533 research outputs found
Void Growth in BCC Metals Simulated with Molecular Dynamics using the Finnis-Sinclair Potential
The process of fracture in ductile metals involves the nucleation, growth,
and linking of voids. This process takes place both at the low rates involved
in typical engineering applications and at the high rates associated with
dynamic fracture processes such as spallation. Here we study the growth of a
void in a single crystal at high rates using molecular dynamics (MD) based on
Finnis-Sinclair interatomic potentials for the body-centred cubic (bcc) metals
V, Nb, Mo, Ta, and W. The use of the Finnis-Sinclair potential enables the
study of plasticity associated with void growth at the atomic level at room
temperature and strain rates from 10^9/s down to 10^6/s and systems as large as
128 million atoms. The atomistic systems are observed to undergo a transition
from twinning at the higher end of this range to dislocation flow at the lower
end. We analyze the simulations for the specific mechanisms of plasticity
associated with void growth as dislocation loops are punched out to accommodate
the growing void. We also analyse the process of nucleation and growth of voids
in simulations of nanocrystalline Ta expanding at different strain rates. We
comment on differences in the plasticity associated with void growth in the bcc
metals compared to earlier studies in face-centred cubic (fcc) metals.Comment: 24 pages, 12 figure
Susceptibility and Percolation in 2D Random Field Ising Magnets
The ground state structure of the two-dimensional random field Ising magnet
is studied using exact numerical calculations. First we show that the
ferromagnetism, which exists for small system sizes, vanishes with a large
excitation at a random field strength dependent length scale. This {\it
break-up length scale} scales exponentially with the squared random
field, . By adding an external field we then study the
susceptibility in the ground state. If , domains melt continuously and
the magnetization has a smooth behavior, independent of system size, and the
susceptibility decays as . We define a random field strength dependent
critical external field value , for the up and down spins to
form a percolation type of spanning cluster. The percolation transition is in
the standard short-range correlated percolation universality class. The mass of
the spanning cluster increases with decreasing and the critical
external field approaches zero for vanishing random field strength, implying
the critical field scaling (for Gaussian disorder) , where and .
Below the systems should percolate even when H=0. This implies that
even for H=0 above the domains can be fractal at low random fields, such
that the largest domain spans the system at low random field strength values
and its mass has the fractal dimension of standard percolation .
The structure of the spanning clusters is studied by defining {\it red
clusters}, in analogy to the ``red sites'' of ordinary site-percolation. The
size of red clusters defines an extra length scale, independent of .Comment: 17 pages, 28 figures, accepted for publication in Phys. Rev.
Random manifolds in non-linear resistor networks: Applications to varistors and superconductors
We show that current localization in polycrystalline varistors occurs on
paths which are, usually, in the universality class of the directed polymer in
a random medium. We also show that in ceramic superconductors, voltage
localizes on a surface which maps to an Ising domain wall. The emergence of
these manifolds is explained and their structure is illustrated using direct
solution of non-linear resistor networks
Disorder Driven Critical Behavior of Periodic Elastic Media in a Crystal Potential
We study a lattice model of a three-dimensional periodic elastic medium at
zero temperature with exact combinatorial optimization methods. A competition
between pinning of the elastic medium, representing magnetic flux lines in the
mixed phase of a superconductor or charge density waves in a crystal, by
randomly distributed impurities and a periodic lattice potential gives rise to
a continuous phase transition from a flat phase to a rough phase. We determine
the critical exponents of this roughening transition via finite size scaling
obtaining , , and find
that they are universal with respect to the periodicity of the lattice
potential. The small order parameter exponent is reminiscent of the random
field Ising critical behavior in 3.Comment: 4 pages, 3 eps-figures include
Continuous Vortices with Broken Symmetry in Rotating Superfluid 3He-A
New NMR measurements are reported on continuous 3He-A vortices in tilted magnetic fields. We introduce a symmetry classification of the continuous vortices with broken axial symmetry. It is found that the discrete internal symmetry may in addition be broken in two inequivalent ways, producing two different continuous vortices. Although NMR may not distinguish between these two vortices, the observed vortex satellite peak is well accounted for by spin waves localized in the soft core of such vortices.Peer reviewe
Status report of the JYFL-ECR ion sources
"Ion beam cocktails" are mixtures of ions with near-identical charge-to-mass ratios. In conjunction with the JYFL-ECRIS, the K130-cyclotron acts as a mass analyzer: the switch from one ion to another within the same cocktail is simple and fast. In the case of the first ion beam cocktail, the oxygen and argon gases were mixed into the gas feed line. At the same time the magnesium and iron ion beams were produced using the MIVOC method. Magnesocene and ferrocene compounds were both mixed into the MIVOC chamber. This capability is especially useful in the study of single event effects (SEE) in space electronics. All gaseous elements from H to Xe can be produced. The non-gaseous elements produced so far are C, Mg, Al, Si, S, Ca, Ti, Cr, Fe, Co, Ni, Cu, Zn and Ge. A major technical modification since the construction (in 1990) of the JYFL-ECRIS was made in January 98: a negatively biased disc replaces now the first plasma stage. After a couple of months experience with the modified source the change was found to be towards a correct direction. The source is now much easier to use and the good operating conditions are well repeated. A real advantage is the new magnetic field settings which are practically the same for all kind of beams, gaseous and solids. Due to the requirements of ion beams with higher charges and heavier elements than the present JYFL-ECRIS can produce, JYFL decided to begin a design and construction project of a new ECR ion source, called as ECRIS 2. The project aims to a source that is based mainly on the design of the 14 GHz AECR-U source at the LBNL. Some modifications made into the similar source under construction at the NSCL/MSU will be utilized here. The new source will be installed horizontally in the basement of the ECRIS laboratory. It requires a new beam-line from the source to the cyclotron injection line, since the old vertically located JYFL-ECRIS will be preserved in operation. The new source is planned to be operational during the year 2000
Percolation in three-dimensional random field Ising magnets
The structure of the three-dimensional random field Ising magnet is studied
by ground state calculations. We investigate the percolation of the minority
spin orientation in the paramagnetic phase above the bulk phase transition,
located at [Delta/J]_c ~= 2.27, where Delta is the standard deviation of the
Gaussian random fields (J=1). With an external field H there is a disorder
strength dependent critical field +/- H_c(Delta) for the down (or up) spin
spanning. The percolation transition is in the standard percolation
universality class. H_c ~ (Delta - Delta_p)^{delta}, where Delta_p = 2.43 +/-
0.01 and delta = 1.31 +/- 0.03, implying a critical line for Delta_c < Delta <=
Delta_p. When, with zero external field, Delta is decreased from a large value
there is a transition from the simultaneous up and down spin spanning, with
probability Pi_{uparrow downarrow} = 1.00 to Pi_{uparrow downarrow} = 0. This
is located at Delta = 2.32 +/- 0.01, i.e., above Delta_c. The spanning cluster
has the fractal dimension of standard percolation D_f = 2.53 at H = H_c(Delta).
We provide evidence that this is asymptotically true even at H=0 for Delta_c <
Delta <= Delta_p beyond a crossover scale that diverges as Delta_c is
approached from above. Percolation implies extra finite size effects in the
ground states of the 3D RFIM.Comment: replaced with version to appear in Physical Review
Disorder, Order, and Domain Wall Roughening in the 2d Random Field Ising Model
Ground states and domain walls are investigated with exact combinatorial
optimization in two-dimensional random field Ising magnets. The ground states
break into domains above a length scale that depends exponentially on the
random field strength squared. For weak disorder, this paramagnetic structure
has remnant long-range order of the percolation type. The domain walls are
super-rough in ordered systems with a roughness exponent close to 6/5.
The interfaces exhibit rare fluctuations and multiscaling reminiscent of some
models of kinetic roughening and hydrodynamic turbulence.Comment: to be published in Phys.Rev.E/Rapid.Com
Energy landscapes in random systems, driven interfaces and wetting
We discuss the zero-temperature susceptibility of elastic manifolds with
quenched randomness. It diverges with system size due to low-lying local
minima. The distribution of energy gaps is deduced to be constant in the limit
of vanishing gaps by comparing numerics with a probabilistic argument. The
typical manifold response arises from a level-crossing phenomenon and implies
that wetting in random systems begins with a discrete transition. The
associated ``jump field'' scales as and for
(1+1) and (2+1) dimensional manifolds with random bond disorder.Comment: Accepted for publication in Phys. Rev. Let
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