61,925 research outputs found
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 . It follows that
the principal null congruence is geodesic and expands isotropically in two
dimensions and does not expand in 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
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