222 research outputs found

    Theory of the quantum Hall effect in graphene

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    We study the quantum Hall effect (QHE) in graphene based on the current injection model. In our model, the presence of disorder, the edge-state picture, extended states and localized states, which are believed to be indispensable ingredients in describing the QHE, do not play an important role. Instead the boundary conditions during the injection into the graphene sheet, which are enforced by the presence of the Ohmic contacts, determine the current-voltage characteristics.Comment: 4 pages, 3 figures, rewritten, role of contacts for boundary conditions in small device

    Twin Pregnancy with Gastroschisis in Both Twins

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    SUMMARYObjectiveGastroschisis is a congenital malformation characterized by an abdominal wall defect located laterally to a normal umbilicus. The cause of gastroschisis is unknown, but most authors consider it exogenous. We describe the case of a woman with a twin pregnancy in which both twins had gastroschisis.Case ReportA 17-year-old primiparous female was referred to our institution because of a twin pregnancy, with one twin diagnosed with gastroschisis at 34 weeks of gestation. Unfortunately, gastroschisis was noted in both twins, but no other anomalies were observed under level II sonographic evaluation. The twins were delivered by cesarean section at 36+weeks of gestation because of preterm labor and breech presentation of one fetus. Both twins presented with a 3-cm abdominal wall defect located to the right side of the umbilicus and a large portion of the bowel protruding that was not covered by membrane. Histopathology of the placenta revealed that the twins were diamniotic monochorionic. Chromosomal analysis of cord blood showed normal karyotype (46, XX) in both newborns.ConclusionThe cause of gastroschisis is unknown, although possible exogenous causes have been studied. The diagnosis of gastroschisis in twin pregnancy is always in late gestation. Therefore, maternal serum alpha feto-protein screening and a detailed prenatal ultrasound evaluation are recommended in multifetal pregnancies

    Direct measurement of the spin gaps in a gated GaAs two-dimensional electron gas.

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    We have performed magnetotransport measurements on gated GaAs two-dimensional electron gases in which electrons are confined in a layer of the nanoscale. From the slopes of a pair of spin-split Landau levels (LLs) in the energy-magnetic field plane, we can perform direct measurements of the spin gap for different LLs. The measured g-factor g is greatly enhanced over its bulk value in GaAs (0.44) due to electron-electron (e-e) interactions. Our results suggest that both the spin gap and g determined from conventional activation energy studies can be very different from those obtained by direct measurements.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

    Large transconductance oscillations in a single-well vertical Aharonov-Bohm interferometer

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    Aharonov-Bohm (AB) interference is reported for the first time in the conductance of a vertical nanostructure based on a single GaAs/AlGaAs quantum well (QW). The two lowest subbands of the well are spatially separated by the Hartree barrier originating from electronic repulsion in the modulation-doped QW and provide AB two-path geometry. Split-gates control the in-plane electronic momentum dispersion. In our system, we have clearly demonstrated AB interference in both electrostatic and magnetic modes. In the latter case the magnetic field was applied parallel to the QW plane, and perpendicular to the 0.02 um^2 AB loop. In the electrostatic mode of operation the single-QW scheme adopted led to large transconductance oscillations with relative amplitudes exceeding 30 %. The relevance of the present design strategy for the implementation of coherent nanoelectronic devices is underlined.Comment: Accepted for publication on Physical Review B Rapid Communication
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