification concerns the coefficient of the dominant exchange coupling of the magnetic impurity with the nearest lattice site. When the system is hole doped, there is also an antiferromagnetic coupling to the nearest neighbors of that site involving additionally a hole. Furthermore, it is found that the magnetic impurity attracts a hole. In the case of electron doping, double occupancies are repelled by the impurity. In contrast to the hole-doped case, we find no magnetic coupling which additionally involves a doubly occupied site.Comment: 16 pages, Revtex 3.

Phase diagram of the one dimensional anisotropic Kondo-necklace model

The one dimensional anisotropic Kondo-necklace model has been studied by several methods. It is shown that a mean field approach fails to gain the correct phase diagram for the Ising type anisotropy. We then applied the spin wave theory which is justified for the anisotropic case. We have derived the phase diagram between the antiferromagnetic long range order and the Kondo singlet phases. We have found that the exchange interaction (J) between the itinerant spins and local ones enhances the quantum fluctuations around the classical long range antiferromagnetic order and finally destroy the ordered phase at the critical value, J_c. Moreover, our results show that the onset of anisotropy in the XY term of the itinerant interactions develops the antiferromagnetic order for J<J_c. This is in agreement with the qualitative feature which we expect from the symmetry of the anisotropic XY interaction. We have justified our results by the numerical Lanczos method where the structure factor at the antiferromagnetic wave vector diverges as the size of system goes to infinity.Comment: 9 pages and 9 eps figure

Field induced quantum phase transition in the anisotropic Kondo necklace model

The anisotropic Kondo necklace model in 2D and 3D is treated as a genuine model for magnetic to Kondo singlet quantum phase transitions in the heavy fermion (HF) compounds. The variation of the quantum critical point (QCP) with anisotropy parameters has been investigated previously in the zero field case [1]. Here we extend the treatment to finite fields using a generalised bond operator representation including all triplet states. The variation of critical tc with external field H and the associated phase diagram is derived. The influence of anisotropies and the different g-factors for localised and itinerant spins on tc(H) is also investigated. It is found that three different types of behaviour may appear: (i) Destruction of antiferromangetism and appearance of a singlet state above a critical field. (ii) The inverese behaviour, namely field induced antiferromagnetism out of the Kondo singlet phase. (iii) Reentrance behaviour of the Kondo singlet phase as function of field strength.Comment: 12 pages, 5 figure

Analytical solution of the Gross-Neveu model at finite density

Recent numerical calculations have shown that the ground state of the Gross-Neveu model at finite density is a crystal. Guided by these results, we can now present the analytical solution to this problem in terms of elliptic functions. The scalar potential is the superpotential of the non-relativistic Lame Hamiltonian. This model can also serve as analytically solvable toy model for a relativistic superconductor in the Larkin-Ovchinnikov-Fulde-Ferrell phase.Comment: 5 pages, no figures, revtex; vs2: appendix with analytical proof of self-consistency adde