1,065 research outputs found
Hidden symmetry and quantum phases in spin-3/2 cold atomic systems
Optical traps and lattices provide a new opportunity to study strongly
correlated high spin systems with cold atoms. In this article, we review the
recent progress on the hidden symmetry properties in the simplest high spin
fermionic systems with hyperfine spin , which may be realized with atoms
of Cs, Be, Ba, Ba, and Hg. A {\it generic}
SO(5) or isomorphically, ) symmetry is proved in such systems with the
s-wave scattering interactions in optical traps, or with the on-site Hubbard
interactions in optical lattices. Various important features from this high
symmetry are studied in the Fermi liquid theory, the mean field phase diagram,
and the sign problem in quantum Monte-Carlo simulations. In the s-wave quintet
Cooper pairing phase, the half-quantum vortex exhibits the global analogue of
the Alice string and non-Abelian Cheshire charge properties in gauge theories.
The existence of the quartetting phase, a four-fermion counterpart of the
Cooper pairing phase, and its competition with other orders are studied in one
dimensional spin-3/2 systems. We also show that counter-intuitively quantum
fluctuations in spin-3/2 magnetic systems are even stronger than those in
spin-1/2 systems
Bond order wave instabilities in doped frustrated antiferromagnets: "Valence bond solids" at fractional filling
We explore both analytically and numerically the properties of doped t-J
models on a class of highly frustrated lattices, such as the kagome and the
pyrochlore lattice. Focussing on a particular sign of the hopping integral and
antiferromagnetic exchange, we find a generic symmetry breaking instability
towards a twofold degenerate ground state at a fractional filling below half
filling. These states show modulated bond strengths and only break lattice
symmetries. They can be seen as a generalization of the well-known valence bond
solid states to fractional filling.Comment: slightly shortened and reorganized versio
Bond-order modulated staggered flux phase for the model on the square lattice
Motivated by the observation of inhomogeneous patterns in some high-T
cuprate compounds, several variational Gutzwiller-projected wave-functions with
built-in charge and bond order parameters are proposed for the extended
model on the square lattice at low doping. First, following a recent
Gutzwiller-projected mean-field approach by one of us (Phys. Rev. B. {\bf 72},
060508(R) (2005)), we investigate, as a function of doping and Coulomb
repulsion, the stability of the staggered flux phase with respect to small
spontaneous modulations of squared unit cells ranging from to
. It is found that a bond-order (BO)
modulation appears spontaneously on top of the staggered flux pattern for hole
doping around 1/8. A related wave-function is then constructed and optimized
accurately and its properties studied extensively using an approximation-free
variational Monte Carlo scheme. Finally, the competition of the BO-modulated
staggered flux wave-function w.r.t. the d-wave RVB wave-function or the
commensurate flux state is investigated. It is found that a short range Coulomb
repulsion penalizes the d-wave superconductor and that a moderate Coulomb
repulsion brings them very close in energy. Our results are discussed in
connection to the STM observations in the under-doped regime of some cuprates.Comment: 10 pages, 9 figure
Asymmetric spin-1/2 two-leg ladders
We consider asymmetric spin-1/2 two-leg ladders with non-equal
antiferromagnetic (AF) couplings J_|| and \kappa J_|| along legs (\kappa <= 1)
and ferromagnetic rung coupling, J_\perp. This model is characterized by a gap
\Delta in the spectrum of spin excitations. We show that in the large J_\perp
limit this gap is equivalent to the Haldane gap for the AF spin-1 chain,
irrespective of the asymmetry of the ladder. The behavior of the gap at small
rung coupling falls in two different universality classes. The first class,
which is best understood from the case of the conventional symmetric ladder at
\kappa=1, admits a linear scaling for the spin gap \Delta ~ J_\perp. The second
class appears for a strong asymmetry of the coupling along legs, \kappa J_|| <<
J_\perp << J_|| and is characterized by two energy scales: the exponentially
small spin gap \Delta ~ J_\perp \exp(-J_|| / J_\perp), and the bandwidth of the
low-lying excitations induced by a Suhl-Nakamura indirect exchange ~ J_\perp^2
/J_|| . We report numerical results obtained by exact diagonalization, density
matrix renormalization group and quantum Monte Carlo simulations for the spin
gap and various spin correlation functions. Our data indicate that the behavior
of the string order parameter, characterizing the hidden AF order in Haldane
phase, is different in the limiting cases of weak and strong asymmetry. On the
basis of the numerical data, we propose a low-energy theory of effective spin-1
variables, pertaining to large blocks on a decimated lattice.Comment: 18 pages, 11 figure
Combined analytical and numerical approach to study magnetization plateaux in doped quasi-one-dimensional antiferromagnets
We investigate the magnetic properties of quasi-one-dimensional quantum
spin-S antiferromagnets. We use a combination of analytical and numerical
techniques to study the presence of plateaux in the magnetization curve. The
analytical technique consists in a path integral formulation in terms of
coherent states. This technique can be extended to the presence of doping and
has the advantage of a much better control for large spins than the usual
bosonization technique. We discuss the appearance of doping-dependent plateaux
in the magnetization curves for spin-S chains and ladders. The analytical
results are complemented by a density matrix renormalization group (DMRG) study
for a trimerized spin-1/2 and anisotropic spin-3/2 doped chains.Comment: 12 pages, 5 figure
Connectivity for the frisbee architecture
In this paper we investigate the kconnectivity
threshold of distributed dense ad hoc
heterogeneous wireless sensor network architecture. We
consider the situation when sensors are deployed in the
surveillance area according to a uniform distribution
perturbed by a Gaussian noise. We derive analytically
the minimum detection range which guarantees an
emerging structure in the network, namely the
connectivity, which becomes larger and larger as the
number of sensors in the network increase. This allows
the target track to be propagated almost surely
throughout the network using the minimum possible
amount ofprime energy. We report the results of some
simulation experiments which further support the
theoretical results
Algorithms for the selection of the active sensors in distributed tracking: Comparison between Frisbee and GNS methods
This paper compares two different
approaches for sensor selection for distributed tracking:
1) The Frisbee method, and 2) Global Node Selection
(GNS). The Frisbee method is based on the proximity of
the nodes to the predicted location of the target; GNS is
based on minimizing the unbiased Cramer Rao lower
bound (CRLB). Both theoretical and experimental
results indicate that the Frisbee method is as effective as
GNS. Furthermore, the Frisbee method is attractive
due to its very light computational load
Antiferromagnetism and hole pair checkerboard in the vortex state of high Tc superconductors
We propose a microscopic state for the vortex phase of BSCO superconductors.
Around the vortex core or above H_{c2}, the d wave hole pairs form a
checkerboard localized in the commensurate antiferromagnetic background. We
discuss this theory in connection with recent STM experiments.Comment: Final versio
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