12,570 research outputs found
Unbiased sampling of globular lattice proteins in three dimensions
We present a Monte Carlo method that allows efficient and unbiased sampling
of Hamiltonian walks on a cubic lattice. Such walks are self-avoiding and visit
each lattice site exactly once. They are often used as simple models of
globular proteins, upon adding suitable local interactions. Our algorithm can
easily be equipped with such interactions, but we study here mainly the
flexible homopolymer case where each conformation is generated with uniform
probability. We argue that the algorithm is ergodic and has dynamical exponent
z=0. We then use it to study polymers of size up to 64^3 = 262144 monomers.
Results are presented for the effective interaction between end points, and the
interaction with the boundaries of the system
Extended two-level quantum dissipative system from bosonization of the elliptic spin-1/2 Kondo model
We study the elliptic spin-1/2 Kondo model (spin-1/2 fermions in one
dimension with fully anisotropic contact interactions with a magnetic impurity)
in the light of mappings to bosonic systems using the fermion-boson
correspondence and associated unitary transformations. We show that for fixed
fermion number, the bosonic system describes a two-level quantum dissipative
system with two noninteracting copies of infinitely-degenerate upper and lower
levels. In addition to the standard tunnelling transitions, and the transitions
driven by the dissipative coupling, there are also bath-mediated transitions
between the upper and lower states which simultaneously effect shifts in the
horizontal degeneracy label. We speculate that these systems could provide new
examples of continuous time quantum random walks, which are exactly solvable.Comment: 7 pages, 1 figur
Rate theory for correlated processes: Double-jumps in adatom diffusion
We study the rate of activated motion over multiple barriers, in particular
the correlated double-jump of an adatom diffusing on a missing-row
reconstructed Platinum (110) surface. We develop a Transition Path Theory,
showing that the activation energy is given by the minimum-energy trajectory
which succeeds in the double-jump. We explicitly calculate this trajectory
within an effective-medium molecular dynamics simulation. A cusp in the
acceptance region leads to a sqrt{T} prefactor for the activated rate of
double-jumps. Theory and numerical results agree
Exact enumeration of Hamiltonian circuits, walks, and chains in two and three dimensions
We present an algorithm for enumerating exactly the number of Hamiltonian
chains on regular lattices in low dimensions. By definition, these are sets of
k disjoint paths whose union visits each lattice vertex exactly once. The
well-known Hamiltonian circuits and walks appear as the special cases k=0 and
k=1 respectively. In two dimensions, we enumerate chains on L x L square
lattices up to L=12, walks up to L=17, and circuits up to L=20. Some results
for three dimensions are also given. Using our data we extract several
quantities of physical interest
Critical properties of joint spin and Fortuin-Kasteleyn observables in the two-dimensional Potts model
The two-dimensional Potts model can be studied either in terms of the
original Q-component spins, or in the geometrical reformulation via
Fortuin-Kasteleyn (FK) clusters. While the FK representation makes sense for
arbitrary real values of Q by construction, it was only shown very recently
that the spin representation can be promoted to the same level of generality.
In this paper we show how to define the Potts model in terms of observables
that simultaneously keep track of the spin and FK degrees of freedom. This is
first done algebraically in terms of a transfer matrix that couples three
different representations of a partition algebra. Using this, one can study
correlation functions involving any given number of propagating spin clusters
with prescribed colours, each of which contains any given number of distinct FK
clusters. For 0 <= Q <= 4 the corresponding critical exponents are all of the
Kac form h_{r,s}, with integer indices r,s that we determine exactly both in
the bulk and in the boundary versions of the problem. In particular, we find
that the set of points where an FK cluster touches the hull of its surrounding
spin cluster has fractal dimension d_{2,1} = 2 - 2 h_{2,1}. If one constrains
this set to points where the neighbouring spin cluster extends to infinity, we
show that the dimension becomes d_{1,3} = 2 - 2 h_{1,3}. Our results are
supported by extensive transfer matrix and Monte Carlo computations.Comment: 15 pages, 3 figures, 2 table
Tracking Data Acquisition System (TDAS) for the 1990's. Volume 6: TDAS navigation system architecture
One-way range and Doppler methods for providing user orbit and time determination are examined. Forward link beacon tracking, with on-board processing of independent navigation signals broadcast continuously by TDAS spacecraft; forward link scheduled tracking; with on-board processing of navigation data received during scheduled TDAS forward link service intervals; and return link scheduled tracking; with ground-based processing of user generated navigation data during scheduled TDAS return link service intervals are discussed. A system level definition and requirements assessment for each alternative, an evaluation of potential navigation performance and comparison with TDAS mission model requirements is included. TDAS satellite tracking is also addressed for two alternatives: BRTS and VLBI tracking
Dynamic rotor mode in antiferromagnetic nanoparticles
We present experimental, numerical, and theoretical evidence for a new mode
of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering
experiments on 8 nm particles of hematite display a loss of diffraction
intensity with temperature, the intensity vanishing around 150 K. However, the
signal from inelastic neutron scattering remains above that temperature,
indicating a magnetic system in constant motion. In addition, the precession
frequency of the inelastic magnetic signal shows an increase above 100 K.
Numerical Langevin simulations of spin dynamics reproduce all measured neutron
data and reveal that thermally activated spin canting gives rise to a new type
of coherent magnetic precession mode. This "rotor" mode can be seen as a
high-temperature version of superparamagnetism and is driven by exchange
interactions between the two magnetic sublattices. The frequency of the rotor
mode behaves in fair agreement with a simple analytical model, based on a high
temperature approximation of the generally accepted Hamiltonian of the system.
The extracted model parameters, as the magnetic interaction and the axial
anisotropy, are in excellent agreement with results from Mossbauer
spectroscopy
Critical exponents of domain walls in the two-dimensional Potts model
We address the geometrical critical behavior of the two-dimensional Q-state
Potts model in terms of the spin clusters (i.e., connected domains where the
spin takes a constant value). These clusters are different from the usual
Fortuin-Kasteleyn clusters, and are separated by domain walls that can cross
and branch. We develop a transfer matrix technique enabling the formulation and
numerical study of spin clusters even when Q is not an integer. We further
identify geometrically the crossing events which give rise to conformal
correlation functions. This leads to an infinite series of fundamental critical
exponents h_{l_1-l_2,2 l_1}, valid for 0 </- Q </- 4, that describe the
insertion of l_1 thin and l_2 thick domain walls.Comment: 5 pages, 3 figures, 1 tabl
Simulations of energetic beam deposition: from picoseconds to seconds
We present a new method for simulating crystal growth by energetic beam
deposition. The method combines a Kinetic Monte-Carlo simulation for the
thermal surface diffusion with a small scale molecular dynamics simulation of
every single deposition event. We have implemented the method using the
effective medium theory as a model potential for the atomic interactions, and
present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to
35 eV. The method is capable of following the growth of several monolayers at
realistic growth rates of 1 monolayer per second, correctly accounting for both
energy-induced atomic mobility and thermal surface diffusion. We find that the
energy influences island and step densities and can induce layer-by-layer
growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag),
which correlates with where the net impact-induced downward interlayer
transport is at a maximum. A high step density is needed for energy induced
layer-by-layer growth, hence the effect dies away at increased temperatures,
where thermal surface diffusion reduces the step density. As part of the
development of the method, we present molecular dynamics simulations of single
atom-surface collisions on flat parts of the surface and near straight steps,
we identify microscopic mechanisms by which the energy influences the growth,
and we discuss the nature of the energy-induced atomic mobility
Phase diagram and critical exponents of a Potts gauge glass
The two-dimensional q-state Potts model is subjected to a Z_q symmetric
disorder that allows for the existence of a Nishimori line. At q=2, this model
coincides with the +/- J random-bond Ising model. For q>2, apart from the usual
pure and zero-temperature fixed points, the ferro/paramagnetic phase boundary
is controlled by two critical fixed points: a weak disorder point, whose
universality class is that of the ferromagnetic bond-disordered Potts model,
and a strong disorder point which generalizes the usual Nishimori point. We
numerically study the case q=3, tracing out the phase diagram and precisely
determining the critical exponents. The universality class of the Nishimori
point is inconsistent with percolation on Potts clusters.Comment: Latex, 7 pages, 3 figures, v2: 1 reference adde
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