Motion and homogenization of vortices in anisotropic Type II superconductors

The motion of vortices in an anisotropic superconductor is considered. For a system of well-separated vortices, each vortex is found to obey a law of motion analogous to the local induction approximation, in which velocity of the vortex depends upon the local curvature and orientation. A system of closely packed vortices is then considered, and a mean field model is formulated in which the individual vortex lines are replaced by a vortex density

May the fourth be with you: a microarchitectural side channel attack on several real-world applications of Curve25519

Session D3: Logical Side ChannelsIn recent years, applications increasingly adopt security primitives designed with better countermeasures against side channel attacks. A concrete example is Libgcrypt’s implementation of ECDH encryption with Curve25519. The implementation employs the Montgomery ladder scalar-by-point multiplication, uses the unified, branchless Montgomery double-and-add formula and implements a constant-time argument swap within the ladder. However, Libgcrypt’s field arithmetic operations are not implemented in a constant-time side-channel-resistant fashion. Based on the secure design of Curve25519, users of the curve are advised that there is no need to perform validation of input points. In this work we demonstrate that when this recommendation is followed, the mathematical structure of Curve25519 facilitates the exploitation of side-channel weaknesses. We demonstrate the effect of this vulnerability on three software applications—encrypted git, email and messaging—that use Libgcrypt. In each case, we show how to craft malicious OpenPGP files that use the Curve25519 point of order 4 as a chosen ciphertext to the ECDH encryption scheme. We find that the resulting interactions of the point at infinity, order-2, and order-4 elements in the Montgomery ladder scalar-by-point multiplication routine create side channel leakage that allows us to recover the private key in as few as 11 attempts to access such malicious files.Daniel Genkin, Luke Valenta, Yuval Yaro

Star Formation Efficiency in the Central 1 kpc Region of Early-Type Spiral Galaxies

It has been reported recently that there are some early-type spiral (Sa--Sab) galaxies having evident star-forming regions which concentrate in their own central 1-kpc. In such central region, is the mechanism of the star formation distinct from that in disks of spiral galaxies? To reveal this, we estimate the star formation efficiency (SFE) in this central 1-kpc star-forming region of some early-type spiral galaxies, taking account of the condition for this 1-kpc region to be self-gravitating. Using two indicators of present star formation rate (H$\alpha$ and infrared luminosity), we estimate the SFE to be a few percents. This is equivalent to the observational SFE in the disks of late-type spiral (Sb--) galaxies. This coincidence may support the universality of the mean SFE of spiral galaxies reported in the recent studies. That is, we find no evidence of distinct mechanism of the star formation in the central 1-kpc region of early-type galaxies. Also, we examine the structure of the central star-forming region, and discuss a method for estimating the mass of star-forming regions.Comment: accepted by A

Self contamination effects in the TAUVEX UV Telescope: Ground testing and computer simulation

The contamination effects due to outgassing from construction materials of the TAUVEX (Tel Aviv University UV Telescope) were evaluated using a combination of ground testing and computer simulations. Tests were performed from the material level of the system level including: (1) High sensitivity CVCM(10(exp -3 percent) measurements of critical materials. (2) Optical degradation measurements of samples specially contaminated by outgassing products at different contamination levels. (3) FTIR studies of chemical composition of outgassed products on above samples. (4) High resolution AFM studies of surface morphology of contaminated surfaces. The expected degradation of TAUVEX performance in mission was evaluated applying a computer simulation code using input parameters determined experimentally in the above tests. The results have served as guidelines for the proper selection of materials, cleanliness requirements, determination of the thermal conditions of the system and bakeout processes

The low-frequency response in the surface superconducting state of ZrB12_{12} single crystal}

The large nonlinear response of a single crystal ZrB$_{12}$ to an ac field (frequency 40 - 2500 Hz) for $H_0>H_{c2}$ has been observed. Direct measurements of the ac wave form and the exact numerical solution of the Ginzburg-Landau equations, as well as phenomenological relaxation equation, permit the study of the surface superconducting states dynamics. It is shown, that the low frequency response is defined by transitions between the metastable superconducting states under the action of an ac field. The relaxation rate which determines such transitions dynamics, is found.Comment: 7 pages, 11 figure

Gauge covariances and nonlinear optical responses

The formalism of the reduced density matrix is pursued in both length and velocity gauges of the perturbation to the crystal Hamiltonian. The covariant derivative is introduced as a convenient representation of the position operator. This allow us to write compact expressions for the reduced density matrix in any order of the perturbation which simplifies the calculations of nonlinear optical responses; as an example, we compute the first and third order contributions of the monolayer graphene. Expressions obtained in both gauges share the same formal structure, allowing a comparison of the effects of truncation to a finite set of bands. This truncation breaks the equivalence between the two approaches: its proper implementation can be done directly in the expressions derived in the length gauge, but require a revision of the equations of motion of the reduced density matrix in the velocity gauge.The work of G.B.V. and D.J.P. is supported by Fundação para a Ciência e Tecnologia (FCT) under the Grants No. PD/BI/129220/2017 and No. PD/BD/135019/2017, respectively. N.M.R.P. acknowledges funding from the European Commission within the project "Graphene-Driven Revolutions in ICT and Beyond" (Ref. No. 696656) and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing Grant No. UID/FIS/04650/2013.info:eu-repo/semantics/publishedVersio

Поляризационные характеристики электролюминесценции, сопровождающей электрический пробой p—n-структур на карбиде кремния

Уперше отримано спектральну залежність ступеня лінійної поляризації електролюмінесценції, що супроводжує електричний пробій сплавних p-n-структур, виготовлених на основі політипів SiC-4Н, 6Н, 15R, а також кубічного карбіду кремнію в діапазоні 1,4-3,8 еВ. Структури розміщували на гранях кристалів, паралельних і перпендикулярних до кристалографічної осі С. Випромінювання виводили з боку тонкої р-області перпендикулярно та під гострим кутом до робочої грані кристала. Виявлено компоненти випромінювання, що мають лінійну поляризацію у площині, паралельній і перпендикулярній до осі С (Е||С, Е⊥С) та паралельній до вектора F напруженості електричного поля у p-n та інтенсивність компонент, пов’язаних із напрямком осі С, мають суттєві відмінності в різних політипах. Загальною рисою для всіх політипів є наявність поляризації (зі ступенем 0,3-0,4) у площині Е||С в області фундаментального поглинання та прилеглій ділянці. Відповідність поляризаційних характеристик випромінювання даним по оптичному поглинанню має місце лише в окремих випадках. Поляризація Е||F досягає ступеня 0,5 та має тенденцію до зростання в бік великих енергій фотона.Spectral dependence of the linear polarization extent of electroluminicsence, which accompanies the electric breakdown of alloyed p-n-structures, made on the base of SiC-4H, 6H, 15R polytypes, and also cubic silicon carbide in the range of 1,4-3,8 eB have been obtained for the first time. The structures have been placed on the crystal faces, which are parallel and perpendicular to the crystallographic axis C. The luminescence was lead out from the side of thin p-region perpendicularly, and also at the sharp angle to the operating crystal face. The components of luminescence which are polarized linearly in the parallel and perpendicular to the crystallographic axis C (E||C, E⊥C) plane, and also parallel to the vector F of electric field tensity in the p-n-junction (E||F) have been revealed. Spectral location and intensity of the component, connected with direction of the axis C, have essential differences in different polytypes. Common feature for all polytypes is the presence of polarization (with the level of 0,3-0,4) in the plane E||C in the region of fundamental absorption and in the adjoining region. Accordance of polarization characteristic of luminicsence to the date on optical absorption takes place only in the separate cases. Polarization E||F reaches the level 0,5 and tends to increase in the side of increasing of photon energy.Впервые получена спектральная зависимость степени линейной поляризации электролюминесценции, сопровождающей электрический пробой сплавных p-n-структур, изготовленных на основе политипов SiC-4Н, 6Н, 15R, а также кубического карбида кремния, в диапазоне 1,4-3,8 еВ. Структуры размещали на гранях кристаллов, параллельных и перпендикулярных кристаллографической оси С. Излучение выводили со стороны тонкой р-области перпендикулярно, а также под острым углом к рабочей грани кристалла. Выявлены компоненты излучения, линейно поляризованные в плоскости, параллельной и перпендикулярной кристаллографической оси С (Е||С, Е⊥С), а также параллельной вектору F напряженности электрического поля в p-n-переходе (Е||F). Спектральное положение и интенсивность компонент, связанных с направлением оси С, имеют существенные отличия в разных политипах. Общей чертой для всех политипов является наличие поляризации (со степенью 0,3-0,4) в плоскости Е||С в области фундаментального поглощения и прилегающей области. Соответствие поляризационных характеристик излучения данным по оптическому поглощению имеет место лишь в отдельных случаях. Поляризация Е||F достигает степени 0,5 и имеет тенденцию к возрастанию в сторону увеличения энергии фотона

Simultaneous Analysis of All SNPs in Genome-Wide and Re-Sequencing Association Studies

Testing one SNP at a time does not fully realise the potential of genome-wide association studies to identify multiple causal variants, which is a plausible scenario for many complex diseases. We show that simultaneous analysis of the entire set of SNPs from a genome-wide study to identify the subset that best predicts disease outcome is now feasible, thanks to developments in stochastic search methods. We used a Bayesian-inspired penalised maximum likelihood approach in which every SNP can be considered for additive, dominant, and recessive contributions to disease risk. Posterior mode estimates were obtained for regression coefficients that were each assigned a prior with a sharp mode at zero. A non-zero coefficient estimate was interpreted as corresponding to a significant SNP. We investigated two prior distributions and show that the normal-exponential-gamma prior leads to improved SNP selection in comparison with single-SNP tests. We also derived an explicit approximation for type-I error that avoids the need to use permutation procedures. As well as genome-wide analyses, our method is well-suited to fine mapping with very dense SNP sets obtained from re-sequencing and/or imputation. It can accommodate quantitative as well as case-control phenotypes, covariate adjustment, and can be extended to search for interactions. Here, we demonstrate the power and empirical type-I error of our approach using simulated case-control data sets of up to 500 K SNPs, a real genome-wide data set of 300 K SNPs, and a sequence-based dataset, each of which can be analysed in a few hours on a desktop workstation

Graphical modeling of binary data using the LASSO: a simulation study

Background: Graphical models were identified as a promising new approach to modeling high-dimensional clinical data. They provided a probabilistic tool to display, analyze and visualize the net-like dependence structures by drawing a graph describing the conditional dependencies between the variables. Until now, the main focus of research was on building Gaussian graphical models for continuous multivariate data following a multivariate normal distribution. Satisfactory solutions for binary data were missing. We adapted the method of Meinshausen and Buhlmann to binary data and used the LASSO for logistic regression. Objective of this paper was to examine the performance of the Bolasso to the development of graphical models for high dimensional binary data. We hypothesized that the performance of Bolasso is superior to competing LASSO methods to identify graphical models. Methods: We analyzed the Bolasso to derive graphical models in comparison with other LASSO based method. Model performance was assessed in a simulation study with random data generated via symmetric local logistic regression models and Gibbs sampling. Main outcome variables were the Structural Hamming Distance and the Youden Index. We applied the results of the simulation study to a real-life data with functioning data of patients having head and neck cancer. Results: Bootstrap aggregating as incorporated in the Bolasso algorithm greatly improved the performance in higher sample sizes. The number of bootstraps did have minimal impact on performance. Bolasso performed reasonable well with a cutpoint of 0.90 and a small penalty term. Optimal prediction for Bolasso leads to very conservative models in comparison with AIC, BIC or cross-validated optimal penalty terms. Conclusions: Bootstrap aggregating may improve variable selection if the underlying selection process is not too unstable due to small sample size and if one is mainly interested in reducing the false discovery rate. We propose using the Bolasso for graphical modeling in large sample sizes

Nonmonotone Barzilai-Borwein Gradient Algorithm for ℓ1\ell_1-Regularized Nonsmooth Minimization in Compressive Sensing

This paper is devoted to minimizing the sum of a smooth function and a nonsmooth $\ell_1$-regularized term. This problem as a special cases includes the $\ell_1$-regularized convex minimization problem in signal processing, compressive sensing, machine learning, data mining, etc. However, the non-differentiability of the $\ell_1$-norm causes more challenging especially in large problems encountered in many practical applications. This paper proposes, analyzes, and tests a Barzilai-Borwein gradient algorithm. At each iteration, the generated search direction enjoys descent property and can be easily derived by minimizing a local approximal quadratic model and simultaneously taking the favorable structure of the $\ell_1$-norm. Moreover, a nonmonotone line search technique is incorporated to find a suitable stepsize along this direction. The algorithm is easily performed, where the values of the objective function and the gradient of the smooth term are required at per-iteration. Under some conditions, the proposed algorithm is shown to be globally convergent. The limited experiments by using some nonconvex unconstrained problems from CUTEr library with additive $\ell_1$-regularization illustrate that the proposed algorithm performs quite well. Extensive experiments for $\ell_1$-regularized least squares problems in compressive sensing verify that our algorithm compares favorably with several state-of-the-art algorithms which are specifically designed in recent years.Comment: 20 page