1,010 research outputs found
Universal features of Thermopower in High Tc systems and Quantum Criticality
In high Tc superconductors a wide ranging connection between the doping
dependence of the transition temperature Tc and the room temperature
thermopower Q has been observed. A "universal correlation" between these two
quantities exists with the thermopower vanishing at optimum doping as noted by
OCTHH (Obertelli, Cooper, Tallon, Honma and Hor). In this work we provide an
interpretation of this OCTHH universality in terms of a possible underlying
quantum critical point (QCP) at Tc. Central to our viewpoint is the recently
noted Kelvin formula relating the thermopower to the density derivative of the
entropy. Perspective on this formula is gained through a model calculation of
the various Kubo formulas in an exactly solved 1-dimensional model with various
limiting procedures of wave vector and frequency.Comment: 12 pages, 8 figure
Microscopic mechanism for the 1/8 magnetization plateau in SrCu_2(BO_3)_2
The frustrated quantum magnet SrCu_2(BO_3)_2 shows a remarkably rich phase
diagram in an external magnetic field including a sequence of magnetization
plateaux. The by far experimentally most studied and most prominent
magnetization plateau is the 1/8 plateau. Theoretically, one expects that this
material is well described by the Shastry-Sutherland model. But recent
microscopic calculations indicate that the 1/8 plateau is energetically not
favored. Here we report on a very simple microscopic mechanism which naturally
leads to a 1/8 plateau for realistic values of the magnetic exchange constants.
We show that the 1/8 plateau with a diamond unit cell benefits most compared to
other plateau structures from quantum fluctuations which to a large part are
induced by Dzyaloshinskii-Moriya interactions. Physically, such couplings
result in kinetic terms in an effective hardcore boson description leading to a
renormalization of the energy of the different plateaux structures which we
treat in this work on the mean-field level. The stability of the resulting
plateaux are discussed. Furthermore, our results indicate a series of stripe
structures above 1/8 and a stable magnetization plateau at 1/6. Most
qualitative aspects of our microscopic theory agree well with a recently
formulated phenomenological theory for the experimental data of SrCu_2(BO_3)_2.
Interestingly, our calculations point to a rather large ratio of the magnetic
couplings in the Shastry-Sutherland model such that non-perturbative effects
become essential for the understanding of the frustrated quantum magnet
SrCu_2(BO_3)_2.Comment: 24 pages, 24 figure
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
Solution of Some Integrable One-Dimensional Quantum Systems
In this paper, we investigate a family of one-dimensional multi-component
quantum many-body systems. The interaction is an exchange interaction based on
the familiar family of integrable systems which includes the inverse square
potential. We show these systems to be integrable, and exploit this
integrability to completely determine the spectrum including degeneracy, and
thus the thermodynamics. The periodic inverse square case is worked out
explicitly. Next, we show that in the limit of strong interaction the "spin"
degrees of freedom decouple. Taking this limit for our example, we obtain a
complete solution to a lattice system introduced recently by Shastry, and
Haldane; our solution reproduces the numerical results. Finally, we emphasize
the simple explanation for the high multiplicities found in this model
The Hall Number, Optical Sum Rule and Carrier Density for the -- model
We revisit the relationship between three classical measures of particle
number, namely the chemical doping , the Hall number and the
particle number inferred from the optical sum rule . We study the
-- model of correlations on a square lattice, as a minimal model for
High systems, using numerical methods to evaluate the low temperature
Kubo conductivites. These measures disagree significantly in this type of
system, owing to Mott Hubbard correlations. The Hall constant has a complex
behavior with several changes of sign as a function of filling , depending
upon the model parameters. Thus depends sensitively on and ,
due to a kind of quantum interference.Comment: Typos removed,9 Figures, (Revised Figure.3 contains comparison with
experiments
Spin-s wavefunctions with algebraic order
We generalize the Gutzwiller wavefunction for s = 1/2 spin chains to
construct a family of wavefunctions for all s > 1/2. Through numerical
simulations, we demonstrate that the spin spin correlation functions for all s
decay as a power law with logarithmic corrections. This is done by mapping the
model to a classical statistical mechanical model, which has coupled Ising spin
chains with long range interactions. The power law exponents are those of the
Wess Zumino Witten models with k = 2s. Thus these simple wavefunctions
reproduce the spin correlations of the family of Hamiltonians obtained by the
Algebraic Bethe Ansatz.Comment: 10 pages, 7 figure
Redundancy Elimination with Coverage Preserving algorithm in Wireless Sensor Network
In Wireless Sensor Network, the sensor nodes are deployed using random or deterministic deployment methods. Many applications prefer random deployment for deploying the sensor nodes. Random deployment is the main cause of redundancy. Detection and elimination of redundant sensor nodes while preserving coverage is very important issue after the sensor nodes are deployed randomly in the region of interest. The redundancy elimination with coverage preserving algorithm is proposed in this paper and the results are presented. The proposed algorithm determines redundant sensor nodes and also the sensor nodes which provide the least coverage of region of interest. If two sensor nodes cover same area or if the Euclidian distance between two nodes is less than 25% of sensing range of a sensor node, the sensor which is not located at optimal position will be deactivated, so that, it reduces the number of optimal nodes required to cover complete region of interest. This in turn increases the lifetime of the network. The simulation results illustrate that the proposed algorithm preserves 100% coverage or region of interest by removing redundant nodes and also the nodes which provide the least coverage of region of interest. It also reduces the number of optimal nodes required to provide 100% coverage of region of interest
Ising pyrochlore magnets: Low temperature properties, ice rules and beyond
Pyrochlore magnets are candidates for spin-ice behavior. We present
theoretical simulations of relevance for the pyrochlore family R2Ti2O7 (R= rare
earth) supported by magnetothermal measurements on selected systems. By
considering long ranged dipole-dipole as well as short-ranged superexchange
interactions we get three distinct behaviours: (i) an ordered doubly degenerate
state, (ii) a highly disordered state with a broad transition to paramagnetism,
(iii) a partially ordered state with a sharp transition to paramagnetism. Thus
these competing interactions can induce behaviour very different from
conventional ``spin ice''. Closely corresponding behaviour is seen in the real
compounds---in particular Ho2Ti2O7 corresponds to case (iii) which has not been
discussed before, rather than (ii) as suggested earlier.Comment: 5 pages revtex, 4 figures; some revisions, additional data,
additional co-authors and a changed title. Basic ideas of paper remain the
same but those who downloaded the original version are requested to get this
more complete versio
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