8,069 research outputs found
Spin systems with dimerized ground states
In view of the numerous examples in the literature it is attempted to outline
a theory of Heisenberg spin systems possessing dimerized ground states (``DGS
systems") which comprises all known examples. Whereas classical DGS systems can
be completely characterized, it was only possible to provide necessary or
sufficient conditions for the quantum case. First, for all DGS systems the
interaction between the dimers must be balanced in a certain sense. Moreover,
one can identify four special classes of DGS systems: (i) Uniform pyramids,
(ii) systems close to isolated dimer systems, (iii) classical DGS systems, and
(iv), in the case of , systems of two dimers satisfying four
inequalities. Geometrically, the set of all DGS systems may be visualized as a
convex cone in the linear space of all exchange constants. Hence one can
generate new examples of DGS systems by positive linear combinations of
examples from the above four classes.Comment: With corrections of proposition 4 and other minor change
Assessing non-linear models for galaxy clustering I: unbiased growth forecasts from multipole expansion
We assess the performance of the Taruya, Nishimichi and Saito (TNS) model for
the halo redshift space power spectrum, focusing on utilising mildly non-linear
scales to constrain the growth rate of structure f. Using simulations with
volume and number density typical of forthcoming Stage IV galaxy surveys, we
determine ranges of validity for the model at redshifts z = 0.5 and z = 1. We
proceed to perform a Bayesian MCMC analysis utilising the monopole, quadrupole,
and hexadecapole spectra, followed by an exploratory Fisher matrix analysis. As
previously noted in other forecasts as well as in real data analyses, we find
that including the hexadecapole can significantly improve the constraints.
However, a restricted range of scales is required for the hexadecapole in order
for the growth parameter estimation to remain unbiased, limiting the
improvement. We consistently quantify these effects by employing the multipole
expansion formalism in both our Fisher and MCMC forecasts.Comment: 12 pages, 7 figures, 2 tables, accepted in OJ
Nonlocal feedback in ferromagnetic resonance
Ferromagnetic resonance in thin films is analyzed under the influence of
spatiotemporal feedback effects. The equation of motion for the magnetization
dynamics is nonlocal in both space and time and includes isotropic, anisotropic
and dipolar energy contributions as well as the conserved Gilbert- and the
non-conserved Bloch-damping. We derive an analytical expression for the
peak-to-peak linewidth. It consists of four separate parts originated by
Gilbert damping, Bloch-damping, a mixed Gilbert-Bloch component and a
contribution arising from retardation. In an intermediate frequency regime the
results are comparable with the commonly used Landau-Lifshitz-Gilbert theory
combined with two-magnon processes. Retardation effects together with Gilbert
damping lead to a linewidth the frequency dependence of which becomes strongly
nonlinear. The relevance and the applicability of our approach to ferromagnetic
resonance experiments is discussed.Comment: 22 pages, 9 figure
Entanglement in a molecular three-qubit system
We study the entanglement properties of a molecular three-qubit system
described by the Heisenberg spin Hamiltonian with anisotropic exchange
interactions and including an external magnetic field. The system exhibits
first order quantum phase transitions by tuning two parameters, and , of
the Hamiltonian to specific values. The three-qubit chain is open ended so that
there are two types of pairwise entanglement : nearest-neighbour (n.n.) and
next-nearest-neighbour (n.n.n.). We calculate the ground and thermal state
concurrences, quantifying pairwise entanglement, as a function of the
parameters , and the temperature . The entanglement threshold and gap
temperatures are also determined as a function of the anisotropy parameter .
The results obtained are of relevance in understanding the entanglement
features of the recently engineered molecular --
complex which serves as a three-qubit system at sufficiently low temperatures.Comment: 9 pages, 13 figures, revtex
Momentum-resolved electron-phonon interaction in lead determined by neutron resonance spin-echo spectroscopy
Neutron resonance spin-echo spectroscopy was used to monitor the temperature
evolution of the linewidths of transverse acoustic phonons in lead across the
superconducting transition temperature, , over an extended range of the
Brillouin zone. For phonons with energies below the superconducting energy gap,
a linewidth reduction of maximum amplitude eV was observed below
. The electron-phonon contribution to the phonon lifetime extracted from
these data is in satisfactory overall agreement with {\it ab-initio}
lattice-dynamical calculations, but significant deviations are found
Exact ground state and kink-like excitations of a two dimensional Heisenberg antiferromagnet
A rare example of a two dimensional Heisenberg model with an exact dimerized
ground state is presented. This model, which can be regarded as a variation on
the kagome lattice, has several features of interest: it has a highly (but not
macroscopically) degenerate ground state; it is closely related to spin chains
studied by earlier authors; in particular, it is probably the first genuinely
two-dimensional quantum system to exhibit domain-wall-like ``kink'' excitations
normally found only in one-dimensional systems. In some limits it decouples
into non-interacting chains, purely dynamically and not because of weakening of
interchain couplings: indeed, paradoxically, this happens in the limit of
strong coupling of the chains.Comment: 4 pages, revtex, 5 figures included via epsfi
Decision-support system for risk management of produced water in the offshore petroleum industry
A decision-support system for produced water management
(DISSPROWM) in offshore operations is being developed. The system
determines the risk and hazards to human and marine species from
non-carcinogenic and carcinogenic pollutants, including radionuclides present
in produced water. The DISSPROWM also evaluates the best available
treatment technology for treating the produced water whose properties are
in the database. The system consists of a Windows-based Graphical User
Interface (GUI) developed with Microsoft Visual Basic, which integrates a
SQL Server database, a risk assessment model and a dilution model for
produced water contaminants. The database contains most produced water
pollutants and their important properties that are required in dispersion and risk
assessment modelling. The database also contains current produced water
regulations and information on some of the selected existing treatment
technologies with typical cost data required in the decision-support system
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