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 $\alpha$-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 $K(\tau,x)$ which depends on a scaled parameter difference $x$. For parameter variations that do not change the symmetry of the system we show by using semiclassical periodic orbit expansions that the small $\tau$ 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.