Driven alloys in the athermal limit

Static molecular simulations of binary alloys under extrinsic forcing show that complex ordered or segregated structures may evolve even in the absence of thermally activated diffusion. This result is in opposition to the standard theoretical framework for so-called "driven alloys," which assumes that extrinsic driving is an ideally randomizing process, and therefore predicts only random atomic configurations in the athermal limit. We propose a qualitative modification to the theory that introduces a new control parameter and use additional Monte Carlo simulations to demonstrate the physical plausibility of this modification. New research directions in nonequilibrium dynamic systems are also suggested by this analysis.     

Open Cosmologies with Rotation

We study a rotating and expanding, Gödel type metric, originally considered by Korotkii and Obukhov [1, 2], showing that, in the limit of large times and nearby distances, it reduces to the open metric of Friedmann. In the epochs when radiation or dust matter dominate the energy density, our solutions are similar to the isotropic ones and, in what concerns processes occurring at small times, the rotation leads only to higher order corrections. At large times, the solution is dominated by a decaying positive cosmological term, with negative pressure, and necessarily describes a quasi-flat universe if the energy conditions have to be satisfied. The absence of closed time-like curves requires a superior limit for the global angular velocity, which appears as a natural explanation for the observed smallness of the present rotation. The conclusion is that the introduction of a global rotation, in addition to be compatible with observation, can enrich the standard model of the Universe, explaining issues like the origin of galaxies rotation and the quasi-flatness problem.     

Strength,plasticity and brittleness of bulk metallic glasses under compression: statistical and geometric effects

To investigate the flaw sensitivity and reliability of bulk metallic glasses (BMGs), compressive testing was performed on a statistically significant number of specimens. Despite the fact that BMGs exhibit little or no macroscopic plasticity before failure (similar to other brittle materials), we observe surprisingly high uniformity in compressive strength. Weibull analysis was employed to study the statistical dispersion in strength, giving very high Weibull moduli of about 25 for an intrinsically brittle glass, and near 75 for an intrinsically malleable one. This high uniformity is encouraging for the use of BMGs in structural applications. Furthermore, we illustrate that subtle imperfections in the test geometry (i.e. miscut or deviations from orthogonality) dramatically affect the compression response. In brittle glasses these act as failure-critical flaws, whereas in malleable glasses they constrain shear bands, lead to tilting and bending during testing, and give rise to misleading macroscopic measurements of plastic deformation.     

Time-Dependent Dephasing and Quantum Transport

The investigation of the phenomenon of dephasing assisted quantum transport, which happens when the presence of dephasing benefits the efficiency of this process, has been mainly focused on Markovian scenarios associated with constant and positive dephasing rates in their respective Lindblad master equations. What happens if we consider a more general framework, where time-dependent dephasing rates are allowed, thereby, permitting the possibility of non-Markovian scenarios? Does dephasing-assisted transport still manifest for non-Markovian dephasing? Here, we address these open questions in a setup of coupled two-level systems. Our results show that the manifestation of non-Markovian dephasing-assisted transport depends on the way in which the incoherent energy sources are locally coupled to the chain. This is illustrated with two different configurations, namely non-symmetric and symmetric. Specifically, we verify that non-Markovian dephasing-assisted transport manifested only in the non-symmetric configuration. This allows us to draw a parallel with the conditions in which time-independent Markovian dephasing-assisted transport manifests. Finally, we find similar results by considering a controllable and experimentally implementable system, which highlights the significance of our findings for quantum technologies.     

Reflection of a slow cesium atomic beam from a naturally magnetized Nd-Fe-B surface

We have demonstrated the partly directed reflection of a slow cesium atomic beam by using the natural magnetic stray field above a Nd-Fe-B surface. From these experiments we determine the reflectivity and a minimum value for the magnetic stray field directly at the surface. Received: 5 July 1999 / Revised version: 6 October 1999 / Published online: 3 November 1999     

Pair-breaking in kondo superconductors

An earlier theory for superconductors with magnetic impurities is extended by taking into account explicitly the energy dependence of pair breaking. Results on the transition temperature and the specific heat jumps are in very good agreement with experimental data on (La_1-cCe_c)Al₂.     

Temperature-induced broadening of the emission lines from a quantum-dot nanostructure

We report on the effect of temperature fluctuations on the midinfrared electroluminescence from a cascade of coupled AlInAs quantum dots and GaAs quantum wells. The observed line width is significantly broadened with increasing temperature. We then present our theoretical results on homogeneous line broadening due to temperature fluctuations for our experimental system. Our numerical simulations clearly indicate that, temperature fluctuations can account for the observed finite width of the emission lines at high-temperatures.     

Molecular mechanics study of conformational trends in simple alcohols and ethers. II. Intramolecular hydrogen bonding

In this article we present an extension of our modified MM2(80) force field MM2MOD in which a potential function for hydrogen bonding in alcohols and ethers is included. The results of applying MM2(85), MM2(87), and MM2MOD on ethylene glycol, 2-methoxyethanol and 1,3-propanediol are reported and compared with available experimental data and ab initio results. It is concluded that hydrogen bonding plays an important role in determining the molecular conformations of these systems. © 1992 by John Wiley & Sons, Inc.     

Fluorescence and intersystem crossing as a function of rotational energy on the S1 state of benzene

The effect of rotational excitation on the electronic relaxation of the S₁ state of “isolated” benzene has been studied by examination of resonance fluorescence generated when narrow-band exciting light is swept through the rotational envelopes of four S₁ ← S₀ absorption bands. To within the ten percent uncertainty in measurement, the fluorescence yields are independent of the position of the exciting light within an absorption band. Thus neither radiative decay nor intersystem crossing to the triplet state displays large sensitivity to molecular rotation, at least when excited state relaxation proceeds from rotational distributions of moderate diversity.     

Ab Initio Calculations on the Vinyl Fluoride Fragmentation Reactions

High level ab initio calculations for the fragmentation reactions of vinyl fluoride were performed. The relative energies calculated at the QCISD(T)/6-311G(2d,2p) level of theory, corrected with MP2/6-311G(2d,2p) zero-point energies (ZPEs), differ significantly from those obtained previously at a lower level of theory. The calculations suggest that both the threeand four-center HF elimination processes are likely to occur, with the three-center elimination favored over the four-center at high energies.