7,757 research outputs found
Fragility of the A-type AF and CE Phases of Manganites: An Exotic Insulator-to-Metal Transition Induced by Quenched Disorder
Using Monte Carlo simulations and the two eg-orbital model for manganites,
the stability of the CE and A-type antiferromagnetic insulating states is
analyzed when quenched disorder in the superexchange JAF between the t2g
localized spins and in the on-site energies is introduced. At vanishing or
small values of the electron-(Jahn-Teller)phonon coupling, the previously
hinted "fragility" of these insulating states is studied in detail, focusing on
their charge transport properties. This fragility is here found to induce a
rapid transition from the insulator to a (poor) metallic state upon the
introduction of disorder. A possible qualitative explanation is presented based
on the close proximity in energy of ferromagnetic metallic phases, and also on
percolative ideas valid at large disorder strength. The scenario is compared
with previously discussed insulator-to-metal transitions in other contexts. It
is argued that the effect unveiled here has unique properties that may define a
new class of giant effects in complex oxides. This particularly severe effect
of disorder must be present in other materials as well, in cases involving
phases that arise as a compromise between very different tendencies, as it
occurs with striped states in the cuprates.Comment: 13 pages, 17 figures, RevTex 4, submitted for publicatio
Signatures of current loop coalescence in solar flares
The nonlinear coalescence instability of current carrying solar loops can explain many of the characteristics of the solar flares such as their impulsive nature, heating and high energy particle acceleration, amplitude oscillations of electromagnetic emission as well as the characteristics of 2-D microwave images obtained during a solar flare. The physical characteristics of the explosive coalescence of currents are presented in detail through computer simulation and theory. Canonical characteristics of the explosive coalescence are: (1) a large amount of impulsive increase of kinetic energies of electrons and ions; (2) simultaneous heating and acceleration of electrons and ions in high and low energy spectra; (3) ensuing quasi-periodic amplitude oscillations in fields and particle quantities; and (4) the double peak (or triple peak) structure in these profiles, participate in the coalescence process, yielding varieties of phenomena
Weight Vectors of the Basic A_1^(1)-Module and the Littlewood-Richardson Rule
The basic representation of \A is studied. The weight vectors are
represented in terms of Schur functions. A suitable base of any weight space is
given. Littlewood-Richardson rule appears in the linear relations among weight
vectors.Comment: February 1995, 7pages, Using AMS-Te
Transport and magnetic properties of Co-doped BaFe_{2}As_{2} epitaxial thin films
We report resistivity, Hall coefficient, current-voltage characteristics, and
magneto-optical imaging measurements of epitaxial Co-doped BaFe_{2}As_{2} thin
films deposited on MgO(001) substrate. The Hall resistivity of the films has a
substantial contribution arising from anomalous Hall effect of ferromagnetic
components. The critical current density (J_{c}) of the films is ~2 MA/cm^{2}
at low temperatures. Differential magneto-optical images of the remanent state
give similar J_{c} values and also exhibit presence of extended defects in the
film.Comment: 9 pages, 4 figure
Electron- and Hole-Doping Effects on -site Ordered NdBaMnO
We have investigated electron- and hole-doping effects on -site ordered
perovskite manganite NdBaMnO, which has the -type (layered)
antiferromagnetic (AFM) ground state. Electrons (holes) are introduced by
partial substitution of Ba (Nd) with Nd (Ba).
Electron-doping generates ferromagnetic (FM) clusters in the -type AFM
matrix. With increasing the electron-doping level, the volume fraction of the
FM phase or the number of the FM clusters is abruptly increasing. In contrast,
the -type AFM phase is robust against the hole-doping, and no FM correlation
is observed in the hole-doped NdBaMnO.Comment: 8 pages, 5 figures, to be published in Journal of the Physical
Society of Japa
Symmetries in M-theory: Monsters, Inc
We will review the algebras which have been conjectured as symmetries in
M-theory. The Borcherds algebras, which are the most general Lie algebras under
control, seem natural candidates.Comment: 6 pages, talk given by PHL at Cargese 200
Spectral Classification and Effective Temperatures of L and T Dwarfs Based of Near-Infrared Spectra
We have obtained near-infrared spectra of L dwarfs, L/T transition objects
and T dwarfs using Subaru. Resulting spectra are examined in detail to see
their dependence on the spectral types. We have obtained bolometric
luminosities of the objects with known parallaxes in our sample, first by
integrating the spectra and second by K band bolometric correction. We derive
the relation between effective temperature and spectral type.Comment: To appear in May 20, 2004 issue of ApJ There is a companion paper by
Tsuji, Nakajima and Yanagisaw
Infrared Features of the Landau Gauge QCD
The infrared features of Landau gauge QCD are studied by the lattice
simulation of and . We
adopt two definitions of the gauge field; 1) linear 2) and
measured the gluon propagator and ghost propagator. Infrared singularity of the
gluon propagator is less than that of tree level result but the gluon
propagator at 0 momentum remains finite. The infrared singularity of ghost
propagator is stronger than the tree level. The QCD running coupling measured
by using the gluon propagator and the ghost propagator has a maximum
at around and decreases as approaches 0.
The data are analyzed in use of formula of the principle of minimal
sensitivity(PMS), the effective charge method and the contour-improved
perturbation method, which suggest necessity of the resummation of perturbation
series in the infrared region together with existence of the infrared fixed
point. Kugo-Ojima parameter saturates at about -0.8 in contrast to the
theoretically expected value -1.Comment: RevTex4, 9 pages, 10 eps figures, Typos corrected. To be published in
Phys. Rev. D(2004
Basic Representations of A_{2l}^(2) and D_{l+1}^(2) and the Polynomial Solutions to the Reduced BKP Hierarchies
Basic representations of A_{2l}^(2) and D_{l+1}^(2) are studied. The weight
vectors are represented in terms of Schur's -functions. The method to get
the polynomial solutions to the reduced BKP hierarchies is shown to be
equivalent to a certain rule in Maya game.Comment: January 1994, 11 page
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