818 research outputs found
Individual scatterers as microscopic origin of equilibration between spin- polarized edge channels in the quantum Hall regime
The equilibration length between spin-polarized edge states in the Quantum
Hall regime is measured as a function of a gate voltage applied to an electrode
on top of the edge channels. Reproducible fluctuations in the coupling are
observed and interpreted as a mesoscopic fingerprint of single spin-flip
scatterers which are turned on and off. A model to analyze macroscopic edge
state coupling in terms of individual scatterers is developed, and
characteristic values for these scatterers in our samples are extracted. For
all samples investigated, the distance between spin-flip scatterers lies
between the Drude and the quantum scattering length.Comment: 4 pages, 2 figure
Separately contacted edge states: A new spectroscopic tool for the investigation of the quantum Hall effect
Using an innovative combination of a quasi-Corbino sample geometry and the
cross-gate technique, we have developed a method that enables us to separately
contact single edge channels in the quantum Hall regime and investigate
equilibration among them. Performing 4-point resistance measurements, we
directly obtain information on the energetic and geometric structure of the
edge region and the equilibration-length for current transport across the
Landau- as well as the spin-gap. Based on an almost free choice in the number
of participating edge channels and their interaction-length a systematic
investigation of the parameter-space becomes possible.Comment: 8 pages, 7 figure
Experimental investigation of the edge states structure at fractional filling factors
We experimentally study electron transport between edge states in the
fractional quantum Hall effect regime. We find an anomalous increase of the
transport across the 2/3 incompressible fractional stripe in comparison with
theoretical predictions for the smooth edge potential profile. We interpret our
results as a first experimental demonstration of the intrinsic structure of the
incompressible stripes arising at the sample edge in the fractional quantum
Hall effect regime.Comment: 5 pages, 5 figures included. Submitted to JETP Letter
Imaging Inter-Edge State Scattering Centers in the Quantum Hall Regime
We use an atomic force microscope tip as a local gate to study the scattering
between edge channels in a 2D electron gas in the quantum Hall regime. The
scattering is dominated by individual, microscopic scattering centers, which we
directly image here for the first time. The tip voltage dependence of the
scattering indicates that tunneling occurs through weak links and localized
states.Comment: 4 pages, 5 figure
Manifestation of the bulk phase transition in the edge energy spectrum in a two dimensional bilayer electron system
We use a quasi-Corbino sample geometry with independent contacts to different
edge states in the quantum Hall effect regime to investigate the edge energy
spectrum of a bilayer electron system at total filling factor . By
analyzing non-linear curves in normal and tilted magnetic fields we
conclude that the edge energy spectrum is in a close connection with the bulk
one. At the bulk phase transition spin-singlet - canted antiferromagnetic phase
curve becomes to be linear, indicating the disappearance or strong
narrowing of the incompressible strip at the edge of the sample.Comment: 5 pages, 5 figure
Composite Fermions, Edge Currents and the Fractional Quantum Hall Effect
We present a theory of composite fermion edge states and their transport
properties in the fractional and integer quantum Hall regimes. We show that the
effective electro-chemical potentials of composite fermions at the edges of a
Hall bar differ, in general, from those of electrons. An expression for the
difference is given. Composite fermion edge states of three different types are
identified. Two of the three types have no analog in previous theories of the
integer or fractional quantum Hall effect. The third type includes the usual
integer edge states. The direction of propagation of the edge states agrees
with experiment. The present theory yields the observed quantized Hall
conductances at Landau level filling fractions p/(mp+-1), for m=0,2,4, p=
1,2,3,... It explains the results of experiments that involve conduction across
smooth potential barriers and through adiabatic constrictions, and of
experiments that involve selective population and detection of fractional edge
channels. The relationship between the present work and Hartree theories of
composite fermion edge structure is discussed.Comment: 19 pages + 6 figures. Self-unpacking uuencoded postscript. To appear
in Physical Review B. Revised version has more details in the Appendix and a
discussion of one more experiment in Section
A Novel Locus and Candidate Gene for Familial Developmental Dyslexia on Chromosome 4q
Objective: Developmental dyslexia is a highly heritable specific reading and writing disability. To identify a possible new locus and candidate gene for this disability, we investigated a four-generation pedigree where transmission of dyslexia is consistent with an autosomal dominant inheritance pattern. Methods: We performed genome wide array-based SNP genotyping and parametric linkage analysis and sequencing analysis of protein-coding exons, exon-intron boundaries and conserved extragenic regions within the haplotype cosegregating with dyslexia in DNA from one affected and one unaffected family member. Cosegregation was confirmed by sequencing all available family members. Additionally, we analyzed 96 dyslexic individuals who had previously shown positive LOD scores on chromosome 4q28 as well as an even larger sample (n = 2591). Results: We found a single prominent linkage interval on chromosome 4q, where sequence analysis revealed a nucleotide variant in the 3' UTR of brain expressed SPRY1 in the dyslexic family member that cosegregated with dyslexia. This sequence alteration might affect the binding efficiency of the IGF2BP1 RNA-binding protein and thus influence the expression level of the SPRY1 gene product. An analysis of 96 individuals from a cohort of dyslexic individuals revealed a second heterozygous variant in this gene, which was absent in the unaffected sister of the proband. An investigation of the region in a much larger sample further found a nominal p-value of 0.0016 for verbal short-term memory (digit span) in 2,591 individuals for a neighboring SNV. After correcting for the local number of analyzed SNVs, and after taking into account linkage disequilibrium, we found this corresponds to a p-value of 0.0678 for this phenotype. Conclusions: We describe a new locus for familial dyslexia and discuss the possibility that SPRY1 might play a role in the etiology of a monogenic form of dyslexia
Regaining momentum for international climate policy beyond Copenhagen
The 'Copenhagen Accord' fails to deliver the political framework for a fair, ambitious and legally-binding international climate agreement beyond 2012. The current climate policy regime dynamics are insufficient to reflect the realities of topical complexity, actor coalitions, as well as financial, legal and institutional challenges in the light of extreme time constraints to avoid 'dangerous' climate change of more than 2°C. In this paper we analyze these stumbling blocks for international climate policy and discuss alternatives in order to regain momentum for future negotiations
Naupliar and Metanaupliar development of Thysanoessa raschii (Malacostraca, Euphausiacea) from Godthåbsfjord, Greenland, with a reinstatement of the ancestral status of the free-living Nauplius in Malacostracan evolution
The presence of a characteristic crustacean larval type, the nauplius, in many crustacean taxa has often been considered one of the few uniting characters of the Crustacea. Within Malacostraca, the largest crustacean group, nauplii are only present in two taxa, Euphauciacea (krill) and Decapoda Dendrobranchiata. The presence of nauplii in these two taxa has traditionally been considered a retained primitive characteristic, but free-living nauplii have also been suggested to have reappeared a couple of times from direct developing ancestors during malacostracan evolution. Based on a re-study of Thysanoessa raschii (Euphausiacea) using preserved material collected in Greenland, we readdress this important controversy in crustacean evolution, and, in the process, redescribe the naupliar and metanaupliar development of T. raschii. In contrast to most previous studies of euphausiid development, we recognize three (not two) naupliar (= ortho-naupliar) stages (N1-N3) followed by a metanauplius (MN). While there are many morphological changes between nauplius 1 and 2 (e.g., appearance of long caudal setae), the changes between nauplius 2 and 3 are few but distinct. They involve the size of some caudal spines (largest in N3) and the setation of the antennal endopod (an extra seta in N3). A wider comparison between free-living nauplii of both Malacostraca and non-Malacostraca revealed similarities between nauplii in many taxa both at the general level (e.g., the gradual development and number of appendages) and at the more detailed level (e.g., unclear segmentation of naupliar appendages, caudal setation, presence of frontal filaments). We recognize these similarities as homologies and therefore suggest that free-living nauplii were part of the ancestral malacostracan type of development. The derived morphology (e.g., lack of feeding structures, no fully formed gut, high content of yolk) of both euphausiid and dendrobranchiate nauplii is evidently related to their non-feeding (lecithotrophic) status
Study of Leading Hadrons in Gluon and Quark Fragmentation
The study of quark jets in e+e- reactions at LEP has demonstrated that the
hadronisation process is reproduced well by the Lund string model. However, our
understanding of gluon fragmentation is less complete. In this study enriched
quark and gluon jet samples of different purities are selected in three-jet
events from hadronic decays of the Z collected by the DELPHI experiment in the
LEP runs during 1994 and 1995. The leading systems of the two kinds of jets are
defined by requiring a rapidity gap and their sum of charges is studied. An
excess of leading systems with total charge zero is found for gluon jets in all
cases, when compared to Monte Carlo Simulations with JETSET (with and without
Bose-Einstein correlations included) and ARIADNE. The corresponding leading
systems of quark jets do not exhibit such an excess. The influence of the gap
size and of the gluon purity on the effect is studied and a concentration of
the excess of neutral leading systems at low invariant masses (<~ 2 GeV/c^2) is
observed, indicating that gluon jets might have an additional hitherto
undetected fragmentation mode via a two-gluon system. This could be an
indication of a possible production of gluonic states as predicted by QCD.Comment: 19 pages, 6 figures, Accepted by Phys. Lett.
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