858 research outputs found
Gauge Theory for Quantum Spin Glasses
The gauge theory for random spin systems is extended to quantum spin glasses
to derive a number of exact and/or rigorous results. The transverse Ising model
and the quantum gauge glass are shown to be gauge invariant. For these models,
an identity is proved that the expectation value of the gauge invariant
operator in the ferromagnetic limit is equal to the one in the classical
equilibrium state on the Nishimori line. As a result, a set of inequalities for
the correlation function are proved, which restrict the location of the ordered
phase. It is also proved that there is no long-range order in the
two-dimensional quantum gauge glass in the ground state. The phase diagram for
the quantum XY Mattis model is determined.Comment: 15 pages, 2 figure
Simulation Studies on the Stability of the Vortex-Glass Order
The stability of the three-dimensional vortex-glass order in random type-II
superconductors with point disorder is investigated by equilibrium Monte Carlo
simulations based on a lattice XY model with a uniform field threading the
system. It is found that the vortex-glass order, which stably exists in the
absence of screening, is destroyed by the screenng effect, corroborating the
previous finding based on the spatially isotropic gauge-glass model. Estimated
critical exponents, however, deviate considerably from the values reported for
the gauge-glass model.Comment: Minor modifications made, a few referenced added; to appear in J.
Phys. Soc. Jpn. Vol.69 No.1 (2000
Anomalous Spin Dynamics observed by High Frequency ESR in Honeycomb Lattice Antiferromagnet InCu2/3V1/3O3
High-frequency ESR results on the S=1/2 Heisenberg hexagonal antiferromagnet
InCu2/3V1/3O3 are reported. This compound appears to be a rare model substance
for the honeycomb lattice antiferromagnet with very weak interlayer couplings.
The high-temperature magnetic susceptibility can be interpreted by the S=1/2
honeycomb lattice antiferromagnet, and it shows a magnetic-order-like anomaly
at TN=38 K. Although, the resonance field of our high-frequency ESR shows the
typical behavior of the antiferromagnetic resonance, the linewidth of our
high-frequency ESR continues to increase below TN, while it tends to decrease
as the temperature in a conventional three-dimensional antiferromagnet
decreases. In general, a honeycomb lattice antiferromagnet is expected to show
a simple antiferromagnetic order similar to that of a square lattice
antiferromagnet theoretically because both antiferromagnets are bipartite
lattices. However, we suggest that the observed anomalous spin dynamics below
TN is the peculiar feature of the honeycomb lattice antiferromagnet that is not
observed in the square lattice antiferromagnet.Comment: 5 pages, 5 figure
Nature of the vortex-glass order in strongly type-II superconductors
The stability and the critical properties of the three-dimensional
vortex-glass order in random type-II superconductors with point disorder is
investigated in the unscreened limit based on a lattice {\it XY} model with a
uniform field. By performing equilibrium Monte Carlo simulations for the system
with periodic boundary conditions, the existence of a stable vortex-glass order
is established in the unscreened limit. Estimated critical exponents are
compared with those of the gauge-glass model.Comment: Error in the reported value of the exponent eta is correcte
Fluctuation Dissipation Ratio in Three-Dimensional Spin Glasses
We present an analysis of the data on aging in the three-dimensional Edwards
Anderson spin glass model with nearest neighbor interactions, which is well
suited for the comparison with a recently developed dynamical mean field
theory. We measure the parameter describing the violation of the
relation among correlation and response function implied by the fluctuation
dissipation theorem.Comment: LaTeX 10 pages + 4 figures (appended as uuencoded compressed
tar-file), THP81-9
Current--Voltage Characteristics of Two--Dimensional Vortex Glass Models
We have performed Monte Carlo simulations to determine current--voltage
characteristics of two different vortex glass models in two dimensions. The
results confirm the conclusions of earlier studies that there is a transition
at . In addition we find that, as , the linear resistance vanishes
exponentially, and the current scale, , where non-linearities appear in
the -- characteristics varies roughly as , quite different from the
predictions of conventional flux creep theory, . The results for
the two models agree quite well with each other, and also agree fairly well
with recent experiments on very thin films of YBCO.Comment: 18 pages with 10 figures available upon request from R. A. Hyman at
[email protected]. The only change in the new version is the
deletion of an unimportant comment.IUCM94-01
Ground-State Phase Diagram of the Two-Dimensional Quantum Heisenberg Mattis Model
The two-dimensional asymmetric Heisenberg Mattis model is
investigated with the exact diagonalization of finite clusters. The N\'eel
order parameter and the spin glass order parameter can be smoothly extrapolated
to the thermodynamic limit in the antiferromagnetic region, as in the pure
Heisenberg antiferromagnet. The critical concentration of the N\'eel phase is
consistent with that of the two-dimensional Ising Mattis model, and the spin
glass order parameter increases monotonously as the ferro-bond concentration
increases. These facts suggest that quantum fluctuation does not play an
essential role in two-dimensional non-frustrated random spin systems.
KEYWORDS: quantum spin system, ground state, randomness, Mattis model, N\'eel
order, spin glass orderComment: 10 pages, LaTeX, 6 compressed/uuencoded postscript figures, J. Phys.
Soc. Jpn. 65 (1996) No. 2 in pres
Organizational learning and emotion: constructing collective meaning in support of strategic themes
Missing in the organizational learning literature is an integrative framework that reflects the emotional as well as the cognitive dynamics involved. Here, we take a step in this direction by focusing in depth over time (five years) on a selected organization which manufactures electronic equipment for the office industry. Drawing on personal construct theory, we define organizational learning as the collective re-construal of meaning in the direction of strategically significant themes. We suggest that emotions arise as members reflect on progress or lack of progress in achieving organizational learning. Our evidence suggests that invalidation – where organizational learning fails to correspond with expectations – gives rise to anxiety and frustration, while validation – where organizational learning is aligned with or exceeds expectations – evokes comfort or excitement. Our work aims to capture the key emotions involved as organizational learning proceeds
Flux Pinning and Phase Transitions in Model High-Temperature Superconductors with Columnar Defects
We calculate the degree of flux pinning by defects in model high-temperature
superconductors (HTSC's). The HTSC is modeled as a three-dimensional network of
resistively-shunted Josephson junctions in an external magnetic field,
corresponding to a HTSC in the extreme Type-II limit. Disorder is introduced
either by randomizing the coupling between grains (Model A disorder) or by
removing grains (Model B disorder). Three types of defects are considered:
point disorder, random line disorder, and periodic line disorder; but the
emphasis is on random line disorder. Static and dynamic properties of the
models are determined by Monte Carlo simulations and by solution of the
analogous coupled overdamped Josephson equations in the presence of thermal
noise. Random line defects considerably raise the superconducting transition
temperature T, and increase the apparent critical current density
J, in comparison to the defect-free crystal. They are more effective
in these respects than a comparable volume density of point defects, in
agreement with the experiments of Civale {\it et al}. Periodic line defects
commensurate with the flux lattice are found to raise T even more than
do random line defects. Random line defects are most effective when their
density approximately equals the flux density. Near T, our static and
dynamic results appear consistent with the anisotropic Bose glass scaling
hypotheses of Nelson and Vinokur, but with possibly different critical indices:Comment: 10 pages, LaTeX(REVTeX v3.0, twocolumn), 11 figures (not included
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