490 research outputs found
Transition from quantum Hall to compressible states in the second Landau level: new light on the =5/2 enigma
Quantum Hall states at filling fraction =5/2 are examined by numerical
diagonalization. Spin-polarized and -unpolarized states of systems with electrons are studied, neglecting effects of Landau level mixing. We find
that the ground state is spin polarized. It is incompressible and has a large
overlap with paired states like the Pfaffian. For a given sample, the energy
gap is about 11 times smaller than at =1/3. Evidence is presented of phase
transitions to compressible states, driven by the interaction strength at short
distance. A reinterpretation of experiments is suggested.Comment: This paper has already appeared in PRL, but has not been on the we
Excitation gaps in fractional quantum Hall states: An exact diagonalization study
We compute energy gaps for spin-polarized fractional quantum Hall states in
the lowest Landau level at filling fractions nu=1/3, 2/5,3/7 and 4/9 using
exact diagonalization of systems with up to 16 particles and extrapolation to
the infinite system-size limit. The gaps calculated for a pure Coulomb
interaction and ignoring finite width effects, disorder and LL mixing agree
with predictions of composite fermion theory provided the logarithmic
corrections to the effective mass are included. This is in contrast with
previous estimates, which, as we show, overestimated the gaps at nu=2/5 and 3/7
by around 15%. We also study the reduction of the gaps as a result of the
non-zero width of the 2D layer. We show that these effects are accurately
accounted for using either Gaussian or z*Gaussian' (zG) trial wavefunctions,
which we show are significantly better variational wavefunctions than the
Fang-Howard wavefunction. For quantum well parameters typical of
heterostructure samples, we find gap reductions of around 20%. The experimental
gaps, after accounting heuristically for disorder,are still around 40% smaller
than the computed gaps. However, for the case of tetracene layers
inmetal-insulator-semiconductor (MIS) devices we find that the measured
activation gaps are close to those we compute. We discuss possible reasons why
the difference between computed and measured activation gaps is larger in GaAs
heterostructures than in MIS devices. Finally, we present new calculations
using systems with up to 18 electrons of the gap at nu=5/2 including width
corrections.Comment: 18 pages, 17 figure
Second Generation of Composite Fermions in the Hamiltonian Theory
In the framework of a recently developed model of interacting composite
fermions restricted to a single level, we calculate the activation gaps of a
second generation of spin-polarized composite fermions. These composite
particles consist each of a composite fermion of the first generation and a
vortex-like excitation and may be responsible for the recently observed
fractional quantum Hall states at unusual filling factors such as
nu=4/11,5/13,5/17, and 6/17. Because the gaps of composite fermions of the
second generation are found to be more than one order of magnitude smaller than
those of the first generation, these states are less visible than the usual
states observed at filling factors nu=p/(2ps+1). Their stability is discussed
in the context of a pseudopotential expansion of the composite-fermion
interaction potential.Comment: 5 pages, 3 figures; after publication in PRB, we have realized that a
factor was missing in one of the expressions; the erroneous results are now
corrected; an erratum has been sent to PR
Quasi-Particle Tunneling in Anti-Pfaffian Quantum Hall State
We study tunneling phenomena at the edge of the anti-Pfaffian quantum Hall
state at the filling factor . The edge current in a single
point-contact is considered. We focus on nonlinear behavior of two-terminal
conductance with the increase in negative split-gate voltage. Expecting the
appearance of the intermediate conductance plateau we calculate the value of
its conductance by using the renormalization group (RG) analysis. Further, we
show that non-perturbative quasi-particle tunneling is effectively described as
perturbative electron tunneling by the instanton method. The two-terminals
conductance is written as a function of the gate voltage. The obtained results
enable us to distinguish the anti-Pfaffian state from the Pfaffian state
experimentally.Comment: 5 pages, 4 figure
The transcriptional response to encystation stimuli in Giardia lamblia is restricted to a small set of genes
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of American Society for Microbiology for personal use, not for redistribution. The definitive version was published in Eukaryotic Cell 9 (2010): 1566-1576, doi:10.1128/EC.00100-10.The protozoan parasite Giardia lamblia undergoes stage-differentiation in the small intestine of
the host to an environmentally resistant and infectious cyst. Encystation involves secretion of
an extracellular matrix comprised of cyst wall proteins (CWPs) and a β(1-3)-GalNAc
homopolymer. Upon induction of encystation, genes coding for CWPs are switched on, and
mRNAs coding for a transcription factor Myb and enzymes involved in cyst wall glycan synthesis
are upregulated. Encystation in vitro is triggered by several protocols, which call for changes in
bile concentrations or availability of lipids, and elevated pH. However, the conditions for
induction are not standardized and we predicted significant protocol-specific side effects. This
makes reliable identification of encystation factors difficult. Here, we exploited the possibility
to induce encystation with two different protocols, which we show to be equally effective, for a
comparative mRNA profile analysis. The standard encystation protocol induced a bipartite
transcriptional response with surprisingly minor involvement of stress genes. A comparative
analysis revealed a core set of only 18 encystation genes and showed that a majority of genes
was indeed upregulated as a side effect of inducing conditions. We also established a Myb
binding sequence as a signature motif in encystation promoters, suggesting coordinated
regulation of these factors.We acknowledge in
particular the “Stiftung zur Förderung der Wissenschaftlichen Forschung an der Universität
Zürich” for financial support for this project. C.S. was supported by the Roche and Novartis
Foundation, and “Stiftung für Forschungsförderung” University of Zurich. Research in the Hehl
laboratory is supported by the Swiss National Science Foundation (grant #31003A-125389)
Spin-Orbit Interaction Enhanced Fractional Quantum Hall States in the Second Landau Level
We study the fractional quantum Hall effect at filling fractions 7/3 and 5/2
in the presence of the spin-orbit interaction, using the exact diagonalization
method and the density matrix renormalization group (DMRG) method in a
spherical geometry. Trial wave functions at these fillings are the Laughlin
state and the Moore-Reed-Pfaffian state. The ground state excitation energy
gaps and pair-correlation functions at fractional filling factor 7/3 and 5/2 in
the second Landau level are calculated. We find that the spin-orbit interaction
stabilizes the fractional quantum Hall states.Comment: 4pages, 4figure
Magnetic phenomena at and near nu =1/2 and 1/4: theory, experiment and interpretation
I show that the hamiltonian theory of Composite Fermions (CF) is capable of
yielding a unified description in fair agreement with recent experiments on
polarization P and relaxation rate 1/T_1 in quantum Hall states at filling nu =
p/(2ps+1), at and near nu = 1/2 and 1/4, at zero and nonzero temperatures. I
show how rotational invariance and two dimensionality can make the underlying
interacting theory behave like a free one in a limited context.Comment: Latex 4 pages, 2 figure
Universal Equilibrium Currents in the Quantum Hall Fluid
The equilibrium current distribution in a quantum Hall fluid that is
subjected to a slowly varying confining potential is shown to generally consist
of strips or channels of current, which alternate in direction, and which have
universal integrated strengths. A measurement of these currents would yield
direct independent measurements of the proper quasiparticle and quasihole
energies in the fractional quantum Hall states.Comment: 4 pages, Revte
Beyond the random phase approximation in the Singwi-Sj\"olander theory of the half-filled Landau level
We study the Chern-Simons system and consider a self-consistent
field theory of the Singwi-Sj\"olander type which goes beyond the random phase
approximation (RPA). By considering the Heisenberg equation of motion for the
longitudinal momentum operator, we are able to show that the zero-frequency
density-density response function vanishes linearly in long wavelength limit
independent of any approximation. From this analysis, we derive a consistency
condition for a decoupling of the equal time density-density and
density-momentum correlation functions. By using the Heisenberg equation of
motion of the Wigner distribution function with a decoupling of the correlation
functions which respects this consistency condition, we calculate the response
functions of the system. In our scheme, we get a density-density
response function which vanishes linearly in the Coulomb case for
zero-frequency in the long wavelength limit. Furthermore, we derive the
compressibility, and the Landau energy as well as the Coulomb energy. These
energies are in better agreement to numerical and exact results, respectively,
than the energies calculated in the RPA.Comment: 9 Revtex pages, 4 eps figures, typos correcte
Composite Fermions and the Energy Gap in the Fractional Quantum Hall Effect
The energy gaps for the fractional quantum Hall effect at filling fractions
1/3, 1/5, and 1/7 have been calculated by variational Monte Carlo using Jain's
composite fermion wave functions before and after projection onto the lowest
Landau level. Before projection there is a contribution to the energy gaps from
the first excited Landau level. After projection this contribution vanishes,
the quasielectron charge becomes more localized, and the Coulomb energy
contribution increases. The projected gaps agree well with previous
calculations, lending support to the composite fermion theory.Comment: 12 pages, Revtex 3.0, 2 compressed and uuencoded postscript figures
appended, NHMFL-94-062
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