27 research outputs found
Charge Stripe in an Antiferromagnet: 1d Band of Composite Excitations
With the help of analytical and numerical studies of the - model we
argue that the charge stripe in an antiferromagnetic insulator should be
understood as a system of holon-spin-polaron excitations condensed at the
self-induced antiphase domain wall. The structure of such a charge excitation
is studied in detail with numerical and analytical results for various
quantities being in a very close agreement. An analytical picture of these
excitations occupying an effective 1D stripe band is also in a very good accord
with numerical data. The emerging concept advocates the primary role of the
kinetic energy in favoring the stripe as a ground state. A comparative analysis
suggests the effect of pairing and collective meandering on the energetics of
the stripe formation to be secondary.Comment: 5 pages, 3 figures, proceedings of SCES'01 conference, Ann Arbor,
2001, to be published in Physica
A numerical and analytical study of two holes doped into the 2D t--J model
Exact diagonalization numerical results are presented for a 32-site square
cluster, with two holes propagating in an antiferromagnetic background
described by the t-J model. We characterize the wave function of the lowest
energy bound state found in this calculation, which has d_{x^2-y^2} symmetry.
Analytical work is presented, based on a Lang-Firsov-type canonical
transformation derived quasiparticle Hamiltonian, that accurately agrees with
numerically determined values for the electron momentum distribution function
and the pair correlation function. We interpret this agreement as strong
support for the validity of this description of the hole quasiparticles.Comment: 3 pages, REVTeX, to appear in the proceedings of the Fifth
International Conference on Spectroscopies in Novel Superconductors,
September 14-18, 1997, Cape Cod, Massachusett
Low-energy limit of the three-band model for electrons in a CuO plane
The three-band model with the O-O direct hopping near to unit filling is
considered. We present the general procedure of reduction of this model to the
low-energy limit. At unit filling the three-band model in the charge-transfer
limit is reduced to the Heisenberg model and we calculate the superexchange
constant. For the case of the small electron doping the three-band model is
reduced to the model and we calculate electron hopping parameters at the
nearest and next neighbors. We derive the structure of corrections to the
model and calculate their magnitude. The values of the hopping parameters for
electron- and hole-doping differ approximately at 40 %.Comment: 10 pp. (LATEX
On the Nagaoka polaron in the t-J model
It is widely believed that a single hole in the two (or three) dimensional
t-J model, for sufficiently small exchange coupling J, creates a ferromagnetic
bubble around itself, a finite J remnant of the ferromagnetic groundstate at
J=0 (the infinite U Hubbard model), first established by Nagaoka. We
investigate this phenomenon in two dimensions using the density matrix
renormalization group, for system sizes up to 9x9. We find that the polaron
forms for J/t<0.02-0.03 (a somewhat larger value than estimated previously).
Although finite-size effects appear large, our data seems consistent with the
expected 1.1(J/t)^{-1/4} variation of polarion radius. We also test the
Brinkman-Rice model of non-retracing paths in a Neel background, showing that
it is quite accurate, at larger J. Results are also presented in the case where
the
Heisenberg interaction is dropped (the t-J^z model). Finally we discuss a
"dressed polaron" picture in which the hole propagates freely inside a finite
region but makes only self-retracing excursions outside this region.Comment: 7 pages, 9 encapsulated figure
Effects of domain walls on hole motion in the two-dimensional t-J model at finite temperature
The t-J model on the square lattice, close to the t-J_z limit, is studied by
quantum Monte Carlo techniques at finite temperature and in the underdoped
regime. A variant of the Hoshen-Koppelman algorithm was implemented to identify
the antiferromagnetic domains on each Trotter slice. The results show that the
model presents at high enough temperature finite antiferromagnetic (AF) domains
which collapse at lower temperatures into a single ordered AF state. While
there are domains, holes would tend to preferentially move along the domain
walls. In this case, there are indications of hole pairing starting at a
relatively high temperature. At lower temperatures, when the whole system
becomes essentially fully AF ordered, at least in finite clusters, holes would
likely tend to move within phase separated regions. The crossover between both
states moves down in temperature as doping increases and/or as the off-diagonal
exchange increases. The possibility of hole motion along AF domain walls at
zero temperature in the fully isotropic t-J is discussed.Comment: final version, to appear in Physical Review
Holes in the t-J_z model: a thorough study
The t-J_z model is the strongly anisotropic limit of the t-J model which
captures some general properties of the doped antiferromagnets (AF). The
absence of spin fluctuations simplifies the analytical treatment of hole motion
in an AF background and allows us to calculate the single- and two-hole spectra
with high accuracy using regular diagram technique combined with real-space
approach. At the same time, numerical studies of this model via exact
diagonalization (ED) on small clusters show negligible finite size effects for
a number of quantities, thus allowing a direct comparison between analytical
and numerical results. Both approaches demonstrate that the holes have tendency
to pair in the p- and d-wave channels at realistic values of t/J. The
interactions leading to pairing and effects selecting p and d waves are
thoroughly investigated. The role of transverse spin fluctuations is considered
using perturbation theory. Based on the results of the present study, we
discuss the pairing problem in the realistic t-J-like model. Possible
implications for preformed pairs formation and phase separation are drawn.Comment: 21 pages, 15 figure
Quantum impurity in an antiferromagnet: non-linear sigma model theory
We present a new formulation of the theory of an arbitrary quantum impurity
in an antiferromagnet, using the O(3) non-linear sigma model. We obtain the low
temperature expansion for the impurity spin susceptibilities of
antiferromagnets with magnetic long-range order in the ground state. We also
consider the bulk quantum phase transition in d=2 to the gapped paramagnet (d
is the spatial dimension): the impurity is described solely by a topological
Berry phase term which is an exactly marginal perturbation to the critical
theory. The physical properties of the quantum impurity near criticality are
obtained by an expansion in (d-1).Comment: 14 pages, 7 figures; (v2) added re
Magnetic field effects and magnetic anisotropy in lightly doped La_{2-x}Sr_xCuO_4
The effects of the application of a magnetic field on the diagonal stripe
spin-glass phase is studied in lightly doped La_{2-x}Sr_xCuO_4 (x=0.014 and
0.024). With increasing magnetic field, the magnetic elastic intensity at the
diagonal incommensurate (DIC) positions (1,\pm\epsilon,0) decreases as opposed
to the increase seen in superconducting samples. This diminution in intensity
with increasing magnetic field originates from a spin reorientation transition,
which is driven by the antisymmetric exchange term in the spin Hamiltonian. On
the other hand, the transition temperature, the incommensurability, and the
peak width of the diagonal incommensurate correlations are not changed with
magnetic field. This result suggests that the magnetic correlations are
determined primarily by the charge disproportionation and that the geometry of
the diagonal incommensurate magnetism is also determined by effects, that is,
stripe formation which are not purely magnetic in origin. The
Dzyaloshinskii-Moriya antisymmetric exchange is nevertheless important in
determining the local spin structure in the DIC stripe phase.Comment: 7 pages, 5 figures, to appear in Phys. Rev.
Bosonic Excitations in Random Media
We consider classical normal modes and non-interacting bosonic excitations in
disordered systems. We emphasise generic aspects of such problems and parallels
with disordered, non-interacting systems of fermions, and discuss in particular
the relevance for bosonic excitations of symmetry classes known in the
fermionic context. We also stress important differences between bosonic and
fermionic problems. One of these follows from the fact that ground state
stability of a system requires all bosonic excitation energy levels to be
positive, while stability in systems of non-interacting fermions is ensured by
the exclusion principle, whatever the single-particle energies. As a
consequence, simple models of uncorrelated disorder are less useful for bosonic
systems than for fermionic ones, and it is generally important to study the
excitation spectrum in conjunction with the problem of constructing a
disorder-dependent ground state: we show how a mapping to an operator with
chiral symmetry provides a useful tool for doing this. A second difference
involves the distinction for bosonic systems between excitations which are
Goldstone modes and those which are not. In the case of Goldstone modes we
review established results illustrating the fact that disorder decouples from
excitations in the low frequency limit, above a critical dimension , which
in different circumstances takes the values and . For bosonic
excitations which are not Goldstone modes, we argue that an excitation density
varying with frequency as is a universal
feature in systems with ground states that depend on the disorder realisation.
We illustrate our conclusions with extensive analytical and some numerical
calculations for a variety of models in one dimension
Electron momentum distribution in underdoped cuprates
We investigate the electron momentum distribution function (EMD) in a weakly
doped two-dimensional quantum antiferromagnet (AFM) as described by the t-J
model. Our analytical results for a single hole in an AFM based on the
self-consistent Born approximation (SCBA) indicate an anomalous momentum
dependence of EMD showing 'hole pockets' coexisting with a signature of an
emerging large Fermi surface. The position of the incipient Fermi surface and
the structure of the EMD is determined by the momentum of the ground state. Our
analysis shows that this result remains robust in the presence of next-nearest
neighbor hopping terms in the model. Exact diagonalization results for small
clusters are with the SCBA reproduced quantitatively.Comment: 5 pages, submitted to PR