Phase Coherence and Superfluid-Insulator Transition in a Disordered Bose-Einstein Condensate

We have studied the effects of a disordered optical potential on the transport and phase coherence of a Bose-Einstein condensate (BEC) of 7Li atoms. At moderate disorder strengths (V_D), we observe inhibited transport and damping of dipole excitations, while in time-of-flight images, random but reproducible interference patterns are observed. In-situ images reveal that the appearance of interference is correlated with density modulation, without complete fragmentation. At higher V_D, the interference contrast diminishes as the BEC fragments into multiple pieces with little phase coherence.Comment: 4 pages, 5 figures, distortions in figures 1 and 4 have been fixed in version 3. This paper has been accepted to PR

Dual Geometric Worm Algorithm for Two-Dimensional Discrete Classical Lattice Models

We present a dual geometrical worm algorithm for two-dimensional Ising models. The existence of such dual algorithms was first pointed out by Prokof'ev and Svistunov \cite{ProkofevClassical}. The algorithm is defined on the dual lattice and is formulated in terms of bond-variables and can therefore be generalized to other two-dimensional models that can be formulated in terms of bond-variables. We also discuss two related algorithms formulated on the direct lattice, applicable in any dimension. These latter algorithms turn out to be less efficient but of considerable intrinsic interest. We show how such algorithms quite generally can be "directed" by minimizing the probability for the worms to erase themselves. Explicit proofs of detailed balance are given for all the algorithms. In terms of computational efficiency the dual geometrical worm algorithm is comparable to well known cluster algorithms such as the Swendsen-Wang and Wolff algorithms, however, it is quite different in structure and allows for a very simple and efficient implementation. The dual algorithm also allows for a very elegant way of calculating the domain wall free energy.Comment: 12 pages, 6 figures, Revtex

Degree of explanation

Partial explanations are everywhere. That is, explanations citing causes that explain some but not all of an effect are ubiquitous across science, and these in turn rely on the notion of degree of explanation. I argue that current accounts are seriously deficient. In particular, they do not incorporate adequately the way in which a cause’s explanatory importance varies with choice of explanandum. Using influential recent contrastive theories, I develop quantitative definitions that remedy this lacuna, and relate it to existing measures of degree of causation. Among other things, this reveals the precise role here of chance, as well as bearing on the relation between causal explanation and causation itself

A Comparative Study of the Valence Electronic Excitations of N_2 by Inelastic X-ray and Electron Scattering

Bound state, valence electronic excitation spectra of N_2 are probed by nonresonant inelastic x-ray and electron scattering. Within the usual theoretical treatments, dynamical structure factors derived from the two probes should be identical. However, we find strong disagreements outside the dipole scattering limit, even at high probe energies. This suggests an unexpectedly important contribution from intra-molecular multiple scattering of the probe electron from core electrons or the nucleus. These effects should grow progressively stronger as the atomic number of the target species increases.Comment: Submitted to Physical Review Letters April 27, 2010. 12 pages including 2 figure pages

First principles theory of chiral dichroism in electron microscopy applied to 3d ferromagnets

Recently it was demonstrated (Schattschneider et al., Nature 441 (2006), 486), that an analogue of the X-ray magnetic circular dichroism (XMCD) experiment can be performed with the transmission electron microscope (TEM). The new phenomenon has been named energy-loss magnetic chiral dichroism (EMCD). In this work we present a detailed ab initio study of the chiral dichroism in the Fe, Co and Ni transition elements. We discuss the methods used for the simulations together with the validity and accuracy of the treatment, which can, in principle, apply to any given crystalline specimen. The dependence of the dichroic signal on the sample thickness, accuracy of the detector position and the size of convergence and collection angles is calculated.Comment: 9 pages, 6 figures, submitted to Physical Review

Knowledge Sharing: Exploring Institutional Policy and Educator Practice Through Eportfolios In Music And Writing

Many higher education institutions have embraced e-Learning and urge, or make compulsory, engagement by academics. Despite this, it is often the educators themselves who take the initiative to engage with innovative e-learning approaches. These approaches, in turn, can influence both peer- and institution-wide thinking about e-Learning. This paper focuses on the introduction or extension of ePortfolios within the creative arts at four Australian universities. Each educator adopted the ePortfolio for a different purpose, and in doing so has influenced, or is at least being monitored by, their university. All four studies have resulted in the growth, development and enrichment of teaching and learning because of the ePortfolio's facility to engage students in such activities as reflection, ongoing student-teacher dialogue, collaborative essay writing, peer evaluation, identity formation, and the documentation of skills, competencies and graduate attributes for career awareness and employability. In sharing this knowledge the studies have also influenced curriculum design and e-learning policy. The academic literature notes institutional interest in ePortfolios in relation to career preparation, demonstrating and assessing student learning, academic advising, and addressing public accountability concerns by facilitating internal and external departmental review and accreditation. Within this paper we discuss the bi-directional impact and sharing of knowledge about ePortfolio use as it occurs between institution and educator. The study findings inform future development of curriculum, policy and practice for creative arts students and academics in a variety of higher education settings. Further, the findings suggest that ePortfolios provide an efficient and transparent means to archive and access student work, and that they facilitate internal and external departmental review and broader institutional assessment. © ACPI

Experimental detection of entanglement via witness operators and local measurements

In this paper we address the problem of detection of entanglement using only few local measurements when some knowledge about the state is given. The idea is based on an optimized decomposition of witness operators into local operators. We discuss two possible ways of optimizing this local decomposition. We present several analytical results and estimates for optimized detection strategies for NPT states of 2x2 and NxM systems, entangled states in 3 qubit systems, and bound entangled states in 3x3 and 2x4 systems.Comment: 24 pages, 2 figures. Contribution to the proceedings of the International Conference on Quantum Information in Oviedo, Spain (July 13-18, 2002). Error in W_W1-witness Eq. (35) corrected as well as minor typos. Reference adde

Photoemission Spectroscopy and the Unusually Robust One Dimensional Physics of Lithium Purple Bronze

Temperature dependent photoemission spectroscopy in Li0.9Mo6O17 contributes to evidence for one dimensional physics that is unusually robust. Three generic characteristics of the Luttinger liquid are observed, power law behavior of the k-integrated spectral function down to temperatures just above the superconducting transition, k-resolved lineshapes that show holon and spinon features, and quantum critical (QC) scaling in the lineshapes. Departures of the lineshapes and the scaling from expectations in the Tomonaga Luttinger model can be partially described by a phenomenological momentum broadening that is presented and discussed. The possibility that some form of 1d physics obtains even down to the superconducting transition temperature is assessed.Comment: submitted to JPCM, Special issue article "Physics in one dimension

Influence of Annealing on the Interface Structure and Strain Relief in Si/Ge Heterostructures on (100) Si

Research work on the general problem of the nature and thermal stability of the Si/Ge semiconductor interface is reviewed. We report on our recent studies of the interface structure in [(Si)m(Ge)n]p superlattices and (Ge)n layers buried in Si as revealed by Raman scattering, extended X-ray absorption fine structure, and X-ray techniques. Strain relaxation and interdiffusion in the superlattices caused by annealing have been investigated, and it is found that considerable strain-enhanced intermixing together with partial relaxation of Ge-Ge bonds occurs even for very short anneal times at 700°C. Further annealing leads to diffusion at a much slower rate and to the eventual formation of an alloy layer. The Ge-Ge bond lengths in as-grown samples are that expected for a fully strained Ge layer. Similar studies of the (Ge)n layers reveal that two-dimensional pseudomorphic growth proceeds up to n = 5, probably mediated by a Si-Ge interface interdiffusion over one or two monolayers of approximately 20%. A n = 12 layer gave evidence of strain relaxation by the introduction of dislocations and clustering. Interdiffusion proceeds rapidly on annealing at 750°C