3,057 research outputs found
Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites
We argue that many properties of the half-doped manganites may be understood
in terms of a new two-(eg electron)-fluid description, which is energetically
favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a
competition between canting of the core spins of Mn promoting mobile carriers
and polaronic trapping of carriers by JT defects, in the presence of CE,
orbital and charge order. We show that this explains several features of the
doping and magnetic field induced insulator-metal transitions, as the
particle-hole asymmetry and the smallness of the transition fields.Comment: 4 pages, 4 figure
Instabilities and Insulator-Metal transitions in Half-Doped Manganites induced by Magnetic-Field and Doping
We discuss the phase diagram of the two-orbital model of half-doped
manganites by calculating self-consistently the Jahn-Teller (JT) distortion
patterns, charge, orbital and magnetic order at zero temperature. We analyse
the instabilities of these phases caused by electron or hole doping away from
half-doping, or by the application of a magnetic-field. For the CE insulating
phase of half-doped manganites, in the intermediate JT coupling regime, we show
that there is a competition between canting of spins (which promotes mobile
carriers) and polaronic self-trapping of carriers by JT defects. This results
in a marked particle-hole asymmetry, with canting winning only on the electron
doped side of half-doping. We also show that the CE phase undergoes a
first-order transition to a ferromagnetic metallic phase when a magnetic-field
is applied, with abrupt changes in the lattice distortion patterns. We discuss
the factors that govern the intriguingly small scale of the transition fields.
We argue that the ferromagnetic metallic phases involved have two types of
charge carriers, localised and band-like, leading to an effective two-fluid
model.Comment: 22 pages, 28 figure
A comparison of the structureborne and airborne paths for propfan interior noise
A comparison is made between the relative levels of aircraft interior noise related to structureborne and airborne paths for the same propeller source. A simple, but physically meaningful, model of the structure treats the fuselage interior as a rectangular cavity with five rigid walls. The sixth wall, the fuselage sidewall, is a stiffened panel. The wing is modeled as a simple beam carried into the fuselage by a large discrete stiffener representing the carry-through structure. The fuselage interior is represented by analytically-derived acoustic cavity modes and the entire structure is represented by structural modes derived from a finite element model. The noise source for structureborne noise is the unsteady lift generation on the wing due to the rotating trailing vortex system of the propeller. The airborne noise source is the acoustic field created by a propeller model consistent with the vortex representation. Comparisons are made on the basis of interior noise over a range of propeller rotational frequencies at a fixed thrust
The Exotic Barium Bismuthates
We review the remarkable properties, including superconductivity,
charge-density-wave ordering, and metal-insulator transitions, of lead- and
potassium-doped barium bismuthate. We discuss some of the early theoretical
studies of these systems. Our recent theoretical work, on the negative-U\/,
extended-Hubbard model for these systems, is also described. Both the large-
and intermediate-U\/ regimes of this model are examined, using mean-field and
random-phase approximations, particularly with a view to fitting various
experimental properties of these bismuthates. On the basis of our studies, we
point out possibilities for exotic physics in these systems. We also emphasize
the different consequences of electronic and phonon-mediated mechanisms for the
negative U.\/ We show that, for an electronic mechanism, the \secin
\,\,phases of these bismuthates must be unique, with their transport properties
{\it dominated by charge Cooperon bound states}. This can explain the
observed difference between the optical and transport gaps. We propose other
experimental tests for this novel mechanism of charge transport and comment on
the effects of disorder.Comment: UUencoded LaTex file, 122 pages, figures available on request To
appear in Int. J. Mod. Phys. B as a review articl
Zero Temperature Insulator-Metal Transition in Doped Manganites
We study the transition at T=0 from a ferromagnetic insulating to a
ferromagnetic metallic phase in manganites as a function of hole doping using
an effective low-energy model Hamiltonian proposed by us recently. The model
incorporates the quantum nature of the dynamic Jahn-Teller(JT) phonons strongly
coupled to orbitally degenerate electrons as well as strong Coulomb correlation
effects and leads naturally to the coexistence of localized (JT polaronic) and
band-like electronic states. We study the insulator-metal transition as a
function of doping as well as of the correlation strength U and JT gain in
energy E_{JT}, and find, for realistic values of parameters, a ground state
phase diagram in agreement with experiments. We also discuss how several other
features of manganites as well as differences in behaviour among manganites can
be understood in terms of our model.Comment: To be published in Europhysics Letter
Complex order parameter symmetry and thermal conductivity
Thermal behaviour of superconductors with complex order parameter symmetry is
studied within a weak coupling theory. It is shown numerically, that the
thermal nature of the different components of complex order parametrs are
qualitatively different. Within the complex order parameter scenario, the
recent experimental observations by Krishna {\it et al.}, [Science {\bf 277},
83 (1997)] on magnetothermal conductivity and by J. Ma {\it et al.},
[Science {\bf 267}, 862 (1995)] on temperature dependent gap anisotropy for
high temperature superconductors can have natural explanation.Comment: 6 pages, 3 figures and macros attached, Europhysics Letters (1998) in
pres
Inference in Probabilistic Logic Programs with Continuous Random Variables
Probabilistic Logic Programming (PLP), exemplified by Sato and Kameya's
PRISM, Poole's ICL, Raedt et al's ProbLog and Vennekens et al's LPAD, is aimed
at combining statistical and logical knowledge representation and inference. A
key characteristic of PLP frameworks is that they are conservative extensions
to non-probabilistic logic programs which have been widely used for knowledge
representation. PLP frameworks extend traditional logic programming semantics
to a distribution semantics, where the semantics of a probabilistic logic
program is given in terms of a distribution over possible models of the
program. However, the inference techniques used in these works rely on
enumerating sets of explanations for a query answer. Consequently, these
languages permit very limited use of random variables with continuous
distributions. In this paper, we present a symbolic inference procedure that
uses constraints and represents sets of explanations without enumeration. This
permits us to reason over PLPs with Gaussian or Gamma-distributed random
variables (in addition to discrete-valued random variables) and linear equality
constraints over reals. We develop the inference procedure in the context of
PRISM; however the procedure's core ideas can be easily applied to other PLP
languages as well. An interesting aspect of our inference procedure is that
PRISM's query evaluation process becomes a special case in the absence of any
continuous random variables in the program. The symbolic inference procedure
enables us to reason over complex probabilistic models such as Kalman filters
and a large subclass of Hybrid Bayesian networks that were hitherto not
possible in PLP frameworks. (To appear in Theory and Practice of Logic
Programming).Comment: 12 pages. arXiv admin note: substantial text overlap with
arXiv:1203.428
A Rapidly-progressing Outbreak of Cholera in a Shelter-home for Mentally-retarded Females, Amta-II Block, Howrah, West Bengal, India
On 13 May 2010, a cluster of diarrhoeal disease cases was reported among the inmates of a shelter-home for mentally-retarded females in Parbaksi village of Howrah district in West Bengal, India. The outbreak was investigated to identify the aetiological agent and source of infection and to propose recommendations. A suspected case of cholera was defined as an acute onset of >3 loose watery stools in a female resident of the shelter-home since 1 May 2010. The demographic and clinical details were collected from the suspected case-patients, and the outbreak was described by time, place, and person. A retrospective cohort study was conducted to identify the risk factors associated with the illness. Of the 101 inmates, 91 (90%) developed diarrhoea, and three patients died (case fatality−3%). Four of the five stool specimens were positive for Vibrio cholerae O1 Ogawa. Drinking of water from the pond-connected tubewell (adjusted odds ratio=25.7, 95% confidence interval 2.7-236.4) was associated with the illness. Relocation of the pond-connected tubewell away from the groundwater tubewell, colour-coding of the tubewells meant for drinking purposes, and regular disinfection of the tubewells were recommended
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