1,242 research outputs found

### 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 UPd$_2$Al$_3$

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

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