3,599 research outputs found
Impurity and boundary effects in one and two-dimensional inhomogeneous Heisenberg antiferromagnets
We calculate the ground-state energy of one and two-dimensional spatially
inhomogeneous antiferromagnetic Heisenberg models for spins 1/2, 1, 3/2 and 2.
Our calculations become possible as a consequence of the recent formulation of
density-functional theory for Heisenberg models. The method is similar to
spin-density-functional theory, but employs a local-density-type approximation
designed specifically for the Heisenberg model, allowing us to explore
parameter regimes that are hard to access by traditional methods, and to
consider complications that are important specifically for nanomagnetic
devices, such as the effects of impurities, finite-size, and boundary geometry,
in chains, ladders, and higher-dimensional systems.Comment: 4 pages, 4 figures, accepted by Phys. Rev.
Antilinear deformations of Coxeter groups, an application to Calogero models
We construct complex root spaces remaining invariant under antilinear
involutions related to all Coxeter groups. We provide two alternative
constructions: One is based on deformations of factors of the Coxeter element
and the other based on the deformation of the longest element of the Coxeter
group. Motivated by the fact that non-Hermitian Hamiltonians admitting an
antilinear symmetry may be used to define consistent quantum mechanical systems
with real discrete energy spectra, we subsequently employ our constructions to
formulate deformations of Coxeter models remaining invariant under these
extended Coxeter groups. We provide explicit and generic solutions for the
Schroedinger equation of these models for the eigenenergies and corresponding
wavefunctions. A new feature of these novel models is that when compared with
the undeformed case their solutions are usually no longer singular for an
exchange of an amount of particles less than the dimension of the
representation space of the roots. The simultaneous scattering of all particles
in the model leads to anyonic exchange factors for processes which have no
analogue in the undeformed case.Comment: 32 page
Projeto de cĂ©lula eletroquĂmica para estudos da remoção de resĂduos orgânicos em solos e água.
Metric operators for non-Hermitian quadratic su(2) Hamiltonians
A class of non-Hermitian quadratic su(2) Hamiltonians having an anti-linear
symmetry is constructed. This is achieved by analysing the possible symmetries
of such systems in terms of automorphisms of the algebra. In fact, different
realisations for this type of symmetry are obtained, including the natural
occurrence of charge conjugation together with parity and time reversal. Once
specified the underlying anti-linear symmetry of the Hamiltonian, the former,
if unbroken, leads to a purely real spectrum and the latter can be mapped to a
Hermitian counterpart by, amongst other possibilities, a similarity
transformation. Here, Lie-algebraic methods which were used to investigate the
generalised Swanson Hamiltonian are employed to identify the class of quadratic
Hamiltonians that allow for such a mapping to the Hermitian counterpart.
Whereas for the linear su(2) system every Hamiltonian of this type can be
mapped to a Hermitian counterpart by a transformation which is itself an
exponential of a linear combination of su(2) generators, the situation is more
complicated for quadratic Hamiltonians. Therefore, the possibility of more
elaborate similarity transformations, including quadratic exponents, is also
explored in detail. The existence of finite dimensional representations for the
su(2) Hamiltonian, as opposed to the su(1,1) studied before, allows for
comparison with explicit diagonalisation results for finite matrices. Finally,
the similarity transformations constructed are compared with the analogue of
Swanson's method for exact diagonalsation of the problem, establishing a simple
relation between both approaches.Comment: 25 pages, 6 figure
Non-Hermitian Hamiltonians of Lie algebraic type
We analyse a class of non-Hermitian Hamiltonians, which can be expressed
bilinearly in terms of generators of a sl(2,R)-Lie algebra or their isomorphic
su(1,1)-counterparts. The Hamlitonians are prototypes for solvable models of
Lie algebraic type. Demanding a real spectrum and the existence of a well
defined metric, we systematically investigate the constraints these
requirements impose on the coupling constants of the model and the parameters
in the metric operator. We compute isospectral Hermitian counterparts for some
of the original non-Hermitian Hamiltonian. Alternatively we employ a
generalized Bogoliubov transformation, which allows to compute explicitly real
energy eigenvalue spectra for these type of Hamiltonians, together with their
eigenstates. We compare the two approaches.Comment: 27 page
The Pauli equation with complex boundary conditions
We consider one-dimensional Pauli Hamiltonians in a bounded interval with
possibly non-self-adjoint Robin-type boundary conditions. We study the
influence of the spin-magnetic interaction on the interplay between the type of
boundary conditions and the spectrum. A special attention is paid to
PT-symmetric boundary conditions with the physical choice of the time-reversal
operator T.Comment: 16 pages, 4 figure
Functional Optimization in Complex Excitable Networks
We study the effect of varying wiring in excitable random networks in which
connection weights change with activity to mold local resistance or
facilitation due to fatigue. Dynamic attractors, corresponding to patterns of
activity, are then easily destabilized according to three main modes, including
one in which the activity shows chaotic hopping among the patterns. We describe
phase transitions to this regime, and show a monotonous dependence of critical
parameters on the heterogeneity of the wiring distribution. Such correlation
between topology and functionality implies, in particular, that tasks which
require unstable behavior --such as pattern recognition, family discrimination
and categorization-- can be most efficiently performed on highly heterogeneous
networks. It also follows a possible explanation for the abundance in nature of
scale--free network topologies.Comment: 7 pages, 3 figure
Pseudo-Hermitian Representation of Quantum Mechanics
A diagonalizable non-Hermitian Hamiltonian having a real spectrum may be used
to define a unitary quantum system, if one modifies the inner product of the
Hilbert space properly. We give a comprehensive and essentially self-contained
review of the basic ideas and techniques responsible for the recent
developments in this subject. We provide a critical assessment of the role of
the geometry of the Hilbert space in conventional quantum mechanics to reveal
the basic physical principle motivating our study. We then offer a survey of
the necessary mathematical tools and elaborate on a number of relevant issues
of fundamental importance. In particular, we discuss the role of the antilinear
symmetries such as PT, the true meaning and significance of the charge
operators C and the CPT-inner products, the nature of the physical observables,
the equivalent description of such models using ordinary Hermitian quantum
mechanics, the pertaining duality between local-non-Hermitian versus
nonlocal-Hermitian descriptions of their dynamics, the corresponding classical
systems, the pseudo-Hermitian canonical quantization scheme, various methods of
calculating the (pseudo-) metric operators, subtleties of dealing with
time-dependent quasi-Hermitian Hamiltonians and the path-integral formulation
of the theory, and the structure of the state space and its ramifications for
the quantum Brachistochrone problem. We also explore some concrete physical
applications of the abstract concepts and tools that have been developed in the
course of this investigation. These include applications in nuclear physics,
condensed matter physics, relativistic quantum mechanics and quantum field
theory, quantum cosmology, electromagnetic wave propagation, open quantum
systems, magnetohydrodynamics, quantum chaos, and biophysics.Comment: 76 pages, 2 figures, 243 references, published as Int. J. Geom. Meth.
Mod. Phys. 7, 1191-1306 (2010
PT-symmetric deformations of Calogero models
We demonstrate that Coxeter groups allow for complex PT-symmetric deformations across the boundaries of all Weyl chambers. We compute the explicit deformations for the A2 and G2-Coxeter group and apply these constructions to Calogero–Moser–Sutherland models invariant under the extended Coxeter groups. The eigenspectra for the deformed models are real and contain the spectra of the undeformed case as subsystems
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