616 research outputs found
Electric Field Control of Soliton Motion and Stacking in Trilayer Graphene
The crystal structure of a material plays an important role in determining
its electronic properties. Changing from one crystal structure to another
involves a phase transition which is usually controlled by a state variable
such as temperature or pressure. In the case of trilayer graphene, there are
two common stacking configurations (Bernal and rhombohedral) which exhibit very
different electronic properties. In graphene flakes with both stacking
configurations, the region between them consists of a localized strain soliton
where the carbon atoms of one graphene layer shift by the carbon-carbon bond
distance. Here we show the ability to move this strain soliton with a
perpendicular electric field and hence control the stacking configuration of
trilayer graphene with only an external voltage. Moreover, we find that the
free energy difference between the two stacking configurations scales
quadratically with electric field, and thus rhombohedral stacking is favored as
the electric field increases. This ability to control the stacking order in
graphene opens the way to novel devices which combine structural and electrical
properties
Band Structure Mapping of Bilayer Graphene via Quasiparticle Scattering
A perpendicular electric field breaks the layer symmetry of Bernal-stacked
bilayer graphene, resulting in the opening of a band gap and a modification of
the effective mass of the charge carriers. Using scanning tunneling microscopy
and spectroscopy, we examine standing waves in the local density of states of
bilayer graphene formed by scattering from a bilayer/trilayer boundary. The
quasiparticle interference properties are controlled by the bilayer graphene
band structure, allowing a direct local probe of the evolution of the band
structure of bilayer graphene as a function of electric field. We extract the
Slonczewski-Weiss-McClure model tight binding parameters as
eV, eV, and eV.Comment: 12 pages, 4 figure
Deep-Learning-Enabled Fast Optical Identification and Characterization of Two-Dimensional Materials
Advanced microscopy and/or spectroscopy tools play indispensable role in
nanoscience and nanotechnology research, as it provides rich information about
the growth mechanism, chemical compositions, crystallography, and other
important physical and chemical properties. However, the interpretation of
imaging data heavily relies on the "intuition" of experienced researchers. As a
result, many of the deep graphical features obtained through these tools are
often unused because of difficulties in processing the data and finding the
correlations. Such challenges can be well addressed by deep learning. In this
work, we use the optical characterization of two-dimensional (2D) materials as
a case study, and demonstrate a neural-network-based algorithm for the material
and thickness identification of exfoliated 2D materials with high prediction
accuracy and real-time processing capability. Further analysis shows that the
trained network can extract deep graphical features such as contrast, color,
edges, shapes, segment sizes and their distributions, based on which we develop
an ensemble approach topredict the most relevant physical properties of 2D
materials. Finally, a transfer learning technique is applied to adapt the
pretrained network to other applications such as identifying layer numbers of a
new 2D material, or materials produced by a different synthetic approach. Our
artificial-intelligence-based material characterization approach is a powerful
tool that would speed up the preparation, initial characterization of 2D
materials and other nanomaterials and potentially accelerate new material
discoveries
Evolution of Flux Noise in Superconducting Qubits with Weak Magnetic Fields
The microscopic origin of magnetic flux noise in superconducting
circuits has remained an open question for several decades despite extensive
experimental and theoretical investigation. Recent progress in superconducting
devices for quantum information has highlighted the need to mitigate sources of
qubit decoherence, driving a renewed interest in understanding the underlying
noise mechanism(s). Though a consensus has emerged attributing flux noise to
surface spins, their identity and interaction mechanisms remain unclear,
prompting further study. Here we apply weak in-plane magnetic fields to a
capacitively-shunted flux qubit (where the Zeeman splitting of surface spins
lies below the device temperature) and study the flux-noise-limited qubit
dephasing, revealing previously unexplored trends that may shed light on the
dynamics behind the emergent noise. Notably, we observe an enhancement
(suppression) of the spin-echo (Ramsey) pure dephasing time in fields up to
. With direct noise spectroscopy, we further observe a
transition from a to approximately Lorentzian frequency dependence below
10 Hz and a reduction of the noise above 1 MHz with increasing magnetic field.
We suggest that these trends are qualitatively consistent with an increase of
spin cluster sizes with magnetic field. These results should help to inform a
complete microscopic theory of flux noise in superconducting circuits
Quantum coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures
Quantum coherence and control is foundational to the science and engineering
of quantum systems. In van der Waals (vdW) materials, the collective coherent
behavior of carriers has been probed successfully by transport measurements.
However, temporal coherence and control, as exemplified by manipulating a
single quantum degree of freedom, remains to be verified. Here we demonstrate
such coherence and control of a superconducting circuit incorporating
graphene-based Josephson junctions. Furthermore, we show that this device can
be operated as a voltage-tunable transmon qubit, whose spectrum reflects the
electronic properties of massless Dirac fermions traveling ballistically. In
addition to the potential for advancing extensible quantum computing
technology, our results represent a new approach to studying vdW materials
using microwave photons in coherent quantum circuits
Genetic association study of QT interval highlights role for calcium signaling pathways in myocardial repolarization.
The QT interval, an electrocardiographic measure reflecting myocardial repolarization, is a heritable trait. QT prolongation is a risk factor for ventricular arrhythmias and sudden cardiac death (SCD) and could indicate the presence of the potentially lethal mendelian long-QT syndrome (LQTS). Using a genome-wide association and replication study in up to 100,000 individuals, we identified 35 common variant loci associated with QT interval that collectively explain ∼8-10% of QT-interval variation and highlight the importance of calcium regulation in myocardial repolarization. Rare variant analysis of 6 new QT interval-associated loci in 298 unrelated probands with LQTS identified coding variants not found in controls but of uncertain causality and therefore requiring validation. Several newly identified loci encode proteins that physically interact with other recognized repolarization proteins. Our integration of common variant association, expression and orthogonal protein-protein interaction screens provides new insights into cardiac electrophysiology and identifies new candidate genes for ventricular arrhythmias, LQTS and SCD
The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III
The Sloan Digital Sky Survey (SDSS) started a new phase in August 2008, with
new instrumentation and new surveys focused on Galactic structure and chemical
evolution, measurements of the baryon oscillation feature in the clustering of
galaxies and the quasar Ly alpha forest, and a radial velocity search for
planets around ~8000 stars. This paper describes the first data release of
SDSS-III (and the eighth counting from the beginning of the SDSS). The release
includes five-band imaging of roughly 5200 deg^2 in the Southern Galactic Cap,
bringing the total footprint of the SDSS imaging to 14,555 deg^2, or over a
third of the Celestial Sphere. All the imaging data have been reprocessed with
an improved sky-subtraction algorithm and a final, self-consistent photometric
recalibration and flat-field determination. This release also includes all data
from the second phase of the Sloan Extension for Galactic Understanding and
Evolution (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars
at both high and low Galactic latitudes. All the more than half a million
stellar spectra obtained with the SDSS spectrograph have been reprocessed
through an improved stellar parameters pipeline, which has better determination
of metallicity for high metallicity stars.Comment: Astrophysical Journal Supplements, in press (minor updates from
submitted version
The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological analysis of the DR12 galaxy sample
We present cosmological results from the final galaxy clustering data set of the Baryon Oscillation Spectroscopic Survey, part of the Sloan Digital Sky Survey III. Our combined galaxy sample comprises 1.2 million massive galaxies over an effective area of 9329 and volume of 18.7 , divided into three partially overlapping redshift slices centred at effective redshifts 0.38, 0.51, and 0.61. We measure the angular diameter distance DM and Hubble parameter H from the baryon acoustic oscillation (BAO) method after applying reconstruction to reduce non-linear effects on the BAO feature. Using the anisotropic clustering of the pre-reconstruction density field, we measure the product DM*H from the Alcock-Paczynski (AP) effect and the growth of structure, quantified by , from redshift-space distortions (RSD). We combine measurements presented in seven companion papers into a set of consensus values and likelihoods, obtaining constraints that are tighter and more robust than those from any one method. Combined with Planck 2015 cosmic microwave background measurements, our distance scale measurements simultaneously imply curvature and a dark energy equation of state parameter w = -1.01+/-0.06, in strong affirmation of the spatially flat cold dark matter model with a cosmological constant (CDM). Our RSD measurements of , at 6 per cent precision, are similarly consistent with this model. When combined with supernova Ia data, we find H0 = 67.3+/-1.0 km/s/Mpc even for our most general dark energy model, in tension with some direct measurements. Adding extra relativistic species as a degree of freedom loosens the constraint only slightly, to H0 = 67.8+/-1.2 km/s/Mpc. Assuming flat CDM we find and H0 = 67.6+/-0.5 km/s/Mpc, and we find a 95% upper limit of on the neutrino mass sum
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