24 research outputs found
Random Field Models for Relaxor Ferroelectric Behavior
Heat bath Monte Carlo simulations have been used to study a four-state clock
model with a type of random field on simple cubic lattices. The model has the
standard nonrandom two-spin exchange term with coupling energy and a random
field which consists of adding an energy to one of the four spin states,
chosen randomly at each site. This Ashkin-Teller-like model does not separate;
the two random-field Ising model components are coupled. When , the
ground states of the model remain fully aligned. When , a
different type of ground state is found, in which the occupation of two of the
four spin states is close to 50%, and the other two are nearly absent. This
means that one of the Ising components is almost completely ordered, while the
other one has only short-range correlations. A large peak in the structure
factor appears at small for temperatures well above the transition
to long-range order, and the appearance of this peak is associated with slow,
"glassy" dynamics. The phase transition into the state where one Ising
component is long-range ordered appears to be first order, but the latent heat
is very small.Comment: 7 pages + 12 eps figures, to appear in Phys Rev
Exawatt-Zettawatt Pulse Generation and Applications
A new amplification method, weaving the three basic compression techniques,
Chirped Pulse Amplification (CPA), Optical Parametric Chirped Pulse
Amplification (OPCPA) and Plasma Compression by Backward Raman Amplification
(BRA) in plasma, is proposed. It is called C3 for Cascaded Conversion
Compression. It has the capability to compress with good efficiency kilojoule
to megajoule, nanosecond laser pulses into femtosecond pulses, to produce
exawatt and beyond peak power. In the future, C3 could be used at large-scale
facilities such as the National Ignition Facility (NIF) or the Laser Megajoule
(LMJ) and open the way to zettawatt level pulses. The beam will be focused to a
wavelength spot size with a f#1. The very small beam size, i.e. few
centimeters, along with the low laser repetition rate laser system will make
possible the use of inexpensive, precision, disposable optics. The resulting
intensity will approach the Schwinger value, thus opening up new possibilities
in fundamental physics.Comment: 13 pages, 4 figure
Magnetic correlations in three-dimensional ising spin glasses
By a recursive method numerically exact free energies are calculated forL×L×M Ising lattices with random bonds andL=4, 4lEMlE10, applying free boundaries in the direction where the lattice is less small and otherwise periodic boundary conditions. Both for the±J model and the gaussian model the specific heat is in fair agreement with Monte Carlo results obtained for much larger lattices. However, the correlation function [langS 0 S R rang T 2 ]av is found to decay exponentially with distanceR [for 1lapRlE9] at temperatures far below the apparent freezing temperatures of the Monte Carlo simulations, implying that there is no nonzero Edwards-Anderson order parameter in equilibrium. This behavior is qualitatively different from Mattis spin glasses (or Ising ferromagnets) where even smaller lattices show pronounced magnetic order at low temperatures. As the Monte Carlo results give evidence for a nonzero Edwards-Anderson order parameter (for not too long observation times), which is fairly independent of lattice size down to sizes of 43, we suggest that Edwards-Anderson ordering is a nonequilibrium phenomenon visible only in studying dynamic properties