Regeneration experiments below 10K in a regenerative-cycle cryocooler

At temperatures below 10K, regenerative cycle cryocoolers are limited by regeneration losses in the helium working fluid which result from the decreasing heat capacity of the regenerating material and the increasing density of helium. Experiments examining several approaches to improving the low-temperature regeneration in a four-stage regenerative cycle cooler constructed primarily of fiberglass materials are discussed. Using an interchangeable fourth stage, the experiments included configurations with multiple regeneration passages, and a static helium volume for increased heat capacity. Experiments using helium-3 as the working fluid and a Malone stage are planned. Results indicate that, using these techniques, it should be possible to construct a regenerative cycle cooler which will operate below 6K

Phase Transitions in liquid Helium 3

The phase transitions of liquid Helium 3 are described by truncations of an exact nonperturbative renormalization group equation. The location of the first order transition lines and the jump in the order parameter are computed quantitatively. At the triple point we find indications for partially universal behaviour. We suggest experiments that could help to determine the effective interactions between fermion pairs.Comment: 4 pages, 6 figures, LaTe

Machine-Checked Proofs For Realizability Checking Algorithms

Virtual integration techniques focus on building architectural models of systems that can be analyzed early in the design cycle to try to lower cost, reduce risk, and improve quality of complex embedded systems. Given appropriate architectural descriptions, assume/guarantee contracts, and compositional reasoning rules, these techniques can be used to prove important safety properties about the architecture prior to system construction. For these proofs to be meaningful, each leaf-level component contract must be realizable; i.e., it is possible to construct a component such that for any input allowed by the contract assumptions, there is some output value that the component can produce that satisfies the contract guarantees. We have recently proposed (in [1]) a contract-based realizability checking algorithm for assume/guarantee contracts over infinite theories supported by SMT solvers such as linear integer/real arithmetic and uninterpreted functions. In that work, we used an SMT solver and an algorithm similar to k-induction to establish the realizability of a contract, and justified our approach via a hand proof. Given the central importance of realizability to our virtual integration approach, we wanted additional confidence that our approach was sound. This paper describes a complete formalization of the approach in the Coq proof and specification language. During formalization, we found several small mistakes and missing assumptions in our reasoning. Although these did not compromise the correctness of the algorithm used in the checking tools, they point to the value of machine-checked formalization. In addition, we believe this is the first machine-checked formalization for a realizability algorithm.Comment: 14 pages, 1 figur

Rubidium and lead abundances in giant stars of the globular clusters M4 and M5

We present measurements of the neutron-capture elements Rb and Pb for bright giants in the globular clusters M4 and M5. The clusters are of similar metallicity ([Fe/H] = -1.2) but M4 is decidedly s-process enriched relative to M5: [Ba/Fe] = +0.6 for M4 but 0.0 for M5. The Rb and Pb abundances were derived by comparing synthetic spectra with high-resolution, high signal-to-noise ratio spectra obtained with MIKE on the Magellan telescope. Abundances of Y, Zr, La, and Eu were also obtained. In M4, the mean abundances from 12 giants are [Rb/Fe] = 0.39 +/- 0.02 (sigma = 0.07), [Rb/Zr] = 0.17 +/- 0.03 (sigma = 0.08), and [Pb/Fe] = 0.30 +/- 0.02 (sigma = 0.07). In M5, the mean abundances from two giants are [Rb/Fe] = 0.00 +/- 0.05 (sigma = 0.06), [Rb/Zr] = 0.08 +/- 0.08 (sigma = 0.11), and [Pb/Fe] = -0.35 +/- 0.02 (sigma = 0.04). Within the measurement uncertainties, the abundance ratios [Rb/Fe], [Pb/Fe] and [Rb/X] for X = Y, Zr, La are constant from star-to-star in each cluster and none of these ratios are correlated with O or Na abundances. While M4 has a higher Rb abundance than M5, the ratios [Rb/X] are similar in both clusters indicating that the nature of the s-products are very similar for each cluster but the gas from which M4's stars formed had a higher concentration of these products.Comment: Accepted for publication in Ap

Attacking Group Protocols by Refuting Incorrect Inductive Conjectures

Automated tools for finding attacks on flawed security protocols often fail to deal adequately with group protocols. This is because the abstractions made to improve performance on fixed 2 or 3 party protocols either preclude the modelling of group protocols all together, or permit modelling only in a fixed scenario, which can prevent attacks from being discovered. This paper describes Coral, a tool for finding counterexamples to incorrect inductive conjectures, which we have used to model protocols for both group key agreement and group key management, without any restrictions on the scenario. We will show how we used Coral to discover 6 previously unknown attacks on 3 group protocols

Truth Table Invariant Cylindrical Algebraic Decomposition by Regular Chains

A new algorithm to compute cylindrical algebraic decompositions (CADs) is presented, building on two recent advances. Firstly, the output is truth table invariant (a TTICAD) meaning given formulae have constant truth value on each cell of the decomposition. Secondly, the computation uses regular chains theory to first build a cylindrical decomposition of complex space (CCD) incrementally by polynomial. Significant modification of the regular chains technology was used to achieve the more sophisticated invariance criteria. Experimental results on an implementation in the RegularChains Library for Maple verify that combining these advances gives an algorithm superior to its individual components and competitive with the state of the art

CuIn1-xAlxSe2 Thin Films and Solar Cells

CuIn[sub 1-x]Al[sub x]Se[sub 2] thin films are investigated for their application as the absorber layer material for high efficiency solar cells. Single-phase CuIn[sub 1-x]Al[sub x]Se[sub 2] films were deposited by four source elemental evaporation with a composition range of 0≤x≤0.6. All these films demonstrate a normalized subband gap transmission \u3e85% with 2 µm film thickness. Band gaps obtained from spectroscopic ellipsometry show an increase with the Al content in the CuIn[sub 1-x]Al[sub x]Se[sub 2] film with a bowing parameter of 0.62. The structural properties investigated using x-ray diffraction measurements show a decrease in lattice spacing as the Al content increases. Devices with efficiencies greater than 10% are fabricated on CuIn[sub 1-x]Al[sub x]Se[sub 2] material over a wide range of Al composition. The best device demonstrated 11% efficiency, and the open circuit voltage increases to 0.73 V

High-Efficiency Solar Cells Based on Cu(InAl)Se[sub 2] Thin Films

A Cu(InAl)Se2solar cell with 16.9% efficiency is demonstrated using a Cu(InAl)Se2thin film deposited by four-source elemental evaporation and a device structure of glass/Mo/Cu(InAl)Se2/CdS/ZnO/indium tin oxide/(Ni/Algrid)/MgF2. A key to high efficiency is improved adhesion between the Cu(InAl)Se2 and the Mo back contact layer, provided by a 5-nm-thick Ga interlayer, which enabled the Cu(InAl)Se2 to be deposited at a 530 °C substrate temperature. Film and device properties are compared to Cu(InGa)Se2 with the same band gap of 1.16 eV. The solar cells have similar behavior, with performance limited by recombination through trap states in the space charge region in the Cu(InAl)Se2 or Cu(InGa)Se2 layer

Adding an Abstraction Barrier to ZF Set Theory

Much mathematical writing exists that is, explicitly or implicitly, based on set theory, often Zermelo-Fraenkel set theory (ZF) or one of its variants. In ZF, the domain of discourse contains only sets, and hence every mathematical object must be a set. Consequently, in ZF, with the usual encoding of an ordered pair ${\langle a, b\rangle}$, formulas like ${\{a\} \in \langle a, b \rangle}$ have truth values, and operations like ${\mathcal P (\langle a, b\rangle)}$ have results that are sets. Such 'accidental theorems' do not match how people think about the mathematics and also cause practical difficulties when using set theory in machine-assisted theorem proving. In contrast, in a number of proof assistants, mathematical objects and concepts can be built of type-theoretic stuff so that many mathematical objects can be, in essence, terms of an extended typed ${\lambda}$-calculus. However, dilemmas and frustration arise when formalizing mathematics in type theory. Motivated by problems of formalizing mathematics with (1) purely set-theoretic and (2) type-theoretic approaches, we explore an option with much of the flexibility of set theory and some of the useful features of type theory. We present ZFP: a modification of ZF that has ordered pairs as primitive, non-set objects. ZFP has a more natural and abstract axiomatic definition of ordered pairs free of any notion of representation. This paper presents axioms for ZFP, and a proof in ZF (machine-checked in Isabelle/ZF) of the existence of a model for ZFP, which implies that ZFP is consistent if ZF is. We discuss the approach used to add this abstraction barrier to ZF