Frustrated Blume-Emery-Griffiths model

A generalised integer S Ising spin glass model is analysed using the replica formalism. The bilinear couplings are assumed to have a Gaussian distribution with ferromagnetic mean = Jo. Incorporation of a quadrupolar interaction term and a chemical potential leads to a richer phase diagram with transitions of first and second order. The first order transition may be interpreted as a phase separation, and contrary to what has been argued previously, it persists in the presence of disorder. Finally, the stability of the replica symmetric solution with respect to fluctuations in replica space is analysed, and the transition lines are obtained both analytically and numerically.Comment: 16 pages, 11 figure

Cut Size Statistics of Graph Bisection Heuristics

We investigate the statistical properties of cut sizes generated by heuristic algorithms which solve approximately the graph bisection problem. On an ensemble of sparse random graphs, we find empirically that the distribution of the cut sizes found by ``local'' algorithms becomes peaked as the number of vertices in the graphs becomes large. Evidence is given that this distribution tends towards a Gaussian whose mean and variance scales linearly with the number of vertices of the graphs. Given the distribution of cut sizes associated with each heuristic, we provide a ranking procedure which takes into account both the quality of the solutions and the speed of the algorithms. This procedure is demonstrated for a selection of local graph bisection heuristics.Comment: 17 pages, 5 figures, submitted to SIAM Journal on Optimization also available at http://ipnweb.in2p3.fr/~martin

The evolution of the self-lensing binary KOI-3278: evidence of extra energy sources during CE evolution

Post-common-envelope binaries (PCEBs) have been frequently used to observationally constrain models of close-compact-binary evolution, in particular common-envelope (CE) evolution. However, recent surveys have detected PCEBs consisting of a white dwarf (WD) exclusively with an M dwarf companion. Thus, we have been essentially blind with respect to PCEBs with more massive companions. Recently, the second PCEB consisting of a WD and a G-type companion, the spectacularly self-lensing binary KOI-3278, has been identified. This system is different from typical PCEBs not only because of the G-type companion, but also because of its long orbital period. Here we investigate whether the existence of KOI-3278 provides new observational constraints on theories of CE evolution. We reconstruct its evolutionary history and predict its future using BSE, clarifying the proper use of the binding energy parameter in this code. We find that a small amount of recombination energy, or any other source of extra energy, is required to reconstruct the evolutionary history of KOI-3278. Using BSE we derive progenitor system parameters of M1,i = 2.450 Msun, M2,i = 1.034 Msun, and Porb,i ~ 1300 d. We also find that in ~9 Gyr the system will go through a second CE phase leaving behind a double WD, consisting of a C/O WD and a He WD with masses of 0.636 Msun and 0.332 Msun, respectively. After IK Peg, KOI-3278 is the second PCEB that clearly requires an extra source of energy, beyond that of orbital energy, to contribute to the CE ejection. Both systems are special in that they have long orbital periods and massive secondaries. This may also indicate that the CE efficiency increases with secondary mass.Comment: Accepted for publication in A&A Letters, 4 pages, 2 figure

White dwarf masses in cataclysmic variables

The white dwarf (WD) mass distribution of cataclysmic variables (CVs) has recently been found to dramatically disagree with the predictions of the standard CV formation model. The high mean WD mass among CVs is not imprinted in the currently observed sample of CV progenitors and cannot be attributed to selection effects. Two possibilities have been put forward: either the WD grows in mass during CV evolution, or in a significant fraction of cases, CV formation is preceded by a (short) phase of thermal time-scale mass transfer (TTMT) in which the WD gains a sufficient amount of mass. We investigate if either of these two scenarios can bring theoretical predictions and observations into agreement. We employed binary population synthesis models to simulate the present intrinsic CV population. We incorporated aspects specific to CV evolution such as an appropriate mass-radius relation of the donor star and a more detailed prescription for the critical mass ratio for dynamically unstable mass transfer. We also implemented a previously suggested wind from the surface of the WD during TTMT and tested the idea of WD mass growth during the CV phase by arbitrarily changing the accretion efficiency. We compare the model predictions with the characteristics of CVs derived from observed samples. We find that mass growth of the WDs in CVs fails to reproduce the observed WD mass distribution. In the case of TTMT, we are able to produce a large number of massive WDs if we assume significant mass loss from the surface of the WD during the TTMT phase. However, the model still produces too many CVs with helium WDs. Moreover, the donor stars are evolved in many of these post-TTMT CVs, which contradicts the observations. We conclude that in our current framework of CV evolution neither TTMT nor WD mass growth can fully explain either the observed WD mass or the period distribution in CVs.Comment: 15 pages, 7 figures, 1 table, accepted for publication in A&A. Replaced and added a reference, corrected typo

A 1.82 m^2 ring laser gyroscope for nano-rotational motion sensing

We present a fully active-controlled He-Ne ring laser gyroscope, operating in square cavity 1.35 m in side. The apparatus is designed to provide a very low mechanical and thermal drift of the ring cavity geometry and is conceived to be operative in two different orientations of the laser plane, in order to detect rotations around the vertical or the horizontal direction. Since June 2010 the system is active inside the Virgo interferometer central area with the aim of performing high sensitivity measurements of environmental rotational noise. So far, continuous not attempted operation of the gyroscope has been longer than 30 days. The main characteristics of the laser, the active remote-controlled stabilization systems and the data acquisition techniques are presented. An off-line data processing, supported by a simple model of the sensor, is shown to improve the effective long term stability. A rotational sensitivity at the level of ten nanoradiants per squareroot of Hz below 1 Hz, very close to the required specification for the improvement of the Virgo suspension control system, is demonstrated for the configuration where the laser plane is horizontal

Non-Markovian data-driven modeling of single-cell motility

Trajectories of human breast cancer cells moving on one-dimensional circular tracks are modeled by thenon-Markovian version of the Langevin equation that includes an arbitrary memory function. When averagedover cells, the velocity distribution exhibits spurious non-Gaussian behavior, while single cells are characterizedby Gaussian velocity distributions. Accordingly, the data are described by a linear memory model whichincludes different random walk models that were previously used to account for various aspects of cell motilitysuch as migratory persistence, non-Markovian effects, colored noise, and anomalous diffusion. The memoryfunction is extracted from the trajectory data without restrictions or assumptions, thus making our approachtruly data driven, and is used for unbiased single-cell comparison. The cell memory displays time-delayedsingle-exponential negative friction, which clearly distinguishes cell motion from the simple persistent randomwalk model and suggests a regulatory feedback mechanism that controls cell migration. Based on the extractedmemory function we formulate a generalized exactly solvable cell migration model which indicates thatnegative friction generates cell persistence over long timescales. The nonequilibrium character of cell motionis investigated by mapping the non-Markovian Langevin equation with memory onto a Markovian model thatinvolves a hidden degree of freedom and is equivalent to the underdamped active Ornstein-Uhlenbeck process

Thickness dependence of linear and quadratic magneto-optical Kerr effect in ultrathin Fe(001) films

Magneto-optical Kerr effect (MOKE) magnetometry is one of the most widely employed techniques for the characterization of ferromagnetic thin-film samples. Some information, such as coercive fields or anisotropy strengths can be obtained without any knowledge of the optical and magneto-optical (MO) properties of the material. On the other hand, a quantitative analysis, which requires a precise knowledge of the material's index of refraction n and the MO coupling constants K and G is often desirable, for instance for the comparison of samples, which are different with respect to ferromagnetic layer thicknesses, substrates, or capping layers. While the values of the parameters n and the linear MO coupling parameter K reported by different authors usually vary considerably, the relevant quadratic MO coupling parameters G of Fe are completely unknown. Here, we report on measurements of the thickness dependence (0-60nm) of the linear and quadratic MOKE in epitaxial bcc-Fe(001) wedge-type samples performed at a commonly used laser wavelength of 670nm. By fitting the thickness dependence we are able to extract a complete set of parameters n, K, (G11 - G12), and G44 for the quantitative description of the MOKE of bcc-Fe(001). We find sizable different n, K, and G parameters for films thinner than about 10nm as compared to thicker films, which is indicative of a thickness dependence of the electronic properties or of surface contributions to the MOKE. The effect size of the quadratic MOKE is found to be about a third of the record values recently reported for Co2FeSi.Comment: 8 pages, 5 figure