690 research outputs found
Coulomb Drag and Magnetotransport in Graphene Double Layers
We review the fabrication and key transport properties of graphene double
layers, consisting of two graphene monolayers placed in close proximity,
independently contacted, and separated by an ultra-thin dielectric. We outline
a simple band structure model relating the layer densities to the applied gate
and inter-layer biases, and show that calculations and experimental results are
in excellent agreement both at zero and in high magnetic fields. Coulomb drag
measurements, which probe the electron-electron scattering between the two
layers reveal two distinct regime: (i) diffusive drag at elevated temperatures,
and (ii) mesoscopic fluctuation-dominated drag at low temperatures. We discuss
the Coulomb drag results within the framework of existing theories.Comment: 9 pages, 6 figure
Tree-guided group lasso for multi-response regression with structured sparsity, with an application to eQTL mapping
We consider the problem of estimating a sparse multi-response regression
function, with an application to expression quantitative trait locus (eQTL)
mapping, where the goal is to discover genetic variations that influence
gene-expression levels. In particular, we investigate a shrinkage technique
capable of capturing a given hierarchical structure over the responses, such as
a hierarchical clustering tree with leaf nodes for responses and internal nodes
for clusters of related responses at multiple granularity, and we seek to
leverage this structure to recover covariates relevant to each
hierarchically-defined cluster of responses. We propose a tree-guided group
lasso, or tree lasso, for estimating such structured sparsity under
multi-response regression by employing a novel penalty function constructed
from the tree. We describe a systematic weighting scheme for the overlapping
groups in the tree-penalty such that each regression coefficient is penalized
in a balanced manner despite the inhomogeneous multiplicity of group
memberships of the regression coefficients due to overlaps among groups. For
efficient optimization, we employ a smoothing proximal gradient method that was
originally developed for a general class of structured-sparsity-inducing
penalties. Using simulated and yeast data sets, we demonstrate that our method
shows a superior performance in terms of both prediction errors and recovery of
true sparsity patterns, compared to other methods for learning a
multivariate-response regression.Comment: Published in at http://dx.doi.org/10.1214/12-AOAS549 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Spin-Polarized to Valley-Polarized Transition in Graphene Bilayers at in High Magnetic Fields
We investigate the transverse electric field () dependence of the =0
quantum Hall state (QHS) in dual-gated graphene bilayers in high magnetic
fields. The longitudinal resistivity () measured at =0 shows an
insulating behavior which is strongest in the vicinity of =0, and at large
-fields. At a fixed perpendicular magnetic field (), the =0 QHS
undergoes a transition as a function of , marked by a minimum,
temperature-independent . This observation is explained by a
transition from a spin polarized =0 QHS at small -fields, to a valley
(layer) polarized =0 QHS at large -fields. The -field value at which
the transition occurs has a linear dependence on Comment: 5 pages, 5 figure
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Electron transport in graphene transistors and heterostructures : towards graphene-based nanoelectronics
textTwo graphene layers placed in close proximity offer a unique system to investigate interacting electron physics as well as to test novel electronic device concepts. In this system, the interlayer spacing can be reduced to value much smaller than that achievable in semiconductor heterostructures, and the zero energy band-gap allows the realization of coupled hole-hole, electron-hole, and electron-electron two-dimensional systems in the same sample. Leveraging the fabrication technique and electron transport study in dual-gated graphene field-effect transistors, we realize independently contacted graphene double layers separated by an ultra-thin dielectric. We probe the resistance and density of each layer, and quantitatively explain their dependence on the backgate and interlayer bias. We experimentally measure the Coulomb drag between the two graphene layers for the first time, by flowing current in one layer and measuring the voltage drop in the opposite layer. The drag resistivity gauges the momentum transfer between the two layers, which, in turn, probes the interlayer electron-electron scattering rate. The temperature dependence of the Coulomb drag above temperatures of 50 K reveals that the ground state in each layer is a Fermi liquid. Below 50 K we observe mesoscopic fluctuations of the drag resistivity, as a result of the interplay between coherent intralayer transport and interlayer interaction. In addition, we develop a technique to directly measure the Fermi energy in an electron system as a function of carrier density using double layer structure. We demonstrate this method in the double layer graphene structure and probe the Fermi energy in graphene both at zero and in high magnetic fields. Last, we realize dual-gated bilayer graphene devices, where we investigate quantum Hall effects at zero energy as a function of transverse electric field and perpendicular magnetic field. Here we observe a development of v = 0 quantum Hall state at large electric fields and in high magnetic fields, which is explained by broken spin and valley spin symmetry in the zero energy Landau levels.Electrical and Computer Engineerin
Smoothing Proximal Gradient Method for General Structured Sparse Learning
We study the problem of learning high dimensional regression models
regularized by a structured-sparsity-inducing penalty that encodes prior
structural information on either input or output sides. We consider two widely
adopted types of such penalties as our motivating examples: 1) overlapping
group lasso penalty, based on the l1/l2 mixed-norm penalty, and 2) graph-guided
fusion penalty. For both types of penalties, due to their non-separability,
developing an efficient optimization method has remained a challenging problem.
In this paper, we propose a general optimization approach, called smoothing
proximal gradient method, which can solve the structured sparse regression
problems with a smooth convex loss and a wide spectrum of
structured-sparsity-inducing penalties. Our approach is based on a general
smoothing technique of Nesterov. It achieves a convergence rate faster than the
standard first-order method, subgradient method, and is much more scalable than
the most widely used interior-point method. Numerical results are reported to
demonstrate the efficiency and scalability of the proposed method.Comment: arXiv admin note: substantial text overlap with arXiv:1005.471
Coulomb Drag of Massless Fermions in Graphene
Using a novel structure, consisting of two, independently contacted graphene
single layers separated by an ultra-thin dielectric, we experimentally measure
the Coulomb drag of massless fermions in graphene. At temperatures higher than
50 K, the Coulomb drag follows a temperature and carrier density dependence
consistent with the Fermi liquid regime. As the temperature is reduced, the
Coulomb drag exhibits giant fluctuations with an increasing amplitude, thanks
to the interplay between coherent transport in the graphene layer and
interaction between the two layers.Comment: 5 pages, 5 figure
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