24 research outputs found
Aharonov-Bohm oscillations and magnetic focusing in ballistic graphene rings
We present low-temperature magnetotransport measurements on graphene rings
encapsulated in hexagonal boron nitride. We investigate phase-coherent
transport and show Aharonov-Bohm (AB) oscillations in quasi-ballistic graphene
rings with hard confinement. In particular, we report on the observation of
, and conductance oscillations. Moreover we show signatures
of magnetic focusing effects at small magnetic fields confirming ballistic
transport. We perform tight binding calculations which allow to reproduce all
significant features of our experimental findings and enable a deeper
understanding of the underlying physics. Finally, we report on the observation
of the AB conductance oscillations in the quantum Hall regime at reasonable
high magnetic fields, where we find regions with enhanced AB oscillation
visibility with values up to %. These oscillations are well explained by
taking disorder into account allowing for a coexistence of hard and soft-wall
confinement.Comment: 28 pages, 8 figure
Graphene quantum dots: Beyond a Dirac billiard
We present realistic simulations of quantum confinement effects in ballistic
graphene quantum dots with linear dimensions of 10 to 40 nm. We determine
wavefunctions and energy level statistics in the presence of disorder resulting
from edge roughness, charge impurities, or short-ranged scatterers. Marked
deviations from a simple Dirac billiard for massless fermions are found. We
find a remarkably stable dependence of the nearest-neighbor level spacing on
edge roughness suggesting that the roughness of fabricated devices can be
potentially characterized by the distribution of measured Coulomb blockade
peaks.Comment: 5 figures, higher resolution available upon reques
Electron-hole crossover in gate-controlled bilayer graphene quantum dots
Electron and hole Bloch states in gapped bilayer graphene exhibit topological
orbital magnetic moments with opposite signs near the band edges, which allows
for tunable valley-polarization in an out-of-plane magnetic field. This
intrinsic property makes electron and hole quantum dots (QDs) in bilayer
graphene interesting for valley and spin-valley qubits. Here we show
measurements of the electron-hole crossover in a bilayer graphene QD,
demonstrating the opposite sign of the orbital magnetic moments associated with
the Berry curvature. Using three layers of metallic top gates, we independently
control the tunneling barriers of the QD while tuning the occupation from the
few-hole regime to the few-electron regime, crossing the displacement-field
controlled band gap. The band gap is around 25 meV, while the charging energies
of the electron and hole dots are between 3-5 meV. The extracted valley
g-factor is around 17 and leads to opposite valley polarization for electron
and hole states at moderate B-fields. Our measurements agree well with
tight-binding calculations for our device
Genetic Data from Nearly 63,000 Women of European Descent Predicts DNA Methylation Biomarkers and Epithelial Ovarian Cancer Risk
DNA methylation is instrumental for gene regulation. Global changes in the epigenetic landscape have been recognized as a hallmark of cancer. However, the role of DNA methylation in epithelial ovarian cancer (EOC) remains unclear. In this study, high-density genetic and DNA methylation data in white blood cells from the Framingham Heart Study (N = 1,595) were used to build genetic models to predict DNA methylation levels. These prediction models were then applied to the summary statistics of a genome-wide association study (GWAS) of ovarian cancer including 22,406 EOC cases and 40,941 controls to investigate genetically predicted DNA methylation levels in association with EOC risk. Among 62,938 CpG sites investigated, genetically predicted methylation levels at 89 CpG were significantly associated with EOC risk at a Bonferroni-corrected threshold of P <7.94 x 10(-7). Of them, 87 were located at GWAS-identified EOC susceptibility regions and two resided in a genomic region not previously reported to be associated with EOC risk. Integrative analyses of genetic, methylation, and gene expression data identified consistent directions of associations across 12 CpG, five genes, and EOC risk, suggesting that methylation at these 12 CpG may influence EOC risk by regulating expression of these five genes, namely MAPT, HOXB3, ABHD8, ARHGAP27, and SKAP1. We identified novel DNA methylation markers associated with EOC risk and propose that methylation at multiple CpG may affect EOC risk via regulation of gene expression. Significance: Identification of novel DNA methylation markers associated with EOC risk suggests that methylation at multiple CpG may affect EOC risk through regulation of gene expression.Peer reviewe
Genomic investigations of unexplained acute hepatitis in children
Since its first identification in Scotland, over 1,000 cases of unexplained paediatric hepatitis in children have been reported worldwide, including 278 cases in the UK1. Here we report an investigation of 38 cases, 66 age-matched immunocompetent controls and 21 immunocompromised comparator participants, using a combination of genomic, transcriptomic, proteomic and immunohistochemical methods. We detected high levels of adeno-associated virus 2 (AAV2) DNA in the liver, blood, plasma or stool from 27 of 28 cases. We found low levels of adenovirus (HAdV) and human herpesvirus 6B (HHV-6B) in 23 of 31 and 16 of 23, respectively, of the cases tested. By contrast, AAV2 was infrequently detected and at low titre in the blood or the liver from control children with HAdV, even when profoundly immunosuppressed. AAV2, HAdV and HHV-6 phylogeny excluded the emergence of novel strains in cases. Histological analyses of explanted livers showed enrichment for T cells and B lineage cells. Proteomic comparison of liver tissue from cases and healthy controls identified increased expression of HLA class 2, immunoglobulin variable regions and complement proteins. HAdV and AAV2 proteins were not detected in the livers. Instead, we identified AAV2 DNA complexes reflecting both HAdV-mediated and HHV-6B-mediated replication. We hypothesize that high levels of abnormal AAV2 replication products aided by HAdV and, in severe cases, HHV-6B may have triggered immune-mediated hepatic disease in genetically and immunologically predisposed children