Tricritical Phenomena at the Cerium γ→α\gamma \to \alpha Transition

The $\gamma \to \alpha$ isostructural transition in the Ce$_{0.9-x}$La$_x$Th$_{0.1}$ system is measured as a function of La alloying using specific heat, magnetic susceptibility, resistivity, thermal expansivity/striction measurements. A line of discontinuous transitions, as indicated by the change in volume, decreases exponentially from 118 K to close to zero with increasing La doping and the transition changes from being first-order to continuous at a critical concentration $0.10 \leq x_c \leq 0.14$. At the tricritical point, the coefficient of the linear $T$ term in the specific heat $\gamma$ and the magnetic susceptibility start to increase rapidly near $x$ = 0.14 and gradually approaches large values at $x$=0.35 signifying that a heavy Fermi-liquid state evolves at large doping. Near $x_c$, the Wilson ratio, $R_W$, has a value of 3.0, signifying the presence of magnetic fluctuations. Also, the low-temperature resistivity shows that the character of the low-temperature Fermi-liquid is changing

Three Questions on Lorentz Violation

We review the basics of the two most widely used approaches to Lorentz violation - the Stardard Model Extension and Noncommutative Field Theory - and discuss in some detail the example of the modified spectrum of the synchrotron radiation. Motivated by touching upon such a fundamental issue as Lorentz symmetry, we ask three questions: What is behind the search for Lorentz violation? Is String Theory a physical theory? Is there an alternative to Supersymmetry?Comment: 16 pages; invited luecture at DICE2006 - Piombino, Italy - September 200

Positive-Operator-Valued Time Observable in Quantum Mechanics

We examine the longstanding problem of introducing a time observable in Quantum Mechanics; using the formalism of positive-operator-valued measures we show how to define such an observable in a natural way and we discuss some consequences.Comment: 13 pages, LaTeX, no figures. Some minor changes, expanded the bibliography (now it is bigger than the one in the published version), changed the title and the style for publication on the International Journal of Theoretical Physic

Origin of complex crystal structures of elements at pressure

We present a unifying theory for the observed complex structures of the sp-bonded elements under pressure based on nearly free electron picture (NFE). In the intermediate pressure regime the dominant contribution to crystal structure arises from Fermi-surface Brillouin zone (FSBZ) interactions - structures which allow this are favoured. This simple theory explains the observed crystal structures, transport properties, the evolution of internal and unit cell parameters with pressure. We illustrate it with experimental data for these elements and ab initio calculation for Li.Comment: 4 pages 5 figure

Density scaling and quasiuniversality of flow-event statistics for athermal plastic flows

Athermal plastic flows were simulated for the Kob-Andersen binary Lennard-Jones system and its repulsive version in which the sign of the attractive terms is changed to a plus. Properties evaluated from simulations at different densities include the distributions of energy drops, stress drops, and strain intervals between the flow events. By reference to hidden scale invariance we show that simulations at a single density in conjunction with an equilibrium-liquid simulation at the same density allows one to predict the plastic flow-event properties at other densities. We furthermore demonstrate quasiuniversality of the flow-event statistics

Godel brane

We consider the brane-world generalisation of the Godel universe and analyse its dynamical interaction with the bulk. The exact homogeneity of the standard Godel spacetime no longer holds, unless the bulk is also static. We show how the anisotropy of the Godel-type brane is dictated by that of the bulk and find that the converse is also true. This determines the precise evolution of the nonlocal anisotropic stresses, without any phenomenological assumptions, and leads to a self-consistent closed set of equations for the evolution of the Godel brane. We also examine the causality of the Godel brane and show that the presence of the bulk cannot prevent the appearance of closed timelike curves.Comment: Revised version, to match paper published in Phys. Rev.

Perturbations on a moving D3-brane and mirage cosmology

We study the evolution of perturbations on a moving probe D3-brane coupled to a 4-form field in an AdS$_5$-Schwarzschild bulk. The unperturbed dynamics are parametrised by a conserved energy $E$ and lead to Friedmann-Robertson-Walker `mirage' cosmology on the brane with scale factor $a(\tau)$. The fluctuations about the unperturbed worldsheet are then described by a scalar field $\phi(\tau,\vec{x})$. We derive an equation of motion for $\phi$, and find that in certain regimes of $a$ the effective mass squared is negative. On an expanding BPS brane with E=0 superhorizon modes grow as $a^4$ whilst subhorizon modes are stable. When the brane contracts, all modes grow. We also briefly discuss the case when $E>0$, BPS anti-branes as well as non-BPS branes. Finally, the perturbed brane embedding gives rise to scalar perturbations in the FRW universe. We show that $\phi$ is proportional to the gauge invariant Bardeen potentials on the brane.Comment: 26 pages, 5 figures, to appear in Phys.Rev.D, comments and minor corrections adde

On Brane World Cosmological Perturbations

We discuss the scalar cosmological perturbations in a 3-brane world with a 5D bulk. We first show explicitly how the effective perturbed Einstein's equations on the brane (involving the Weyl fluid) are encoded into Mukohyama's master equation. We give the relation between Mukohyama's master variable and the perturbations of the Weyl fluid, we also discuss the relation between the former and the perturbations of matter and induced metric on the brane. We show that one can obtain a boundary condition on the brane for the master equation solely expressible in term of the master variable, in the case of a perfect fluid with adiabatic perturbations on a Randall-Sundrum (RS) or Dvali-Gabadadze-Porrati (DGP) brane. This provides an easy way to solve numerically for the evolution of the perturbations as well as should shed light on the various approximations done in the literature to deal with the Weyl degrees of freedom.Comment: 36 pages, 1 figur

(1+3) Covariant Dynamics of Scalar Perturbations in Braneworlds

We discuss the dynamics of linear, scalar perturbations in an almost Friedmann-Robertson-Walker braneworld cosmology of Randall-Sundrum type II using the 1+3 covariant approach. We derive a complete set of frame-independent equations for the total matter variables, and a partial set of equations for the non-local variables which arise from the projection of the Weyl tensor in the bulk. The latter equations are incomplete since there is no propagation equation for the non-local anisotropic stress. We supplement the equations for the total matter variables with equations for the independent constituents in a cold dark matter cosmology, and provide solutions in the high and low-energy radiation-dominated phase under the assumption that the non-local anisotropic stress vanishes. These solutions reveal the existence of new modes arising from the two additional non-local degrees of freedom. Our solutions should prove useful in setting up initial conditions for numerical codes aimed at exploring the effect of braneworld corrections on the cosmic microwave background (CMB) power spectrum. As a first step in this direction, we derive the covariant form of the line of sight solution for the CMB temperature anisotropies in braneworld cosmologies, and discuss possible mechanisms by which braneworld effects may remain in the low-energy universe.Comment: 22 pages replaced with additional references and minor corrections in Revtex4, and accepted for publication in Phys. Rev.