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Stretched Exponential Relaxation Arising from a Continuous Sum of Exponential Decays
Stretched exponential relaxation of a quantity n versus time t according to n
= n_0 exp[-(lambda* t)^beta] is ubiquitous in many research fields, where
lambda* is a characteristic relaxation rate and the stretching exponent beta is
in the range 0 < beta < 1. Here we consider systems in which the stretched
exponential relaxation arises from the global relaxation of a system containing
independently exponentially relaxing species with a probability distribution
P(lambda/lambda*,beta) of relaxation rates lambda. We study the properties of
P(lambda/lambda*,beta) and their dependence on beta. Physical interpretations
of lambda* and beta, derived from consideration of P(lambda/lambda*,beta) and
its moments, are discussed.Comment: 8 pages, 10 figures; version to be published in Phys. Rev.
Antiferromagnetism in EuCu2As2 and EuCu1.82Sb2 Single Crystals
Single crystals of EuCu2As2 and EuCu2Sb2 were grown from CuAs and CuSb
self-flux, respectively. The crystallographic, magnetic, thermal and electronic
transport properties of the single crystals were investigated by
room-temperature x-ray diffraction (XRD), magnetic susceptibility \chi versus
temperature T, isothermal magnetization M versus magnetic field H, specific
heat Cp(T) and electrical resistivity \rho(T) measurements. EuCu2As2
crystallizes in the body-centered tetragonal ThCr2Si2-type structure (space
group I4/mmm), whereas EuCu2Sb2 crystallizes in the related primitive
tetragonal CaBe2Ge2-type structure (space group P4/nmm). The energy-dispersive
x-ray spectroscopy and XRD data for the EuCu2Sb2 crystals showed the presence
of vacancies on the Cu sites, yielding the actual composition EuCu1.82Sb2. The
\rho(T) and Cp(T) data reveal metallic character for both EuCu2As2 and
EuCu1.82Sb2. Antiferromagnetic (AFM) ordering is indicated from the \chi(T),
Cp(T), and \rho(T) data for both EuCu2As2 (T_N = 17.5 K) and EuCu1.82Sb2 (T_N =
5.1 K). In EuCu1.82Sb2, the ordered-state \chi(T) and M(H) data suggest either
a collinear A-type AFM ordering of Eu+2 spins S=7/2 or a planar noncollinear
AFM structure, with the ordered moments oriented in the tetragonal ab plane in
either case. This ordered-moment orientation for the A-type AFM is consistent
with calculations with magnetic dipole interactions. The anisotropic \chi(T)
and isothermal M(H) data for EuCu2As2, also containing Eu+2 spins S=7/2,
strongly deviate from the predictions of molecular field theory for collinear
AFM ordering and the AFM structure appears to be both noncollinear and
noncoplanar.Comment: 21 pages, 22 figures, 4 Table
Pneumatic inflatable end effector
The invention relates to an end effector device for robot or teleoperated type space vehicle which includes an inflatable balloon member carried on the end of tubular member which has a hollow center or conduit through which a suitable pressurized fluid is supplied. The device may be inserted into a variety of shaped openings or truss-type structures for handling in space
Laser net - A concept for monitoring wingtip vortices on runways
Network of laser beams passes over runway to photodetectors on opposite side, magnitude of beam deflection indicates magnitude of density gradient encountered. Visual display of beam deflections affects go, no-go decision for takeoff and landing
Physical properties of EuPd2As2 single crystals
The physical properties of self-flux grown EuPd2As2 single crystals have been
investigated by magnetization M, magnetic susceptibility chi, specific heat Cp,
and electrical resistivity rho measurements versus temperature T and magnetic
field H. The crystal structure was determined by powder x-ray diffraction
measurements, which confirmed the ThCr2Si2-type body-centered tetragonal
structure (space group I4/mmm) reported previously. The rho(T) data indicate
that state of EuPd2As2 is metallic. Long-range antiferromagnetic (AFM) ordering
is apparent from the chi(T), Cp(T), and rho(T) measurements. For H \parallel c
the chi(T) indicates two transitions at TN1 = 11.0 K and TN2 = 5.5 K, whereas
for H \perp c only one transition is observed at TN1 = 11.0 K. Between TN1 and
TN2 the anisotropic chi(T) data suggest a planar noncollinear AFM structure,
whereas at T < TN2 the chi(T) and M(H,T) data suggest a spin reorientation
transition in which equal numbers of spins cant in opposite directions out of
the ab plane. We estimate the critical field at 2 K at which all Eu moments
become aligned with the field to be about 22 T. The magnetic entropy at 25 K
estimated from the Cp(T) measurements is about smaller than expected,
possibly due to an inaccuracy in the lattice heat capacity contribution. An
upturn in rho at T < TN1 suggests superzone energy gap formation below TN1.
This behavior of rho(T < TN1) is not sensitive to applied magnetic fields up to
H = 12 T.Comment: 11 pages, 10 figures, 2 tables and 52 references; To appear in J.
Phys.: Condens. Matte
Metallic behavior induced by potassium doping of the trigonal antiferromagnetic insulator EuMn2As2
We report magnetic susceptibility \chi, isothermal magnetization M, heat
capacity C_p and electrical resistivity \rho measurements on undoped EuMn2As2
and K-doped Eu0.96K0.04Mn2As2 and Eu0.93K0.07Mn2As2 single crystals with the
trigonal CaAl2Si2-type structure as a function of temperature T and magnetic
field H. EuMn2As2 has an insulating ground state with an activation energy of
52 meV and exhibits antiferromagnetic (AFM) ordering of the Eu+2 spins S=7/2 at
T_N1 = 15 K from C_p(T) and \chi(T) data with a likely spin-reorientation
transition at T_N2 = 5.0 K. The Mn+2 3d5 spins-5/2 exhibit AFM ordering at T_N
= 142 K from all three types of measurements. The M(H) isotherm and \chi(T)
data indicate that the Eu AFM structure is both noncollinear and noncoplanar.
The AFM structure of the Mn spins is also unclear. A 4% substitution of K for
Eu in Eu0.96K0.04Mn2As2 is sufficient to induce a metallic ground state.
Evidence is found for a difference in the AFM structure of the Eu moments in
the metallic crystals from that of undoped EuMn2As2 versus both T and H. For
metallic Eu0.96K0.04Mn2As2 and Eu0.93K0.07Mn2As2, an anomalous S-shape T
dependence of \rho related to the Mn magnetism is found. Upon cooling from 200
K, \rho exhibits a strong negative curvature, reaches maximum positive slope at
the Mn T_N ~ 150 K, and then continues to decrease but more slowly below T_N.
This suggests that dynamic short-range AFM order of the Mn spins above the Mn
T_N strongly suppresses the resistivity, contrary to the conventional decrease
of \rho that is only observed upon cooling below T_N of an antiferromagnet.Comment: 21 pages, 22 figures, 4 Table
A numerical solution of the Navier-Stokes equations for chemically nonequilibrium, merged stagnation shock layers on spheres and two-dimensional cylinders in air
Results of solving the Navier-Stokes equations for chemically nonequilibrium, merged stagnation shock layers on spheres and two-dimensional cylinders are presented. The effects of wall catalysis and slip are also examined. The thin shock layer assumption is not made, and the thick viscous shock is allowed to develop within the computational domain. The results show good comparison with existing data. Due to the more pronounced merging of shock layer and boundary layer for the sphere, the heating rates for spheres become higher than those for cylinders as the altitude is increased
Decorating Random Quadrangulations
On various regular lattices (simple cubic, body centred cubic..) decorating
an edge with an Ising spin coupled by bonds of strength L to the original
vertex spins and competing with a direct anti-ferromagnetic bond of strength
alpha L can give rise to three transition temperatures for suitable alpha. The
system passes through ferromagnetic, paramagnetic, anti-ferromagnetic and
paramagnetic phases respectively as the temperature is increased.
For the square lattice on the other hand multiple decoration is required to
see this effect. We note here that a single decoration suffices for the Ising
model on planar random quadrangulations (coupled to 2D quantum gravity). Other
random bipartite lattices such as the Penrose tiling are more akin to the
regular square lattice and require multiple decoration to have any affect.Comment: 6 pages + 5 figure
Potts Models with (17) Invisible States on Thin Graphs
The order of a phase transition is usually determined by the nature of the
symmetry breaking at the phase transition point and the dimension of the model
under consideration. For instance, q-state Potts models in two dimensions
display a second order, continuous transition for q = 2,3,4 and first order for
higher q.
Tamura et al recently introduced Potts models with "invisible" states which
contribute to the entropy but not the internal energy and noted that adding
such invisible states could transmute continuous transitions into first order
transitions. This was observed both in a Bragg-Williams type mean-field
calculation and 2D Monte-Carlo simulations. It was suggested that the invisible
state mechanism for transmuting the order of a transition might play a role
where transition orders inconsistent with the usual scheme had been observed.
In this paper we note that an alternative mean-field approach employing
3-regular random ("thin") graphs also displays this change in the order of the
transition as the number of invisible states is varied, although the number of
states required to effect the transmutation, 17 invisible states when there are
2 visible states, is much higher than in the Bragg-Williams case. The
calculation proceeds by using the equivalence of the Potts model with 2 visible
and r invisible states to the Blume-Emery-Griffiths (BEG) model, so a
by-product is the solution of the BEG model on thin random graphs.Comment: (2) Minor typos corrected, references update
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