"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, $H_{ex}$, of a few tesla in low resistance Be films with sheet resistance $R\ll R_Q$, where $R_Q=h/e^2$ is the quantum resistance. We show that $H_{ex}$ 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 $R\gg R_Q$.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 $^{129}$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

Evidence for massive bulk Dirac Fermions in Pb1−x_{1-x}Snx_xSe 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 $S_{xx}$. In the lowest LL, $S_{xx}$ 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

High-Field Shubnikov-de Haas Oscillations in the Topological Insulator Bi2_2Te2_2Se

We report measurements of the surface Shubnikov de Haas oscillations (SdH) on crystals of the topological insulator Bi$_2$Te$_2$Se. In crystals with large bulk resistivity ($\sim$4 $\Omega$cm at 4 K), we observe $\sim$15 surface SdH oscillations (to the $n$ = 1 Landau Level) in magnetic fields $B$ up to 45 Tesla. Extrapolating to the limit $1/B\to 0$, we confirm the $\frac12$-shift expected from a Dirac spectrum. The results are consistent with a very small surface Lande $g$-factor.Comment: Text expanded, slight changes in text, final version; Total 6 pages, 6 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 $\rho_{xx}$ is B-linear up to 35 T, while the Hall angle exhibits an unusual profile approaching a step-function. The conductivities $\sigma_{xx}$ and $\sigma_{xy}$ 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