69 research outputs found
Probing the plateau-insulator quantum phase transition in the quantum Hall regime
We report quantum Hall experiments on the plateau-insulator transition in a
low mobility In_{.53} Ga_{.47} As/InP heterostructure. The data for the
longitudinal resistance \rho_{xx} follow an exponential law and we extract a
critical exponent \kappa= .55 \pm .05 which is slightly different from the
established value \kappa = .42 \pm .04 for the plateau transitions. Upon
correction for inhomogeneity effects, which cause the critical conductance
\sigma_{xx}^* to depend marginally on temperature, our data indicate that the
plateau-plateau and plateau- insulator transitions are in the same universality
class.Comment: 4 pages, 4 figures (.eps
The key role of smooth impurity potential in formation of hole spectrum for p-Ge/Ge_{1-x}Si_x heterostructures in the quantum Hall regime
We have measured the temperature (0.1 <= T <= 15 K) and magnetic field (0 <=
B <= 12 T) dependences of longitudinal and Hall resistivities for the
p-Ge_0.93Si_0.07/Ge multilayers with different Ge layer widths 10 <= d_w <= 38
nm and hole densities p_s = (1-5)10^11 cm^-2. Two models for the long-range
random impurity potential (the model with randomly distributed charged centers
located outside the conducting layer and the model of the system with a spacer)
are used for evaluation of the impurity potential fluctuation characteristics:
the random potential amplitude, nonlinear screening length in vicinity of
integer filling factors nu = 1 and nu = 2 and the background density of state
(DOS). The described models are suitable for explanation of the unusually high
value of DOS at nu = 1 and nu = 2, in contrast to the short-range impurity
potential models. For half-integer filling factors the linear temperature
dependence of the effective QHE plateau-to-plateau transition width nu_0(T) is
observed in contrast to scaling behavior for systems with short-range disorder.
The finite T -> 0 width of QHE transitions may be due to an effective low
temperature screening of smooth random potential owing to Coulomb repulsion of
electrons.Comment: Accepted for publication in Nanotechnolog
Sn delta-doping in GaAs
We have prepared a number of GaAs structures delta-doped by Sn using the
well-known molecular beam epitaxy growth technique. The samples obtained for a
wide range of Sn doping densities were characterised by magnetotransport
experiments at low temperatures and in high magnetic fields up to 38 T.
Hall-effect and Shubnikov-de Haas measurements show that the electron densities
reached are higher than for other delta-dopants, like Si and Be. The maximum
carrier density determined by the Hall effect equals 8.4x10^13 cm^-2. For all
samples several Shubnikov-de Haas frequencies were observed, indicating the
population of multiple subbands. The depopulation fields of the subbands were
determined by measuring the magnetoresistance with the magnetic field in the
plane of the delta-layer. The experimental results are in good agreement with
selfconsistent bandstructure calculations. These calculation shows that in the
sample with the highest electron density also the conduction band at the L
point is populated.Comment: 11 pages text (ps), 9 figures (ps), submitted to Semicon. Science
Tech
Phase Diagram of Integer Quantum Hall Effect
The phase diagram of integer quantum Hall effect is numerically determined in
the tight-binding model, which can account for overall features of recently
obtained experimental phase diagram. In particular, the quantum Hall plateaus
are terminated by two distinct insulating phases, characterized by the Hall
resistance with classic and quantized values, respectively, which is also in
good agreement with experiments.Comment: 4 pages, RevTex, 4 PostScript figures; one new figure is added; minor
modifications in the tex
Integer quantum Hall transition in the presence of a long-range-correlated quenched disorder
We theoretically study the effect of long-ranged inhomogeneities on the
critical properties of the integer quantum Hall transition. For this purpose we
employ the real-space renormalization-group (RG) approach to the network model
of the transition. We start by testing the accuracy of the RG approach in the
absence of inhomogeneities, and infer the correlation length exponent nu=2.39
from a broad conductance distribution. We then incorporate macroscopic
inhomogeneities into the RG procedure. Inhomogeneities are modeled by a smooth
random potential with a correlator which falls off with distance as a power
law, r^{-alpha}. Similar to the classical percolation, we observe an
enhancement of nu with decreasing alpha. Although the attainable system sizes
are large, they do not allow one to unambiguously identify a cusp in the
nu(alpha) dependence at alpha_c=2/nu, as might be expected from the extended
Harris criterion. We argue that the fundamental obstacle for the numerical
detection of a cusp in the quantum percolation is the implicit randomness in
the Aharonov-Bohm phases of the wave functions. This randomness emulates the
presence of a short-range disorder alongside the smooth potential.Comment: 10 pages including 6 figures, revised version as accepted for
publication in PR
Promiscuous Expression of Ξ±-Tubulin II in Maturing Male and Female Plasmodium falciparum Gametocytes
BACKGROUND: Antimalarial interventions designed to impact on the transmissible sexual stages of Plasmodium falciparum are evaluated by measurement of peripheral gametocyte carriage in vivo and infectivity to mosquitoes. Drug or vaccine-elicited effects may differentially affect the relative abundance of mature male and female sexual forms, and this can be measured by estimation of sex ratios before and after intervention in vivo and in vitro. Measuring the impact of anti-gametocyte drugs on sexual commitment of immature gametocyte stages in vitro is not currently possible as male and female parasites cannot be distinguished by morphology alone prior to stage IV. METHODOLOGY/PRINCIPAL FINDINGS: We have modified an existing immunofluorescence-based approach for distinguishing male and female gametocytes during development in vitro, by using highly synchronised magnetically-enriched gametocyte preparations at different stages of maturity. Antibodies recognising Ξ±-tubulin II (males) and Pfg377 (females) were used to attempt to discriminate the sexes. Transcription of these two proteins was not coordinated during in vitro development, with pfg377 transcripts accumulating only late in development, immediately prior to immunofluorescent signals from the PfG377 protein appearing in stage IV gametocytes. Contrary to previous descriptions of this protein as male-specific in P. falciparum, Ξ±-tubulin II recognised both male and female gametocytes at stages I to IV, but evidence of differential expression levels of this protein in late stage male and female gametocytes was found. Using antibodies recognising PfG377 as the primary marker and Ξ±-tubulin II as a secondary marker, robust estimates of sex ratio in in vitro cultures were obtained for gametocytes at stage IV or later, and validated by light microscopic counts. However, sex ratio estimation was not possible for early stage gametocytes due to the promiscuity of Ξ±-tubulin II protein expression, and the relatively late accumulation of PfG377 during the development process. CONCLUSIONS/SIGNIFICANCE: This approach is a feasible method for the evaluation of drug impacts on late-stage gametocyte sex ratio in in vitro studies. Additional sex-specific antigens need to be evaluated for sex ratio estimation in early stage gametocyte preparations
Gene Disruption of Plasmodium falciparum p52 Results in Attenuation of Malaria Liver Stage Development in Cultured Primary Human Hepatocytes
Difficulties with inducing sterile and long lasting protective immunity against malaria with subunit vaccines has renewed interest in vaccinations with attenuated Plasmodium parasites. Immunizations with sporozoites that are attenuated by radiation (RAS) can induce strong protective immunity both in humans and rodent models of malaria. Recently, in rodent parasites it has been shown that through the deletion of a single gene, sporozoites can also become attenuated in liver stage development and, importantly, immunization with these sporozoites results in immune responses identical to RAS. The promise of vaccination using these genetically attenuated sporozoites (GAS) depends on translating the results in rodent malaria models to human malaria. In this study, we perform the first essential step in this transition by disrupting, p52, in P. falciparum an ortholog of the rodent parasite gene, p36p, which we had previously shown can confer long lasting protective immunity in mice. These P. falciparum P52 deficient sporozoites demonstrate gliding motility, cell traversal and an invasion rate into primary human hepatocytes in vitro that is comparable to wild type sporozoites. However, inside the host hepatocyte development is arrested very soon after invasion. This study reveals, for the first time, that disrupting the equivalent gene in both P. falciparum and rodent malaria Plasmodium species generates parasites that become similarly arrested during liver stage development and these results pave the way for further development of GAS for human use
Gene-Specific Signatures of Elevated Non-Synonymous Substitution Rates Correlate Poorly across the Plasmodium Genus
BACKGROUND: Comparative genome analyses of parasites allow large scale investigation of selective pressures shaping their evolution. An acute limitation to such analysis of Plasmodium falciparum is that there is only very partial low-coverage genome sequence of the most closely related species, the chimpanzee parasite P. reichenowi. However, if orthologous genes have been under similar selective pressures throughout the Plasmodium genus then positive selection on the P. falciparum lineage might be predicted to some extent by analysis of other lineages. PRINCIPAL FINDINGS: Here, three independent pairs of closely related species in different sub-generic clades (P. falciparum and P. reichenowi; P. vivax and P. knowlesi; P. yoelii and P. berghei) were compared for a set of 43 candidate ligand genes considered likely to be under positive directional selection and a set of 102 control genes for which there was no selective hypothesis. The ratios of non-synonymous to synonymous substitutions (dN/dS) were significantly elevated in the candidate ligand genes compared to control genes in each of the three clades. However, the rank order correlation of dN/dS ratios for individual candidate genes was very low, less than the correlation for the control genes. SIGNIFICANCE: The inability to predict positive selection on a gene in one lineage by identifying elevated dN/dS ratios in the orthologue within another lineage needs to be noted, as it reflects that adaptive mutations are generally rare events that lead to fixation in individual lineages. Thus it is essential to complete the genome sequences of particular species of phylogenetic importance, such as P. reichenowi
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