Thermodynamic investigation of an insulator irradiated by a low-energy electron beam

The surface of an insulating material irradiated by a beam of low energy electrons charges positively if the yield of secondary electron is greater than unity. For such a dynamical equilibrium, the thermodynamic properties have been investigated by measuring the surface potential in response to a temperature oscillation of the material. It is shown that an oscillation amplitude of 0.4 K at 530 K induces an oscillation of the surface potential of about 0.5 volts. The frequency dependence indicates a monotonous decrease in the response with decreasing frequency, extrapolating to zero at zero frequency. We propose that this modification of the surface charge is driven by the temperature dependence of a gas of charged particles in equilibrium with the vacuum level

Design of an experimental setup to achieve sinusoidal temperature oscillation in ultrahigh vacuum

We designed an experimental setup to produce a temperature oscillation at the surface of a sample, in ultrahigh vacuum. The heating device, a tungsten wire, uses infrared radiation to heat the sample. A regular sine wave cannot produce a harmonic power oscillation, due to the nonlinear character of the Stefan-Boltzmann law of radiation. To achieve it we generate a complex wave form to take into account the thermal behavior of the heating filament as well as its electronic transport properties. An example of temperature oscillation is shown in the case of a tantalum sample, at a 0.12 Hz frequency. It exhibits harmonic behavior with an oscillation amplitude of about 1 K. This method opens the field to new experiments in surface science, to study reversible surface phase transitions

Improving the coherence of a low-energy electron beam by modulation

In a standard low-energy electron generator the beam is formed by particles traveling close to the axis of symmetry. However some electron trajectories are unstable and strongly dependent on the initial conditions. Numerical ray tracing shows that ultimate beam coherence is limited by these trajectories that pass far from the symmetry axis. This contribution can be partly eliminated by modulating the initial conditions and selecting the modulated response. This is exemplified with a low-energy electron beam used for electron diffraction, where the beam current modulation produces a modulated diffraction pattern that displays noteworthy improvement ͑sevenfold͒ in wave vector resolution

Triplet superconducting pairing and density-wave instabilities in organic conductors

Using a renormalization group approach, we determine the phase diagram of an extended quasi-one-dimensional electron gas model that includes interchain hopping, nesting deviations and both intrachain and interchain repulsive interactions. We find a close proximity of spin-density- and charge-density-wave phases, singlet d-wave and triplet f-wave superconducting phases. There is a striking correspondence between our results and recent puzzling experimental findings in the Bechgaard salts, including the coexistence of spin-density-wave and charge-density-wave phases and the possibility of a triplet pairing in the superconducting phase.Comment: 4 pages, 5 eps figure

Verifying the Safety of a Flight-Critical System

This paper describes our work on demonstrating verification technologies on a flight-critical system of realistic functionality, size, and complexity. Our work targeted a commercial aircraft control system named Transport Class Model (TCM), and involved several stages: formalizing and disambiguating requirements in collaboration with do- main experts; processing models for their use by formal verification tools; applying compositional techniques at the architectural and component level to scale verification. Performed in the context of a major NASA milestone, this study of formal verification in practice is one of the most challenging that our group has performed, and it took several person months to complete it. This paper describes the methodology that we followed and the lessons that we learned.Comment: 17 pages, 5 figure

Automatic Verification of Erlang-Style Concurrency

This paper presents an approach to verify safety properties of Erlang-style, higher-order concurrent programs automatically. Inspired by Core Erlang, we introduce Lambda-Actor, a prototypical functional language with pattern-matching algebraic data types, augmented with process creation and asynchronous message-passing primitives. We formalise an abstract model of Lambda-Actor programs called Actor Communicating System (ACS) which has a natural interpretation as a vector addition system, for which some verification problems are decidable. We give a parametric abstract interpretation framework for Lambda-Actor and use it to build a polytime computable, flow-based, abstract semantics of Lambda-Actor programs, which we then use to bootstrap the ACS construction, thus deriving a more accurate abstract model of the input program. We have constructed Soter, a tool implementation of the verification method, thereby obtaining the first fully-automatic, infinite-state model checker for a core fragment of Erlang. We find that in practice our abstraction technique is accurate enough to verify an interesting range of safety properties. Though the ACS coverability problem is Expspace-complete, Soter can analyse these verification problems surprisingly efficiently.Comment: 12 pages plus appendix, 4 figures, 1 table. The tool is available at http://mjolnir.cs.ox.ac.uk/soter

SO(4) Theory of Competition between Triplet Superconductivity and Antiferromagnetism in Bechgaard Salts

Motivated by recent experiments with Bechgaard salts, we investigate the competition between antiferromagnetism and triplet superconductivity in quasi one-dimensional electron systems. We unify the two orders in an SO(4) symmetric framework, and demonstrate the existence of such symmetry in one-dimensional Luttinger liquids. SO(4) symmetry, which strongly constrains the phase diagram, can explain coexistence regions between antiferromagnetic, superconducting, and normal phases, as observed in (TMTSF)$_2$PF$_6$. We predict a sharp neutron scattering resonance in superconducting samples.Comment: 5 pages, 3 figures; Added discussion of applicability of SO(4) symmetry for strongly anisotropic Fermi liquids; Added reference

Spin-triplet superconductivity in quasi-one dimension

We consider a system with electron-phonon interaction, antiferromagnetic fluctuations and disconnected open Fermi surfaces. The existence of odd-parity superconductivity in this circumstance is shown for the first time. If it is applied to the quasi-one-dimensional systems like the organic conductors (TMTSF)_2X we obtain spin-triplet superconductivity with nodeless gap. Our result is also valid in higher dimensions(2d and 3d).Comment: 2 page

Validation of Convex Optimization Algorithms and Credible Implementation for Model Predictive Control

Advanced real-time embedded algorithms are growing in complexity and length, related to the growth in autonomy, which allows vehicles to plan paths of their own. However, this promise cannot happen without proper attention to the considerably stronger operational constraints that real time, safety-critical applications must meet. This paper discusses the formal verification for optimization algorithms with a particular emphasis on receding-horizon controllers. Following a brief historical overview, a prototype autocoder for embedded convex optimization algorithms is discussed. Options for encoding code properties and proofs, and their applicability and limitations is detailed as well

A Study of Concurrency Bugs and Advanced Development Support for Actor-based Programs

The actor model is an attractive foundation for developing concurrent applications because actors are isolated concurrent entities that communicate through asynchronous messages and do not share state. Thereby, they avoid concurrency bugs such as data races, but are not immune to concurrency bugs in general. This study taxonomizes concurrency bugs in actor-based programs reported in literature. Furthermore, it analyzes the bugs to identify the patterns causing them as well as their observable behavior. Based on this taxonomy, we further analyze the literature and find that current approaches to static analysis and testing focus on communication deadlocks and message protocol violations. However, they do not provide solutions to identify livelocks and behavioral deadlocks. The insights obtained in this study can be used to improve debugging support for actor-based programs with new debugging techniques to identify the root cause of complex concurrency bugs.Comment: - Submitted for review - Removed section 6 "Research Roadmap for Debuggers", its content was summarized in the Future Work section - Added references for section 1, section 3, section 4.3 and section 5.1 - Updated citation