3,225 research outputs found
Bethe ansatz solution of the anisotropic correlated electron model associated with the Temperley-Lieb algebra
A recently proposed strongly correlated electron system associated with the
Temperley-Lieb algebra is solved by means of the coordinate Bethe ansatz for
periodic and closed boundary conditions.Comment: 21 page
Integrable multiparametric quantum spin chains
Using Reshetikhin's construction for multiparametric quantum algebras we
obtain the associated multiparametric quantum spin chains. We show that under
certain restrictions these models can be mapped to quantum spin chains with
twisted boundary conditions. We illustrate how this general formalism applies
to construct multiparametric versions of the supersymmetric t-J and U models.Comment: 17 pages, RevTe
Violation of the string hypothesis and Heisenberg XXZ spin chain
In this paper we count the numbers of real and complex solutions to Bethe
constraints in the two particle sector of the XXZ model. We find exact number
of exceptions to the string conjecture and total number of solutions which is
required for completeness.Comment: 15 pages, 7 Postscript figure
Evidence for multiband superconductivity in the heavy fermion compound UNi2Al3
Epitaxial thin films of the heavy fermion superconductor UNi2Al3 with
Tc{max}=0.98K were investigated. The transition temperature Tc depends on the
current direction which can be related to superconducting gaps opening at
different temperatures. Also the influence of the magnetic ordering at TN=5K on
R(T) is strongly anisotropic indicating different coupling between the magnetic
moments and itinerant charge carriers on the multi-sheeted Fermi surface. The
upper critical field Hc2(T) suggests an unconventional spin-singlet
superconducting state.Comment: 4 pages, 6 figures revised version: inset of fig. 2 changed, fig. 3
added accepted for pub. in Phys. Rev. Lett. (estimated 9/04
Magnetic Susceptibility of an integrable anisotropic spin ladder system
We investigate the thermodynamics of a spin ladder model which possesses a
free parameter besides the rung and leg couplings. The model is exactly solved
by the Bethe Ansatz and exhibits a phase transition between a gapped and a
gapless spin excitation spectrum. The magnetic susceptibility is obtained
numerically and its dependence on the anisotropy parameter is determined. A
connection with the compounds KCuCl3, Cu2(C5H12N2)2Cl4 and (C5H12N)2CuBr4 in
the strong coupling regime is made and our results for the magnetic
susceptibility fit the experimental data remarkably well.Comment: 12 pages, 12 figures included, submitted to Phys. Rev.
The nested SU(N) off-shell Bethe ansatz and exact form factors
The form factor equations are solved for an SU(N) invariant S-matrix under
the assumption that the anti-particle is identified with the bound state of N-1
particles. The solution is obtained explicitly in terms of the nested off-shell
Bethe ansatz where the contribution from each level is written in terms of
multiple contour integrals.Comment: This work is dedicated to the 75th anniversary of H. Bethe's
foundational work on the Heisenberg chai
Reference instruments based on spectrometric measurement with Lucas Cells
The Bundesamt für Strahlenschutz (Berlin, Germany) and the Paul Scherrer Institute (Villigen, Switzerland) both operate accredited calibration laboratories for radon gas activity concentration. Both the institutions use Lucas Cells as detector in their reference instrumentation due to the low dependence of this detector type on variations in environmental conditions. As a further measure to improve the quality of the reference activity concentration, a spectrometric method of data evaluation has been applied. The electric pulses from the photomultiplier tube coupled to the Lucas Cells are subjected to a pulse height analysis. The stored pulse height spectra are analysed retrospectively to compensate for fluctuations in the electric parameters of the instrumentation during a measurement. The reference instrumentation of both the laboratories is described with the respective spectrum evaluation procedures. The methods of obtaining traceability to the primary calibration laboratories of Germany and Switzerland and data of performance tests are presente
Integrable Hamiltonians with symmetry from the Fateev-Zamolodchikov model
A special case of the Fateev-Zamolodchikov model is studied resulting in a
solution of the Yang-Baxter equation with two spectral parameters. Integrable
models from this solution are shown to have the symmetry of the Drinfeld double
of a dihedral group. Viewing this solution as a descendant of the zero-field
six-vertex model allows for the construction of functional relations and Bethe
ansatz equations
A Majorana Fermion t-J Model in One Dimension
We study a rotation invariant Majorana fermion model in one dimension using
diagrammatic perturbation theory and numerical diagonalization of small
systems. The model is inspired by a Majorana representation of the
antiferromagnetic spin-1/2 chain, and it is similar in form to the t-J model of
electrons, except that the Majorana fermions carry spin-1 and Z_2 charge. We
discuss the implications of our results for the low-energy excitations of the
spin-1/2 chain. We also discuss a generalization of our model from 3 species of
Majorana fermions to N species; the SO(4) symmetric model is particularly
interesting.Comment: 29 LaTeX pages, 11 postscript figure
Learning to Communicate: A Machine Learning Framework for Heterogeneous Multi-Agent Robotic Systems
We present a machine learning framework for multi-agent systems to learn both
the optimal policy for maximizing the rewards and the encoding of the high
dimensional visual observation. The encoding is useful for sharing local visual
observations with other agents under communication resource constraints. The
actor-encoder encodes the raw images and chooses an action based on local
observations and messages sent by the other agents. The machine learning agent
generates not only an actuator command to the physical device, but also a
communication message to the other agents. We formulate a reinforcement
learning problem, which extends the action space to consider the communication
action as well. The feasibility of the reinforcement learning framework is
demonstrated using a 3D simulation environment with two collaborating agents.
The environment provides realistic visual observations to be used and shared
between the two agents.Comment: AIAA SciTech 201
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