462 research outputs found
Anharmonicity of flux lattices and thermal fluctuations in layered superconductors
We study elasticity of a perpendicular flux lattice in a layered
superconductor with Josephson coupling between layers. We find that the energy
contains ln(flux displacement) terms, so that elastic constants cannot be
strictly defined. Instead we define effective elastic constants by a thermal
average. The tilt modulus has terms with ln(T) which for weak fields, i.e.
Josephson length smaller than the flux line spacing, lead to displacement
square average proportional to T/ln(T). The expansion parameter indicates that
the dominant low temperature phase transition is either layer decoupling at
high fields or melting at low fields.Comment: 15 pages, 2 eps figures, Revtex, submitted to Phys. Rev. B.
Sunj-class: superconductivit
Second magnetization peak in flux lattices: the decoupling scenario
The second peak phenomena of flux lattices in layered superconductors is
described in terms of a disorder induced layer decoupling transition. For weak
disorder the tilt mudulus undergoes an apparent discontinuity which leads to an
enhanced critical current and reduced domain size in the decoupled phase. The
Josephson plasma frequency is reduced by decoupling and by Josephson glass
pinning; in the liquid phase it varies as 1/[BT(T+T_0)] where T is temperature,
B is field and T_0 is the disorder dependent temperature of the multicritical
point.Comment: 5 pages, 1 eps figure, Revtex. Minor changes, new reference
Sliding Phases in XY-Models, Crystals, and Cationic Lipid-DNA Complexes
We predict the existence of a totally new class of phases in weakly coupled,
three-dimensional stacks of two-dimensional (2D) XY-models. These ``sliding
phases'' behave essentially like decoupled, independent 2D XY-models with
precisely zero free energy cost associated with rotating spins in one layer
relative to those in neighboring layers. As a result, the two-point spin
correlation function decays algebraically with in-plane separation. Our
results, which contradict past studies because we include higher-gradient
couplings between layers, also apply to crystals and may explain recently
observed behavior in cationic lipid-DNA complexes.Comment: 4 pages of double column text in REVTEX format and 1 postscript
figur
W(h)ither Fossils? Studying Morphological Character Evolution in the Age of Molecular Sequences
A major challenge in the post-genomics era will be to integrate molecular sequence data from extant organisms with morphological data from fossil and extant taxa into a single, coherent picture of phylogenetic relationships; only then will these phylogenetic hypotheses be effectively applied to the study of morphological character evolution. At least two analytical approaches to solving this problem have been utilized: (1) simultaneous analysis of molecular sequence and morphological data with fossil taxa included as terminals in the analysis, and (2) the molecular scaffold approach, in which morphological data are analyzed over a molecular backbone (with constraints that force extant taxa into positions suggested by sequence data). The perceived obstacles to including fossil taxa directly in simultaneous analyses of morphological and molecular sequence data with extant taxa include: (1) that fossil taxa are missing the molecular sequence portion of the character data; (2) that morphological characters might be misleading due to convergence; and (3) character weighting, specifically how and whether to weight characters in the morphological partition relative to characters in the molecular sequence data partition. The molecular scaffold has been put forward as a potential solution to at least some of these problems. Using examples of simultaneous analyses from the literature, as well as new analyses of previously published morphological and molecular sequence data matrices for extant and fossil Chiroptera (bats), we argue that the simultaneous analysis approach is superior to the molecular scaffold approach, specifically addressing the problems to which the molecular scaffold has been suggested as a solution. Finally, the application of phylogenetic hypotheses including fossil taxa (whatever their derivation) to the study of morphological character evolution is discussed, with special emphasis on scenarios in which fossil taxa are likely to be most enlightening: (1) in determining the sequence of character evolution; (2) in determining the timing of character evolution; and (3) in making inferences about the presence or absence of characteristics in fossil taxa that may not be directly observable in the fossil record.
Published By: Missouri Botanical Garde
Correlation Effect on Peierls Transition
The effect of correlation on Peierls transition, which is accompanied by a
dimerization, t_d, of a bond alternation for transfer energy, has been examined
for a half-filled one-dimensional electron system with on-site repulsive
interaction (U). By applying the renormalization group method to the
interaction of the bosonized Hamiltonian, the dimerization has been calculated
variationally and self-consistently with a fixed electron-phonon coupling
constant (\lambda) and it is shown that t_d takes a maximum as a function of U.
The result is examined in terms of charge gap and spin gap and is compared with
that of the numerical simulation by Hirsch [Phys. Rev. Lett 51 (1983) 296].
Relevance to the spin Peierls transition in organic conductors is discussed.Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Jpn. 71 No.3
(2002
Thermal metal in network models of a disordered two-dimensional superconductor
We study the universality class for localization which arises from models of
non-interacting quasiparticles in disordered superconductors that have neither
time-reversal nor spin-rotation symmetries. Two-dimensional systems in this
category, which is known as class D, can display phases with three different
types of quasiparticle dynamics: metallic, localized, or with a quantized
(thermal) Hall conductance. Correspondingly, they can show a variety of
delocalization transitions. We illustrate this behavior by investigating
numerically the phase diagrams of network models with the appropriate symmetry,
and for the first time show the appearance of the metallic phase.Comment: 5 pages, 3 figure
Abrupt Change of Josephson Plasma Frequency at the Phase Boundary of the Bragg Glass in Bi_2Sr_2CaCu_2O_{8+\delta}
We report the first detailed and quantitative study of the Josephson coupling
energy in the vortex liquid, Bragg glass and vortex glass phases of
Bi_2Sr_2CaCu_2O_{8+\delta} by the Josephson plasma resonance. The measurements
revealed distinct features in the T- and H-dependencies of the plasma frequency
for each of these three vortex phases. When going across either
the Bragg-to-vortex glass or the Bragg-to-liquid transition line,
shows a dramatic change. We provide a quantitative discussion on the properties
of these phase transitions, including the first order nature of the
Bragg-to-vortex glass transition.Comment: 5pages, 4figure
Modulated Phases in Spin-Peierls Systems
Lattice modulations in the high magnetic field phase and close to impurities
in spin-Peierls systems are considered and compared to experiment. Necessary
extensions of existing theories are proposed. The influence of zero-point
fluctuations on magnetic amplitudes is shown.Comment: 10 pages, 4 figures included, to appear in Advances in Solid State
Physics/Festkoerperprobleme Spring Conference 1999 of the DP
Freezing transitions and the density of states of 2D random Dirac Hamiltonians
Using an exact mapping to disordered Coulomb gases, we introduce a novel
method to study two dimensional Dirac fermions with quenched disorder in two
dimensions which allows to treat non perturbative freezing phenomena. For
purely random gauge disorder it is known that the exact zero energy eigenstate
exhibits a freezing-like transition at a threshold value of disorder
. Here we compute the dynamical exponent which
characterizes the critical behaviour of the density of states around zero
energy, and find that it also exhibits a phase transition. Specifically, we
find that (and ) with for and
for . For a finite system size we find large
sample to sample fluctuations with a typical .
Adding a scalar random potential of small variance , as in the
corresponding quantum Hall system, yields a finite noncritical whose scaling exponent exhibits two transitions, one
at and the other at . These transitions are shown
to be related to the one of a directed polymer on a Cayley tree with random
signs (or complex) Boltzmann weights. Some observations are made for the strong
disorder regime relevant to describe transport in the quantum Hall system
Temperature-doping phase diagram of layered superconductors
The superconducting properties of a layered system are analyzed for the cases
of zero- and non-zero angular momentum of the pairs. The effective
thermodynamic potential for the quasi-2D XY-model for the gradients of the
phase of the order parameter is derived from the microscopic superconducting
Hamiltonian. The dependence of the superconducting critical temperature T_c on
doping, or carrier density, is studied at different values of coupling and
inter-layer hopping. It is shown that the critical temperature T_c of the
layered system can be lower than the critical temperature of the
two-dimensional Berezinskii-Kosterlitz-Thouless transition T_BKT at some values
of the model parameters, contrary to the case when the parameters of the
XY-model do not depend on the microscopic Hamiltonian parameters.Comment: To be published in Phys. Rev.
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