2,324 research outputs found
Quantum Spin Glasses
Ising spin glasses in a transverse field exhibit a zero temperature quantum
phase transition, which is driven by quantum rather than thermal fluctuations.
They constitute a universality class that is significantly different from the
classical, thermal phase transitions. Most interestingly close to the
transition in finite dimensions a quantum Griffiths phase leads to drastic
consequences for various physical quantities: for instance diverging magnetic
susceptibilities are observable over a whole range of transverse field values
in the disordered phase.Comment: 10 pages LaTeX (Springer Lecture Notes style file included), 1
eps-figure; Review article for XIV Sitges Conference: Complex Behavior of
Glassy System
Quantum Annealing and Analog Quantum Computation
We review here the recent success in quantum annealing, i.e., optimization of
the cost or energy functions of complex systems utilizing quantum fluctuations.
The concept is introduced in successive steps through the studies of mapping of
such computationally hard problems to the classical spin glass problems. The
quantum spin glass problems arise with the introduction of quantum
fluctuations, and the annealing behavior of the systems as these fluctuations
are reduced slowly to zero. This provides a general framework for realizing
analog quantum computation.Comment: 22 pages, 7 figs (color online); new References Added. Reviews of
Modern Physics (in press
Theory of paramagnetic scattering in highly frustrated magnets with long-range dipole-dipole interactions: The case of the Tb2Ti2O7, pyrochlore antiferromagnet
Highly frustrated antiferromagnets composed of magnetic rare-earth moments
are currently attracting much experimental and theoretical interest. Rare-earth
ions generally have small exchange interactions and large magnetic moments.
This makes it necessary to understand in detail the role of long-range magnetic
dipole-dipole interactions in these systems, in particular in the context of
spin-spin correlations that develop in the paramagnetic phase, but are often
unable to condense into a conventional long-range magnetic ordered phase. This
scenario is most dramatically emphasized in the frustrated pyrochlore
antiferromagnet material Tb2Ti207 which does not order down to 50 mK despite an
antiferromagnetic Curie-Weiss temperature Tcw ~ -20 K. In this paper we report
results from mean-field theory calculations of the paramagnetic elastic
neutron-scattering in highly frustrated magnetic systems with long-range
dipole-dipole interactions, focusing on the Tb2Ti207 system. Modeling Tb2Ti207
as an antiferromagnetic Ising pyrochlore, we find that the mean-field
paramagnetic scattering is inconsistent with the experimentally observed
results. Through simple symmetry arguments we demonstrate that the observed
paramagnetic correlations in Tb2Ti207 are precluded from being generated by any
spin Hamiltonian that considers only Ising spins, but are qualitatively
consistent with Heisenberg-like moments. Explicit calculations of the
paramagnetic scattering pattern for both Ising and Heisenberg models,
which include finite single-ion anisotropy, support these claims. We offer
suggestions for reconciling the need to restore spin isotropy with the Ising
like structure suggested by the single-ion properties of Tb3+.Comment: Revtex4, 18 pages, 3 eps figures (2 color figures). Change in title
and emphasis on Tb2Ti2O7 only. Spin-ice material removed, to appear in a
later publicatio
Quantum phase transition in spin glasses with multi-spin interactions
We examine the phase diagram of the -interaction spin glass model in a
transverse field. We consider a spherical version of the model and compare with
results obtained in the Ising case. The analysis of the spherical model, with
and without quantization, reveals a phase diagram very similar to that obtained
in the Ising case. In particular, using the static approximation, reentrance is
observed at low temperatures in both the quantum spherical and Ising models.
This is an artifact of the approximation and disappears when the imaginary time
dependence of the order parameter is taken into account. The resulting phase
diagram is checked by accurate numerical investigation of the phase boundaries.Comment: 20 Pages including 10 figures, Revte
Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors
We discuss the analysis of mixed-state magnetization data of type-II
superconductors using a recently developed scaling procedure. It is based on
the fact that, if the Ginzburg-Landau parameter kappa does not depend on
temperature, the magnetic susceptibility is a universal function of H/H_c2(T),
leading to a simple relation between magnetizations at different temperatures.
Although this scaling procedure does not provide absolute values of the upper
critical fieldH_c2(T), its temperature variation can be established rather
accurately. This provides an opportunity to validate theoretical models that
are usually employed for the evaluation of H_c2(T) from equilibrium
magnetization data. In the second part of the paper we apply this scaling
procedure for a discussion of the notorious first order phase transition in the
mixed state of high temperature superconductors. Our analysis, based on
experimental magnetization data available in the literature, shows that the
shift of the magnetization accross the transition may adopt either sign,
depending on the particular chosen sample. We argue that this observation is
inconsistent with the interpretation that this transition always represents the
melting transition of the vortex lattice.Comment: 18 pages, 12 figure
Thermodynamics of the vortex liquid in heavy ion-irradiated superconductors
It is shown that the large effect of heavy ion-irradiation on the
thermodynamical properties of the anisotropic superconductor
YBaCuO extends well into the superconducting
fluctuation regime. The presence of the induced amorphous columnar defects
shifts the specific heat maximum at the normal-to-superconducting transition.
This effect is similar to that recently put into evidence in cubic
KBaBiO (). In both compounds, vortex pinning
manifests itself as a sharp angular dependence of the \em equilibrium \rm
torque. In YBaCuO, pinning by the defects appears at
the temperature of the specific heat maximum, well above the
magnetic irreversibility line . In isotropic
KBaBiO, the onset of the pinning-related torque anomaly
tracks the onset of the specific heat anomaly and the irreversibility line. In
YBaCuO, fluctuations of the amplitude of the order
parameter (and not vortex line wandering) are ultimately responsible for the
vanishing of pinning. In KBaBiO, vortex pinning disappears
only at the superconducting-to-normal transition. The results indicate that in
both compounds, the pinning energy at the ``Bose glass'' transition is large
with respect to the total free energy gain in the superconducting state. By
implication, the mechanism of this latter transition should be reconsidered.Comment: 9 pages, 9 figures, resubmitted to PRB 23-09-200
Qualitative research practices and family business scholarship:a review and future research agenda
In spite of various calls for a wider application of qualitative research in the family business field, it is our contention that the full potential of qualitative inquiry is not being fully realized. Part of the reason for this relates to the tendency to promote methods choice and diversity rather than addressing the foundational questions and processes which underlie qualitative research choices. These tendencies obscure attention to the reasons why researchers choose qualitative methods and the kinds of foundational issues about family businesses that are brought to light through qualitative research. To address this, we undertake an analysis of the most-cited articles using qualitative methods from an annotated bibliography of family business studies. From this, we identify the strengths and weaknesses of extant qualitative studies in family business research and argue for the need to re-orientate calls in family business research towards the foundational questions (rather than methods) that underline qualitative inquiry
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