62,836 research outputs found
Coexisting tuneable fractions of glassy and equilibrium long-range-order phases in manganites
Antiferromagnetic-insulating(AF-I) and the ferromagnetic-metallic(FM-M)
phases coexist in various half-doped manganites over a range of temperature and
magnetic field, and this is often believed to be an essential ingredient to
their colossal magnetoresistence. We present magnetization and resistivity
measurements on Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) and Pr(0.5)Sr(0.5)MnO(3)
showing that the fraction of the two coexisting phases at low-temperature in
any specified measuring field H, can be continuously controlled by following
designed protocols traversing field-temperature space; for both materials the
FM-M fraction rises under similar cooling paths. Constant-field temperature
variations however show that the former sample undergoes a 1st order transition
from AF-I to FM-M with decreasing T, while the latter undergoes the reverse
transition. We suggest that the observed path-dependent phase-separated states
result from the low-T equilibrium phase coexisting with supercooled glass-like
high temperature phase, where the low-T equilibrium phases are actually
homogeneous FM-M and AF-I phases respectively for the two materials
Conformal Theory of M2, D3, M5 and `D1+D5' Branes
The bosonic actions for M2, D3 and M5 branes in their own d-dimensional
near-horizon background are given in a manifestly SO(p+1,2) x SO(d-p-1)
invariant form (p=2,3,5). These symmetries result from a breakdown of ISO(d,2)
(with d=10 for D3 and d=11 for M2 and M5) symmetry by the Wess-Zumino term and
constraints. The new brane actions, reduce after gauge-fixing and solving
constraints to (p+1) dimensional interacting field theories with a non-linearly
realized SO(p+1,2) conformal invariance. We also present an interacting
two-dimensional conformal field theory on a D-string in the near-horizon
geometry of a D1+D5 configuration.Comment: 32 pages, two figures, Latex. A version to appear in JHEP. A comment
is added on infinite dimensional Kac-Moody type symmetry of D1+D5 system
observed by Brandt, Gomis, Sim'o
Physical Parameters for the Afterglows of GRB 980703, 990123, 990510, and 991216 Determined from Modeling of Multi-Frequency Data
We model the radio, optical, and X-ray emission for the afterglows of GRB
980703, 990123, 990510, and 991216, within the framework of relativistic jets,
to determine their physical parameters. The models that yield acceptable fits
to the data have jet energies mostly between 10^{50} to 10^{51} erg and initial
opening angles between 1 deg and 4 deg. The external medium density is
uncertain by at least one order of magnitude in each case, being around
10^{-3}/cm^3 for GRB 980703 and 990123, ~0.1/cm^3 for GRB 990510, and ~3/cm^3
for GRB 991216. If the jets are uniform (i.e. there are no angular gradients of
the energy per solid angle) then the 20 keV -- 1 MeV radiative efficiency
during the GRB phase must have been at least 2-3% for GRB 990510, 20% for GRB
990123, and 30% for GRB 991216.Comment: accepted for publication by the ApJ, vol. 554. 11 pages, color
figures. Last figures replaced with probability distributions of model
parameter
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