28 research outputs found
A continuous isotropic-nematic liquid crystalline transition of F-actin solutions
The phase transition from the isotropic (I) to nematic (N) liquid crystalline
suspension of F-actin of average length m or above was studied by local
measurements of optical birefringence and protein concentration. Both
parameters were detected to be continuous in the transition region, suggesting
that the I-N transition is higher than 1st order. This finding is consistent
with a recent theory by Lammert, Rokhsar & Toner (PRL, 1993, 70:1650),
predicting that the I-N transition may become continuous due to suppression of
disclinations. Indeed, few line defects occur in the aligned phase of F-actin.
Individual filaments in solutions of a few mg/ml F-actin undergo fast
translational diffusion along the filament axis, whereas both lateral and
rotational diffusions are suppressed.Comment: 4 pages with 4 figures. Submitted to Physical Review Letter
Superconductivity in hole-doped C60 from electronic correlations
We derive a model for the highest occupied molecular orbital band of a C60
crystal which includes on-site electron-electron interactions. The form of the
interactions are based on the icosahedral symmetry of the C60 molecule together
with a perturbative treatment of an isolated C60 molecule. Using this model we
do a mean-field calculation in two dimensions on the [100] surface of the
crystal. Due to the multi-band nature we find that electron-electron
interactions can have a profound effect on the density of states as a function
of doping. The doping dependence of the transition temperature can then be
qualitatively different from that expected from simple BCS theory based on the
density of states from band structure calculations
The Phonon Drag Effect in Single-Walled Carbon Nanotubes
A variational solution of the coupled electron-phonon Boltzmann equations is
used to calculate the phonon drag contribution to the thermopower in a 1-D
system. A simple formula is derived for the temperature dependence of the
phonon drag in metallic, single-walled carbon nanotubes. Scattering between
different electronic bands yields nonzero values for the phonon drag as the
Fermi level varies.Comment: 8 pages, 4 figure
Electronic Structure of Disclinated Graphene in an Uniform Magnetic Field
The electronic structure in the vicinity of the 1-heptagonal and 1-pentagonal
defects in the carbon graphene plane is investigated. Using a continuum gauge
field-theory model the local density of states around the Fermi energy is
calculated for both cases. In this model, the disclination is represented by an
SO(2) gauge vortex and corresponding metric follows from the elasticity
properties of the graphene membrane. To enhance the interval of energies, a
self-consistent perturbation scheme is used. The Landau states are investigated
and compared with the predicted values.Comment: keywords: graphene, heptagonal defect, elasticity, carbon nanohorns,
13 page
Supersymmetry in carbon nanotubes in a transverse magnetic field
Electron properties of Carbon nanotubes in a transverse magnetic field are
studied using a model of a massless Dirac particle on a cylinder. The problem
possesses supersymmetry which protects low energy states and ensures stability
of the metallic behavior in arbitrarily large fields. In metallic tubes we find
suppression of the Fermi velocity at half-filling and enhancement of the
density of states. In semiconducting tubes the energy gap is suppressed. These
features qualitatively persist (although to a smaller degree) in the presence
of electron interactions. The possibilities of experimental observation of
these effects are discussed.Comment: A new section on electron interaction effects added and explanation
on roles of supersymmetry expanded. Revtex4, 6 EPS figure file
Fractionalization patterns in strongly correlated electron systems: Spin-charge separation and beyond
We discuss possible patterns of electron fractionalization in strongly
interacting electron systems. A popular possibility is one in which the charge
of the electron has been liberated from its Fermi statistics. Such a
fractionalized phase contains in it the seed of superconductivity. Another
possibility occurs when the spin of the electron, rather than its charge, is
liberated from its Fermi statistics. Such a phase contains in it the seed of
magnetism, rather than superconductivity. We consider models in which both of
these phases occur and study possible phase transitions between them. We
describe other fractionalized phases, distinct from these, in which fractions
of the electron themselves fractionalize, and discuss the topological
characterization of such phases. These ideas are illustrated with specific
models of p-wave superconductors, Kondo lattices, and coexistence between
d-wave superconductivity and antiferromagnetism.Comment: 28 pages, 11 fig
Nonperturbative renormalization group approach to frustrated magnets
This article is devoted to the study of the critical properties of classical
XY and Heisenberg frustrated magnets in three dimensions. We first analyze the
experimental and numerical situations. We show that the unusual behaviors
encountered in these systems, typically nonuniversal scaling, are hardly
compatible with the hypothesis of a second order phase transition. We then
review the various perturbative and early nonperturbative approaches used to
investigate these systems. We argue that none of them provides a completely
satisfactory description of the three-dimensional critical behavior. We then
recall the principles of the nonperturbative approach - the effective average
action method - that we have used to investigate the physics of frustrated
magnets. First, we recall the treatment of the unfrustrated - O(N) - case with
this method. This allows to introduce its technical aspects. Then, we show how
this method unables to clarify most of the problems encountered in the previous
theoretical descriptions of frustrated magnets. Firstly, we get an explanation
of the long-standing mismatch between different perturbative approaches which
consists in a nonperturbative mechanism of annihilation of fixed points between
two and three dimensions. Secondly, we get a coherent picture of the physics of
frustrated magnets in qualitative and (semi-) quantitative agreement with the
numerical and experimental results. The central feature that emerges from our
approach is the existence of scaling behaviors without fixed or pseudo-fixed
point and that relies on a slowing-down of the renormalization group flow in a
whole region in the coupling constants space. This phenomenon allows to explain
the occurence of generic weak first order behaviors and to understand the
absence of universality in the critical behavior of frustrated magnets.Comment: 58 pages, 15 PS figure
Competing orders in a magnetic field: spin and charge order in the cuprate superconductors
We describe two-dimensional quantum spin fluctuations in a superconducting
Abrikosov flux lattice induced by a magnetic field applied to a doped Mott
insulator. Complete numerical solutions of a self-consistent large N theory
provide detailed information on the phase diagram and on the spatial structure
of the dynamic spin spectrum. Our results apply to phases with and without
long-range spin density wave order and to the magnetic quantum critical point
separating these phases. We discuss the relationship of our results to a number
of recent neutron scattering measurements on the cuprate superconductors in the
presence of an applied field. We compute the pinning of static charge order by
the vortex cores in the `spin gap' phase where the spin order remains
dynamically fluctuating, and argue that these results apply to recent scanning
tunnelling microscopy (STM) measurements. We show that with a single typical
set of values for the coupling constants, our model describes the field
dependence of the elastic neutron scattering intensities, the absence of
satellite Bragg peaks associated with the vortex lattice in existing neutron
scattering observations, and the spatial extent of charge order in STM
observations. We mention implications of our theory for NMR experiments. We
also present a theoretical discussion of more exotic states that can be built
out of the spin and charge order parameters, including spin nematics and phases
with `exciton fractionalization'.Comment: 36 pages, 33 figures; for a popular introduction, see
http://onsager.physics.yale.edu/superflow.html; (v2) Added reference to new
work of Chen and Ting; (v3) reorganized presentation for improved clarity,
and added new appendix on microscopic origin; (v4) final published version
with minor change
The degenerate 3-band Hubbard model with "anti-Hund's rule" interactions; a model for AxC60
We consider the orbitally degenerate 3-band Hubbard model with on-site
interactions which favor low spin and low orbital angular momentum using
standard second order perturbation theory in the large Hubbard-U limit. At even
integer filling this model is a Mott insulator with a non-degenerate ground
state that allows for a simple description of particle-hole excitations as well
as gapped spin and orbital modes. We find that the Mott gap is generally
indirect and that the single particle spectrum at low doping reappears close to
even filling but rescaled by a factor 2/3 or 1/3. The model captures the basic
phenomenology of the Mott insulating and metallic fullerides AxC60. This
includes the existence of a smaller spin gap and larger charge gap at even
integer filling, the fact that odd integer stoichiometries are generally
metallic while even are insulating, as well as the rapid suppression of the
density of states and superconducting transition temperatures with doping away
from x=3.Comment: Revised with additional reference