10 research outputs found
Quantum Symmetry of Hubbard Model Unraveled
Superconducting quantum symmetries in extended single-band 1-dimensional
Hubbard models are shown to originate from the classical (pseudo-)spin SO(4)
symmetry of a class of models of which the standard Hubbard model is a special
case. Extending the notion of symmetry to include quantum groups allows us to
introduce extra parameters but the corresponding quantum symmetric models are
restricted to one dimension. All models discussed are related by generalized
Lang-Firsov transformations, some have symmetries away from half filling. The
most general model with symmetric next-neighbour interaction terms and
classical SO(4) symmetry is given explicitly.Comment: 5 pages, revtex; 3 references added, hidden symmetries mentioned in
introductio
The Recent Excitement in High-Density QCD
Over the past few months, the theory of QCD at high density has been advanced
considerably. It provides new perspectives on, and controlled realizations of,
confinement and chiral symmetry breaking. Here I survey the recent
developments, and suggest a few directions for future work.Comment: LaTeX, 16 pages, 2 figures. Invited talk at PANIC `99, Uppsala,
Sweden, June 199
First order magnetic transition in CeFe alloys: Phase-coexistence and metastability
First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in
doped-CeFe alloys is studied with micro-Hall probe technique. Clear visual
evidence of magnetic phase-coexistence on micrometer scales and the evolution
of this phase-coexistence as a function of temperature, magnetic field and time
across the first order FM-AFM transition is presented. Such phase-coexistence
and metastability arise as natural consequence of an intrinsic
disorder-influenced first order transition. Generality of this phenomena
involving other classes of materials is discussed.Comment: 11 pages of text and 3 figure
Energy level statistics of the two-dimensional Hubbard model at low filling
The energy level statistics of the Hubbard model for square
lattices (L=3,4,5,6) at low filling (four electrons) is studied numerically for
a wide range of the coupling strength. All known symmetries of the model
(space, spin and pseudospin symmetry) have been taken into account explicitly
from the beginning of the calculation by projecting into symmetry invariant
subspaces. The details of this group theoretical treatment are presented with
special attention to the nongeneric case of L=4, where a particular complicated
space group appears. For all the lattices studied, a significant amount of
levels within each symmetry invariant subspaces remains degenerated, but except
for L=4 the ground state is nondegenerate. We explain the remaining
degeneracies, which occur only for very specific interaction independent
states, and we disregard these states in the statistical spectral analysis. The
intricate structure of the Hubbard spectra necessitates a careful unfolding
procedure, which is thoroughly discussed. Finally, we present our results for
the level spacing distribution, the number variance , and the
spectral rigidity , which essentially all are close to the
corresponding statistics for random matrices of the Gaussian ensemble
independent of the lattice size and the coupling strength. Even very small
coupling strengths approaching the integrable zero coupling limit lead to the
Gaussian ensemble statistics stressing the nonperturbative nature of the
Hubbard model.Comment: 31 pages (1 Revtex file and 10 postscript figures
High Precision Renormalization Group Study of the Roughening Transition
We confirm the Kosterlitz-Thouless scenario of the roughening transition for
three different Solid-On-Solid models: the Discrete Gaussian model, the
Absolute-Value-Solid-On-Solid model and the dual transform of the XY model with
standard (cosine) action. The method is based on a matching of the
renormalization group flow of the candidate models with the flow of a bona fide
KT model, the exactly solvable BCSOS model. The Monte Carlo simulations are
performed using efficient cluster algorithms. We obtain high precision
estimates for the critical couplings and other non-universal quantities. For
the XY model with cosine action our critical coupling estimate is
. For the roughening coupling of the Discrete Gaussian
and the Absolute-Value-Solid-On-Solid model we find and
, respectively.Comment: 46 pages, PostScript file (compressed and uuencoded), preprints
CERN-TH.7182/94, HU-RI-2/94, and MS-TPI-94-
Instability, Intermixing and Electronic Structure at the Epitaxial LaAlO3/SrTiO3(001) Heterojunction
The question of stability against diffusional mixing at the prototypical
LaAlO3/SrTiO3(001) interface is explored using a multi-faceted experimental and
theoretical approach. We combine analytical methods with a range of
sensitivities to elemental concentrations and spatial separations to
investigate interfaces grown using on-axis pulsed laser deposition. We also
employ computational modeling based on the density function theory as well as
classical force fields to explore the energetic stability of a wide variety of
intermixed atomic configurations relative to the idealized, atomically abrupt
model. Statistical analysis of the calculated energies for the various
configurations is used to elucidate the relative thermodynamic stability of
intermixed and abrupt configurations. We find that on both experimental and
theoretical fronts, the tendency toward intermixing is very strong. We have
also measured and calculated key electronic properties such as the presence of
electric fields and the value of the valence band discontinuity at the
interface. We find no measurable electric field in either the LaAlO3 or SrTiO3,
and that the valence band offset is near zero, partitioning the band
discontinuity almost entirely to the conduction band edge. Moreover, we find
that it is not possible to account for these electronic properties
theoretically without including extensive intermixing in our physical model of
the interface. The atomic configurations which give the greatest electrostatic
stability are those that eliminate the interface dipole by intermixing, calling
into question the conventional explanation for conductivity at this interface -
electronic reconstruction. Rather, evidence is presented for La indiffusion and
doping of the SrTiO3 below the interface as being the cause of the observed
conductivity