36,392 research outputs found
Comparison of Dissipative Particle Dynamics and Langevin thermostats for out-of-equilibrium simulations of polymeric systems
In this work we compare and characterize the behavior of Langevin and
Dissipative Particle Dynamics (DPD) thermostats in a broad range of
non-equilibrium simulations of polymeric systems. Polymer brushes in relative
sliding motion, polymeric liquids in Poiseuille and Couette flows, and
brush-melt interfaces are used as model systems to analyze the efficiency and
limitations of different Langevin and DPD thermostat implementations. Widely
used coarse-grained bead-spring models under good and poor solvent conditions
are employed to assess the effects of the thermostats. We considered
equilibrium, transient, and steady state examples for testing the ability of
the thermostats to maintain constant temperature and to reproduce the
underlying physical phenomena in non-equilibrium situations. The common
practice of switching-off the Langevin thermostat in the flow direction is also
critically revisited. The efficiency of different weight functions for the DPD
thermostat is quantitatively analyzed as a function of the solvent quality and
the non-equilibrium situation.Comment: 12 pages, introduction improved, references added, to appear in Phys.
Rev.
Coil-helix transition of polypeptide at water-lipid interface
We present the exact solution of a microscopic statistical mechanical model
for the transformation of a long polypeptide between an unstructured coil
conformation and an -helix conformation. The polypeptide is assumed to
be adsorbed to the interface between a polar and a non-polar environment such
as realized by water and the lipid bilayer of a membrane. The interfacial
coil-helix transformation is the first stage in the folding process of helical
membrane proteins. Depending on the values of model parameters, the
conformation changes as a crossover, a discontinuous transition, or a
continuous transition with helicity in the role of order parameter. Our model
is constructed as a system of statistically interacting quasiparticles that are
activated from the helix pseudo-vacuum. The particles represent links between
adjacent residues in coil conformation that form a self-avoiding random walk in
two dimensions. Explicit results are presented for helicity, entropy, heat
capacity, and the average numbers and sizes of both coil and helix segments.Comment: 22 pages, 12 figures, accepted for publication by JSTA
Semiclassical universality of parametric spectral correlations
We consider quantum systems with a chaotic classical limit that depend on an
external parameter, and study correlations between the spectra at different
parameter values. In particular, we consider the parametric spectral form
factor which depends on a scaled parameter difference . For
parameter variations that do not change the symmetry of the system we show by
using semiclassical periodic orbit expansions that the small expansion
of the form factor agrees with Random Matrix Theory for systems with and
without time reversal symmetry.Comment: 18 pages, no figure
A SiGe HEMT Mixer IC with Low Conversion Loss
The authors present the first SiGe HEMT mixer integrated circuit. The active mixer stage, operating up to 10GHz RF, has been designed and realized using a 0.1µ µµ µm gate length transistor technology. The design is based on a new large-signal simulation model developed for the SiGe HEMT. Good agreement between simulation and measurement is reached. The mixer exhibits 4.0dB and 4.7dB conversion loss when down-converting 3.0GHz and 6.0GHz signals, respectively, to an intermediate frequency of 500MHz using high-side injection of 5dBm local oscillator power. Conversion loss is less than 8dB for RF frequencies up to 10GHz with a mixer linearity of –8.8dBm input related 1dB compression point
Avalanches in mean-field models and the Barkhausen noise in spin-glasses
We obtain a general formula for the distribution of sizes of "static
avalanches", or shocks, in generic mean-field glasses with
replica-symmetry-breaking saddle points. For the Sherrington-Kirkpatrick (SK)
spin-glass it yields the density rho(S) of the sizes of magnetization jumps S
along the equilibrium magnetization curve at zero temperature. Continuous
replica-symmetry breaking allows for a power-law behavior rho(S) ~ 1/(S)^tau
with exponent tau=1 for SK, related to the criticality (marginal stability) of
the spin-glass phase. All scales of the ultrametric phase space are implicated
in jump events. Similar results are obtained for the sizes S of static jumps of
pinned elastic systems, or of shocks in Burgers turbulence in large dimension.
In all cases with a one-step solution, rho(S) ~ S exp(-A S^2). A simple
interpretation relating droplets to shocks, and a scaling theory for the
equilibrium analog of Barkhausen noise in finite-dimensional spin glasses are
discussed.Comment: 6 pages, 1 figur
Strong charge fluctuations manifested in the high-temperature Hall coefficient of high-T_c cuprates
By measuring the Hall coefficient R_H up to 1000 K in La_2CuO_4,
Pr_{1.3}La_{0.7}CuO_4, and La_{2-x}Sr_xCuO_4 (LSCO), we found that the
temperature (T) dependence of R_H in LSCO for x = 0 - 0.05 at high temperature
undoubtedly signifies a gap over which the charge carriers are thermally
activated, which in turn indicates that in lightly-doped cuprates strong charge
fluctuations are present at high temperature and the carrier number is not a
constant. At higher doping (x = 0.08 - 0.21), the high-temperature R_H(T)
behavior is found to be qualitatively the same, albeit with a weakened
temperature dependence, and we attempt to understand its behavior in terms of a
phenomenological two-carrier model where the thermal activation is considered
for one of the two species. Despite the crude nature of the model, our analysis
gives a reasonable account of R_H both at high temperature and at 0 K for a
wide range of doping, suggesting that charge fluctuations over a gap remain
important at high temperature in LSCO deep into the superconducting doping
regime. Moreover, our model gives a perspective to understand the seemingly
contradicting high-temperature behavior of R_H and the in-plane resistivity
near optimum doping in a consistent manner. Finally, we discuss possible
implications of our results on such issues as the scattering-time separation
and the large pseudogap.Comment: 9 pages, 8 figures; final version, to appear in Phys. Rev.
- …