808 research outputs found
Ferroelectricity in strained CaSrTiO from first principles
We present a density functional theory investigation of strained
CaSrTiO. We have determined the structure and polarization
for a number of arrangements of Ca and Sr in a 222 supercell.
The a and b lattice vectors are strained to match the lattice constants of the
rotated Si(001) face. To set the context for the CSTO study, we also include
simulations of the Si(001) constrained structures for CaTiO and SrTiO.
Our primary findings are that all CaSrTiO structures
examined except one are ferroelectric, exhibiting polarizations ranging from
0.08 C/m for the lowest energy configuration to about 0.26 C/m for the
higher energy configurations. We find that the configurations with larger
polarizations have lower c/a ratios. The net polarization of the cell is the
result of Ti-O ferroelectric displacements regulated by A-site cations.Comment: 13 pages, 4 figure
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
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
Luttinger Liquid Instability in the One Dimensional t-J Model
We study the t-J model in one dimension by numerically projecting the true
ground state from a Luttinger liquid trial wave function. We find the model
exhibits Luttinger liquid behavior for most of the phase diagram in which
interaction strength and density are varied. However at small densities and
high interaction strengths a new phase with a gap to spin excitations and
enhanced superconducting correlations is found. We show this phase is a
Luther-Emery liquid and study its correlation functions.Comment: REVTEX, 11 pages. 4 Figures available on request from
[email protected]
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
Comment on ``Stripes and the t-J Model''
This is a comment being submitted to Physical Review Letters on a recent
letter by Hellberg and Manousakis on stripes in the t-J model.Comment: One reference correcte
Matrix Element Distribution as a Signature of Entanglement Generation
We explore connections between an operator's matrix element distribution and
its entanglement generation. Operators with matrix element distributions
similar to those of random matrices generate states of high multi-partite
entanglement. This occurs even when other statistical properties of the
operators do not conincide with random matrices. Similarly, operators with some
statistical properties of random matrices may not exhibit random matrix element
distributions and will not produce states with high levels of multi-partite
entanglement. Finally, we show that operators with similar matrix element
distributions generate similar amounts of entanglement.Comment: 7 pages, 6 figures, to be published PRA, partially supersedes
quant-ph/0405053, expands quant-ph/050211
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
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
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