681 research outputs found
Fictitious fluxes in doped antiferromagnets
In a tight binding model of charged spin-1/2 electrons on a square lattice, a
fully polarized ferromagnetic spin configuration generates an apparent U(1)
flux given by times the skyrmion charge density of the ferromagnetic
order parameter. We show here that for an antiferromagnet, there are two
``fictitious'' magnetic fields, one staggered and one unstaggered. The
staggered topological flux per unit cell can be varied between
with a negligible change in the value of the effective
nearest neighbor coupling constant whereas the magnitude of the unstaggered
flux is strongly coupled to the magnitude of the second neighbor effective
coupling.Comment: RevTeX, 5 pages including 4 figure
Behaviour of the energy gap near a commensurate-incommensurate transition in double layer quantum Hall systems at nu=1
The charged excitations in the system of the title are vortex-antivortex
pairs in the spin-texture described in the theory by Yang et al which, in the
commensurate phase, are bound together by a ``string''. It is shown that their
excitation energy drops as the string lengthens as the parallel magnetic field
approaches the critical value, then goes up again in the incommensurate phase.
This produces a sharp downward cusp at the critical point. An alternative
description based on the role of disorder in the tunnelling and which appears
not to produce a minimum in the excitation energy is also discussed. It is
suggested that a similar transition could also occur in compressible
Fermi-liquid-like states.Comment: latex file, 17 page
Tunneling gap of laterally separated quantum Hall states
We use a method of matched asymptotics to determine the energy gap of two
counter-propagating, strongly interacting, quantum Hall edge states. The
microscopic edge state dispersion and Coulomb interactions are used to
precisely constrain the short-distance behavior of an integrable field theory,
which then determines the low energy spectrum. We discuss the relationship of
our results to the tunneling measurements of Kang et al., Nature 403, 59
(2000).Comment: 4 pages, 1 figur
Negative magnetoresistance in the nearest-neighbour hopping conduction in granular gold film
The low temperature (0.5-55 K) conduction of semicontinuous gold film vacuum
deposited at T \approx 50 K is studied. The film is near the percolation
threshold (thickness 3.25 nm). Its resistance is extremely sensitive to the
applied voltage U. At low enough U the film behaves as an insulator
(two-dimensional granular metal). In this state the dependences R(T) \propto
\exp (1/T) (for T \leq 20 K) and R(U) \propto \exp (1/U)) (for T \leq 1 K and U
> 0.1 V) are observed. Magnetoresistance (MR) is negative and can be described
by \Delta R(H)/R(0) \propto -H^2/T. This negative MR which manifests itself for
nearest-neighbour hopping is rather uncommon and, up to now, has not been
clarified. The possible mechanisms of such case of negative MR are discussed.Comment: 9 pages, LATEX, 6 figures. To be published in Physica B. Fig.4 is JPG
file, in case of troubles with it, appeal for help and advice to:
[email protected]
Invariant structure of the hierarchy theory of fractional quantum Hall states with spin
We describe the invariant structure common to abelian fractional quantum Hall
systems with spin. It appears in a generalization of the lattice description of
the polarized hierarchy that encompasses both partially polarized and
unpolarized ground state systems. We formulate, using the spin-charge
decomposition, conditions that should be satisfied so that the description is
SU(2) invariant. In the case of the spin- singlet hierarchy construction, we
find that there are as many SU(2) symmetries as there are levels in the
construction. We show the existence of a spin and charge lattice for the
systems with spin. The ``gluing'' of the charge and spin degrees of freedom in
their bulk is described by the gluing theory of lattices.Comment: 21 pages, LaTex, Submitted to Phys. Rev.
Ground state of graphite ribbons with zigzag edges
We study the interaction effects on the ground state of nanographite ribbons
with zigzag edges. Within the mean-field approximation, we found that there are
two possible phases: the superconducting (SC) phase and the excitonic insulator
(EI). The two phases are separated by a first-order transition point. After
taking into account the low-lying fluctuations around the mean-field solutions,
the SC phase becomes a spin liquid phase with one gapless charge mode.
On the other hand, all excitations in the EI phase, especially the spin
excitations, are gapped.Comment: 6 pages, 3 figure
Integer quantum Hall effect for hard-core bosons and a failure of bosonic Chern-Simons mean-field theories for electrons at half-filled Landau level
Field-theoretical methods have been shown to be useful in constructing simple
effective theories for two-dimensional (2D) systems. These effective theories
are usually studied by perturbing around a mean-field approximation, so the
question whether such an approximation is meaningful arises immediately. We
here study 2D interacting electrons in a half-filled Landau level mapped onto
interacting hard-core bosons in a magnetic field. We argue that an interacting
hard-core boson system in a uniform external field such that there is one flux
quantum per particle (unit filling) exhibits an integer quantum Hall effect. As
a consequence, the mean-field approximation for mapping electrons at
half-filling to a boson system at integer filling fails.Comment: 13 pages latex with revtex. To be published in Phys. Rev.
Bosonization Theory of Excitons in One-dimensional Narrow Gap Semiconductors
Excitons in one-dimensional narrow gap semiconductors of anti-crossing
quantum Hall edge states are investigated using a bosonization method. The
excitonic states are studied by mapping the problem into a non-integrable
sine-Gordon type model. We also find that many-body interactions lead to a
strong enhancement of the band gap. We have estimated when an exciton
instability may occur.Comment: 4pages, 1 figure, to appear in Phys. Rev. B Brief Report
Charge and Current in the Quantum Hall Matrix Model
We extend the quantum Hall matrix model to include couplings to external
electric and magnetic fields. The associated current suffers from matrix
ordering ambiguities even at the classical level. We calculate the linear
response at low momenta -- this is unambigously defined. In particular, we
obtain the correct fractional quantum Hall conductivity, and the expected
density modulations in response to a weak and slowly varying magnetic field.
These results show that the classical quantum Hall matrix models describe
important aspects of the dynamics of electrons in the lowest Landau level. In
the quantum theory the ordering ambiguities are more severe; we discuss
possible strategies, but we have not been able to construct a good density
operator, satisfying the pertinent lowest Landau level commutator algebra.Comment: 12 pages, no figures; a logical error below the proposed density
operator (46) in version 1 is corrected, and the claim that this density
operator satisfy the magnetic algebra (2) is withdrawn. Some formulations
have been changed and a few misprints correcte
The order of the metal to superconductor transition
We present results from large-scale Monte Carlo simulations on the full
Ginzburg-Landau (GL) model, including fluctuations in the amplitude and the
phase of the matter-field, as well as fluctuations of the non-compact
gauge-field of the theory. {}From this we obtain a precise critical value of
the GL parameter \kct separating a first order metal to superconductor
transition from a second order one, \kct = (0.76\pm 0.04)/\sqrt{2}. This
agrees surprisingly well with earlier analytical results based on a disorder
theory of the superconductor to metal transition, where the value
\kct=0.798/\sqrt{2} was obtained. To achieve this, we have done careful
infinite volume and continuum limit extrapolations. In addition we offer a
novel interpretation of \kct, namely that it is also the value separating
\typeI and \typeII behaviour.<Comment: Minor corrections, present version accepted for publication in PR
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