39 research outputs found
Hubbard model versus t-J model: The one-particle spectrum
The origin of the apparent discrepancies between the one-particle spectra of
the Hubbard and t-J models is revealed: Wavefunction corrections, in addition
to the three-site terms, should supplement the bare t-J. In this way a
quantitative agreement between the two models is obtained, even for the
intermediate- values appropriate for the high-Tc cuprate superconductors.
Numerical results for clusters of up to 20 sites are presented. The momentum
dependence of the observed intensities in the photoemission spectra of
Sr2CuO2Cl2 are well described by this complete strong-coupling approach.Comment: 4 two-column RevTeX pages, including 4 Postscript figures. Uses epsf.
Accepted for publication in Physical Review B, Rapid Communicatio
Hole-hole correlations in the limit of the Hubbard model and the stability of the Nagaoka state
We use exact diagonalisation in order to study the infinite - limit of
the two dimensional Hubbard model. As well as looking at single-particle
correlations, such as , we also study {\it N-particle correlation
functions} which compare the relative positions of {\it all} the particles in
different models. In particular we study 16 and 18-site clusters and compare
the charge correlations in the Hubbard model with those of spinless fermions
and hard-core bosons. We find that although low densities of holes favour a
`locally-ferromagnetic' fermionic description, the correlations at larger
densities resemble those of pure hard-core bosons surprisingly well .Comment: 15 pages, REVTE
Resonant scattering on impurities in the Quantum Hall Effect
We develop a new approach to carrier transport between the edge states via
resonant scattering on impurities, which is applicable both for short and long
range impurities. A detailed analysis of resonant scattering on a single
impurity is performed. The results are used for study of the inter-edge
transport by multiple resonant hopping via different impurities' sites. It is
shown that the total conductance can be found from an effective Schroedinger
equation with constant diagonal matrix elements in the Hamiltonian, where the
complex non-diagonal matrix elements are the amplitudes of a carrier hopping
between different impurities. It is explicitly demonstrated how the complex
phase leads to Aharonov-Bohm oscillations in the total conductance. Neglecting
the contribution of self-crossing resonant-percolation trajectories, one finds
that the inter-edge carrier transport is similar to propagation in
one-dimensional system with off-diagonal disorder. We demonstrated that each
Landau band has an extended state , while all other states are
localized. The localization length behaves as .Comment: RevTex 41 pages; 3 Postscript figure on request; Final version
accepted for publication in Phys. Rev. B. A new section added contained a
comparison with other method
Aging and Immortality in a Cell Proliferation Model
We investigate a model of cell division in which the length of telomeres
within the cell regulate their proliferative potential. At each cell division
the ends of linear chromosomes change and a cell becomes senescent when one or
more of its telomeres become shorter than a critical length. In addition to
this systematic shortening, exchange of telomere DNA between the two daughter
cells can occur at each cell division. We map this telomere dynamics onto a
biased branching diffusion process with an absorbing boundary condition
whenever any telomere reaches the critical length. As the relative effects of
telomere shortening and cell division are varied, there is a phase transition
between finite lifetime and infinite proliferation of the cell population.
Using simple first-passage ideas, we quantify the nature of this transition.Comment: 6 pages, 1 figure, 2-column revtex4 format; version 2: final
published form; contains various improvements in response to referee comment
Relation between flux formation and pairing in doped antiferromagnets
We demonstrate that patterns formed by the current-current correlation
function are landmarks which indicate that spin bipolarons form in doped
antiferromagnets. Holes which constitute a spin bipolaron reside at opposite
ends of a line (string) formed by the defects in the antiferromagnetic spin
background. The string is relatively highly mobile, because the motion of a
hole at its end does not raise extensively the number of defects, provided that
the hole at the other end of the line follows along the same track. Appropriate
coherent combinations of string states realize some irreducible representations
of the point group C_4v. Creep of strings favors d- and p-wave states. Some
more subtle processes decide the symmetry of pairing. The pattern of the
current correlation function, that defines the structure of flux, emerges from
motion of holes at string ends and coherence factors with which string states
appear in the wave function of the bound state. Condensation of bipolarons and
phase coherence between them puts to infinity the correlation length of the
current correlation function and establishes the flux in the system.Comment: 5 pages, 6 figure
Hole dynamics in a quantum antiferromagnet beyond the retraceable path approximation
The one-hole spectral weight for two chains and two dimensional lattices is
studied numerically using a new method of analysis of the spectral function
within the Lanczos iteration scheme: the Lanczos spectra decoding method. This
technique is applied to the model for , directly in the
infinite size lattice. By a careful investigation of the first 13 Lanczos steps
and the first 26 ones for the two dimensional and the two chain cases
respectively, we get several new features of the one-hole spectral weight. A
sharp incoherent peak with a clear momentum dispersion is identified, together
with a second broad peak at higher energy. The spectral weight is finite up to
the Nagaoka energy where it vanishes in a non-analytic way. Thus the lowest
energy of one hole in a quantum antiferromagnet is degenerate with the Nagaoka
energy in the thermodynamic limit.Comment: RevTeX 3.0, SISSA preprint 156/93/CM/MB, 10 pages + postscript file
appended, contains more accurate calculations in Fig.
Even and odd-frequency pairing correlations in 1-D t-J-h model: a comparative study
An equal time version of odd-frequency pairing for a generalized model
is introduced. It is shown that the composite operators describing binding of
Cooper pairs with magnetization fluctuations naturally appear in this approach.
The pairing correlations in both BCS and odd-frequency channels are
investigated exactly in 1D systems with up to 16 sites. Our results indicate
that at some range of parameters odd-frequency correlations become comparable,
however smaller than BCS pairing correlations. It is speculated that the spin
and density fluctuations in the frustrated model lead to the enhancement of the
odd gap susceptibilities. 4 postscript figure files are attached at the bottom
of the tex file.Comment: 6 pages + 4 figure
Formation of an Edge Striped Phase in Fractional Quantum Hall Systems
We have performed an exact diagonalization study of up to N=12 interacting
electrons on a disk at filling for both Coulomb and
short-range interaction for which Laughlin wave function is the exact solution.
For Coulomb interaction and we find persistent radial oscillations
in electron density, which are not captured by the Laughlin wave function. Our
results srongly suggest formation of a chiral edge striped phase in quantum
Hall systems. The amplitude of the charge density oscillations decays slowly,
perhaps as a square root of the distance from the edge; thus the spectrum of
edge excitations is likely to be affected.Comment: 4 pages, 3 Figs. include
Single-hole properties in the - and strong-coupling models
We report numerical results for the single-hole properties in the -
model and the strong-coupling approximation to the Hubbard model in two
dimensions. Using the hopping basis with over states we discuss (for an
infinite system) the bandwidth, the leading Fourier coefficients in the
dispersion, the band masses, and the spin-spin correlations near the hole. We
compare our results with those obtained by other methods. The band minimum is
found to be at () for the - model for , and for the strong-coupling model for . The bandwidth
in both models is approximately at large , in rough agreement with
loop-expansion results but in disagreement with other results. The
strong-coupling bandwidth for t/J\agt6 can be obtained from the - model
by treating the three-site terms in first-order perturbation theory. The
dispersion along the magnetic zone face is flat, giving a large
parallel/perpendicular band mass ratio.Comment: 1 RevTeX file with epsf directives to include 8 .eps figures 8 figure
files encoded using uufile
An Exact Diagonalization Demonstration of Incommensurability and Rigid Band Filling for N Holes in the t-J Model
We have calculated S(q) and the single particle distribution function
for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site
lattice with periodic boundary conditions; we justify the use of this lattice
in compariosn to those of having the full square symmetry of the bulk. This new
cluster has a high density of vec k points along the diagonal of reciprocal
space, viz. along k = (k,k). The results clearly demonstrate that when the
single hole problem has a ground state with a system momentum of vec k =
(pi/2,pi/2), the resulting ground state for N holes involves a shift of the
peak of the system's structure factor away from the antiferromagnetic state.
This shift effectively increases continuously with N. When the single hole
problem has a ground state with a momentum that is not equal to k =
(pi/2,pi/2), then the above--mentioned incommensurability for N holes is not
found. The results for the incommensurate ground states can be understood in
terms of rigid--band filling: the effective occupation of the single hole k =
(pi/2,pi/2) states is demonstrated by the evaluation of the single particle
momentum distribution function . Unlike many previous studies, we show
that for the many hole ground state the occupied momentum states are indeed k =
(+/- pi/2,+/- pi/2) states.Comment: Revtex 3.0; 23 pages, 1 table, and 13 figures, all include