1,458 research outputs found
Subcritical instabilities in a convective fluid layer under a quasi-1D heating
The study and characterization of the diversity of spatiotemporal patterns
generated when a rectangular layer of fluid is locally heated beneath its free
surface is presented. We focus on the instability of a stationary cellular
pattern of wave number which undergoes a globally subcritical transition
to traveling waves by parity-breaking symmetry. The experimental results show
how the emerging traveling mode () switches on a resonant triad
(, , ) within the cellular pattern yielding a ``mixed''
pattern. The nature of this transition is described quantitatively in terms of
the evolution of the fundamental modes by complex demodulation techniques. The
B\' enard-Marangoni convection accounts for the different dynamics depending on
the depth of the fluid layer and on the vertical temperature difference. The
existence of a hysteresis cycle has been evaluated quantitatively. When the
bifurcation to traveling waves is measured in the vicinity of the codimension-2
bifurcation point, we measure a decrease of the subcritical interval in which
the traveling mode becomes unstable. From the traveling wave state the system
under goes a {\it new} global secondary bifurcation to an alternating pattern
which doubles the wavelength () of the primary cellular pattern, this
result compares well with theoretical predictions [P. Coullet and G. Ioss, {\em
Phys. Rev. Lett.} {\bf 64}, 8 66 (1990)]. In this cascade of bifurcations
towards a defect dynamics, bistability due to the subcritical behavior of our
system is the reason for the coexistence of two different modulated patterns
connected by a front. These fronts are stationary for a finite interval of the
control parameters.Comment: 13 pages, 12 figure
MSW mediated neutrino decay and the solar neutrino problem
We investigate the solar neutrino problem assuming simultaneous presence of
MSW transitions in the sun and neutrino decay on the way from sun to earth. We
do a global -analysis of the data on total rates in Cl, Ga and
Superkamiokande (SK) experiments and the SK day-night spectrum data and
determine the changes in the allowed region in the \dm - \tan^2\theta plane
in presence of decay. We also discuss the implications for unstable neutrinos
in the SNO experiment.Comment: Final version to appear in Phys. Rev.
Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment
In this paper, we review theoretical and experimental research on rare region
effects at quantum phase transitions in disordered itinerant electron systems.
After summarizing a few basic concepts about phase transitions in the presence
of quenched randomness, we introduce the idea of rare regions and discuss their
importance. We then analyze in detail the different phenomena that can arise at
magnetic quantum phase transitions in disordered metals, including quantum
Griffiths singularities, smeared phase transitions, and cluster-glass
formation. For each scenario, we discuss the resulting phase diagram and
summarize the behavior of various observables. We then review several recent
experiments that provide examples of these rare region phenomena. We conclude
by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative
Ising chain fixed, references added, v3: final version as publishe
A multiconfigurational time-dependent Hartree-Fock method for excited electronic states. I. General formalism and application to open-shell states
The solution of the time-dependent Schrodinger equation for systems of interacting electrons is generally a prohibitive task, for which approximate methods are necessary. Popular approaches, such as the time-dependent Hartree-Fock (TDHF) approximation and time-dependent density functional theory (TDDFT), are essentially single-configurational schemes. TDHF is by construction incapable of fully accounting for the excited character of the electronic states involved in many physical processes of interest; TDDFT, although exact in principle, is limited by the currently available exchange-correlation functionals. On the other hand, multiconfigurational methods, such as the multiconfigurational time-dependent Hartree-Fock (MCTDHF) approach, provide an accurate description of the excited states and can be systematically improved. However, the computational cost becomes prohibitive as the number of degrees of freedom increases, and thus, at present, the MCTDHF method is only practical for few-electron systems. In this work, we propose an alternative approach which effectively establishes a compromise between efficiency and accuracy, by retaining the smallest possible number of configurations that catches the essential features of the electronic wavefunction. Based on a time-dependent variational principle, we derive the MCTDHF working equation for a multiconfigurational expansion with fixed coefficients and specialise to the case of general open-shell states, which are relevant for many physical processes of interest. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3600397
Analog of Rabi oscillations in resonant electron-ion systems
Quantum coherence between electron and ion dynamics, observed in organic semiconductors by means of ultrafast spectroscopy, is the object of recent theoretical and computational studies. To simulate this kind of quantum coherent dynamics, we have introduced in a previous article [L. Stella, M. Meister, A. J. Fisher, and A. P. Horsfield, J. Chem. Phys. 127, 214104 (2007)] an improved computational scheme based on Correlated Electron-Ion Dynamics (CEID). In this article, we provide a generalization of that scheme to model several ionic degrees of freedom and many-body electronic states. To illustrate the capability of this extended CEID, we study a model system which displays the electron-ion analog of the Rabi oscillations. Finally, we discuss convergence and scaling properties of the extended CEID along with its applicability to more realistic problems. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3589165
Non-Fermi liquid behavior and Griffiths phase in {\it f}-electron compounds
We study the interplay among disorder, RKKY and Kondo interactions in {\it
f}-electron alloys. We argue that the non-Fermi liquid behavior observed in
these systems is due to the existence of a Griffiths phase close to a quantum
critical point. The existence of this phase provides a unified picture of a
large class of materials. We also propose new experiments that can test these
ideas.Comment: 4 pages, 1 Figure. NEW version of the original manuscript. A single
framework for NFL behavior in different kinds of alloys is presented. Final
version finally allowed to appear on the glorious Physical Review Letter
Rare region effects at classical, quantum, and non-equilibrium phase transitions
Rare regions, i.e., rare large spatial disorder fluctuations, can
dramatically change the properties of a phase transition in a quenched
disordered system. In generic classical equilibrium systems, they lead to an
essential singularity, the so-called Griffiths singularity, of the free energy
in the vicinity of the phase transition. Stronger effects can be observed at
zero-temperature quantum phase transitions, at nonequilibrium phase
transitions, and in systems with correlated disorder. In some cases, rare
regions can actually completely destroy the sharp phase transition by smearing.
This topical review presents a unifying framework for rare region effects at
weakly disordered classical, quantum, and nonequilibrium phase transitions
based on the effective dimensionality of the rare regions. Explicit examples
include disordered classical Ising and Heisenberg models, insulating and
metallic random quantum magnets, and the disordered contact process.Comment: Topical review, 68 pages, 14 figures, final version as publishe
Odd Frequency Pairing in the Kondo Lattice
We discuss the possibility that heavy fermion superconductors involve
odd-frequency pairing of the kind first considered by Berezinskii. Using a toy
model for odd frequency triplet pairing in the Kondo lattice we are able to
examine key properties of this new type of paired state. To make progress
treating the strong constraint in the Kondo lattice model we use the
technical trick of a Majorana representation of the local moments, which
permits variational treatments of the model without a Gutzwiller approximation.
The simplest mean field theory involves the development of bound states between
the local moments and conduction electrons, characterized by a spinor order
parameter. We show that this state is a stable realization of odd frequency
triplet superconductivity with surfaces of gapless excitations whose spin and
charge coherence factors vanish linearly in the quasiparticle energy. A
NMR relaxation rate coexists with a linear specific heat. We discuss possible
extensions of our toy model to describe heavy fermion superconductivity.Comment: 67 page
Local fluctuations in quantum critical metals
We show that spatially local, yet low-energy, fluctuations can play an
essential role in the physics of strongly correlated electron systems tuned to
a quantum critical point. A detailed microscopic analysis of the Kondo lattice
model is carried out within an extended dynamical mean-field approach. The
correlation functions for the lattice model are calculated through a
self-consistent Bose-Fermi Kondo problem, in which a local moment is coupled
both to a fermionic bath and to a bosonic bath (a fluctuating magnetic field).
A renormalization-group treatment of this impurity problem--perturbative in
, where is an exponent characterizing the spectrum
of the bosonic bath--shows that competition between the two couplings can drive
the local-moment fluctuations critical. As a result, two distinct types of
quantum critical point emerge in the Kondo lattice, one being of the usual
spin-density-wave type, the other ``locally critical.'' Near the locally
critical point, the dynamical spin susceptibility exhibits scaling
with a fractional exponent. While the spin-density-wave critical point is
Gaussian, the locally critical point is an interacting fixed point at which
long-wavelength and spatially local critical modes coexist. A Ginzburg-Landau
description for the locally critical point is discussed. It is argued that
these results are robust, that local criticality provides a natural description
of the quantum critical behavior seen in a number of heavy-fermion metals, and
that this picture may also be relevant to other strongly correlated metals.Comment: 20 pages, 12 figures; typos in figure 3 and in the main text
corrected, version as publishe
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