2,978 research outputs found
Fast computation of the Kohn-Sham susceptibility of large systems
For hybrid systems, such as molecules grafted onto solid surfaces, the
calculation of linear response in time dependent density functional theory is
slowed down by the need to calculate, in N^4 operations, the susceptibility of
N non interacting Kohn-Sham reference electrons. We show how this
susceptibility can be calculated N times faster within finite precision. By
itself or in combination with previous methods, this should facilitate the
calculation of TDDFT response and optical spectra of hybrid systems.Comment: submitted 25/1/200
Self-consistent Green function approach for calculations of electronic structure in transition metals
We present an approach for self-consistent calculations of the many-body
Green function in transition metals. The distinguishing feature of our approach
is the use of the one-site approximation and the self-consistent quasiparticle
wave function basis set, obtained from the solution of the Schrodinger equation
with a nonlocal potential. We analyze several sets of skeleton diagrams as
generating functionals for the Green function self-energy, including GW and
fluctuating exchange sets. Their relative contribution to the electronic
structure in 3d-metals was identified. Calculations for Fe and Ni revealed
stronger energy dependence of the effective interaction and self-energy of the
d-electrons near the Fermi level compared to s and p electron states.
Reasonable agreement with experimental results is obtained
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
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.
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
Local Communication Protocols for Learning Complex Swarm Behaviors with Deep Reinforcement Learning
Swarm systems constitute a challenging problem for reinforcement learning
(RL) as the algorithm needs to learn decentralized control policies that can
cope with limited local sensing and communication abilities of the agents.
While it is often difficult to directly define the behavior of the agents,
simple communication protocols can be defined more easily using prior knowledge
about the given task. In this paper, we propose a number of simple
communication protocols that can be exploited by deep reinforcement learning to
find decentralized control policies in a multi-robot swarm environment. The
protocols are based on histograms that encode the local neighborhood relations
of the agents and can also transmit task-specific information, such as the
shortest distance and direction to a desired target. In our framework, we use
an adaptation of Trust Region Policy Optimization to learn complex
collaborative tasks, such as formation building and building a communication
link. We evaluate our findings in a simulated 2D-physics environment, and
compare the implications of different communication protocols.Comment: 13 pages, 4 figures, version 2, accepted at ANTS 201
The su(N) XX model
The natural su(N) generalization of the XX model is introduced and analyzed.
It is defined in terms of the characterizing properties of the usual XX model:
the existence of two infinite sequences of mutually commuting conservation laws
and the existence of two infinite sequences of mastersymmetries. The
integrability of these models, which cannot be obtained in a degenerate limit
of the su(N)-XXZ model, is established in two ways: by exhibiting their R
matrix and from a direct construction of the commuting conservation laws. We
then diagonalize the conserved laws by the method of the algebraic Bethe
Ansatz. The resulting spectrum is trivial in a certain sense; this provides
another indication that the su(N) XX model is the natural generalization of the
su(2) model. The application of these models to the construction of an
integrable ladder, that is, an su(N) version of the Hubbard model, is
mentioned.Comment: 16 pages, TeX and harvmac (option b). Minor corrections, accepted for
publication in Nuclear Physics
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