291 research outputs found
Liquid-liquid transition in supercooled silicon determined by first-principles simulation
First principles molecular dynamics simulations reveal a liquid-liquid phase
transition in supercooled elemental silicon. Two phases coexist below
. The low density phase is nearly tetra-coordinated, with a
pseudogap at the Fermi surface, while the high density phase is more highly
coordinated and metallic in nature. The transition is observed through the
formation of van der Waals loops in pressure-volume isotherms below .Comment: 9 pages 4 figure
Tricritical Phenomena at the Cerium Transition
The isostructural transition in the
CeLaTh system is measured as a function of La alloying
using specific heat, magnetic susceptibility, resistivity, thermal
expansivity/striction measurements. A line of discontinuous transitions, as
indicated by the change in volume, decreases exponentially from 118 K to close
to zero with increasing La doping and the transition changes from being
first-order to continuous at a critical concentration . At the tricritical point, the coefficient of the linear term in the
specific heat and the magnetic susceptibility start to increase
rapidly near = 0.14 and gradually approaches large values at =0.35
signifying that a heavy Fermi-liquid state evolves at large doping. Near ,
the Wilson ratio, , has a value of 3.0, signifying the presence of
magnetic fluctuations. Also, the low-temperature resistivity shows that the
character of the low-temperature Fermi-liquid is changing
Gaussian excitations model for glass-former dynamics and thermodynamics
We describe a model for the thermodynamics and dynamics of glass-forming
liquids in terms of excitations from an ideal glass state to a Gaussian
manifold of configurationally excited states. The quantitative fit of this
three parameter model to the experimental data on excess entropy and heat
capacity shows that ``fragile'' behavior, indicated by a sharply rising excess
heat capacity as the glass transition is approached from above, occurs in
anticipation of a first-order transition -- usually hidden below the glass
transition -- to a ``strong'' liquid state of low excess entropy. The dynamic
model relates relaxation to a hierarchical sequence of excitation events each
involving the probability of accumulating sufficient kinetic energy on a
separate excitable unit. Super-Arrhenius behavior of the relaxation rates, and
the known correlation of kinetic with thermodynamic fragility, both follow from
the way the rugged landscape induces fluctuations in the partitioning of energy
between vibrational and configurational manifolds. A relation is derived in
which the configurational heat capacity, rather than the configurational
entropy of the Adam Gibbs equation, controls the temperature dependence of the
relaxation times, and this gives a comparable account of the experimental
observations.Comment: 21 pp., 17 fig
X-Ray Light Curves of Gamma-ray Bursts Detected with the All-Sky Monitor on RXTE
We present X-ray light curves (1.5-12 keV) for fifteen gamma-ray bursts
(GRBs) detected by the All-Sky Monitor on the Rossi X-ray Timing Explorer. We
compare these soft X-ray light curves with count rate histories obtained by the
high-energy (>12 keV) experiments BATSE, Konus-Wind, the BeppoSAX Gamma-Ray
Burst Monitor, and the burst monitor on Ulysses. We discuss these light curves
within the context of a simple relativistic fireball and synchrotron shock
paradigm, and we address the possibility of having observed the transition
between a GRB and its afterglow. The light curves show diverse morphologies,
with striking differences between energy bands. In several bursts, intervals of
significant emission are evident in the ASM energy range with little or no
corresponding emission apparent in the high-energy light curves. For example,
the final peak of GRB 970815 as recorded by the ASM is only detected in the
softest BATSE energy bands. We also study the duration of bursts as a function
of energy. Simple, singly-peaked bursts seem consistent with the E^{-0.5} power
law expected from an origin in synchrotron radiation, but durations of bursts
that exhibit complex temporal structure are not consistent with this
prediction. Bursts such as GRB 970828 that show many short spikes of emission
at high energies last significantly longer at low energies than the synchrotron
cooling law would predict.Comment: 15 pages with 20 figures and 2 tables. In emulateapj format. Accepted
by ApJ
- …