133,767 research outputs found

### Exact form factors for the Josephson tunneling current and relative particle number fluctuations in a model of two coupled Bose-Einstein condensates

Form factors are derived for a model describing the coherent Josephson
tunneling between two coupled Bose-Einstein condensates. This is achieved by
studying the exact solution of the model in the framework of the algebraic
Bethe ansatz. In this approach the form factors are expressed through
determinant representations which are functions of the roots of the Bethe
ansatz equations.Comment: 11 pages, latex, no figures, final version to appear in Lett. Math.
Phy

### Dynamics of a hole in the large--U Hubbard model: a Feynman diagram approach

We study the dynamics of a single hole in an otherwise half--filled
two--dimensional Hubbard model by introducing a nonlocal Bogolyubov
transformation in the antiferromagnetic state. This allows us to rewrite the
Hamiltonian in a form that makes a separation between high--energy processes
(involving double--occupancy) and low--energy physics possible. A diagrammatic
scheme is developped that allows for a systematic study of the different
processes delocalizing a carrier in the antiferromagnetic state. In particular,
the so--called Trugman process, important if transverse spin fluctuations are
neglected, is studied and is shown to be dominated by the leading vertex
corrections. We analyze the dynamics of a single hole both in the Ising limit
and with spin fluctuations. The results are compared with previous theories as
well as with recent exact small--cluster calculations, and we find good
agreement. The formalism establishes a link between weak and strong coupling
methodologies.Comment: Latex 34pages, Orsay Preprint, submitted to Phys. Rev.

### Exact solvability in contemporary physics

We review the theory for exactly solving quantum Hamiltonian systems through
the algebraic Bethe ansatz. We also demonstrate how this theory applies to
current studies in Bose-Einstein condensation and metallic grains which are of
nanoscale size.Comment: 23 pages, no figures, to appear in ``Classical and Quantum Nonlinear
Integrable Systems'' ed. A. Kund

### Minimax estimation with thresholding and its application to wavelet analysis

Many statistical practices involve choosing between a full model and reduced
models where some coefficients are reduced to zero. Data were used to select a
model with estimated coefficients. Is it possible to do so and still come up
with an estimator always better than the traditional estimator based on the
full model? The James-Stein estimator is such an estimator, having a property
called minimaxity. However, the estimator considers only one reduced model,
namely the origin. Hence it reduces no coefficient estimator to zero or every
coefficient estimator to zero. In many applications including wavelet analysis,
what should be more desirable is to reduce to zero only the estimators smaller
than a threshold, called thresholding in this paper. Is it possible to
construct this kind of estimators which are minimax? In this paper, we
construct such minimax estimators which perform thresholding. We apply our
recommended estimator to the wavelet analysis and show that it performs the
best among the well-known estimators aiming simultaneously at estimation and
model selection. Some of our estimators are also shown to be asymptotically
optimal.Comment: Published at http://dx.doi.org/10.1214/009053604000000977 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org

### Ground-State Fidelity and Kosterlitz-Thouless Phase Transition for Spin 1/2 Heisenberg Chain with Next-to-the-Nearest-Neighbor Interaction

The Kosterlitz-Thouless transition for the spin 1/2 Heisenberg chain with the
next-to-the-nearest-neighbor interaction is investigated in the context of an
infinite matrix product state algorithm, which is a generalization of the
infinite time-evolving block decimation algorithm [G. Vidal, Phys. Rev. Lett.
\textbf{98}, 070201 (2007)] to accommodate both the
next-to-the-nearest-neighbor interaction and spontaneous dimerization. It is
found that, in the critical regime, the algorithm automatically leads to
infinite degenerate ground-state wave functions, due to the finiteness of the
truncation dimension. This results in \textit{pseudo} symmetry spontaneous
breakdown, as reflected in a bifurcation in the ground-state fidelity per
lattice site. In addition, this allows to introduce a pseudo-order parameter to
characterize the Kosterlitz-Thouless transition.Comment: 4 pages, 4 figure

### Highlights of the TEXONO Research Program on Neutrino and Astroparticle Physics

This article reviews the research program and efforts for the TEXONO
Collaboration on neutrino and astro-particle physics. The ``flagship'' program
is on reactor-based neutrino physics at the Kuo-Sheng (KS) Power Plant in
Taiwan. A limit on the neutrino magnetic moment of \munuebar < 1.3 X 10^{-10}
\mub} at 90% confidence level was derived from measurements with a high purity
germanium detector. Other physics topics at KS, as well as the various R&D
program, are discussedComment: 10 pages, 9 figures, Proceedings of the International Symposium on
Neutrino and Dark Matter in Nuclear Physics (NDM03), Nara, Japan, June 9-14,
200

### Solution space heterogeneity of the random K-satisfiability problem: Theory and simulations

The random K-satisfiability (K-SAT) problem is an important problem for
studying typical-case complexity of NP-complete combinatorial satisfaction; it
is also a representative model of finite-connectivity spin-glasses. In this
paper we review our recent efforts on the solution space fine structures of the
random K-SAT problem. A heterogeneity transition is predicted to occur in the
solution space as the constraint density alpha reaches a critical value
alpha_cm. This transition marks the emergency of exponentially many solution
communities in the solution space. After the heterogeneity transition the
solution space is still ergodic until alpha reaches a larger threshold value
alpha_d, at which the solution communities disconnect from each other to become
different solution clusters (ergodicity-breaking). The existence of solution
communities in the solution space is confirmed by numerical simulations of
solution space random walking, and the effect of solution space heterogeneity
on a stochastic local search algorithm SEQSAT, which performs a random walk of
single-spin flips, is investigated. The relevance of this work to glassy
dynamics studies is briefly mentioned.Comment: 11 pages, 4 figures. Final version as will appear in Journal of
Physics: Conference Series (Proceedings of the International Workshop on
Statistical-Mechanical Informatics, March 7-10, 2010, Kyoto, Japan

### Quantum fluctuations in the spiral phase of the Hubbard model

We study the magnetic excitations in the spiral phase of the two--dimensional
Hubbard model using a functional integral method. Spin waves are strongly
renormalized and a line of near--zeros is observed in the spectrum around the
spiral pitch $\pm{\bf Q}$. The possibility of disordered spiral states is
examined by studying the one--loop corrections to the spiral order parameter.
We also show that the spiral phase presents an intrinsic instability towards an
inhomogeneous state (phase separation, CDW, ...) at weak doping. Though phase
separation is suppressed by weak long--range Coulomb interactions, the CDW
instability only disappears for sufficiently strong Coulomb interaction.Comment: Figures are NOW appended via uuencoded postscript fil

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