4,408 research outputs found
2-Dimensional Polymers Confined in a Strip
Single two dimensional polymers confined to a strip are studied by Monte
Carlo simulations. They are described by N-step self-avoiding random walks on a
square lattice between two parallel hard walls with distance 1 << D << N^\nu
(\nu = 3/4 is the Flory exponent). For the simulations we employ the
pruned-enriched-Rosenbluth method (PERM) with Markovian anticipation. We
measure the densities of monomers and of end points as functions of the
distance from the walls, the longitudinal extent of the chain, and the forces
exerted on the walls. Their scaling with D and the universal ratio between
force and monomer density at the wall are compared to theoretical predictions.Comment: 5 pages RevTex, 7 figures include
Spin-state crossover and hyperfine interactions of ferric iron in MgSiO perovskite
Using density functional theory plus Hubbard calculations, we show that
the ground state of (Mg,Fe)(Si,Fe)O perovskite, a major mineral phase in
the Earth's lower mantle, has high-spin ferric iron () at both the
dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site
iron undergoes a spin-state crossover to the low-spin state (), while
the A-site iron remains in the high-spin state. Our calculation shows that the
B-site spin-state crossover in the pressure range of 40-70 GPa is accompanied
by a noticeable volume reduction and an increase in quadrupole splitting,
consistent with recent X-ray diffraction and M\"ossbauer spectroscopy
measurements. The volume reduction leads to a significant softening in the bulk
modulus, which suggests a possible source of seismic velocity anomalies in the
lower mantle.Comment: 11 pages, 4 figures, 1 tabl
Monte Carlo Protein Folding: Simulations of Met-Enkephalin with Solvent-Accessible Area Parameterizations
Treating realistically the ambient water is one of the main difficulties in
applying Monte Carlo methods to protein folding. The solvent-accessible area
method, a popular method for treating water implicitly, is investigated by
means of Metropolis simulations of the brain peptide Met-Enkephalin. For the
phenomenological energy function ECEPP/2 nine atomic solvation parameter (ASP)
sets are studied that had been proposed by previous authors. The simulations
are compared with each other, with simulations with a distance dependent
electrostatic permittivity , and with vacuum simulations
(). Parallel tempering and a recently proposed biased Metropolis
technique are employed and their performances are evaluated. The measured
observables include energy and dihedral probability densities (pds), integrated
autocorrelation times, and acceptance rates. Two of the ASP sets turn out to be
unsuitable for these simulations. For all other sets, selected configurations
are minimized in search of the global energy minima. Unique minima are found
for the vacuum and the system, but for none of the ASP models.
Other observables show a remarkable dependence on the ASPs. In particular,
autocorrelation times vary dramatically with the ASP parameters. Three ASP sets
have much smaller autocorrelations at 300 K than the vacuum simulations,
opening the possibility that simulations can be speeded up vastly by
judiciously chosing details of the forceComment: 10 pages; published in "NIC Symposium 2004", eds. D. Wolf at el.
(NIC, Juelich, 2004
Polymers Confined between Two Parallel Plane Walls
Single three dimensional polymers confined to a slab, i.e. to the region
between two parallel plane walls, are studied by Monte Carlo simulations. They
are described by -step walks on a simple cubic lattice confined to the
region . The simulations cover both regions (where is the Flory radius, with ), as
well as the cross-over region in between. Chain lengths are up to ,
slab widths up to D=120. In order to test the analysis program and to check for
finite size corrections, we actually studied three different models: (a)
Ordinary random walks (mimicking -polymers); (b) Self-avoiding walks
(SAW); and (c) Domb-Joyce walks with the self-repulsion tuned to the point
where finite size corrections for free (unrestricted) chains are minimal. For
the simulations we employ the pruned-enriched-Rosenbluth method (PERM) with
Markovian anticipation. In addition to the partition sum (which gives us a
direct estimate of the forces exerted onto the walls), we measure the density
profiles of monomers and of end points transverse to the slab, and the radial
extent of the chain parallel to the walls. All scaling laws and some of the
universal amplitude ratios are compared to theoretical predictions.Comment: 8 pages, 14 figures include
Nuclear-spin-induced localization of the edge states in two-dimensional topological insulators
We investigate the influence of nuclear spins on the resistance of helical
edge states of two-dimensional topological insulators (2DTIs). Via the
hyperfine interaction, nuclear spins allow electron backscattering, otherwise
forbidden by time reversal symmetry. We identify two backscattering mechanisms,
depending on whether the nuclear spins are ordered or not. Their temperature
dependence is distinct but both give resistance, which increases with the edge
length, decreasing temperature, and increasing strength of the
electron-electron interaction. Overall, we find that the nuclear spins will
typically shut down the conductance of the 2DTI edges at zero temperature.Comment: 5 pages, 3 figures, revised version accepted for publication in Phys.
Rev.
Effects of nuclear spins on the transport properties of the edge of two-dimensional topological insulators
The electrons in the edge channels of two-dimensional topological insulators
can be described as a helical Tomonaga-Luttinger liquid. They couple to nuclear
spins embedded in the host materials through the hyperfine interaction, and are
therefore subject to elastic spin-flip backscattering on the nuclear spins. We
investigate the nuclear-spin-induced edge resistance due to such backscattering
by performing a renormalization-group analysis. Remarkably, the effect of this
backscattering mechanism is stronger in a helical edge than in nonhelical
channels, which are believed to be present in the trivial regime of InAs/GaSb
quantum wells. In a system with sufficiently long edges, the disordered nuclear
spins lead to an edge resistance which grows exponentially upon lowering the
temperature. On the other hand, electrons from the edge states mediate an
anisotropic Ruderman-Kittel-Kasuya-Yosida nuclear spin-spin interaction, which
induces a spiral nuclear spin order below the transition temperature. We
discuss the features of the spiral order, as well as its experimental
signatures. In the ordered phase, we identify two backscattering mechanisms,
due to charge impurities and magnons. The backscattering on charge impurities
is allowed by the internally generated magnetic field, and leads to an
Anderson-type localization of the edge states. The magnon-mediated
backscattering results in a power-law resistance, which is suppressed at zero
temperature. Overall, we find that in a sufficiently long edge the nuclear
spins, whether ordered or not, suppress the edge conductance to zero as the
temperature approaches zero.Comment: 20 pages, 11 figures; revised version accepted for publication in
Phys. Rev.
Growth Algorithms for Lattice Heteropolymers at Low Temperatures
Two improved versions of the pruned-enriched-Rosenbluth method (PERM) are
proposed and tested on simple models of lattice heteropolymers. Both are found
to outperform not only the previous version of PERM, but also all other
stochastic algorithms which have been employed on this problem, except for the
core directed chain growth method (CG) of Beutler & Dill. In nearly all test
cases they are faster in finding low-energy states, and in many cases they
found new lowest energy states missed in previous papers. The CG method is
superior to our method in some cases, but less efficient in others. On the
other hand, the CG method uses heavily heuristics based on presumptions about
the hydrophobic core and does not give thermodynamic properties, while the
present method is a fully blind general purpose algorithm giving correct
Boltzmann-Gibbs weights, and can be applied in principle to any stochastic
sampling problem.Comment: 9 pages, 9 figures. J. Chem. Phys., in pres
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