3,026 research outputs found
Band-structure topologies of graphene: spin-orbit coupling effects from first principles
The electronic band structure of graphene in the presence of spin-orbit
coupling and transverse electric field is investigated from first principles
using the linearized augmented plane-wave method. The spin-orbit coupling opens
a gap at the -point of the magnitude of 24 eV (0.28 K). This
intrinsic splitting comes 96% from the usually neglected and higher
orbitals. The electric field induces an additional (extrinsic)
Bychkov-Rashba-type splitting of 10 eV (0.11 K) per V/nm, coming from the
- mixing. A 'mini-ripple' configuration with every other atom is
shifted out of the sheet by less than 1% differs little from the intrinsic
case.Comment: 4 pages, 4 figure
Shot Noise in Graphene
We report measurements of current noise in single- and multi-layer graphene
devices. In four single-layer devices, including a p-n junction, the Fano
factor remains constant to within +/-10% upon varying carrier type and density,
and averages between 0.35 and 0.38. The Fano factor in a multi-layer device is
found to decrease from a maximal value of 0.33 at the charge-neutrality point
to 0.25 at high carrier density. These results are compared to theoretical
predictions for shot noise in ballistic and disordered graphene.Comment: related papers available at http://marcuslab.harvard.ed
Evidence for Steep Luminosity Functions in Clusters of Galaxies
Luminosity Functions have been obtained for very faint dwarf galaxies in the
cores of four rich clusters of galaxies (Abell 2052, 2107, 2199 and 2666). It
is found that the luminosity function of dwarf galaxies rises very steeply in
these clusters, with a power-law slope of \alpha -2.2 (down to absolute
limiting magnitudes M_I = -13 and M_B = -11 for H_0 = 75 km/s/Mpc). A
steepening of the luminosity function at faint magnitudes may in fact be a
common feature of both cluster and field populations. Such a result may explain
the observed excess counts of faint, intermediate redshift galaxies in the
Universe, without resorting to more exotic phenomena. An alternate explanation
is that star formation in dwarf galaxies is less affected by gas loss in the
richest clusters, because of the dense, hot intracluster medium found in such
environments.Comment: 15 pages, 2 figures (attached). Uuencoded compressed Postscript
tarfile. Accepted by the Astrophysical Journal
On the consistency of scale among experiments, theory, and simulation
As a tool for addressing problems of scale, we consider an evolving approach
known as the thermodynamically constrained averaging theory (TCAT), which has
broad applicability to hydrology. We consider the case of modeling of two-fluid-phase flow in porous media, and we focus on issues of scale as they
relate to various measures of pressure, capillary pressure, and state
equations needed to produce solvable models. We apply TCAT to perform
physics-based data assimilation to understand how the internal behavior
influences the macroscale state of two-fluid porous medium systems. A
microfluidic experimental method and a lattice Boltzmann simulation method
are used to examine a key deficiency associated with standard approaches. In
a hydrologic process such as evaporation, the water content will ultimately
be reduced below the irreducible wetting-phase saturation
determined from experiments. This is problematic since the derived closure
relationships cannot predict the associated capillary pressures for these
states. We demonstrate that the irreducible wetting-phase saturation is an
artifact of the experimental design, caused by the fact that the boundary
pressure difference does not approximate the true capillary pressure. Using
averaging methods, we compute the true capillary pressure for fluid
configurations at and below the irreducible wetting-phase saturation. Results
of our analysis include a state function for the capillary pressure expressed
as a function of fluid saturation and interfacial area
Tunable Noise Cross-Correlations in a Double Quantum Dot
We report measurements of the cross-correlation between current noise
fluctuations in two capacitively coupled quantum dots in the Coulomb blockade
regime. The sign of the cross-spectral density is found to be tunable by gate
voltage and source-drain bias. Good agreement is found with a model of
sequential tunneling through the dots in the presence of inter-dot capacitive
coupling.Comment: related papers available at http://marcuslab.harvard.ed
Exploratory analysis of methods for automated classification of laboratory test orders into syndromic groups in veterinary medicine
Background: Recent focus on earlier detection of pathogen introduction in human and animal populations has led to the development of surveillance systems based on automated monitoring of health data. Real- or near real-time monitoring of pre-diagnostic data requires automated classification of records into syndromes-syndromic surveillance-using algorithms that incorporate medical knowledge in a reliable and efficient way, while remaining comprehensible to end users. Methods: This paper describes the application of two of machine learning (Naïve Bayes and Decision Trees) and rule-based methods to extract syndromic information from laboratory test requests submitted to a veterinary diagnostic laboratory. Results: High performance (F1-macro = 0.9995) was achieved through the use of a rule-based syndrome classifier, based on rule induction followed by manual modification during the construction phase, which also resulted in clear interpretability of the resulting classification process. An unmodified rule induction algorithm achieved an F1-micro score of 0.979 though this fell to 0.677 when performance for individual classes was averaged in an unweighted manner (F1-macro), due to the fact that the algorithm failed to learn 3 of the 16 classes from the training set. Decision Trees showed equal interpretability to the rule-based approaches, but achieved an F1-micro score of 0.923 (falling to 0.311 when classes are given equal weight). A Naïve Bayes classifier learned all classes and achieved high performance (F1-micro = 0.994 and F1-macro =. 955), however the classification process is not transparent to the domain experts. Conclusion: The use of a manually customised rule set allowed for the development of a system for classification of laboratory tests into syndromic groups with very high performance, and high interpretability by the domain experts. Further research is required to develop internal validation rules in order to establish automated methods to update model rules without user input
Black Hole Evaporation in an Expanding Universe
We calculate the quantum radiation power of black holes which are asymptotic
to the Einstein-de Sitter universe at spatial and null infinities. We consider
two limiting mass accretion scenarios, no accretion and significant accretion.
We find that the radiation power strongly depends on not only the asymptotic
condition but also the mass accretion scenario. For the no accretion case, we
consider the Einstein-Straus solution, where a black hole of constant mass
resides in the dust Friedmann universe. We find negative cosmological
correction besides the expected redshift factor. This is given in terms of the
cubic root of ratio in size of the black hole to the cosmological horizon, so
that it is currently of order but could have been significant at the formation epoch of
primordial black holes. Due to the cosmological effects, this black hole has
not settled down to an equilibrium state. This cosmological correction may be
interpreted in an analogy with the radiation from a moving mirror in a flat
spacetime. For the significant accretion case, we consider the Sultana-Dyer
solution, where a black hole tends to increase its mass in proportion to the
cosmological scale factor. In this model, we find that the radiation power is
apparently the same as the Hawking radiation from the Schwarzschild black hole
of which mass is that of the growing mass at each moment. Hence, the energy
loss rate decreases and tends to vanish as time proceeds. Consequently, the
energy loss due to evaporation is insignificant compared to huge mass accretion
onto the black hole. Based on this model, we propose a definition of
quasi-equilibrium temperature for general conformal stationary black holes.Comment: Accepted for publication in Class.Quant.Grav., 18 pages and 3 figure
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