Inherent speedup limitations in multiple time step/particle mesh Ewald algorithms

Multiple time step (MTS) algorithms present an effective integration approach to reduce the computational cost of dynamics simulations. By using force splitting to allow larger time steps for the more slowly varying force components, computational savings can be realized. The Particle-Mesh-Ewald (PME) method has been independently devised to provide an effective and efficient treatment of the long-range electrostatics interactions. Here we examine the performance of a combined MTS/PME algorithm previously developed for AMBER on a large polymerase beta/DNA complex containing 40,673 atoms. Our goal is to carefully combine the robust features of the Langevin/MTS (LN) methodology implemented in CHARMM-which uses position rather than velocity Verlet with stochasticity to make possible outer time steps of 150 fs-with the PME formulation. The developed MTS/PME integrator removes fast terms from the reciprocal-space Ewald component by using switch functions. We analyze the advantages and limitations of the resulting scheme by comparing performance to the single time step leapfrog Verlet integrator currently used in AMBER by evaluating different time-step protocols using three assessors for accuracy, speedup, and stability, all applied to long (i.e., nanosecond) simulations to ensure proper energy conservation. We also examine the performance of the algorithm on a parallel, distributed shared-memory computer (SGI Origin 2000 with 8 300-MHz R12000 processors). Good energy conservation and stability behavior can be demonstrated, for Newtonian protocols with outer time steps of up to 8 fs and Langevin protocols with outer time steps of up to 16 fs. Still, we emphasize the inherent limitations imposed by the incorporation of MTS methods into the PME formulation that may not be widely appreciated. Namely, the limiting factor on the largest outer time-step size, and hence speedup, is an intramolecular cancellation error inherent to PME. This error stems from the excluded-nonbonded correction term contained in the reciprocal-space component. This cancellation error varies in time and introduces artificial frequencies to the governing dynamics motion. Unfortunately, we find that this numerical PME error cannot be easily eliminated by refining the PME parameters (grid resolution and/or order of interpolating polynomial). We suggest that methods other than PME for fast electrostatics may allow users to reap the full advantages from MTS algorithms.     

Modelling structural changes in the near-surface region in amorphous silica

A Monte Carlo model for structure formation in amorphous silica is developed. General trends in structural change from the surface to the bulk material at different temperatures are revealed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 1, pp. 40–43, January–February, 1995.     

Dispersionless modes and the superconductivity of ultrathin films

A self-consistent analytical solution of the problem of the superconductivity of ultrathin metal films is found within the tight-binding model for normal-metal electrons with a simple example of a film of three atomic layers. Superconductivity is not destroyed in atomically thin films if the energies of the electron subsystem lie near the Fermi surface at least for some values of the quasimomentum component along the film. A substantial increase in the critical temperature of an ultrathin metal film as compared to its bulk value is possible if the electron excitation spectrum contains low-energy modes with an anomalously weak dispersion.     

Effect of liquid media on fatigue strength of polyamides

The results of an experimental study of polyamide fatigue strength are presented. Multiple flexing tests were performed on specimens rotating at 3000 cycles/min with a symmetrical load cycle in various media, including air (with and without blowing), volatile liquids (petroleum ether, ethanol, water), glycerin, and transformer oil. Fatigue strength is determined from (N — log N) curves for 10⁶ cycles. Dissipation of heat is found to be the main factor determining the effect of slightly aggressive media on fatigue strength. Values of the ratio fatigue strength/static strength (coefficient K) are presented.Mekhanika polimerov, Vol. 1, No. 1, pp. 124–127, 1965     

0-pi transitions in Josephson junctions with antiferromagnetic interlayers

We show that the dc Josephson current through superconductor-antiferromagnet-superconductor (S-AF-S) junctions manifests a remarkable atomic-scale dependence on the interlayer thickness. At low temperatures the junction is either a 0 or pi junction depending on whether the AF interlayer consists of an even or odd number of atomic layers. This is associated with different symmetries of the AF interlayers in the two cases. In the junction with odd AF interlayers an additional pi- 0 transition can take place as a function of temperature. This originates from the interplay of spin-split Andreev bound states. Experimental implications of these theoretical findings are discussed.     

Low-energy subgap states and the magnetic flux periodicity in d-wave superconducting rings

Wave functions of low-energy quasiparticle subgap states in d-wave superconducting rings, threaded by an Aharonov-Bohm magnetic flux, are found analytically. The respective energies are closest to the midgap position at small magnetic fluxes and deviate from the Fermi surface due to the Doppler shift, produced by the supercurrent. The Doppler-shifted zero-energy states result in a paramagnetic response of the ring at small fluxes. The states exist only for even angular momenta of the center of mass of Cooper pairs, in agreement with recent numerical studies of the problem. This macroscopic quantum effect in d-wave rings results in broken h/2e periodicity, retaining only the h/e periodic behavior of the supercurrent with varying magnetic flux.     

Josephson Spin-Valve Realization in the Magnetic Nodal-Line Topological Semimetal Fe3GeTe2

JETP Letters - Three-dimensional van der Waals ferromagnet Fe3GeTe2 (FGT) is regarded as a candidate for the magnetic topological nodal line semimetal. We investigate lateral electron transport...     

S-N-D corner junctions in magnetic field

The dependence of the critical current of a highly transparent S-N-D corner junction on the applied magnetic field is determined for different orientations of a d-wave superconductor relative to the interface plane. It is shown that this dependence exhibits characteristic plateaus in a certain range of magnetic fields at low temperatures. These plateaus do not appear in the S-N-S corner junctions, indicating the presence of a superconductor with a sign-variable order parameter.