Do Athermal Amorphous Solids Exist?

We study the elastic theory of amorphous solids made of particles with finite range interactions in the thermodynamic limit. For the elastic theory to exist one requires all the elastic coefficients, linear and nonlinear, to attain a finite thermodynamic limit. We show that for such systems the existence of non-affine mechanical responses results in anomalous fluctuations of all the nonlinear coefficients of the elastic theory. While the shear modulus exists, the first nonlinear coefficient B_2 has anomalous fluctuations and the second nonlinear coefficient B_3 and all the higher order coefficients (which are non-zero by symmetry) diverge in the thermodynamic limit. These results put a question mark on the existence of elasticity (or solidity) of amorphous solids at finite strains, even at zero temperature. We discuss the physical meaning of these results and propose that in these systems elasticity can never be decoupled from plasticity: the nonlinear response must be very substantially plastic.Comment: 11 pages, 11 figure

Some Comments on Gravitational Entropy and the Inverse Mean Curvature Flow

The Geroch-Wald-Jang-Huisken-Ilmanen approach to the positive energy problem to may be extended to give a negative lower bound for the mass of asymptotically Anti-de-Sitter spacetimes containing horizons with exotic topologies having ends or infinities of the form $\Sigma_g \times {\Bbb R}$, in terms of the cosmological constant. We also show how the method gives a lower bound for for the mass of time-symmetric initial data sets for black holes with vectors and scalars in terms of the mass, $|Z(Q,P)|$ of the double extreme black hole with the same charges. I also give a lower bound for the area of an apparent horizon, and hence a lower bound for the entropy in terms of the same function $|Z(Q,P)|$. This shows that the so-called attractor behaviour extends beyond the static spherically symmetric case. and underscores the general importance of the function $|Z(Q,P)|$. There are hints that higher dimensional generalizations may involve the Yamabe conjectures.Comment: 13pp. late

Photoproduction off the nucleon revisited: Evidence for a narrow N(1688) resonance?

Revised analysis of $\Sigma$ beam asymmetry for the $\eta$ photoproduction on the free proton reveals a structure at $W\sim 1.69$ GeV. Fit of the experimental data based on the E429 solution of the SAID partial wave analysis suggests a narrow ($\Gamma \leq 25$ MeV) resonance. Possible candidates are $P_{11}, P_{13}$, or $D_{13}$ resonances. The result is considered in conjunction with the recent evidence for a bump-like structure at $W\sim 1.67 - 1.68$ GeV in the quasi-free $\eta$ photoproduction on the neutron.Comment: Contribution to the Workshop on the Physics of the Excited Nucleons NSTAR2007, Bonn, Germany, Sept. 5 - 8 2007. To be published in Eur.Phys.J.

New Physics Effects From B Meson Decays

In this talk, we point out some of the present and future possible signatures of physics beyond the Standard Model from B-meson decays, taking R-parity conserving and violating supersymmetry as illustrative examples.Comment: Talk given at the Sixth Workshop on High Energy Particle Phenomenology (WHEPP-6), Chennai (Madras), India. Includes 2 epsf figure

Double polarization hysteresis loop induced by the domain pinning by defect dipoles in HoMnO3 epitaxial thin films

We report on antiferroelectriclike double polarization hysteresis loops in multiferroic HoMnO3 thin films below the ferroelectric Curie temperature. This intriguing phenomenon is attributed to the domain pinning by defect dipoles which were introduced unintentionally during film growth process. Electron paramagnetic resonance suggests the existence of Fe1+ defects in thin films and first principles calculations reveal that the defect dipoles would be composed of oxygen vacancy and Fe1+ defect. We discuss migration of charged point defects during film growth process and formation of defect dipoles along ferroelectric polarization direction, based on the site preference of point defects. Due to a high-temperature low-symmetry structure of HoMnO3, aging is not required to form the defect dipoles in contrast to other ferroelectrics (e.g., BaTiO3).Comment: 4 figure

Hysteresis and the dynamic phase transition in thin ferromagnetic films

Hysteresis and the non-equilibrium dynamic phase transition in thin magnetic films subject to an oscillatory external field have been studied by Monte Carlo simulation. The model under investigation is a classical Heisenberg spin system with a bilinear exchange anisotropy in a planar thin film geometry with competing surface fields. The film exhibits a non-equilibrium phase transition between dynamically ordered and dynamically disordered phases characterized by a critical temperature Tcd, whose location of is determined by the amplitude H0 and frequency w of the applied oscillatory field. In the presence of competing surface fields the critical temperature of the ferromagnetic-paramagnetic transition for the film is suppressed from the bulk system value, Tc, to the interface localization-delocalization temperature Tci. The simulations show that in general Tcd < Tci for the model film. The profile of the time-dependent layer magnetization across the film shows that the dynamically ordered and dynamically disordered phases coexist within the film for T < Tcd. In the presence of competing surface fields, the dynamically ordered phase is localized at one surface of the film.Comment: PDF file, 21 pages including 8 figure pages; added references,typos added; to be published in PR

Orbitally driven spin-singlet dimerization in SS=1 La4_{4}Ru2_{2}O10_{10}

Using x-ray absorption spectroscopy at the Ru-$L_{2,3}$ edge we reveal that the Ru$^{4+}$ ions remain in the $S$=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U (LSDA+U) band structure calculations that the crystal fields in the low temperature phase are not strong enough to stabilize the $S$=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La$_{4}$Ru$_{2}$O$_{10}$ appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi 2-dimensional $S$=1 system.Comment: 5 pages, 4 figures, and 1 tabl

Extended Lifetime in Computational Evolution of Isolated Black Holes

Solving the 4-d Einstein equations as evolution in time requires solving equations of two types: the four elliptic initial data (constraint) equations, followed by the six second order evolution equations. Analytically the constraint equations remain solved under the action of the evolution, and one approach is to simply monitor them ({\it unconstrained} evolution). The problem of the 3-d computational simulation of even a single isolated vacuum black hole has proven to be remarkably difficult. Recently, we have become aware of two publications that describe very long term evolution, at least for single isolated black holes. An essential feature in each of these results is {\it constraint subtraction}. Additionally, each of these approaches is based on what we call "modern," hyperbolic formulations of the Einstein equations. It is generally assumed, based on computational experience, that the use of such modern formulations is essential for long-term black hole stability. We report here on comparable lifetime results based on the much simpler ("traditional") $\dot g$ - $\dot K$ formulation. We have also carried out a series of {\it constrained} 3-d evolutions of single isolated black holes. We find that constraint solution can produce substantially stabilized long-term single hole evolutions. However, we have found that for large domains, neither constraint-subtracted nor constrained $\dot g$ - $\dot K$ evolutions carried out in Cartesian coordinates admit arbitrarily long-lived simulations. The failure appears to arise from features at the inner excision boundary; the behavior does generally improve with resolution.Comment: 20 pages, 6 figure

Just how long can you live in a black hole and what can be done about it?

We study the problem of how long a journey within a black hole can last. Based on our observations, we make two conjectures. First, for observers that have entered a black hole from an asymptotic region, we conjecture that the length of their journey within is bounded by a multiple of the future asymptotic ``size'' of the black hole, provided the spacetime is globally hyperbolic and satisfies the dominant-energy and non-negative-pressures conditions. Second, for spacetimes with ${\Bbb R}^3$ Cauchy surfaces (or an appropriate generalization thereof) and satisfying the dominant energy and non-negative-pressures conditions, we conjecture that the length of a journey anywhere within a black hole is again bounded, although here the bound requires a knowledge of the initial data for the gravitational field on a Cauchy surface. We prove these conjectures in the spherically symmetric case. We also prove that there is an upper bound on the lifetimes of observers lying ``deep within'' a black hole, provided the spacetime satisfies the timelike-convergence condition and possesses a maximal Cauchy surface. Further, we investigate whether one can increase the lifetime of an observer that has entered a black hole, e.g., by throwing additional matter into the hole. Lastly, in an appendix, we prove that the surface area $A$ of the event horizon of a black hole in a spherically symmetric spacetime with ADM mass $M_{\text{ADM}}$ is always bounded by $A \le 16\pi M_{\text{ADM}}^2$, provided that future null infinity is complete and the spacetime is globally hyperbolic and satisfies the dominant-energy condition.Comment: 20 pages, REVTeX 3.0, 6 figures included, self-unpackin