A simple topological model with continuous phase transition

In the area of topological and geometric treatment of phase transitions and symmetry breaking in Hamiltonian systems, in a recent paper some general sufficient conditions for these phenomena in $\mathbb{Z}_2$-symmetric systems (i.e. invariant under reflection of coordinates) have been found out. In this paper we present a simple topological model satisfying the above conditions hoping to enlighten the mechanism which causes this phenomenon in more general physical models. The symmetry breaking is testified by a continuous magnetization with a nonanalytic point in correspondence of a critical temperature which divides the broken symmetry phase from the unbroken one. A particularity with respect to the common pictures of a phase transition is that the nonanalyticity of the magnetization is not accompanied by a nonanalytic behavior of the free energy.Comment: 17 pages, 7 figure

Topological conditions for discrete symmetry breaking and phase transitions

In the framework of a recently proposed topological approach to phase transitions, some sufficient conditions ensuring the presence of the spontaneous breaking of a Z_2 symmetry and of a symmetry-breaking phase transition are introduced and discussed. A very simple model, which we refer to as the hypercubic model, is introduced and solved. The main purpose of this model is that of illustrating the content of the sufficient conditions, but it is interesting also in itself due to its simplicity. Then some mean-field models already known in the literature are discussed in the light of the sufficient conditions introduced here

VLA Imaging of the Disk Surrounding the Nearby Young Star TW Hya

The TW Hya system is perhaps the closest analog to the early solar nebula. We have used the Very Large Array to image TW Hya at wavelengths of 7mm and 3.6 cm with resolutions 0.1 arcseconds (about 5 AU) and 1.0 arcseconds (about 50 AU), respectively. The 7mm emission is extended and appears dominated by a dusty disk of radius larger than 50 AU surrounding the star. The 3.6 cm emission is unresolved and likely arises from an ionized wind or gyrosynchrotron activity. The dust spectrum and spatially resolved 7mm images of the TW Hya disk are fitted by a simple model with temperature and surface density described by radial power laws, $T(r)\propto r^{-0.5}$ and $\Sigma(r) \propto r^{-1}$. These properties are consistent with an irradiated gaseous accretion disk of mass $\sim0.03~{\rm M_{\odot}}$ with an accretion rate $\sim10^{-8}~{\rm M_{\odot}yr^{-1}}$ and viscosity parameter $\alpha = 0.01$. The estimates of mass and mass accretion rates are uncertain as the gas-to-dust ratio in the TW Hya disk may have evolved from the standard interstellar value.Comment: 13 pages, 3 figures, accepted by ApJ Letter

Energy thresholds for discrete breathers

Discrete breathers are time-periodic, spatially localized solutions of the equations of motion for a system of classical degrees of freedom interacting on a lattice. An important issue, not only from a theoretical point of view but also for their experimental detection, are their energy properties. We considerably enlarge the scenario of possible energy properties presented by Flach, Kladko, and MacKay [Phys. Rev. Lett. 78, 1207 (1997)]. Breather energies have a positive lower bound if the lattice dimension is greater than or equal to a certain critical value d_c. We show that d_c can generically be greater than two for a large class of Hamiltonian systems. Furthermore, examples are provided for systems where discrete breathers exist but do not emerge from the bifurcation of a band edge plane wave. Some of these systems support breathers of arbitrarily low energy in any spatial dimension.Comment: 4 pages, 4 figure

The Phase Effect in Electronic Stopping: A Survey of the Contributing Physical Processes

The phase effect in electronic stopping deals with the question whether the energy loss of an ion due to the interaction with electrons depends on the state of aggregation of the target. It is commonly accepted that charge changing collisions of the projectile and changes in the electronic states of the target contribute to the phase effect In addition, the energy loss measurements might possibly be influenced by different impact parameter selection in the two experiments (solid and gas phase). Quantitative results of our calculations show that generally the impact parameter selection inherently present in a transmission experiment is quenched by the inevitable multiple scattering of the projectiles. Thus, electronic excitation and ionization in the projectile and the target are the only processes that contribute significantly to the phase effect

Effective Sample Size: Quick Estimation of the Effect of Related Samples in Genetic Case-Control Association Analyses

Correlated samples have been frequently avoided in case-control
genetic association
 studies in part because the methods for handling them are either not
easily implemented or not widely known. We
advocate one method for case-control association analysis of correlated
samples -- the effective sample size method -- as a simple and
accessible approach that does not require specialized computer programs.
The effective sample size method captures the variance inflation
of allele frequency estimation exactly, and can be used to modify the
chi-square test statistic, p-value, and 95% confidence interval of
odds-ratio simply by replacing the apparent number of allele counts with the
effective ones. For genotype frequency estimation, although a single
effective sample size is unable to completely characterize the variance inflation,
an averaged one can satisfactorily approximate the simulated result.
The effective sample size method is applied to the rheumatoid arthritis
siblings data collected from the North American Rheumatoid Arthritis Consortium (NARAC)
to establish a significant association with the interferon-induced
helicasel gene (IFIH1) previously being identified as a type 1 diabetes
susceptibility locus. Connections between the effective sample size
method and other methods, such as generalized estimation equation,
variance of eigenvalues for correlation matrices, and genomic controls,
are also discussed.
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Hellmut Fritzsche

In a letter he wrote in June 2018, the month of his passing, Hellmut Fritzsche, professor emeritus at the University of Chicago, reflected on his life and marriage: “[My wife] Sybille and I were so very fortunate to have been selected in 1950 to the group of European students who were invited by the US government to come to the United States for one year. This is how we met. How immensely admirable was the generosity of the United States! We both had a rich and rewarding life. We are grateful for the opportunities given to us and for the wonderful life and inspiring circle of friends who questioned and supported us intellectually.” Those of us in that circle were richly rewarded for having experienced Hellmut’s inquisitive mind and vibrant nature

Hellmut Fritzsche

In a letter he wrote in June 2018, the month of his passing, Hellmut Fritzsche, professor emeritus at the University of Chicago, reflected on his life and marriage: “[My wife] Sybille and I were so very fortunate to have been selected in 1950 to the group of European students who were invited by the US government to come to the United States for one year. This is how we met. How immensely admirable was the generosity of the United States! We both had a rich and rewarding life. We are grateful for the opportunities given to us and for the wonderful life and inspiring circle of friends who questioned and supported us intellectually.” Those of us in that circle were richly rewarded for having experienced Hellmut’s inquisitive mind and vibrant nature

The Possibilist Transactional Interpretation and Relativity

A recent ontological variant of Cramer's Transactional Interpretation, called "Possibilist Transactional Interpretation" or PTI, is extended to the relativistic domain. The present interpretation clarifies the concept of 'absorption,' which plays a crucial role in TI (and in PTI). In particular, in the relativistic domain, coupling amplitudes between fields are interpreted as amplitudes for the generation of confirmation waves (CW) by a potential absorber in response to offer waves (OW), whereas in the nonrelativistic context CW are taken as generated with certainty. It is pointed out that solving the measurement problem requires venturing into the relativistic domain in which emissions and absorptions take place; nonrelativistic quantum mechanics only applies to quanta considered as 'already in existence' (i.e., 'free quanta'), and therefore cannot fully account for the phenomenon of measurement, in which quanta are tied to sources and sinks.Comment: Final version with some minor corrections as published in Foundations of Physics. This paper has significant overlap with Chapter 6 of my book on the Transactional Interpretation, forthcoming from Cambridge University Press: http://www.cambridge.org/us/knowledge/isbn/item6860644/?site_locale=en_US (Additional preview material is available at rekastner.wordpress.com) Comments welcom

The close T Tauri binary system V4046 Sgr: Rotationally modulated X-ray emission from accretion shocks

We report initial results from a quasi-simultaneous X-ray/optical observing campaign targeting V4046 Sgr, a close, synchronous-rotating classical T Tauri star (CTTS) binary in which both components are actively accreting. V4046 Sgr is a strong X-ray source, with the X-rays mainly arising from high-density (n_e ~ 10^(11-12) cm^(-3)) plasma at temperatures of 3-4 MK. Our multiwavelength campaign aims to simultaneously constrain the properties of this X-ray emitting plasma, the large scale magnetic field, and the accretion geometry. In this paper, we present key results obtained via time-resolved X-ray grating spectra, gathered in a 360 ks XMM-Newton observation that covered 2.2 system rotations. We find that the emission lines produced by this high-density plasma display periodic flux variations with a measured period, 1.22+/-0.01 d, that is precisely half that of the binary star system (2.42 d). The observed rotational modulation can be explained assuming that the high-density plasma occupies small portions of the stellar surfaces, corotating with the stars, and that the high-density plasma is not azimuthally symmetrically distributed with respect to the rotational axis of each star. These results strongly support models in which high-density, X-ray-emitting CTTS plasma is material heated in accretion shocks, located at the base of accretion flows tied to the system by magnetic field lines.Comment: paper accepted by Ap