1,110 research outputs found
Off-diagonal Interactions, Hund's Rules and Pair-binding in Hubbard Molecules
We have studied the effect of including nearest-neighbor, electron-electron
interactions, in particular the off-diagonal (non density-density) terms, on
the spectra of truncated tetrahedral and icosahedral ``Hubbard molecules,''
focusing on the relevance of these systems to the physics of doped C.
Our perturbation theoretic and exact diagonalization results agree with
previous work in that the density-density term suppresses pair-binding.
However, we find that for the parameter values of interest for the
off-diagonal terms {\em enhance} pair-binding, though not enough to offset the
suppression due to the density-density term. We also find that the critical
interaction strengths for the Hund's rules violating level crossings in
C, C and C are quite insensitive to the
inclusion of these additional interactions.Comment: 20p + 5figs, Revtex 3.0, UIUC preprint P-94-10-08
Effects of Next-Nearest-Neighbor Hopping on the Hole Motion in an Antiferromagnetic Background
In this paper we study the effect of next-nearest-neighbor hopping on the
dynamics of a single hole in an antiferromagnetic (N\'{e}el) background. In the
framework of large dimensions the Green function of a hole can be obtained
exactly. The exact density of states of a hole is thus calculated in large
dimensions and on a Bethe lattice with large coordination number. We suggest a
physically motivated generalization to finite dimensions (e.g., 2 and 3). In
we present also the momentum dependent spectral function. With varying
degree, depending on the underlying lattice involved, the discrete spectrum for
holes is replaced by a continuum background and a few resonances at the low
energy end. The latter are the remanents of the bound states of the
model. Their behavior is still largely governed by the parameters and .
The continuum excitations are more sensitive to the energy scales and
.Comment: To appear in Phys. Rev. B, Revtex, 23 pages, 10 figures available on
request from [email protected]
Robustness of a local Fermi Liquid against Ferromagnetism and Phase Separation
We study the properties of Fermi Liquids with the microscopic constraint of a
local self-energy. In this case the forward scattering sum-rule imposes strong
limitations on the Fermi-Liquid parameters, which rule out any Pomeranchek
instabilities. For both attractive and repulsive interactions, ferromagnetism
and phase separation are suppressed. Superconductivity is possible in an s-wave
channel only. We also study the approach to the metal-insulator transition, and
find a Wilson ratio approaching 2. This ratio and other properties of
Sr_{1-x}La_xTiO_3 are all consistent with the local Fermi Liquid scenario.Comment: 4 pages (twocolumn format), can compile with or without epsf.sty
latex style file -- Postscript files: fig1.ps and fig2.p
Rigorous results on superconducting ground states for attractive extended Hubbard models
We show that the exact ground state for a class of extended Hubbard models
including bond-charge, exchange, and pair-hopping terms, is the Yang
"eta-paired" state for any non-vanishing value of the pair-hopping amplitude,
at least when the on-site Coulomb interaction is attractive enough and the
remaining physical parameters satisfy a single constraint. The ground state is
thus rigorously superconducting. Our result holds on a bipartite lattice in any
dimension, at any band filling, and for arbitrary electron hopping.Comment: 12 page
Exact ground-state correlation functions of the one-dimensional strongly correlated electron models with the resonating-valence-bond ground state
We investigate the one-dimensional strongly correlated electron models which
have the resonating-valence-bond state as the exact ground state. The
correlation functions are evaluated exactly using the transfer matrix method
for the geometric representations of the valence-bond states. In this method,
we only treat matrices with small dimensions. This enables us to give
analytical results. It is shown that the correlation functions decay
exponentially with distance. The result suggests that there is a finite
excitation gap, and that the ground state is insulating. Since the
corresponding non-interacting systems may be insulating or metallic, we can say
that the gap originates from strong correlation. The persistent currents of the
present models are also investigated and found to be exactly vanishing.Comment: 59 pages, REVTeX 3.0, Figures are available on reques
Network Analysis of Biochemical Logic for Noise Reduction and Stability: A System of Three Coupled Enzymatic AND Gates
We develop an approach aimed at optimizing the parameters of a network of
biochemical logic gates for reduction of the "analog" noise buildup.
Experiments for three coupled enzymatic AND gates are reported, illustrating
our procedure. Specifically, starch - one of the controlled network inputs - is
converted to maltose by beta-amylase. With the use of phosphate (another
controlled input), maltose phosphorylase then produces glucose. Finally,
nicotinamide adenine dinucleotide (NAD+) - the third controlled input - is
reduced under the action of glucose dehydrogenase to yield the optically
detected signal. Network functioning is analyzed by varying selective inputs
and fitting standardized few-parameters "response-surface" functions assumed
for each gate. This allows a certain probe of the individual gate quality, but
primarily yields information on the relative contribution of the gates to noise
amplification. The derived information is then used to modify our experimental
system to put it in a regime of a less noisy operation.Comment: 31 pages, PD
Magnetic Properties of the t-J Model in the Dynamical Mean-Field Theory
We present a theory for the spin correlation function of the t-J model in the
framework of the dynamical mean-field theory. Using this mapping between the
lattice and a local model we are able to obtain an intuitive expression for the
non-local spin susceptibility, with the corresponding local correlation
function as input. The latter is calculated by means of local Goldstone
diagrams following closely the procedures developed and successfully applied
for the (single impurity) Anderson model.We present a systematic study of the
magnetic susceptibility and compare our results with those of a Hubbard model
at large U. Similarities and differences are pointed out and the magnetic phase
diagram of the t-J model is discussed.Comment: 28 pages LaTeX, postscript figures as compressed and uuencoded file
included fil
Plaquette operators used in the rigorous study of ground-states of the Periodic Anderson Model in dimensions
The derivation procedure of exact ground-states for the periodic Anderson
model (PAM) in restricted regions of the parameter space and D=2 dimensions
using plaquette operators is presented in detail. Using this procedure, we are
reporting for the first time exact ground-states for PAM in 2D and finite value
of the interaction, whose presence do not require the next to nearest neighbor
extension terms in the Hamiltonian. In order to do this, a completely new type
of plaquette operator is introduced for PAM, based on which a new localized
phase is deduced whose physical properties are analyzed in detail. The obtained
results provide exact theoretical data which can be used for the understanding
of system properties leading to metal-insulator transitions, strongly debated
in recent publications in the frame of PAM. In the described case, the lost of
the localization character is connected to the break-down of the long-range
density-density correlations rather than Kondo physics.Comment: 34 pages, 5 figure
Ferromagnetism in the large-U Hubbard model
We study the Hubbard model on a hypercubic lattice with regard to the
possibility of itinerant ferromagnetism. The Dynamical Mean Field theory is
used to map the lattice model on an effective local problem, which is treated
with help of the Non Crossing Approximation. By investigating spin dependent
one-particle Green's functions and the magnetic susceptibility, a region with
nonvanishing ferromagnetic polarization is found in the limit . The
-T-phase diagram as well as thermodynamic quantities are discussed. The
dependence of the Curie temperature on the Coulomb interaction and the
competition between ferromagnetism and antiferromagnetism are studied in the
large limit of the Hubbard model.Comment: 4 pages, 5 figures, accepted for publication in Physical Review B,
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