Ettingshausen effect due to Majorana modes

The presence of Majorana zero-energy modes at vortex cores in a topological superconductor implies that each vortex carries an extra entropy $s_0$, given by $(k_{B}/2)\ln 2$, that is independent of temperature. By utilizing this special property of Majorana modes, the edges of a topological superconductor can be cooled (or heated) by the motion of the vortices across the edges. As vortices flow in the transverse direction with respect to an external imposed supercurrent, due to the Lorentz force, a thermoelectric effect analogous to the Ettingshausen effect is expected to occur between opposing edges. We propose an experiment to observe this thermoelectric effect, which could directly probe the intrinsic entropy of Majorana zero-energy modes.Comment: 16 pages, 3 figure

Safety-Aware Apprenticeship Learning

Apprenticeship learning (AL) is a kind of Learning from Demonstration techniques where the reward function of a Markov Decision Process (MDP) is unknown to the learning agent and the agent has to derive a good policy by observing an expert's demonstrations. In this paper, we study the problem of how to make AL algorithms inherently safe while still meeting its learning objective. We consider a setting where the unknown reward function is assumed to be a linear combination of a set of state features, and the safety property is specified in Probabilistic Computation Tree Logic (PCTL). By embedding probabilistic model checking inside AL, we propose a novel counterexample-guided approach that can ensure safety while retaining performance of the learnt policy. We demonstrate the effectiveness of our approach on several challenging AL scenarios where safety is essential.Comment: Accepted by International Conference on Computer Aided Verification (CAV) 201

Role of hydrogen in volatile behaviour of defects in SiO2-based electronic devices

Charge capture and emission by point defects in gate oxides of metal–oxide–semiconductor field-effect transistors (MOSFETs) strongly affect reliability and performance of electronic devices. Recent advances in experimental techniques used for probing defect properties have led to new insights into their characteristics. In particular, these experimental data show a repeated dis- and reappearance (the so-called volatility) of the defect-related signals. We use multiscale modelling to explain the charge capture and emission as well as defect volatility in amorphous SiO2 gate dielectrics. We first briefly discuss the recent experimental results and use a multiphonon charge capture model to describe the charge-trapping behaviour of defects in silicon-based MOSFETs. We then link this model to ab initio calculations that investigate the three most promising defect candidates. Statistical distributions of defect characteristics obtained from ab initio calculations in amorphous SiO2 are compared with the experimentally measured statistical properties of charge traps. This allows us to suggest an atomistic mechanism to explain the experimentally observed volatile behaviour of defects. We conclude that the hydroxyl-E′ centre is a promising candidate to explain all the observed features, including defect volatility

Zero-voltage conductance peak from weak antilocalization in a Majorana nanowire

We show that weak antilocalization by disorder competes with resonant Andreev reflection from a Majorana zero-mode to produce a zero-voltage conductance peak of order e^2/h in a superconducting nanowire. The phase conjugation needed for quantum interference to survive a disorder average is provided by particle-hole symmetry - in the absence of time-reversal symmetry and without requiring a topologically nontrivial phase. We identify methods to distinguish the Majorana resonance from the weak antilocalization effect.Comment: 13 pages, 8 figures. Addendum, February 2014: Appendix B shows results for weak antilocalization in the circular ensemble. (This appendix is not in the published version.

Conformal mapping and shot noise in graphene

Ballistic transport through a collection of quantum billiards in undoped graphene is studied analytically within the conformal mapping technique. The billiards show pseudodiffusive behavior, with the conductance equal to that of a classical conductor characterized by the conductivity $\sigma_0=4e^2/\pi h$, and the Fano factor $F=1/3$. By shrinking at least one of the billiard openings, we observe a tunneling behavior, where the conductance shows a power-law decay with the system size, and the shot noise is Poissonian (F=1). In the crossover region between tunneling and pseudodiffusive regimes, the conductance $G\approx (1-F)\times se^2/h$. The degeneracy $s=8$ for the Corbino disk, which preserves the full symmetry of the Dirac equation, $s=4$ for billiards bounded with smooth edges which break the symplectic symmetry, and $s=2$ when abrupt edges lead to strong intervalley scattering. An alternative, analytical or numerical technique, is utilized for each of the billiards to confirm the applicability of the conformal mapping for various boundary conditions.Comment: RevTeX, 16 pages, 15 figure

Coordination and chemical effects on the structural, electronic and magnetic properties in Mn pnictides

Simple structures of MnX binary compounds, namely hexagonal NiAs and zincblende, are studied as a function of the anion (X = Sb, As, P) by means of the all-electron FLAPW method within local spin density and generalized gradient approximations. An accurate analysis of the structural, electronic and magnetic properties reveals that the cubic structure greatly favours the magnetic alignment in these compounds leading to high magnetic moments and nearly half-metallic behaviour for MnSb and MnAs. The effect of the anion chemical species is related to both its size and the possible hybridization with the Mn $d$ states; both contributions are seen to hinder the magnitude of the magnetic moment for small and light anions. Our results are in very good agreement with experiment - where available - and show that the generalized gradient approximation is essential to correctly recover both the equilibrium volume and magnetic moment.Comment: 18 pages and 4 figures, Latex-file, submitted to Phys.Rev.

Spelling in adolescents with dyslexia: errors and modes of assessment

In this study we focused on the spelling of high-functioning students with dyslexia. We made a detailed classification of the errors in a word and sentence dictation task made by 100 students with dyslexia and 100 matched control students. All participants were in the first year of their bachelor’s studies and had Dutch as mother tongue. Three main error categories were distinguished: phonological, orthographic, and grammatical errors (on the basis of morphology and language-specific spelling rules). The results indicated that higher-education students with dyslexia made on average twice as many spelling errors as the controls, with effect sizes of d ≥ 2. When the errors were classified as phonological, orthographic, or grammatical, we found a slight dominance of phonological errors in students with dyslexia. Sentence dictation did not provide more information than word dictation in the correct classification of students with and without dyslexia

Compressibility and Electronic Structure of MgB2 up to 8 GPa

The lattice parameters of MgB2 up to pressures of 8 GPa were determined using high-resolution x-ray powder diffraction in a diamond anvil cell. The bulk modulus, B0, was determined to be 151 +-5 GPa. Both experimental and first-principles calculations indicate nearly isotropic mechanical behavior under pressure. This small anisotropy is in contrast to the 2 dimensional nature of the boron pi states. The pressure dependence of the density of states at the Fermi level and a reasonable value for the average phonon frequency account within the context of BCS theory for the reduction of Tc under pressure.Comment: REVTeX file. 4 pages, 4 figure

Impact ionization in GaAs: a screened exchange density functional approach

Results are presented of a fully ab-initio calculation of impact ionization rates in GaAs within the density functional theory framework, using a screened-exchange formalism and the highly precise all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculated impact ionization rates show a marked orientation dependence in {\bf k} space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather soft edge and a threshold energy greater than the direct band gap. The consistency with available Monte Carlo and empirical pseudopotential calculations shows the reliability of our approach and paves the way to ab-initio calculations of pair production rates in new and more complex materials.Comment: 11 pages, 4 figures, Submitted to Phys. Rev.

Advanced Modeling of Charge Trapping: RTN, 1/f noise, SILC, and BTI (Invited Paper)

Abstract-In the course of years, several models have been put forward to explain noise phenomena, bias temperature instability (BTI), and gate leakage currents amongst other reliability issues. Mostly, these models have been developed independently and without considering that they may be caused by the same physical phenomenon. However, new experimental techniques have emerged, which are capable of studying these reliability issue on a microscopic level. One of them is the time-dependent defect spectroscopy (TDDS). Its intensive use has led to several interesting findings, including the fact that the recoverable component of BTI is due to reaction-limited processes. As a consequence, a quite detailed picture of the processes governing BTI has emerged. Interestingly, this picture has also been found to match the observations made for other reliability issues, such as random telegraph noise, 1/f noise, as well as gate leakage currents. Furthermore, the findings based on TDDS have lead to the development of capture/emission time (CET) maps, which can be used to understand the dynamic response of the defects given their widely distributed parameters