Widely separated binary systems of very low mass stars

In this paper we review some recent detections of wide binary brown dwarf systems and discuss them in the context of the multiplicity properties of very low-mass stars and brown dwarfs.Comment: 2 pages, 1 figure (new version with minor corrections); to appear in the proceedings of the workshop "Ultra-low mass star formation and evolution", to be published in A

Discovery of Radio Emission from the Tight M8 Binary: LP 349-25

We present radio observations of 8 ultracool dwarfs with a narrow spectral type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8 binary LP 349-25 was detected. LP 349-25 is the tenth ultracool dwarf system detected in radio and its trigonometric parallax pi = 67.6 mas, recently measured by Gatewood et al., makes it the furthest ultracool system detected by the Very Large Array to date, and the most radio-luminous outside of obvious flaring activity or variability. With a separation of only 1.8 AU, masses of the components of LP 349-25 can be measured precisely without any theoretical assumptions (Forveille et al.), allowing us to clarify their fully-convective status and hence the kind of magnetic dynamo in these components which may play an important role to explain our detection of radio emission from these objects. This also makes LP 349-25 an excellent target for further studies with better constraints on the correlations between X-ray, radio emission and stellar parameters such as mass, age, temperature, and luminosity in ultracool dwarfs.Comment: accepted by ApJ, referee's comments included, typo in equation 1 correcte

Unexpected phase locking of magnetic fluctuations in the multi-k magnet USb

The spin waves in the multi-k antiferromagnet USb soften and become quasielastic well below the antiferromagnetic ordering temperature TN. This occurs without a magnetic or structural transition. It has been suggested that this change is in fact due to dephasing of the different multi-k components: a switch from 3-k to 1-k behavior. In this work, we use inelastic neutron scattering with tridirectional polarization analysis to probe the quasielastic magnetic excitations and reveal that the 3-k structure does not dephase. More surprisingly, the paramagnetic correlations also maintain the same clear phase correlations well above TN (up to at least 1.4TN)

Finite temperature Casimir pistons for electromagnetic field with mixed boundary conditions and its classical limit

In this paper, the finite temperature Casimir force acting on a two-dimensional Casimir piston due to electromagnetic field is computed. It was found that if mixed boundary conditions are assumed on the piston and its opposite wall, then the Casimir force always tends to restore the piston towards the equilibrium position, regardless of the boundary conditions assumed on the walls transverse to the piston. In contrary, if pure boundary conditions are assumed on the piston and the opposite wall, then the Casimir force always tend to pull the piston towards the closer wall and away from the equilibrium position. The nature of the force is not affected by temperature. However, in the high temperature regime, the magnitude of the Casimir force grows linearly with respect to temperature. This shows that the Casimir effect has a classical limit as has been observed in other literatures.Comment: 14 pages, 3 figures, accepted by Journal of Physics

Investigating the benefits of molecular profiling of advanced non-small cell lung cancer tumors to guide treatments.

In this study we utilized data on patient responses to guided treatments, and we evaluated their benefit for a non-small cell lung cancer cohort. The recommended therapies used were predicted using tumor molecular profiles that involved a range of biomarkers but primarily used immunohistochemistry markers. A dataset describing 91 lung non-small cell lung cancer patients was retrospectively split into two. The first group's drugs were consistent with a treatment plan whereby all drugs received agreed with their tumor's molecular profile. The second group each received one or more drug that was expected to lack benefit. We found that there was no significant difference in overall survival or mortality between the two groups. Patients whose treatments were predicted to be of benefit survived for an average of 402 days, compared to 382 days for those that did not (P = 0.7934). In the matched treatment group, 48% of patients were deceased by the time monitoring had finished compared to 53% in the unmatched group (P = 0.6094). The immunohistochemistry biomarker for the ERCC1 receptor was found to be a marker that could be used to predict future survival; ERCC1 loss was found to be predictive of poor survival

A two measure model of dark energy and dark matter

In this work we construct a unified model of dark energy and dark matter. This is done with the following three elements: a gravitating scalar field, phi with a non-conventional kinetic term, as in the string theory tachyon; an arbitrary potential, V(phi); two measures -- a metric measure (sqrt{-g}) and a non-metric measure (Phi). The model has two interesting features: (i) For potentials which are unstable and would give rise to tachyonic scalar field, this model can stabilize the scalar field. (ii) The form of the dark energy and dark matter that results from this model is fairly insensitive to the exact form of the scalar field potential.Comment: 8 pages,no figures, revtex, typos corrected to match published versio

Machine Learning and Portfolio Optimization

The portfolio optimization model has limited impact in practice due to estimation issues when applied with real data. To address this, we adapt two machine learning methods, regularization and cross-validation, for portfolio optimization. First, we introduce performance-based regularization (PBR), where the idea is to constrain the sample variances of the estimated portfolio risk and return, which steers the solution towards one associated with less estimation error in the performance. We consider PBR for both mean-variance and mean-CVaR problems. For the mean-variance problem, PBR introduces a quartic polynomial constraint, for which we make two convex approximations: one based on rank-1 approximation and another based on a convex quadratic approximation. The rank-1 approximation PBR adds a bias to the optimal allocation, and the convex quadratic approximation PBR shrinks the sample covariance matrix. For the mean-CVaR problem, the PBR model is a combinatorial optimization problem, but we prove its convex relaxation, a QCQP, is essentially tight. We show that the PBR models can be cast as robust optimization problems with novel uncertainty sets and establish asymptotic optimality of both Sample Average Approximation (SAA) and PBR solutions and the corresponding efficient frontiers. To calibrate the right hand sides of the PBR constraints, we develop new, performance-based k-fold cross-validation algorithms. Using these algorithms, we carry out an extensive empirical investigation of PBR against SAA, as well as L1 and L2 regularizations and the equally-weighted portfolio. We find that PBR dominates all other benchmarks for two out of three of Fama-French data sets

Screening nuclear field fluctuations in quantum dots for indistinguishable photon generation

A semiconductor quantum dot can generate highly coherent and indistinguishable single photons. However, intrinsic semiconductor dephasing mechanisms can reduce the visibility of two-photon interference. For an electron in a quantum dot, a fundamental dephasing process is the hyperfine interaction with the nuclear spin bath. Here we directly probe the consequence of the fluctuating nuclear spins on the elastic and inelastic scattered photon spectra from a resident electron in a single dot. We find the nuclear spin fluctuations lead to detuned Raman scattered photons which are distinguishable from both the elastic and incoherent components of the resonance fluorescence. This significantly reduces two-photon interference visibility. However, we demonstrate successful screening of the nuclear spin noise which enables the generation of coherent single photons that exhibit high visibility two-photon interference.Comment: 5 pages, 4 figures + Supplementary Informatio