789 research outputs found
On the soliton width in the incommensurate phase of spin-Peierls systems
We study using bosonization techniques the effects of frustration due to
competing interactions and of the interchain elastic couplings on the soliton
width and soliton structure in spin-Peierls systems. We compare the predictions
of this study with numerical results obtained by exact diagonalization of
finite chains. We conclude that frustration produces in general a reduction of
the soliton width while the interchain elastic coupling increases it. We
discuss these results in connection with recent measurements of the soliton
width in the incommensurate phase of CuGeO_3.Comment: 4 pages, latex, 2 figures embedded in the tex
Hole-Pairs in a Spin Liquid: Influence of Electrostatic Hole-Hole Repulsion
The stability of hole bound states in the t-J model including short-range
Coulomb interactions is analyzed using computational techniques on ladders with
up to sites. For a nearest-neighbors (NN) hole-hole repulsion,
the two-holes bound state is surprisingly robust and breaks only when the
repulsion is several times the exchange . At hole doping the
pairs break only for a NN-repulsion as large as . Pair-pair
correlations remain robust in the regime of hole binding. The results support
electronic hole-pairing mechanisms on ladders based on holes moving in
spin-liquid backgrounds. Implications in two dimensions are also presented. The
need for better estimations of the range and strength of the Coulomb
interaction in copper-oxides is remarked.Comment: Revised version with new figures. 4 pages, 5 figure
Superexchange in the quarter- filled two- leg ladder system NaV2O5
A theory of superexchange in the mixed valent layer compound NaV2O5 is
presented which provides a consistent description of exchange both in the
disordered and charge ordered state. Starting from results of band structure
calculations for NaV2O5 first an underlying electronic model for a ladder unit
in the Trellis lattice is formulated. By using the molecular orbital
representation for intra-rung electronic states a second-order perturbation
procedure is developed and an effective spin-chain model for a ladder is
derived. Variation of the resulting superexchange integral J is examined
numerically as the ladder system evolves from a charge disordered to the
extreme ('zig-zag') charge ordered state. It is found that the effective intra-
ladder superexchange is always antiferromagnetic.Comment: 18 pages Revtex, 7 Postscript figure
Charge and spin ordering, and charge transport properties in a two-dimensional inhomogeneous t-J model
We study a two-dimensional t-J model close to the Ising limit in which charge
inhomogeneity is stabilized by an on-site potential e_s, by using
diagonalization in a restricted Hilbert space and finite temperature Quantum
Monte Carlo. Both site and bond centered stripes are considered and their
similitudes and differences are analyzed. The amplitude of charge inhomogeneity
is studied as e_s -> 0. Moreover, we show that the anti-phase domain ordering
occurs at a much lower temperature than the formation of charge inhomogeneities
and charge localization. Hole-hole correlations indicate a metallic behavior of
the stripes with no signs of hole attraction. Kinetic energies and current
susceptibilities are computed and indications of charge localization are
discussed. The study of the doping dependence in the range 0.083 < x < 0.167
suggests that these features are characteristic of the whole underdoped region.Comment: minor changes, to be published in Physical Review
Gas-plasma compressional wave coupling by momentum transfer
Pressure disturbances in a gas-plasma mixed fluid will result in a hybrid response, with magnetosonic plasma waves coupled to acoustic waves in the neutral gas. In the analytical and numerical treatment presented here, we demonstrate the evolution of the total fluid medium response under a variety of conditions, with the gas-plasma linkage achieved by additional coupling terms in the momentum equations of each species. The significance of this treatment lies in the consideration of density perturbations in such fluids: there is no 'pure' mode response, only a collective one in which elements of the characteristics of each component are present. For example, an initially isotropic gas sound wave can trigger an anisotropic magnetic response in the plasma, with the character of each being blended in the global evolution. Hence sound waves do not remain wholly isotropic, and magnetic responses are less constrained by pure magnetoplasma dynamics
Diagonalization in Reduced Hilbert Spaces using a Systematically Improved Basis: Application to Spin Dynamics in Lightly Doped Ladders
A method is proposed to improve the accuracy of approximate techniques for
strongly correlated electrons that use reduced Hilbert spaces. As a first step,
the method involves a change of basis that incorporates exactly part of the
short distance interactions. The Hamiltonian is rewritten in new variables that
better represent the physics of the problem under study. A Hilbert space
expansion performed in the new basis follows. The method is successfully tested
using both the Heisenberg model and the model with holes on 2-leg ladders
and chains, including estimations for ground state energies, static
correlations, and spectra of excited states. An important feature of this
technique is its ability to calculate dynamical responses on clusters larger
than those that can be studied using Exact Diagonalization. The method is
applied to the analysis of the dynamical spin structure factor on
clusters with sites and 0 and 2 holes. Our results confirm
previous studies (M. Troyer, H. Tsunetsugu, and T. M. Rice, Phys. Rev. ,
251 (1996)) which suggested that the state of the lowest energy in the spin-1
2-holes subspace corresponds to the bound state of a hole pair and a
spin-triplet. Implications of this result for neutron scattering experiments
both on ladders and planes are discussed.Comment: 9 pages, 8 figures, Revtex + psfig; changed conten
Binding of holes and pair spectral function in the t-J model
Clusters of the two-dimensionnal t--J model with 2 holes and up to 26 sites
are diagonalized using a Lanczos algorithm. The behaviour of the binding energy
with system size suggests the existence of a finite critical value of J above
which binding occurs in the bulk. Only the d-wave pair field operator acting on
the Heisenberg GS has a finite overlap with the 2 hole ground state for all the
clusters considered. The related spectral function associated with the
propagation of a d-wave (spin singlet) pair of holes in the antiferromagnetic
background is calculated. The quasiparticle peak at the bottom of the spectrum
as well as some structure appearing above the peak survive with increasing
cluster size. Although no simple scaling law was found for the quasiparticle
weight the data strongly suggest that this weight is finite in the bulk limit
and is roughly proportional to the antiferromagnetic coupling J (for J<1).Comment: Report LPQTH-93/01, 18 pages (REVTEX), 8 postscript files include
Mixing of magnetic and phononic excitations in incommensurate Spin-Peierls systems
We analyze the excitation spectra of a spin-phonon coupled chain in the
presence of a soliton. This is taken as a microscopic model of a Spin-Peierls
material placed in a high magnetic field. We show, by using a semiclassical
approximation in the bosonized representation of the spins that a trapped
magnetic state obtained in the adiabatic approximation is destroyed by
dynamical phonons. Low energy states are phonons trapped by the soliton. When
the magnetic gap is smaller than the phonon frequencies the only low energy
state is a mixed magneto-phonon state with the energy of the gap. We emphasize
that our results are relevant for the Raman spectra of the inorganic
Spin-Peierls material CuGeO.Comment: 5 pages, latex, 2 figures embedded in the tex
Bogoliubov Quasiparticle Excitations in the Two-Dimensional t-J Model
Using a proposed numerical technique for calculating anomalous Green's
functions characteristic of superconductivity, we show that the low-lying
excitations in a wide parameter and doping region of the two-dimensional
model are well described by the picture of dressed Bogoliubov
quasiparticles in the BCS pairing theory. The pairing occurs predominantly in
-wave channel and the energy gap has a size
between quarter and half fillings. Opening of
the superconducting gap in the photoemission and inverse-photoemission spectrum
is demonstrated.Comment: 6 pages, RevTe
Domain excitations in spin-Peierls systems
We study a model of a Spin-Peierls material consisting of a set of
antiferromagnetic Heisenberg chains coupled with phonons and interacting among
them via an inter-chain elastic coupling. The excitation spectrum is analyzed
by bosonization techniques and the self-harmonic approximation. The elementary
excitation is the creation of a localized domain structure where the dimerized
order is the opposite to the one of the surroundings. It is a triplet
excitation whose formation energy is smaller than the magnon gap. Magnetic
internal excitations of the domain are possible and give the further
excitations of the system. We discuss these results in the context of recent
experimental measurements on the inorganic Spin-Peierls compound CuGeOComment: 5 pages, 2 figures, corrected version to appear in Phys. Rev.
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