686 research outputs found
Monte Carlo simulation method for Laughlin-like states in a disk geometry
We discuss an alternative accurate Monte Carlo method to calculate the
ground-state energy and related quantities for Laughlin states of the
fractional quantum Hall effect in a disk geometry. This alternative approach
allows us to obtain accurate bulk regime (thermodynamic limit) values for
various quantities from Monte Carlo simulations with a small number of
particles (much smaller than that needed with standard Monte Carlo approaches).Comment: 13 pages, 6 figures, 2 table
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
Stability and effective masses of composite-fermions in the first and second Landau Level
We propose a measure of the stability of composite fermions (CF's) at
even-denominator Landau-level filling fractions. Assuming Landau-level mixing
effects are not strong, we show that the CF liquid at in the
Landau level cannot exist and relate this to the absence of a hierarchy of
incompressible states for filling fractions . We find that
a polarized CF liquid should exist at . We also show that, for CF
states, the variation with system size of the ground state energy of
interacting electrons follows that for non-interacting particles in zero
magnetic field. We use this to estimate the CF effective masses.Comment: 9 pages, Revtex, PSIZ-TP-940
Localized quasiholes and the Majorana fermion in fractional quantum Hall state at nu=5/2 via direct diagonalization
Using exact diagonalization in the spherical geometry, we investigate systems
of localized quasiholes at nu=5/2 for interactions interpolating between the
pure Coulomb and the three-body interaction for which the Moore-Read state is
the exact ground state. We show that the charge e/4 quasihole can be easily
localized by means of a delta-function pinning potential. Using a tuned smooth
pinning potential, the quasihole radius can be limited to approximately three
magnetic length units. For systems of two quasiholes, adiabatic continuity
between the Moore-Read and the Coulomb limit holds for the ground state, while
for four quasiholes, the lowest two energy states exhibit adiabatic continuity.
This implies the existence of a Majorana fermion for pure Coulomb interaction.
We also present preliminary results in the Coulomb limit for braiding in
systems containing four quasiholes, with up to 14 electrons, diagonalizing in
the full spin-polarized sector of the second Landau-level Hilbert space.Comment: 9 pages, 8 figure
Interface steps in field effect devices
The charge doped into a semiconductor in a field effect transistor (FET) is
generally confined to the interface of the semiconductor. A planar step at the
interface causes a potential drop due to the strong electric field of the FET,
which in turn is screened by the doped carriers. We analyze the dipolar
electronic structure of a single step in the Thomas-Fermi approximation and
find that the transmission coefficient through the step is exponentially
suppressed by the electric field and the induced carrier density as well as by
the step height. In addition, the field enhancement at the step edge can
facilitate the electric breakthrough of the insulating layer. We suggest that
these two effects may lead to severe problems when engineering FET devices with
very high doping. On the other hand steps can give rise to interesting physics
in superconducting FETs by forming weak links and potentially creating atomic
size Josephson junctions.Comment: 6 pages, 4 figures, submitted to J. Appl. Phy
Microscopic non-equilibrium theory of quantum well solar cells
We present a microscopic theory of bipolar quantum well structures in the
photovoltaic regime, based on the non-equilibrium Green's function formalism
for a multi band tight binding Hamiltonian. The quantum kinetic equations for
the single particle Green's functions of electrons and holes are
self-consistently coupled to Poisson's equation, including inter-carrier
scattering on the Hartree level. Relaxation and broadening mechanisms are
considered by the inclusion of acoustic and optical electron-phonon interaction
in a self consistent Born approximation of the scattering self energies.
Photogeneration of carriers is described on the same level in terms of a self
energy derived from the standard dipole approximation of the electron-photon
interaction. Results from a simple two band model are shown for the local
density of states, spectral response, current spectrum, and current-voltage
characteristics for generic single quantum well systems.Comment: 10 pages, 6 figures; corrected typos, changed caption Fig. 1,
replaced Fig.
Composite Fermions in Negative Effective Magnetic Field: A Monte-Carlo Study
The method of Jain and Kamilla [PRB {\bf 55}, R4895 (1997)] allows numerical
generation of composite fermion trial wavefunctions for large numbers of
electrons in high magnetic fields at filling fractions of the form nu=p/(2mp+1)
with m and p positive integers. In the current paper we generalize this method
to the case where the composite fermions are in an effective (mean) field with
opposite sign from the actual physical field, i.e. when p is negative. We
examine both the ground state energies and the low energy neutral excitation
spectra of these states. Using particle-hole symmetry we can confirm the
correctness of our method by comparing results for the series m=1 with p>0
(previously calculated by others) to our results for the conjugate series m=1
with p <0. Finally, we present similar results for ground state energies and
low energy neutral excitations for the states with m=2 and p <0 which were not
previously addressable, comparing our results to the m=1 case and the p > 0,
m=2 cases.Comment: 11 page
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
Spondylodiscitis as the first manifestation of Whipple's disease -a removal worker with chronic low back pain
Whipple's disease is a rare systemic infectious disease caused by the actinobacterium Tropheryma whipplei. Spondylodiscitis is an extremely rare manifestation of the infection and has previously been described in only three case reports. We present a 55-year-old man with persistent lumbago and signs of systemic illness, but without any gastrointestinal symptoms or arthralgia. The signal response in the lumbar spine in magnetic resonance tomography, both native and after intravenous gadolinium administration, was compatible with spondylodiscitis at the L4/L5 level. Culture of a specimen obtained by radiographically guided disc puncture and repeated blood cultures remained sterile. Tropheryma whipplei was detected by PCR amplification in material obtained from the disc specimen, from a biopsy of the terminal ileum and from the stool. The histology of duodenum, terminal ileum, colon and disc material was normal and, in particular, showed no PAS-positive inclusions in macrophages. Long-term antibiotic treatment with sulphamethoxazole and trimethoprim was successful, with marked improvement of the low back pain and normalisation of the systemic inflammatory signs. The possibility of Whipple's disease must be suspected in the case of a ‘culture-negative' spondylodiscitis even if there are no gastrointestinal symptoms and no arthralgia presen
The Dynactin Complex Enhances the Speed of Microtubule-Dependent Motions of Adenovirus Both Towards and Away from the Nucleus
Unlike transport vesicles or organelles, human adenovirus (HAdV) directly binds to the microtubule minus end-directed motor dynein for transport to the nucleus. The dynein cofactor dynactin enhances nuclear transport of HAdV and boosts infection. To determine if dynactin has a specific role in cytoplasmic trafficking of incoming HAdV on microtubules, we used live cell spinning disc confocal microscopy at 25 Hz acquisition frequency and automated tracking of single virus particles at 20–50 nm spatial resolution. Computational dissection by machine-learning algorithms extracted specific motion patterns of viral trajectories. We found that unperturbed cells supported two kinds of microtubule-dependent motions, directed motions (DM) and fast drifts (FD). DM had speeds of 0.2 to 2 μm/s and run lengths of 0.4 up to 7 μm, while FD were slower and less extensive at 0.02 to 0.4 μm/s and 0.05 to 2.5 μm. Dynactin interference by overexpression of p50/dynamitin or a coiled-coil domain of p150/Glued reduced the speeds and amounts of both center- and periphery-directed DM but not FD, and inhibited infection. These results indicate that dynactin enhances adenovirus infection by increasing the speed and efficiency of dynein-mediated virus motion to the nucleus, and, surprisingly, also supports a hereto unknown motor activity for virus transport to the cell periphery
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