Doped magnetic moments in a disordered electron system: insulator-metal transition, spin glass and `cmr'

Recent experiments on the amorphous magnetic semiconductor Gd_x Si_{1-x}, Phys. Rev. Lett. 77, 4652 (1996), ibid 83, 2266 (1999), ibid 84, 5411 (2000), ibid 85, 848 (2000), have revealed an insulator-metal transition (i-m-t), as a function of doping and magnetic field, a spin glass state at low temperature, and colossal magnetoresistance close to the i-m-t. There are also signatures of strong electron-electron interaction close to the i-m-t. Motivated by these results we examine the role of doped magnetic moments in a strongly disordered electron system. In this paper we study a model of electrons coupled to structural disorder and (classical) magnetic moments, through an essentially exact combination of spin Monte Carlo and fermion exact diagonalisation. Our preliminary results, ignoring electron-electron interactions, highlights the interplay of structural and magnetic `disorder' which is primarily responsible for the observed features in magnetism and transport.Comment: 12 pages, two column revtex, with 11 embedded figure

Superconducting proximity effect in a diffusive ferromagnet with spin-active interfaces

We reconsider the problem of the superconducting proximity effect in a diffusive ferromagnet bounded by tunneling interfaces, using spin-dependent boundary conditions. This introduces for each interface a phase-shifting conductance Gphi which results from the spin dependence of the phase shifts acquired by the electrons upon scattering on the interface. We show that Gphi strongly affects the density of states and supercurrents predicted for superconducting/ferromagnetic hybrid circuits. We show the relevance of this effect by identifying clear signatures of Gphi in the data of T. Kontos et al [Phys. Rev. Lett. 86, 304 (2001), ibid. 89, 137007 (2002)].Comment: submitted to Phys. Rev. Let

Infra-red absorption spectra and molecular structure of triatomic molecules

This report is written as a résumé of investigations of infra -red absorption spectra of triatomic molecules carried out during tenure of a Senior Research Award. It might have been written in thesis form giving details of experimental and other work, but much of this has already been published, and copies of the papers have been sent the Department; these papers run into some hundred pages of print, and a thesis would merely form unnecessary repetition occupying almostdouble thib number of typewritten pages. A resume which brings together the results, and gives the now well established methods of interpreting infra -red absorption spectra of triatomic molecules, seems therefore the more satisfactory form for the report.Published work will be referred to as follows: 'Investigations in the Infra -Red Region of the Spectrum' (1). Part I (Froc. Roy. Soc., 1930, A, 130, 133). || (2). Part II. (ibis, 1930, A, 130, 142). || (3). Part III. (ibid, 1931, A, 132, 236). || (4). Part IV. (ibid, 1931, A, 132, 252). || (5). Part V. (ibid, 1932, A, 135, 375). || (6). Part VI. (ibid, 1932, A, 137, 622).- || (7). Fart VII. (ibid, 1932, A, 138, 531). || (8). Part VIII. (ibid, 1933, A, 140, 193). || (a). Part IX. (ibid, 1933, A, 142, 129). || (b). Part X (ibid, 1934, A, 145, 338). || (9). 'Infra -Red and Raman Bands of Carbon Dioxide, Carbonyl Sulphide, and Carbon Disulphide'. (Z. Physik, 1932, 79, 35). || (10). 'Molecular Configuration of N20.' (Phys. Rev., 1932, 39, 534). || (11). 'Infra -Red Absorption Spectrum of Nitrogen Dioxide'. || (Nature, 1933, 131, 239). (12). 'The Asymmetrical Rotator and its Infra -Red Spectrum'. (Trans. Faraday Soc., 1930, XXVI, 197). || (13). 'A Method of Eliminating Magnetic Disturbance of High Sensitivity Galvanometers'. (J. Sci. Inst., 1930, VII). || (14). 'Structure of Triatomic I.Iolecules'. A. B. D. Cassie (Nature, 1933, 131, 438). || (15). 'Investigations of the Infra -Red Absorption Spectra of Sulphur Dioxide, Carbon Disulphide, and Carbonyl Sulphide'. A thesis for the Ph.D. degree at London University, by A. B. D. Cassie, June 1931. || (16). 'Investigations in the Infra -Red Region of the Spectrum Part XI - The Absorption Spectrum and Molecular Configuration of Boron Trichioride, and the Effect of Strain on Plane Groups of the Type XYs. (Proc. Roy. Soc., 1935, A, 148, 87).This work has been published in collaboration with Mr. C. R. Bailey, Lecturer at University College, London, apart from references (13), (14), (15), and (16)

Bending and Base-Stacking Interactions in Double-Stranded Semiflexible Polymer

Simple expressions for the bending and the base-stacking energy of double-stranded semiflexible biopolymers (such as DNA and actin) are derived. The distribution of the folding angle between the two strands is obtained by solving a Schr\"{o}dinger equation variationally. Theoretical results based on this model on the extension versus force and extension versus degree of supercoiling relations of DNA chain are in good agreement with the experimental observations of Cluzel {\it et al.} [Science {\bf 271}, 792 (1996)], Smith {\it et al.} [{\it ibid.} {\bf 271}, 795 (1996)], and Strick {\it et al.} [{\it ibid.} {\bf 271}, 1835 (1996)].Comment: 8 pages in Revtex format, with 4 EPS figure

Finite-precision measurement does not nullify the Kochen-Specker theorem

It is proven that any hidden variable theory of the type proposed by Meyer [Phys. Rev. Lett. {\bf 83}, 3751 (1999)], Kent [{\em ibid.} {\bf 83}, 3755 (1999)], and Clifton and Kent [Proc. R. Soc. London, Ser. A {\bf 456}, 2101 (2000)] leads to experimentally testable predictions that are in contradiction with those of quantum mechanics. Therefore, it is argued that the existence of dense Kochen-Specker-colorable sets must not be interpreted as a nullification of the physical impact of the Kochen-Specker theorem once the finite precision of real measurements is taken into account.Comment: REVTeX4, 5 page

Theory of Suspension Segregation in Partially Filled Horizontal Rotating Cylinders

It is shown that a suspension of particles in a partially-filled, horizontal, rotating cylinder is linearly unstable towards axial segregation and an undulation of the free surface at large enough particle concentrations. Relying on the shear-induced diffusion of particles, concentration-dependent viscosity, and the existence of a free surface, our theory provides an explanation of the experiments of Tirumkudulu et al., Phys. Fluids 11, 507-509 (1999); ibid. 12, 1615 (2000).Comment: Accepted for publication in Phys Fluids (Lett) 10 pages, two eps figure

Bianchi identities in higher dimensions

A higher dimensional frame formalism is developed in order to study implications of the Bianchi identities for the Weyl tensor in vacuum spacetimes of the algebraic types III and N in arbitrary dimension $n$. It follows that the principal null congruence is geodesic and expands isotropically in two dimensions and does not expand in $n-4$ spacelike dimensions or does not expand at all. It is shown that the existence of such principal geodesic null congruence in vacuum (together with an additional condition on twist) implies an algebraically special spacetime. We also use the Myers-Perry metric as an explicit example of a vacuum type D spacetime to show that principal geodesic null congruences in vacuum type D spacetimes do not share this property.Comment: 25 pages, v3: Corrections to Appendix B as given in Erratum-ibid.24:1691,2007 are now incorporated (A factor of 2 was missing in certain Bianchi equations.

Proofs for some conjectures of Rajaratnam and Takawira on the peakedness of handoff traffic

The purpose of this paper is to supplement a recent paper by M. Rajaratnam and F. Takawira (see ibid., vol.49, p.817-34, 2000), which deals with a model for the performance analysis of cellular mobile networks. The key performance measure is a second-order characteristic (peakedness) of the traffic stream that serves as a model for handoff traffic. We show that this quantity may be obtained by evaluating an explicit formula rather than by solving a set of equations. This result enables us to verify some conjectures formulated by Rajaratnam and Takawira on the basis of numerical experiments. We also show the uniqueness of the solution to a system of nonlinear equations, required in the performance analysis, as conjectured by Rajaratnam and Takawira

Inelastic effects in molecular junctions in the Coulomb and Kondo regimes: Nonequilibrium equation-of-motion approach

Inelastic effects in the Coulomb blockade and Kondo regimes of electron transport through molecular junctions are considered within a simple nonequilibrium equation-of-motion (EOM) approach. The scheme is self-consistent, and can qualitatively reproduce the main experimental observations of vibrational features in Coulomb blockade [H.Park et al., Nature 407, 57 (2000)] and Kondo [L.H.Yu et al., Phys. Rev. Lett. 93, 266802 (2004)] regimes. Considerations similar to the equilibrium EOM approach by Meir et al. [Phys. Rev. Lett. 66, 3048 (1991); ibid. 70, 2601 (1993)] are used on the Keldysh contour to account for the nonequilibrium nature of the junction, and dressing by appropriate Franck-Condon (FC) factors is used to account for vibrational features. Results of the equilibrium EOM scheme by Meir et al. are reproduced in the appropriate limit.Comment: 12 pages, 5 figure