Spectral functions and optical conductivity of spinless fermions on a checkerboard lattice

We study the dynamical properties of spinless fermions on the checkerboard lattice. Our main interest is the limit of large nearest-neighbor repulsion $V$ as compared with hopping $|t|$. The spectral functions show broad low-energy excitation which are due to the dynamics of fractionally charged excitations. Furthermore, it is shown that the fractional charges contribute to the electrical current density.Comment: 9 Pages, 9 Figure

Marginal Fermi Liquid Theory in the Hubbard Model

We find Marginal Fermi Liquid (MFL) like behavior in the Hubbard model on a square lattice for a range of hole doping and on-site interaction parameter U. Thereby we use a self-consistent projection operator method. It enables us to compute the momentum and frequency dependence of the single-particle excitations with high resolution. The Fermi surface is found to be hole-like in the underdoped and electron-like in the overdoped regime. When a comparison is possible we find consistency with finite temperature quantum Monte Carlo results. We also find a discontinuous change with doping concentration from a MFL to Fermi liquid behavior resulting from a collapse of the lower Hubbard band. This renders Luttinger's theorem inapplicable in the underdoped regime.Comment: 8 pages, 6 figure

Superconductivity in a magnetically ordered background

Borocarbide compounds with the formula RNi2B2C show interesting superconducting and magnetic properties and the coexistence of the two phenomena. BCS theory is extended to systems with underlying commensurate magnetic order. In the case of helical phases the technique may be extended to any Q-vector and there exists a well defined limit for incommensurate values. The way magnetic order influences superconductivity depends crucially on the details of both the magnetic structure and the electron bands, but some qualitative criteria may be given. As an example we give a brief analysis of the compound HoNi2B2C.Comment: 3 pages, 1 figure, proceedings to the conference "Anomalous Complex Superconductors", Crete 199

Charge carrier correlation in the electron-doped t-J model

We study the t-t'-t''-J model with parameters chosen to model an electron-doped high temperature superconductor. The model with one, two and four charge carriers is solved on a 32-site lattice using exact diagonalization. Our results demonstrate that at doping levels up to x=0.125 the model possesses robust antiferromagnetic correlation. When doped with one charge carrier, the ground state has momenta (\pm\pi,0) and (0,\pm\pi). On further doping, charge carriers are unbound and the momentum distribution function can be constructed from that of the single-carrier ground state. The Fermi surface resembles that of small pockets at single charge carrier ground state momenta, which is the expected result in a lightly doped antiferromagnet. This feature persists upon doping up to the largest doping level we achieved. We therefore do not observe the Fermi surface changing shape at doping levels up to 0.125

Correlated electrons in Fe-As compounds: a quantum chemical perspective

State-of-the-art quantum chemical methods are applied to the study of the multiorbital correlated electronic structure of a Fe-As compound, the recently discovered LiFeAs. Our calculations predict a high-spin, S=2, ground-state configuration for the Fe ions, which shows that the on-site Coulomb interactions are substantial. Also, orbital degeneracy in the (xz,yz) sector and a three-quarter filling of these levels suggest the presence of strong fluctuations and are compatible with a low metallic conductivity in the normal state. The lowest electron-removal states have As 4p character, in analogy to the ligand hole states in p-type cuprate superconductors

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

Quasiclassical Hamiltonians for large-spin systems

We propose a method for obtaining effective classical Hamiltonians \cal H for many-body quantum spin systems with large spins. This method uses the coherent-state representation of the partition function Z and the cumulant expansion in powers of 1/S. For the quantum Hamiltonian \hat H of a Heisenberg form, the 1/S corrections in \cal H have a non-Heisenberg many-spin form. The effective Hamiltonian \cal H can be treated by methods familiar for classical systems. The non-Heisenberg terms in \cal H may be responsible for such effects as spin-Peierls transition and uplifting of the classical degeneracy by quantum fluctuations.Comment: 8 Pages, 2 Figures, submitted to EPJ