Y. Ohta

Tight-binding calculations of the electronic structure and magnetic properties in ordered TPt3 (T=Ti, V, Cr, Mn, Fe and Co) alloys

Tight-binding-type self-consistent spin-polarised band calculations are performed to study the electronic structure and magnetic properties of ordered TPt3 (T=Ti, V, Cr, Mn, Fe and Co) alloys with Cu3Au structure. The global trends of the magnetic moments and low-temperature specific heat coefficients are explained successfully. Ferrimagnetism in VPt3 and CrPt3 is explained in terms of the hybridisation between 3d states in the V or Cr band and 5d states in the Pt band. The antiferromagnetic state with collinear magnetic moment on T atoms in the (110) subsheets and with no moment on Pt atoms is shown to be the most stable for FePt3 among these alloys.

ANOMALOUS SPIN AND CHARGE DYNAMICS OF THE T-J MODEL AT LOW DOPING

We present an exact diagonalization study of the dynamical spin and density correlation function of the 2D t-J model for hole doping < 25%. Both correlation functions show a remarkably regular, but completely different scaling behaviour with both hole concentration and parameter values: the density correlation function is consistent with that of bosons corresponding to the doped holes and condensed into the lowest state of the noninteracting band of width 8t, the spin correlation function is consistent with Fermions in a band of width J. We show that the spin bag picture gives a natural explanation for this unusual behaviour.

SPIN BAGS IN THE DOPED T-J MODEL

We present a nonperturbative method for deriving a quasiparticle description of the low-energy excitations in the [ital t]-[ital J] model for strongly correlated electrons. Using the exact diagonalization technique we evaluated exactly the spectral functions of composite operators, which describe an electron or hole dressed by antiferromagnetic spin fluctuations as expected in the string or spin bag picture. For hole doping up to 1/8, use of the composite operators leads to a drastic simplification of the single-particle spectral function: at half-filling it takes free-particle form, for the doped case it resembles a system of weakly interacting fermions corresponding to the doped holes. We conclude that for all doping levels studied, the elementary electronic excitations next to the Fermi level are adequately described by the antiferromagnetic spin-fluctuation picture.

A block recursion method with complex wave vectors

A new block recursion method using complex wave vectors is developed to calculate the s-p and p-d elements of the inter-site Green functions. A numerical example shows satisfactory results compared with those obtained in conventional band calculations.

Physical Parameters in High-Temperature Superconductors

Transition metal oxides are classified into two categories: the charge transfer (CT) materials and the Mott-Hubbard (MH) ones. High temperature oxides are known to be in the CT materials. This is because of the subtle balance of the energy levels in copper and oxygen ions and of the unique crystal structure. In this lecture, we examine relations between crystal structure and electronic structure in a variety of the oxides and discuss what the key physical parameters are in the high temperature superconductors. Special attention is paid to the crucial difference in the electronic properties between CT and MH materials. Systematic variation in various electronic properties with materials provides us with valuable information on the mechanism of high temperature superconductivity.

Electronic structure and crystal structure in high Tc oxides

Abstract The electronic structure of cuprates consists of the orbitals of both copper and oxygen and is dependent sensitively on the crystal structure. Correlation between the energy level structure and superconducting transition temperature T c is examined in the ionic and cluster models. It is found that the energy level of the apex oxygen is of crucial importance for the electronic state of the CuO 2 plane and controls the optimum T c s as well as carrier doping. A cluster model analysis is made on the properties of doped carriers.

Theoretical aspects of magnetoelasticity in transition metals and alloys

Abstract Our recent theoretical studies are summarized. The magnetovolume effect in Fe and Ni is discussed and the effects on the volume striction of spin fluctuations are estimated in the paramagnetic state of Fe, Ni, Cr and ZrZn2. The pressure dependence of the Curie temperature and first-order transition temperature and forced magnetostriction are calculated in a combined model of localized spins and itinerant d-electrons and the observed results for dilute Pd and Pt alloys with Fe, Co and Ni atoms and rare earth-Co2 compounds are explained. Anisotropic magnetoelastic coupling constants in Fe and deformations of energy levels, Fermi surfaces and density of states due to shear strains in V are calculated in a tight-binding parametrisation scheme. A topological change of the Fermi surfaces due to shear strains is obtained in V. The electronic contribution to elastic shear constants is also calculated. The electronic Gruneisen constant is calculated for Nb and a comparison with experiment is carried out.

Spin and charge dynamics of the 2-dimensional T-J model at intermediate electron-densities - absence of spin-charge separation

We present an exact diagonalization study of the dynamical spin and density correlation functions in small clusters of the {ital t}-{ital J} model, focusing on the regime of intermediate and low electron densities, {rho}{sub {ital e}}{lt}0.5. In two dimensions (2D) both correlation functions agree remarkably well with the convolution of the single-particle spectral function, i.e., the simplest estimate possible within a Fermi-liquid picture. Deviations from the convolution are shown to originate from symmetry-related selection rules, which are unaccounted for in the convolution estimate. For all fillngs under consideration, we show that the low-energy peaks originate from particle-hole excitations between the Fermi momenta, as expected for a Fermi liquid. We contrast this with the behavior in 1D, where spin and density correlation function show the differences characteristic of spin-charge separation and where neither correlation function is approximated well by the convolution.

Electronic structure of the Hubbard and extended Hubbard models: An exact diagonalization study

Abstract An exact diagonalization technique for small clusters is used to study the electronic structure of the Hubbard model with nearest-neighbor repulsive interaction. The phase diagram of the model is extracted from the calculated charge and spin correlation functions and single-particle excitation spectra. It is shown that the small Fermi surface is realized in a parameter and doping region, which may provide a complementary understanding of the large Fermi surface believed to exist in the Hubbard model.

Ingap state in doped and undoped cuprates

Abstract The dispersive electronic state emerging upon carrier-doping in cuprates, i.e., the so-called ingap state, is derived in the three-band and one-band Hubbard models by means of the exact diagonalization of finite-size clusters. The dispersive state near the charge transfer gap at half filling undergoes a strongly momentum-dependent transfer of the spectral weight by carrier doping, and evolves into the new ingap state at the edge of the gap with a free-electron-like dispersion characteristic of a large Fermi surface consistent with Luttingers sum rule. It is shown that the extended Hubbard model with strong nearest-neighbor repulsive interaction, on the other hand, forms a small Fermi surface by carrier doping and provides a counter-example of the doped cuprates.