418 research outputs found
Stringent limitations on reductive perturbation studies of nonplanar acoustic solitons in plasmas
More than fifty years ago, the Korteweg-de Vries equation was shown to describe not only solitary surface waves on shallow water, but also nonlinear ion-acoustic waves. Because of the algorithmic ease of using reductive perturbation theory, intensive research followed on a wide range of wave types. Soon, the formalism was extended to nonplanar modes by introducing a stretching designed to accommodate spherically and cylindrically symmetric ion-acoustic waves. Over the last two decades many authors followed this approach, but almost all have ignored the severe restrictions in parameter space imposed by the Ansatz. In addition, for other steps in the formalism, the justification is often not spelled out, leading to effects that are physically undesirable or ambiguous. Hence, there is a need to critically assess this approach to nonplanar modes and to use it with the utmost care, respecting the restrictions on its validity. Only inward propagation may be meaningfully studied and respect for weak nonlinearities of at most 1/10 implies that one cannot get closer to the axis or centre of symmetry than about 30 Debye lengths. Thus, one is in a regime where the modes are quasi-planar and not particularly interesting. Most papers disregard these constraints and hence reach questionable conclusions
Hole-pair hopping in arrangements of hole-rich/hole-poor domains in a quantum antiferromagnet
We study the motion of holes in a doped quantum antiferromagnet in the
presence of arrangements of hole-rich and hole-poor domains such as the
stripe-phase in high- cuprates. When these structures form, it becomes
energetically favorable for single holes, pairs of holes or small bound-hole
clusters to hop from one hole-rich domain to another due to quantum
fluctuations. However, we find that at temperature of approximately 100 K, the
probability for bound hole-pair exchange between neighboring hole-rich regions
in the stripe phase, is one or two orders of magnitude larger than single-hole
or multi-hole droplet exchange. As a result holes in a given hole-rich domain
penetrate further into the antiferromagnetically aligned domains when they do
it in pairs. At temperature of about 100 K and below bound pairs of holes hop
from one hole-rich domain to another with high probability. Therefore our main
finding is that the presence of the antiferromagnetic hole-poor domains act as
a filter which selects, from the hole-rich domains (where holes form a
self-bound liquid), hole pairs which can be exchanged throughout the system.
This fluid of bound hole pairs can undergo a superfluid phase ordering at the
above mentioned temperature scale.Comment: Revtex, 6 two-column pages, 4 figure
Variational state based on the Bethe ansatz solution and a correlated singlet liquid state in the one-dimensional t-J model
The one-dimensional t-J model is investigated by the variational Monte Carlo
method. A variational wave function based on the Bethe ansatz solution is newly
proposed, where the spin-charge separation is realized, and a long-range
correlation factor of Jastrow-type is included. In most regions of the phase
diagram, this wave function provides an excellent description of the
ground-state properties characterized as a Tomonaga-Luttinger liquid; Both of
the amplitude and exponent of correlation functions are correctly reproduced.
For the spin-gap phase, another trial state of correlated singlet pairs with a
Jastrow factor is introduced. This wave function shows generalized Luther-Emery
liquid behavior, exhibiting enhanced superconducting correlations and
exponential decay of the spin correlation function. Using these two variational
wave functions, the whole phase diagram is determined. In addition, relations
between the correlation exponent and variational parameters in the trial
functions are derived.Comment: REVTeX 3.0, 27 pages. 7 figures available upon request
([email protected]). To be published in Phys. Rev. B 5
Phase Separation of the Two-Dimensional t-J model
The boundary of phase separation of the two-dimensional t-J model is
investigated by the power-Lanczos method and Maxwell construction. The method
is similar to a variational approach and it determines the lower bound of the
phase separation boundary with in the limit . In
the physical interesting regime of high T_c superconductors where
there is no phase separation.Comment: LaTex 5 pages, 4 figure
Green's Function Monte Carlo for Lattice Fermions: Application to the t-J Model
We develop a general numerical method to study the zero temperature
properties of strongly correlated electron models on large lattices. The
technique, which resembles Green's Function Monte Carlo, projects the ground
state component from a trial wave function with no approximations. We use this
method to determine the phase diagram of the two-dimensional t-J model, using
the Maxwell construction to investigate electronic phase separation. The shell
effects of fermions on finite-sized periodic lattices are minimized by keeping
the number of electrons fixed at a closed-shell configuration and varying the
size of the lattice. Results obtained for various electron numbers
corresponding to different closed-shells indicate that the finite-size effects
in our calculation are small. For any value of interaction strength, we find
that there is always a value of the electron density above which the system can
lower its energy by forming a two-component phase separated state. Our results
are compared with other calculations on the t-J model. We find that the most
accurate results are consistent with phase separation at all interaction
strengths.Comment: 22 pages, 22 figure
Stripes and the t-J model
We investigate the two-dimensional t-J model at a hole doping of x = 1/8 and
J/t = 0.35 with exact diagonalization. The low-energy states are uniform (not
striped). We find numerous excited states with charge density wave structures,
which may be interpreted as striped phases. Some of these are consistent with
neutron scattering data on the cuprates and nickelates.Comment: 4 pages; 4 eps figures included in text; Revte
Formation of clusters in the ground state of the model on a two leg ladder
We investigate the ground state properties of the model on a two leg
ladder with anisotropic couplings () along rungs and
() along legs. We have implemented a cluster approach based
on 4-site plaqettes. In the strong asymmetric cases and
the ground state energy is well described by plaquette
clusters with charges . The interaction between the clusters favours the
condensation of plaquettes with maximal charge -- a signal for phase
separation. The dominance of Q=2 plaquettes explains the emergence of tightly
bound hole pairs. We have presented the numerical results of exact
diagonalization to support our cluster approach.Comment: 11 pages, 9 figures, RevTex
Phase separation in t-J ladders
The phase separation boundary of isotropic t-J ladders is analyzed using
density matrix renormalization group techniques. The complete boundary to phase
separation as a function of J/t and doping is determined for a chain and for
ladders with two, three and four legs. Six-chain ladders have been analyzed at
low hole doping. We use a direct approach in which the phase separation
boundary is determined by measuring the hole density in the part of the system
which contains both electrons and holes. In addition we examine the binding
energy of multi-hole clusters. An extrapolation in the number of legs suggests
that the lowest J/t for phase separation to occur in the two dimensional t-J
model is J/t~1.Comment: 8 pages in revtex format including 13 embedded figures, one reference
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Singlet pairing in the double chain t-J model
Applying the bosonization procedure to constrained fermions in the framework
of the one dimensional t-J model we discuss a scenario of singlet
superconductivity in a lightly doped double chain where all spin excitations
remain gapful.Comment: 13 pages, TeX, C Version 3.
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