EPR of cadmium ferric voltaite above the ferrimagnetic transition

A single isotropic EPR line of Fe³⁺ in synthetic cadmium ferric voltaite, (NH₄)₂Cd₅Fe₃Al(SO₄)₁₂ · 18H₂O, was observed in a wide temperature range from 295 to 1.57°K. The ferrimagnetic transition temperature of CdFe voltaite was determined to be ～ 0.7°K using the temperature dependence of the g-factor and the line width. The cubic crystal field parameter, a, for Fe³⁺ in CdFe voltaite is extracted from the EPR line width measurement using the exchange-narrowed line width model of Anderson and Weiss. The parameter a for Fe³⁺ in CdFe voltaite at 4.2°K is 157 × 10^-4cm^-1 which is consistent with the corresponding values for Fe³⁺ in other cubic structures.     

Non-ergodicity of Two Particles Interacting via a Smooth Potential

We examine two point particles interacting via a smooth Lennard-Jones-type potential of finite range on a two-dimensional torus. We find situations under which this system contains a stable, elliptic periodic orbit and hence is not ergodic. This result is in contrast to the case of hard spheres interacting via inelastic collision, which are always ergodic for two particles, are conjectured to be ergodic for arbitrarily many particles, and can never contain elliptic periodic orbits.     

Potentials on the two-torus for which the Hamiltonian flow is ergodic

We consider the motion of a test particle in a compound central potential field on a two-dimensional torus. We discuss three different classes of potentials (attracting, repelling, and mixed) that lead to Hamiltonian systems which have positive Lyapunov exponent almost everywhere and are ergodic. Included among the mixed potentials are smooth potentials without singularities.Partially supported by NSF grant DMS 8806067     

Using integrability to produce chaos: Billiards with positive entropy

A new open class of convex 2 dimensional planar billiards with positive Lyapunov exponent almost everywhere is constructed. We introduce the notion of a focusing arc and show that such arcs can be used to build billiard systems with positive Lyapunov exponents. We prove that under smallC ⁶ perturbations, focusing arcs remain focusing and thereby show that perturbations of the Bunimovich stadium billiard have positive Lyapunov exponents.Partially supported by NSF grant DMS 8806067     

Predicting points of zero charge of oxides and hydroxides

Pauling's electrostatic valence principle has been applied to describe the surface-charging mechanism of oxides in an aqueous environment. This approach has led to the development of an equation with which one can predict the points of zero charge (PZC) of oxides and hydroxides from crystallographic data. The equation proposed in the present work is an improved version of Parks' PZC equation. The improvements are the following: (i) The equation does not incorporate correction terms to take the cation coordination numbers into account; (ii) the PZC of a complex oxide can be predicted directly from crystallographic data instead of from “assumed” PZCs of its component oxides; and (iii) one can use the mean metal-oxygen bond distance of a crystal instead of the ionic radii tabulated by Parks, which are not consistent with the up-to-date values.     

Phorboid 20-homovanillates induce apoptosis through a VR1-independent mechanism

BACKGROUND: Vanilloids, such as capsaicin and resiniferatoxin (RTX), are recognized at the cell surface by vanilloid receptor type 1 (VR1), which has recently been cloned. VR1 mediates the effects of capsaicin and RTX in VR1-expressing cells, but vanilloids can induce apoptosis through a pathway not mediated by VR1. Phorboid 20-homovanillates can be used to investigate cell death induced by vanilloids. RESULTS: 12,13-Diacylphorbol-20 homovanillates were prepared by the sequential esterification of the natural polyol. Phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV) induced apoptosis in Jurkat cells to the same extent as RTX. Apoptosis was preceded by an increase in intracellular reactive oxygen species and by the loss of mitochondrial transmembrane potential. PPAHV-induced apoptosis was mediated by a pathway involving caspase-3 activation and was initiated at the S phase of the cell cycle. The cell-death pathway triggered by VR1 activation was studied in 293T cells transfected with the cloned rat vanilloid receptor. In this system, capsaicin and PPAHV induced cell death by an apparent necrotic mechanism, which was selectively inhibited by the competitive vanilloid receptor antagonist capsazepine. Interestingly, phorbol-12, 13-bisnonanoate-20-homovanillate, an analogue of PPAHV, induced cell death in VR1-transfected cells but could not trigger apoptosis in the Jurkat cell line. CONCLUSIONS: Vanilloids can induce cell death through different signalling pathways. The cell death induced in a VR1-independent manner has the hallmark of apoptosis, whereas the cell death mediated by vanilloids binding to VR1 is seemingly necrotic. Phorboid homovanillates that have antitumour and anti-inflammatory activities but lack the undesirable side effects of the natural vanilloids could be developed as potential drugs.     

A New Proof of the Existence of Embedded Surfaces with Anosov Geodesic Flow

We give a new proof of the existence of compact surfaces embedded in ?³ with Anosov geodesic flows. This proof starts with a noncompact model surface whose geodesic flow is shown to be Anosov using a uniformly strictly invariant cone condition. Using a sequence of explicit maps based on the standard torus embedding, we produce compact embedded surfaces that can be seen as small perturbations of the Anosov model system and hence are themselves Anosov.