Out of Equilibrium Solutions in the XYXY-Hamiltonian Mean Field model

Out of equilibrium magnetised solutions of the $XY$-Hamiltonian Mean Field ($XY$-HMF) model are build using an ensemble of integrable uncoupled pendula. Using these solutions we display an out-of equilibrium phase transition using a specific reduced set of the magnetised solutions

Interstellar extinction and the distribution of stellar populations in the direction of the ultra-deep Chandra Galactic field

We studied the stellar population in the central 6.6x6.6arcmin,region of the ultra-deep (1Msec) Chandra Galactic field - the "Chandra bulge field" (CBF) approximately 1.5 degrees away from the Galactic Center - using the Hubble Space Telescope ACS/WFC blue (F435W) and red (F625W) images. We mainly focus on the behavior of red clump giants - a distinct stellar population, which is known to have an essentially constant intrinsic luminosity and color. By studying the variation in the position of the red clump giants on a spatially resolved color-magnitude diagram, we confirm the anomalous total-to-selective extinction ratio, as reported in previous work for other Galactic bulge fields. We show that the interstellar extinction in this area is = 4 on average, but varies significantly between ~3-5 on angular scales as small as 1 arcminute. Using the distribution of red clump giants in an extinction-corrected color-magnitude diagram, we constrain the shape of a stellar-mass distribution model in the direction of this ultra-deep Chandra field, which will be used in a future analysis of the population of X-ray sources. We also show that the adopted model for the stellar density distribution predicts an infrared surface brightness in the direction of the "Chandra bulge field" in good agreement (i.e. within ~15%) with the actual measurements derived from the Spitzer/IRAC observations.Comment: 9 pages, 9 figures. Accepted for publication in A&

Large deviations for ideal quantum systems

We consider a general d-dimensional quantum system of non-interacting particles, with suitable statistics, in a very large (formally infinite) container. We prove that, in equilibrium, the fluctuations in the density of particles in a subdomain of the container are described by a large deviation function related to the pressure of the system. That is, untypical densities occur with a probability exponentially small in the volume of the subdomain, with the coefficient in the exponent given by the appropriate thermodynamic potential. Furthermore, small fluctuations satisfy the central limit theorem.Comment: 28 pages, LaTeX 2

Photometric variability in the old open cluster M 67. II. General Survey

We use differential CCD photometry to search for variability in BVI among 990 stars projected in and around the old open cluster M 67. In a previous paper we reported results for 22 cluster members that are optical counterparts to X-ray sources; this study focuses on the other stars in our observations. A variety of sampling rates were employed, allowing variability on time scales ranging from \sim 0.3 hours to \sim 20 days to be studied. Among the brightest sources studied, detection of variability as small as sigma approx 10 mmag is achieved (with > 3 sigma confidence); for the typical star observed, sensitivity to variability at levels sigma approx 20 mmag is achieved. The study is unbiased for stars with 12.5 < B < 18.5, 12.5 < V < 18.5, and 12 < I < 18 within a radius of about 10 arcmin from the cluster centre. In addition, stars with 10 < BVI < 12.5 were monitored in a few small regions in the cluster. We present photometry for all 990 sources studied, and report the variability characteristics of those stars found to be variable at a statistically significant level. Among the variables, we highlight several sources that merit future study, including stars located on the cluster binary sequence, stars on the giant branch, blue stragglers, and a newly discovered W UMa system.Comment: 12 pages, including 6 figures and 5 tables. Tables 1 and 3 only available in electronic version of paper. Accepted by A&

Gravitational diffraction radiation

We show that if the visible universe is a membrane embedded in a higher-dimensional space, particles in uniform motion radiate gravitational waves because of spacetime lumpiness. This phenomenon is analogous to the electromagnetic diffraction radiation of a charge moving near to a metallic grating. In the gravitational case, the role of the metallic grating is played by the inhomogeneities of the extra-dimensional space, such as a hidden brane. We derive a general formula for gravitational diffraction radiation and apply it to a higher-dimensional scenario with flat compact extra dimensions. Gravitational diffraction radiation may carry away a significant portion of the particle's initial energy. This allows to set stringent limits on the scale of brane perturbations. Physical effects of gravitational diffraction radiation are briefly discussed.Comment: 5 pages, 2 figures, RevTeX4. v2: References added. Version to appear in Phys. Rev.

Where the <i>really</i> hard problems Aren’t

Not all problem instances in combinatorial optimization are equally hard. One famous study "Where the Really Hard Problems Are" shows that for three decision problems and one optimization problem, computational costs can vary dramatically for equally sized instances. Moreover, runtimes could be predicted from an "order parameter", which is a property of the problem instance itself. For the only optimization problem in the study, the asymmetric traveling salesman problem (ATSP), the proposed order parameter was the standard deviation in the probability distribution used for generating distance matrices. For greater standard deviations, most randomly generated instances turned out to be easily solved to optimality, whereas smaller standard deviations produced harder instances. In this replication study, we show these findings can be contested. Most likely, the difference in instance hardness stems from a roundoff error that was possibly overlooked. This gives rise to a sudden emergence of minimum-cost tours, a feature that is readily exploited by most branch and bound algorithms. This new contradiction renders the earlier proposed order parameter unsuitable and changes the perspective on the fundamentals of ATSP instance hardness for this kind of algorithm

Rigorous investigation of the reduced density matrix for the ideal Bose gas in harmonic traps by a loop-gas-like approach

In this paper, we rigorously investigate the reduced density matrix (RDM) associated to the ideal Bose gas in harmonic traps. We present a method based on a sum-decomposition of the RDM allowing to treat not only the isotropic trap, but also general anisotropic traps. When focusing on the isotropic trap, the method is analogous to the loop-gas approach developed by W.J. Mullin in [38]. Turning to the case of anisotropic traps, we examine the RDM for some anisotropic trap models corresponding to some quasi-1D and quasi-2D regimes. For such models, we bring out an additional contribution in the local density of particles which arises from the mesoscopic loops. The close connection with the occurrence of generalized-BEC is discussed. Our loop-gas-like approach provides relevant information which can help guide numerical investigations on highly anisotropic systems based on the Path Integral Monte Carlo (PIMC) method.Comment: v3: Minor modifications of v2. v2: Major modifications: the former version (v1) has been completely rewritten. New results concerning the anisotropic traps and generalized Bose-Einstein condensation have been added. The connection with the loop-gas approach is further discussed. 40 page

Flipping after a pandemic: A case study

The idea of ‘flipping’ a course, i.e., delivering all content before class time and instead focusing on active learning opportunities, is not a new one. Studies have shown that these classrooms can increase student engagement and performance, while decreasing the required number of face-to-face hours (Karabulut‐Ilgu et al., 2018). However, it has also been shown that students state a lowered preference for these activities, believing they learn better in passive environments (Deslauriers et al., 2019). This mismatch of student preference and actual performance is particularly important as the COVID-19 pandemic has seen extremely low attendance rates across most science lectures and tutorials worldwide. In this study, the method of content delivery was flipped in a single unit from 3 lectures and one tutorial a week to 1 workshop a week and all content delivered before class time. The laboratory content remained the same. In particular, we used: lightboard videos made with Mayer’s Multimedia principles (Mayer, 2002) in mind, a blended online delivery platform with interactive H5P embedded questions, and full contextualised problem sets with weekly in-class quizzes. Using a range of questionnaires and student/staff interviews, alongside marks analysis of the cohort, we have found:    1. High attendance rates.    2. Students preferred the new mode.    3. Tutors stated an increase in the ‘level’ of student questions.    4. Marks surprisingly remained the same! REFERENCES Deslauriers, L., McCarty, L. S., Miller, K., Callaghan, K., &amp; Kestin, G. (2019). Measuring actual learning versus feeling of learning in response to being actively engaged in the classroom. Proceedings of the National Academy of Sciences, 116(39), 19251-19257. Mayer, R. E. (2002). Multimedia learning. In Psychology of learning and motivation (Vol. 41, pp. 85-139). Academic Press. Karabulut‐Ilgu, A., Jaramillo Cherrez, N., &amp; Jahren, C. T. (2018). A systematic review of research on the flipped learning method in engineering education. British Journal of Educational Technology, 49(3), 398-411

Volkov-Pankratov states in topological graphene nanoribbons

In topological systems, a modulation in the gap onset near interfaces can lead to the appearance of massive edge states, as were first described by Volkov and Pankratov. In this work, we study graphene nanoribbons in the presence of intrinsic spin-orbit coupling smoothly modulated near the system edges. We show that this space modulation leads to the appearance of Volkov-Pankratov states, in addition to the topologically protected ones. We obtain this result by means of two complementary methods, one based on the effective low-energy Dirac equation description and the other on a fully numerical tight-binding approach, finding excellent agreement between the two. We then show how transport measurements might reveal the presence of Volkov-Pankratov states, and discuss possible graphenelike structures in which such states might be observed