476 research outputs found
Optical properties of random alloys : Application to Cu_{50}Au_{50} and Ni_{50}Pt_{50}
In an earlier paper [K. K. Saha and A. Mookerjee, Phys. Rev. B 70 (2004) (in
press) or, cond-mat/0403456] we had presented a formulation for the calculation
of the configuration-averaged optical conductivity in random alloys. Our
formulation is based on the augmented-space theorem introduced by one of us [A.
Mookerjee, J. Phys. C: Solid State Phys. 6, 1340 (1973)]. In this communication
we shall combine our formulation with the tight-binding linear muffin-tin
orbitals (TB-LMTO) technique to study the optical conductivities of two alloys
Cu_{50}Au_{50} and Ni_{50}Pt_{50}.Comment: 5 pages, 7 figure
65 The CD4 peptide analogue 802-2 inhibits proliferation of both TH1 and TH2 cells in response to allogeneic stimulation
Structure, bonding and magnetism in cobalt clusters
The structural, electronic and magnetic properties of Co clusters
(20) have been investigated using density functional theory within the
pseudopotential plane wave method. An unusual hexagonal growth pattern has been
observed in the intermediate size range, 20. The cobalt atoms are
ferromagnetically ordered and the calculated magnetic moments are found to be
higher than that of corresponding hcp bulk value, which are in good agreement
with the recent Stern-Gerlach experiments. The average coordination number is
found to dominate over the average bond length to determine the effective
hybridization and consequently the cluster magnetic moment.Comment: 12 pages and 9 figure
A theoretical study of the cluster glass-Kondo-magnetic disordered alloys
The physics of disordered alloys, such as typically the well known case of
CeNi1-xCux alloys, showing an interplay among the Kondo effect, the spin glass
state and a magnetic order, has been studied firstly within an average
description like in the Sherrington-Kirkpatrick model. Recently, a theoretical
model (PRB 74, 014427 (2006)) involving a more local description of the
intersite interaction has been proposed to describe the phase diagram of
CeNi1-xCux. This alloy is an example of the complex interplay between Kondo
effect and frustration in which there is in particular the onset of a
cluster-glass state. Although the model given in Ref. PRB 74, 014427 (2006) has
reproduced the different phases relatively well, it is not able to describe the
cluster-glass state. We study here the competition between the Kondo effect and
a cluster glass phase within a Kondo Lattice model with an inter-cluster random
Gaussian interaction. The inter-cluster term is treated within the cluster
mean-field theory for spin glasses, while, inside the cluster, an exact
diagonalisation is performed including inter-site ferromagnetic and intra-site
Kondo interactions. The cluster glass order parameters and the Kondo
correlation function are obtained for different values of the cluster size, the
intra-cluster ferromagnetic coupling and the Kondo intra-site coupling. We
obtain, for instance, that the increase of the Kondo coupling tends to destroy
the cluster glass phase.Comment: 6 pages, 2 figures, Accepted for publication in Physica
Lattice thermal conductivity of disordered NiPd and NiPt alloys
Numerical calculations of lattice thermal conductivity are reported for the
binary alloys NiPd and NiPt. The present work is a continuation of an earlier
paper by us [PRB, 72, 214207 (2005)]which had developed a theoretical framework
for the calculation of configuration-averaged lattice thermal conductivity and
thermal diffusivity in disordered alloys. The formulation was based on the
augmented space theorem combined with a scattering diagram technique. In this
paper we shall show dependence of the lattice thermal conductivity on a series
of variables like phonon frequency, temperature and alloy composition. The
temperature dependence of and its realtion to the measured thermal
conductivity is discussed. The concentration dependence of appears to
justify the notion of a minimum thermal conductivity as discussed by Kittel,
Slack and others. We also study the frequency and composition dependence of the
thermal diffusivity averaged over modes. A numerical estimate of this quantity
gives an idea about the location of mobility edge and the fraction of states in
the frequency spectrum which is delocalized.Comment: 23 pages, 18 figure
An augmented space recursion study of the electronic structure of rough epitaxial overlayers
In this communication we propose the use of the Augmented Space Recursion as
an ideal methodology for the study of electronic and magnetic structures of
rough surfaces, interfaces and overlayers. The method can take into account
roughness, short-ranged clustering effects, surface dilatation and
interdiffusion. We illustrate our method by an application of Fe overlayer on
Ag (100) surface.Comment: 22 pages, Latex, 6 postscript figure
Phonons in random alloys: the itinerant coherent-potential approximation
We present the itinerant coherent-potential approximation(ICPA), an analytic,
translationally invariant and tractable form of augmented-space-based,
multiple-scattering theory in a single-site approximation for harmonic phonons
in realistic random binary alloys with mass and force-constant disorder.
We provide expressions for quantities needed for comparison with experimental
structure factors such as partial and average spectral functions and derive the
sum rules associated with them. Numerical results are presented for Ni_{55}
Pd_{45} and Ni_{50} Pt_{50} alloys which serve as test cases, the former for
weak force-constant disorder and the latter for strong. We present results on
dispersion curves and disorder-induced widths. Direct comparisons with the
single-site coherent potential approximation(CPA) and experiment are made which
provide insight into the physics of force-constant changes in random alloys.
The CPA accounts well for the weak force-constant disorder case but fails for
strong force-constant disorder where the ICPA succeeds.Comment: 19 pages, 12 eps figures, uses RevTex
Spectral statistics near the quantum percolation threshold
The statistical properties of spectra of a three-dimensional quantum bond
percolation system is studied in the vicinity of the metal insulator
transition. In order to avoid the influence of small clusters, only regions of
the spectra in which the density of states is rather smooth are analyzed. Using
finite size scaling hypothesis, the critical quantum probability for bond
occupation is found to be while the critical exponent for the
divergence of the localization length is estimated as . This
later figure is consistent with the one found within the universality class of
the standard Anderson model.Comment: REVTeX, 4 pages, 5 figures, all uuencoded, accepted for publication
in PRB (Rapid Communication
Development of a Time Efficient Protocol for Cross-Limb Comparisons of Muscle Mitochondrial Capacity Using NIRS
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