846 research outputs found
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
Superconductivity in CoO Layers and the Resonating Valence Bond Mean Field Theory of the Triangular Lattice t-J model
Motivated by the recent discovery of superconductivity in two dimensional
CoO layers, we present some possibly useful results of the RVB mean field
theory applied to the triangular lattice. Away from half filling, the order
parameter is found to be complex, and yields a fully gapped quasiparticle
spectrum. The sign of the hopping plays a crucial role in the analysis, and we
find that superconductivity is as fragile for one sign as it is robust for the
other. NaCoOHO is argued to belong to the robust case, by
comparing the LDA Fermi surface with an effective tight binding model. The high
frequency Hall constant in this system is potentially interesting, since it is
pointed out to increase linearly with temperature without saturation for T
T.Comment: Published in Physical Review B, total 1 tex + 9 eps files. Erratum
added as separate tex file on November 7, 2003, a numerical factor corrected
in the erratum on Dec 3, 200
The Kelvin Formula for Thermopower
Thermoelectrics are important in physics, engineering, and material science
due to their useful applications and inherent theoretical difficulty,
especially in strongly correlated materials. Here we reexamine the framework
for calculating the thermopower, inspired by ideas of Lord Kelvin from 1854. We
find an approximate but concise expression, which we term as the Kelvin formula
for the the Seebeck coefficient. According to this formula, the Seebeck
coefficient is given as the particle number derivative of the entropy
, at constant volume and temperature ,
. This formula is shown to be competitive compared to other
approximations in various contexts including strongly correlated systems. We
finally connect to a recent thermopower calculation for non-Abelian fractional
quantum Hall states, where we point out that the Kelvin formula is exact.Comment: 6 pages, 2 figure
Uncertainty Principle Enhanced Pairing Correlations in Projected Fermi Systems Near Half Filling
We point out the curious phenomenon of order by projection in a class of
lattice Fermi systems near half filling. Enhanced pairing correlations of
extended s-wave Cooper pairs result from the process of projecting out s-wave
Cooper pairs, with negligible effect on the ground state energy. The Hubbard
model is a particularly nice example of the above phenomenon, which is revealed
with the use of rigorous inequalities including the Uncertainty Principle
Inequality. In addition, we present numerical evidence that at half filling, a
related but simplified model shows ODLRO of extended s-wave Cooper pairs.Comment: RevTex 11 pages + 1 ps figure. Date 19 September 1996, Ver.
What Does The Korringa Ratio Measure?
We present an analysis of the Korringa ratio in a dirty metal, emphasizing
the case where a Stoner enhancement of the uniform susceptibilty is present. We
find that the relaxation rates are significantly enhanced by disorder, and that
the inverse problem of determining the bare density of states from a study of
the change of the Knight shift and relaxation rates with some parameter, such
as pressure, has rather constrained solutions, with the disorder playing an
important role. Some preliminary applications to the case of chemical
substitution in the RbKC family of superconductors is
presented and some other relevant systems are mentioned.Comment: 849, Piscataway, New Jersey 08855 24 June 199
Ground state of the spin-1/2 Heisenberg antiferromagnet on an Archimedean 4-6-12 lattice
An investigation of the N\'eel Long Range Order (NLRO) in the ground state of
antiferromagnetic Heisenberg spin system on the two-dimensional, uniform,
bipartite lattice consisting of squares, hexagons and dodecagons is presented.
Basing on the analysis of the order parameter and the long-distance correlation
function the NLRO is shown to occur in this system. Exact diagonalization and
variational (Resonating Valence Bond) methods are applied.Comment: 4 pages, 6 figure
Quantum-Mechanical Position Operator and Localization in Extended Systems
We introduce a fundamental complex quantity, , which allows us to
discriminate between a conducting and non-conducting thermodynamic phase in
extended quantum systems. Its phase can be related to the expectation value of
the position operator, while its modulus provides an appropriate definition of
a localization length. The expressions are valid for {\it any} fractional
particle filling. As an illustration we use to characterize insulator
to ``superconducting'' and Mott transitions in one-dimensional lattice models
with infinite on-site Coulomb repulsion at quarter filling.Comment: 4 pages, REVTEX, 1 ps figure
Near-equivalence of the role of structural unpinning number, basicity and reciprocal average electronegativity in determining the conductivity of glasses
The chemical approach made to investigate the origin of fast ion conduction in Agl-based fast ion conducting (FIC) glasses has been extended to various ionically conducting systems containing Na+ ion. An index known as structural unpinning number (SUN), S, has been defined for this purpose based on the unscreened nuclear charge on the cation and the average electronegativity of all the anions. Variation of the log(conductivity), at a given temperature, as a function of structural unpinning number, optical basicity, λ, and the reciprocal average electronegativity of all the anions, l/χa, has been examined for a number of Na+-ion conducting glasses and a nearly identical variation has been noticed in all the cases. The equivalence of these chemical parameters as determinants of the conductivity behavior of glasses has thus been established and the origin of this equivalence has been discussed
Numerical renormalization group study of the correlation functions of the antiferromagnetic spin- Heisenberg chain
We use the density-matrix renormalization group technique developed by White
\cite{white} to calculate the spin correlation functions
for isotropic Heisenberg rings up to
sites. The correlation functions for large and are found to obey
the scaling relation
proposed by Kaplan et al. \cite{horsch} , which is used to determine
. The asymptotic correlation function and
the magnetic structure factor show logarithmic corrections
consistent with , where is related
to the cut-off dependent coupling constant , as
predicted by field theoretical treatments.Comment: Accepted in Phys. Rev. B. 4 pages of text in Latex + 5 figures in
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