Theory of Kondo lattices and its application to high-temperature superconductivity and pseudo-gaps in cuprate oxides

A theory of Kondo lattices is developed for the t-J model on a square lattice. The spin susceptibility is described in a form consistent with a physical picture of Kondo lattices: Local spin fluctuations at different sites interact with each other by a bare intersite exchange interaction, which is mainly composed of two terms such as the superexchange interaction, which arises from the virtual exchange of spin-channel pair excitations of electrons across the Mott-Hubbard gap, and an exchange interaction arising from that of Gutzwiller's quasi-particles. The bare exchange interaction is enhanced by intersite spin fluctuations developed because of itself. The enhanced exchange interaction is responsible for the development of superconducting fluctuations as well as the Cooper pairing between Gutzwiller's quasi-particles. On the basis of the microscopic theory, we develop a phenomenological theory of low-temperature superconductivity and pseudo-gaps in the under-doped region as well as high-temperature superconductivity in the optimal-doped region. Anisotropic pseudo-gaps open mainly because of d\gamma-wave superconducting low-energy fluctuations: Quasi-particle spectra around (\pm\pi/a,0) and (0,\pm\pi/a), with a the lattice constant, or X points at the chemical potential are swept away by strong inelastic scatterings, and quasi-particles are well defined only around (\pm\pi/2a,\pm\pi/2a) on the Fermi surface or line. As temperatures decrease in the vicinity of superconducting critical temperatures, pseudo-gaps become smaller and the well-defined region is extending toward X points. The condensation of d\gamma-wave Cooper pairs eventually occurs at low enough temperatures when the pair breaking by inelastic scatterings becomes small enough.Comment: 15 pages, 14 figure

Anisotropic Superexchange for nearest and next nearest coppers in chain, ladder and lamellar cuprates

We present a detailed calculation of the magnetic couplings between nearest-neighbor and next-nearest-neighbor coppers in the edge-sharing geometry, ubiquitous in many cuprates. In this geometry, the interaction between nearest neighbor coppers is mediated via two oxygens, and the Cu-O-Cu angle is close to 90 degrees. The derivation is based on a perturbation expansion of a general Hubbard Hamiltonian, and produces numerical estimates for the various magnetic energies. In particular we find the dependence of the anisotropy energies on the angular deviation away from the 90 degrees geometry of the Cu-O-Cu bonds. Our results are required for the correct analysis of the magnetic structure of various chain, ladder and lamellar cuprates.Comment: 13 pages, Latex, 7 figure

Weak antiferromagnetism due to Dzyaloshinskii-Moriya interaction in Ba3_3Cu2_2O4_4Cl2_2

The antiferromagnetic insulating cuprate Ba$_3$Cu$_2$O$_4$Cl$_2$ contains folded CuO$_2$ chains with four magnetic copper ions ($S=1/2$) per unit cell. An underlying multiorbital Hubbard model is formulated and the superexchange theory is developed to derive an effective spin Hamiltonian for this cuprate. The resulting spin Hamiltonian involves a Dzyaloshinskii-Moriya term and a more weak symmetric anisotropic exchange term besides the isotropic exchange interaction. The corresponding Dzyaloshinskii-Moriya vectors of each magnetic Cu-Cu bond in the chain reveal a well defined spatial order. Both, the superexchange theory and the complementary group theoretical consideration, lead to the same conclusion on the character of this order. The analysis of the ground-state magnetic properties of the derived model leads to the prediction of an additional noncollinear modulation of the antiferromagnetic structure. This weak antiferromagnetism is restricted to one of the Cu sublattices.Comment: 13 pages, 1 table, 4 figure

Ab initio investigation of VOSeO3, a spin gap system with coupled spin dimers

Motivated by an early experimental study of VOSeO3, which suggested that it is a quasi-2D system of weakly coupled spin dimers with a small spin gap, we have investigated the electronic structure of this material via density-functional calculations. These ab initio results indicate that the system is better thought of as an alternating spin-1/2 chain with moderate interchain interactions, an analog of (VO)2P2O7. The potential interest of this system for studies in high magnetic field given the presumably small value of the spin gap is emphasized.Comment: 4 pages, 5 figure

Oxygen Moment Formation and Canting in Li2CuO2

The possibilities of oxygen moment formation and canting in the quasi-1D cuprate Li2CuO2 are investigated using single crystal neutron diffraction at 2 K. The observed magnetic intensities could not be explained without the inclusion of a large ordered oxygen moment of 0.11(1) Bohr magnetons. Least-squares refinement of the magnetic structure of Li2CuO2 in combination with a spin-density Patterson analysis shows that the magnetization densities of the Cu and O atoms are highly aspherical, forming quasi-1D ribbons of localised Cu and O moments. Magnetic structure refinements and low-field magnetization measurements both suggest that the magnetic structure of Li2CuO2 at 2 K may be canted. A possible model for the canted configuration is proposed.Comment: 10 pages, 8 figures (screen resolution

CuSiO_3 : a quasi - one - dimensional S=1/2 antiferromagnetic chain system

CuSiO_3, isotypic to the spin - Peierls compound CuGeO_3, was discovered recently as a metastable decomposition product of the silicate mineral dioptase, Cu_6Si_6O_{18}\cdot6H_2O. We investigated the physical properties of CuSiO_3 using susceptibility, magnetization and specific heat measurements on powder samples. The magnetic susceptibility \chi(T) is reproduced very well above T = 8 K by theoretical calculations for an S=1/2 antiferromagnetic Heisenberg linear chain without frustration (\alpha = 0) and a nearest - neighbor exchange coupling constant of J/k_{B} = 21 K, much weaker than in CuGeO_3. Below 8 K the susceptibility exhibits a substantial drop. This feature is identified as a second - order phase transition at T_{0} = 7.9 K by specific heat measurements. The influence of magnetic fields on T_{0} is weak, and ac - magnetization measurements give strong evidence for a spin - flop - phase at \mu_0H_{SF} ~ 3 T. The origin of the magnetic phase transition at T_{0} = 7.9 K is discussed in the context of long - range antiferromagnetic order (AF) versus spin - Peierls(SP)order. Susceptibility and specific heat results support the AF ordered ground state. Additional temperature dependent ^{63,65}Cu nuclear quadrupole resonance experiments have been carried out to probe the Cu^{2+} electronic state and the spin dynamics in CuSiO_3

Theory of spin wave excitations of metallic A-type antiferromagnetic manganites

The spin dynamic of the metallic A-type antiferromagnetic manganites is studied. An effective nearest-neighbour Heisenberg spin wave dispersion is derived from the double exchange model taking into account the superexchange interaction between the core spins. The result of inelastic neutron scattering experiment on ${Nd}_{0.45}{Sr}_{0.55}{Mn} {O}_{3}$ is qualitatively reproduced. Comparing theory with experimental data two main parameters of the model: nearest-neighbour electron transfer amplitude and superexchange coupling between the core spins are estimated.Comment: to appear in Phys. Rev.

Phase diagrams of La1−xCaxMnO3\rm La_{1-x}Ca_xMnO_3 in Double Exchange Model with added antiferromagnetic and Jahn-Teller interaction

The phase diagram of the multivalent manganites $\rm La_{1-x}Ca_xMnO_3$, in space of temperature and doping $x$, is a challenge for the theoretical physics. It is an important test for the model used to study these compounds and the method of calculation. To obtain theoretically this diagram for $x<0.5$, we consider the two-band Double Exchange Model for manganites with added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to calculate Curie and N\'{e}el temperatures we derive an effective Heisenberg model for a vector which describes the local orientation of the total magnetization of the system. The exchange constants of this model are different for different space directions and depend on the density of $e_g$ electrons, antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well known phase transitions from A-type antiferromagnetism to ferromagnetism at low $x$ and C-type antiferromagnetism to G-type antiferromagnetism at large $x$, we argue that the antiferromagnetic exchange constants should depend on the lattice direction. We show that ferromagnetic to A-type antiferromagnetic transition results from the Jahn-Teller distortion. Accounting adequately for the magnon-magnon interaction, Curie and N\'{e}el temperatures are calculated. The results are in very good agreement with the experiment and provide values for the model parameters, which best describe the behavior of the critical temperature for $x<0.5$.Comment: 13 pages, 5 figure

Dynamical mean field theory for transition temperature and optics of CMR manganites

A tight binding parametrization of local spin density functional band theory is combined with a dynamical mean field treatment of correlations to obtain a theory of the magnetic transition temperature, optical conductivity and T=0 spinwave stiffness of a minimal model for the pseudocubic metallic $CMR$ manganites such a $La_{1-X}Sr_{x}MnO_{3}$. The results indicate that previous estimates of $T_{c}$ obtained by one of us (Phys. Rev. \textbf{B61} 10738-49 (2000)) are in error, that in fact the materials are characterized by Hunds coupling $J\approx 1.5eV$, and that magnetic-order driven changes in the kinetic energy may not be the cause of the observed 'colossal' magnetoresistive and multiphase behavior in the manganites, raising questions about our present understanding of these materials.Comment: Published version; 10 pages, 9 figure

Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu2Te2O5X2 (X=Br,Cl)

Motivated by recent discussion on possible quantum critical behavior in the coupled Cu-tetrahedra system Cu2Te2O5Br2, we present a comparative ab initio study of the electronic properties of Cu2Te2O5Br2 and the isostructural Cu2Te2O5Cl2. A detailed investigation of the copper-copper interaction pathes reveals that the halogen-ions play an important role in the inter-tetrahedral couplings via X_4-rings (X=Br, Cl). We find that, contrary to initial indications, both systems show a similar electronic behavior with long range exchange pathes mediated by the X_4-rings.Comment: 5 pages, 7 figure