17,505 research outputs found
"Virus hunting" using radial distance weighted discrimination
Motivated by the challenge of using DNA-seq data to identify viruses in human
blood samples, we propose a novel classification algorithm called "Radial
Distance Weighted Discrimination" (or Radial DWD). This classifier is designed
for binary classification, assuming one class is surrounded by the other class
in very diverse radial directions, which is seen to be typical for our virus
detection data. This separation of the 2 classes in multiple radial directions
naturally motivates the development of Radial DWD. While classical machine
learning methods such as the Support Vector Machine and linear Distance
Weighted Discrimination can sometimes give reasonable answers for a given data
set, their generalizability is severely compromised because of the linear
separating boundary. Radial DWD addresses this challenge by using a more
appropriate (in this particular case) spherical separating boundary.
Simulations show that for appropriate radial contexts, this gives much better
generalizability than linear methods, and also much better than conventional
kernel based (nonlinear) Support Vector Machines, because the latter methods
essentially use much of the information in the data for determining the shape
of the separating boundary. The effectiveness of Radial DWD is demonstrated for
real virus detection.Comment: Published at http://dx.doi.org/10.1214/15-AOAS869 in the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Exchange Field-Mediated Magnetoresistance in the Correlated Insulator Phase of Be Films
We present a study of the proximity effect between a ferromagnet and a
paramagnetic metal of varying disorder. Thin beryllium films are deposited onto
a 5 nm-thick layer of the ferromagnetic insulator EuS. This bilayer arrangement
induces an exchange field, , of a few tesla in low resistance Be films
with sheet resistance , where is the quantum resistance.
We show that survives in very high resistance films and, in fact,
appears to be relatively insensitive to the Be disorder. We exploit this fact
to produce a giant low-field magnetoresistance in the correlated insulator
phase of Be films with .Comment: To be published in Physical Review Letter
Imaginary Potential Induced Quantum Coherence for Bose-Einstein Condensates
The role of complex potentials in single-body Schr\H{o}dinger equation has
been studied intensively. We study the quantum coherence for degenerate Bose
gases in complex potentials, when the exchange symmetry of identical bosons is
considered. For initially independent Bose-Einstein condensates, it is shown
that even very weak imaginary potential can induce perfect quantum coherence
between different condensates. The scheme to observe imaginary potential
induced quantum coherence is discussed.Comment: 4 pages, 4 figure
Non-linear amplification of small spin precession using long range dipolar interactions
In measurements of small signals using spin precession the precession angle
usually grows linearly in time. We show that non-linear interactions between
particles can lead to an exponentially growing spin precession angle, resulting
in an amplification of small signals and raising them above the noise level of
a detection system. We demonstrate amplification by a factor of greater than 8
of a spin precession signal due to a small magnetic field gradient in a
spherical cell filled with hyperpolarized liquid Xe. This technique can
improve the sensitivity in many measurements that are limited by the noise of
the detection system, rather then the fundamental spin-projection noise.Comment: 4 pages, 4 figure
High-Field Shubnikov-de Haas Oscillations in the Topological Insulator BiTeSe
We report measurements of the surface Shubnikov de Haas oscillations (SdH) on
crystals of the topological insulator BiTeSe. In crystals with large
bulk resistivity (4 cm at 4 K), we observe 15 surface SdH
oscillations (to the = 1 Landau Level) in magnetic fields up to 45
Tesla. Extrapolating to the limit , we confirm the -shift
expected from a Dirac spectrum. The results are consistent with a very small
surface Lande -factor.Comment: Text expanded, slight changes in text, final version; Total 6 pages,
6 figure
Evidence for massive bulk Dirac Fermions in PbSnSe from Nernst and thermopower experiments
The lead chalcogenides (Pb,Sn)Te and (Pb,Sn)Se are the first examples of
topological crystalline insulators (TCI) predicted \cite{Fu,Hsieh} (and
confirmed \cite{Hasan,Story,Takahashi}) to display topological surface Dirac
states (SDS) that are protected by mirror symmetry. A starting premise
\cite{Hsieh} is that the SDS arise from bulk states describable as massive
Dirac states \cite{Wallis,Svane}, but this assumption is untested. Here we show
that the thermoelectric response of the bulk states display features specific
to the Dirac spectrum. We show that, in the quantum limit, the lowest Landau
Level (LL) is singly spin-degenerate, whereas higher levels are doubly
degenerate. The abrupt change in spin degeneracy leads to a large step-decrease
in the thermopower . In the lowest LL, displays a striking
linear increase vs. magnetic field. In addition, the Nernst signal undergoes an
anomalous sign change when the bulk gap inverts at 180 K.Comment: 16 pages, 8 figure
Anomalous conductivity tensor in the Dirac semimetal Na_3Bi
Na3Bi is a Dirac semimetal with protected nodes that may be sensitive to the
breaking of time-reversal invariance in a magnetic field B. We report
experiments which reveal that both the conductivity and resistivity tensors
exhibit robust anomalies in B. The resistivity is B-linear up to 35
T, while the Hall angle exhibits an unusual profile approaching a
step-function. The conductivities and share
identical power-law dependences at large B. We propose that these significant
deviations from conventional transport result from an unusual sensitivity of
the transport lifetime to B. Comparison with Cd3As2 is made.Comment: 8 pages, 5 figure
The Application of ALOS/PALSAR InSAR to Measure Subsurface Penetration Depths in Deserts
Spaceborne Synthetic Aperture Radar (SAR) interferometry has been utilised to acquire
high-resolution Digital Elevation Models (DEMs) with wide coverage, particularly for persistently
cloud-covered regions where stereophotogrammetry is hard to apply. Since the discovery of sand
buried drainage systems by the Shuttle Imaging Radar-A (SIR-A) L-band mission in 1982, radar
images have been exploited to map subsurface features beneath a sandy cover of extremely low loss
and low bulk humidity in some hyper-arid regions such as from the Japanese Earth Resources Satellite
1 (JERS-1) and Advanced Land Observing Satellite/Phased Array type L-band Synthetic Aperture
Radar (ALOS/PALSAR). Therefore, we hypothesise that a Digital Elevation Model (DEM) derived
by InSAR in hyper-arid regions is likely to represent a subsurface elevation model, especially for
lower frequency radar systems, such as the L-band system (1.25 GHz). In this paper, we compare the
surface appearance of radar images (L-band and C-band) with that of optical images to demonstrate
their different abilities to show subsurface features. Moreover, we present an application of L-band
InSAR to measure penetration depths in the eastern Sahara Desert. We demonstrate how the retrieved
L-band InSAR DEM appears to be of a consistently 1–2 m lower elevation than the C-band Shuttle
Radar Topography Mission (SRTM) DEM over sandy covered areas, which indicates the occurrence
of penetration and confirms previous studies
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