9,956 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
Entangled spin clusters: some special features
In this paper, we study three specific aspects of entanglement in small spin
clusters. We first study the effect of inhomogeneous exchange coupling strength
on the entanglement properties of the S=1/2 antiferromagnetic linear chain
tetramer compound NaCuAsO_{4}. The entanglement gap temperature, T_{E}, is
found to have a non-monotonic dependence on the value of , the exchange
coupling inhomogeneity parameter. We next determine the variation of T_{E} as a
function of S for a spin dimer, a trimer and a tetrahedron. The temperature
T_{E} is found to increase as a function of S, but the scaled entanglement gap
temperature t_{E} goes to zero as S becomes large. Lastly, we study a spin-1
dimer compound to illustrate the quantum complementarity relation. We show that
in the experimentally realizable parameter region, magnetization and
entanglement plateaus appear simultaneously at low temperatures as a function
of the magnetic field. Also, the sharp increase in one quantity as a function
of the magnetic field is accompanied by a sharp decrease in the other so that
the quantum complementarity relation is not violated.Comment: 17 pages, 6 figures. Accepted in Phys. Rev.
Spanning Properties of Theta-Theta Graphs
We study the spanning properties of Theta-Theta graphs. Similar in spirit
with the Yao-Yao graphs, Theta-Theta graphs partition the space around each
vertex into a set of k cones, for some fixed integer k > 1, and select at most
one edge per cone. The difference is in the way edges are selected. Yao-Yao
graphs select an edge of minimum length, whereas Theta-Theta graphs select an
edge of minimum orthogonal projection onto the cone bisector. It has been
established that the Yao-Yao graphs with parameter k = 6k' have spanning ratio
11.67, for k' >= 6. In this paper we establish a first spanning ratio of
for Theta-Theta graphs, for the same values of . We also extend the class of
Theta-Theta spanners with parameter 6k', and establish a spanning ratio of
for k' >= 5. We surmise that these stronger results are mainly due to a
tighter analysis in this paper, rather than Theta-Theta being superior to
Yao-Yao as a spanner. We also show that the spanning ratio of Theta-Theta
graphs decreases to 4.64 as k' increases to 8. These are the first results on
the spanning properties of Theta-Theta graphs.Comment: 20 pages, 6 figures, 3 table
Robust coupling of superconducting order parameter in a mesoscale NbN-Fe-NbN epitaxial structure
We report an unconventional and promising route to self-assemble distributed
superconductor-ferromagnet-superconductor (S-F-S) Josephson Junctions on single
crystal [100] MgO. These structures consist of [110] epitaxial nano-plaquettes
of Fe covered with superconducting NbN films of varying thickness. The S-F-S
structures are characterized by strong magnetoresistance (MR) anisotropy for
the in-plane and out-of-plane magnetic fields. The stronger in-plane MR
suggests decoherence of S-F-S junctions whose critical current follows a
(1-T/Tc) and (1-T/Tc)1/2 dependence for T Tc and T<<Tc respectively, in
accordance with the theory of supercurrent transport in such junctions.Comment: 9 pages, 4 figure
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
Scaling of entanglement between separated blocks in spin chains at criticality
We compute the entanglement between separated blocks in certain spin models
showing that at criticality this entanglement is a function of the ratio of the
separation to the length of the blocks and can be written as a product of a
power law and an exponential decay. It thereby interpolates between the
entanglement of individual spins and blocks of spins. It captures features of
correlation functions at criticality as well as the monogamous nature of
entanglement. We exemplify invariant features of this entanglement to
microscopic changes within the same universality class. We find this
entanglement to be invariant with respect to simultaneous scale transformations
of the separation and the length of the blocks. As a corollary, this study
estimates the entanglement between separated regions of those quantum fields to
which the considered spin models map at criticality.Comment: 4 pages, 3 figures; comments welcom
Single hole doped strongly correlated ladder with a static impurity
We consider a strongly correlated ladder with diagonal hopping and exchange
interactions described by type hamiltonian. We study the dynamics of a
single hole in this model in the presence of a static non-magnetic (or
magnetic) impurity. In the case of a non-magnetic (NM) impurity we solve the
problem analytically both in the triplet (S=1) and singlet (S=0) sectors. In
the triplet sector the hole doesn't form any bound state with the impurity.
However, in the singlet sector the hole forms bound states of different
symmetries with increasing values. Binding energies of those
impurity-hole bound states are compared with the binding energy of a pair of
holes in absence of any impurity. In the case of magnetic impurity the
analytical eigenvalue equations are solved for a large (50 X 2) lattice. In
this case also, with increasing values, impurity-hole bound states of
different symmetries are obtained. Binding of the hole with the impurity is
favoured for the case of a ferromagnetic (FM) impurity than in the case of
antiferromagnetic (AFM) impurity. However binding energy is found to be maximum
for the NM impurity. Comparison of binding energies and various impurity-hole
correlation functions indicates a pair breaking mechanism by NM impurity.Comment: 15 Pages, 6 figure
The green potential of RFID projects: A case-based analysis
Findings from 13 case studies on RFID's use in green projects reveal its potential not only to enhance environmental sustainability but also to reduce costs and generate revenue by creating new commercial opportunities. © 2006 IEEE.published_or_final_versio
Predicting the survival or failure of click-and-mortar corporations
With the boom in e-business, several corporations have emerged in the late nineties that have primarily conducted their business through the Internet and the Web. They have come to be known as the dotcoms or click-and-mortar corporations. The success of these companies has been short lived and many of these companies have failed rapidly in a short span of 4-5 years. This research is an investigation of the burst of the dotcom bubble from a financial perspective. Data from the financial statements of several survived and failed dotcom companies is used to compute financial ratios, which are analyzed using two data mining techniques - discriminant analysis (DA) and neural networks (NN) to find out whether they can predict the financial fate of companies. Neural networks perform better than discriminant analysis in predicting survival or failure of click-and-mortar corporations. The key financial ratios that play a major role in the process of prediction are identified. © 2005 IEEE.published_or_final_versio
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