643 research outputs found
Recursion method and one-hole spectral function of the Majumdar-Ghosh model
We consider the application of the recursion method to the calculation of
one-particle Green's functions for strongly correlated systems and propose a
new way how to extract the information about the infinite system from the exact
diagonalisation of small clusters. Comparing the results for several cluster
sizes allows us to establish those Lanczos coefficients that are not affected
by the finite size effects and provide the information about the Green's
function of the macroscopic system. The analysis of this 'bulk-related' subset
of coefficients supplemented by alternative analytic approaches allows to infer
their asymptotic behaviour and to propose an approximate analytical form for
the 'terminator' of the Green's function continued fraction expansion for the
infinite system. As a result, the Green's function acquires the branch cut
singularity corresponding to the incoherent part of the spectrum. The method is
applied to the spectral function of one-hole in the Majumdar-Ghosh model (the
one-dimensional model at ). For this model,
the branch cut starts at finite energy , but there is no upper bound
of the spectrum, corresponding to a linear increase of the recursion
coefficients. Further characteristics of the spectral function are band gaps in
the middle of the band and bound states below or within the gaps.
The band gaps arise due to the period doubling of the unit cell and show up as
characteristic oscillations of the recursion coefficients on top of the linear
increase.Comment: 12 pages, 7 figure
Non-rigid hole band in the extended t-J model
The dispersion of one hole in an extended - model with additional
hopping terms to second and third nearest neighbours and a frustration term in
the exchange part has been investigated. Two methods, a Green's function
projection technique describing a magnetic polaron of minimal size and the
exact diagonalization of a lattice, have been applied, showing reasonable
agreement among each other. Using additional hopping integrals which are
characteristic for the CuO plane in cuprates we find in the nonfrustrated
case an isotropic minimum of the dispersion at the point in
-space in good coincidence with recent angle-resolved photoemission results
for the insulating compound SrCuOCl. Including frustration or
finite temperature which shall simulate the effect of doping, the dispersion is
drastically changed such that a flat region and an extended saddle point may be
observed between and in agreement with experimental
results for the optimally doped cuprates.Comment: 14 pages, LaTeX, 6 figures on request, submitted to Zeitschrift fuer
Physi
Lieb-Mattis ferrimagnetism in diluted magnetic semiconductors
We show the possibility of long-range ferrimagnetic ordering with a
saturation magnetisation of the order of 1 Bohr magneton per spin for
arbitrarily low concentration of magnetic impurities in semiconductors,
provided that the impurities form a superstructure satisfying the conditions of
the Lieb-Mattis theorem. Explicit examples of such superstructures are given
for the wurtzite lattice, and the temperature of ferrimagnetic transition is
estimated from a high-temperature expansion. Exact diagonalization studies show
that small fragments of the structure exhibit enhanced magnetic response and
isotropic superparamagnetism at low temperatures. A quantum transition in a
high magnetic field is considered and similar superstructures in cubic
semiconductors are discussed as well.Comment: 6 pages,4 figure
- …