Residual entropy in a model for the unfolding of single polymer chains

We study the unfolding of a single polymer chain due to an external force. We use a simplified model which allows to perform all calculations in closed form without assuming a Boltzmann-Gibbs form for the equilibrium distribution. Temperature is then defined by calculating the Legendre transform of the entropy under certain constraints. The application of the model is limited to flexible polymers. It exhibits a gradual transition from compact globule to rod. The boundary line between these two phases shows reentrant behavior. This behavior is explained by the presence of residual entropy.Comment: 5 pages, 4 figures, extended version of arXiv:cond-mat/061225

Numerical simulation of grain-size effects on creep crack growth by means of grain elements

The effect of grain size on creep crack growth is investigated by means of a numerical technique in which the actual crack growth process is simulated in a discrete manner by grain elements and grain boundary elements. The grain elements account for the creep deformation of individual grains, while grain boundary cavitation and sliding are accounted for by grain boundary elements between the grains. This grain-element technique allows for an independent study of multiple grain size effects: a (direct) size effect related to the specimen size/grain size ratio or an (indirect) effect related to the effect of grain size on nucleation rate and creep resistance. Preliminary numerical results are presented concerning the direct effect of grain size, which predict that the crack growth rate and brittleness increase with grain size.

Microstructural modelling of creep crack growth from a blunted crack

The effect of crack tip blunting on the initial stages of creep crack growth is investigated by means of a planar microstructural model in which grains are represented discretely. The actual linking-up process of discrete microcracks with the macroscopic crack is simulated, with full account of the underlying physical mechanisms such as the nucleation, growth and coalescence of grain boundary cavities accompanied by grain boundary sliding. Results are presented for C*-controlled mode I crack growth under small-scale damage conditions. Particular attention is focused on creep constrained vs. unconstrained growth. Also the effect of grain boundary shear stresses on linking-up is investigated through shear-modified nucleation and growth models. The computations show a general trend that while an initially sharp crack tends to propagate away from the original crack plane, crack tip blunting reduces the crack growth direction. Under unconstrained conditions this can be partly rationalized by the strain rate and facet stress distribution corresponding to steady-state creep.

Plasticity size effects in tension and compression of single crystals

The effect of size and loading conditions on the tension and compression stress–strain response of micron-sized planar crystals is investigated using discrete dislocation plasticity. The crystals are taken to have a single active slip system and both small-strain and finite-strain analyses are carried out. When rotation of the tensile axis is constrained, the build-up of geometrically necessary dislocations results in a weak size dependence but a strong Bauschinger effect. On the other hand, when rotation of the tensile axis is unconstrained, there is a strong size dependence, with the flow strength increasing with decreasing specimen size, and a negligible Bauschinger effect. Below a certain specimen size, the flow strength of the crystals is set by the nucleation strength of the initially present Frank–Read sources. The main features of the size dependence are the same for the small-strain and finite-strain analyses. However, the predicted hardening rates differ and the finite-strain analyses give rise to some tension–compression asymmetry.

Current-induced torques in textured Rashba ferromagnets

In systems with small spin-orbit coupling, current-induced torques on the magnetization require inhomogeneous magnetization textures. For large spin-orbit coupling, such torques exist even without gradients in the magnetization direction. Here, we consider current-induced torques in ferromagnetic metals with both Rashba spin-orbit coupling and inhomogeneous magnetization. We first phenomenologically construct all torques that are allowed by the symmetries of the system, to first order in magnetization-direction gradients and electric field. Second, we use a Boltzmann approach to calculate the spin torques that arise to second order in the spin-orbit coupling. We apply our results to current-driven domain walls and find that the domain-wall mobility is strongly affected by torques that result from the interplay between spin-orbit coupling and inhomogeneity of the magnetization texture.Comment: 9 pages, 3 figure

Many-body fermionic excitations in Weyl semimetals due to elastic gauge fields

We study the single-particle spectrum of three-dimensional Weyl semimetals taking into account electron-phonon interactions that are the result of straining the material. We find that a well-defined fermionic excitation appears in addition to the standard peak corresponding to quasiparticle states as suggested by Landau-Fermi liquid theory. Contrary to the case of Dirac systems interacting via the Coulomb interaction, these satellite peaks appear even at lowest order in perturbation theory. The new excitations are anisotropic, as opposed to the single-particle spectrum, and their behavior is dictated by the Debye frequency, which naturally regulates the electron-phonon coupling.Comment: 10 pages, 2 figures, 5 pages supplemental materia

Leading by Example? Investment Decisions in a Mixed Sequential-Simultaneous Public Bad Experiment

This paper investigates the effect of having a leader in a laboratory public bad experiment with five subjects in each group.The control treatment is a standard public bad experiment, while in the leader treatments the design is such that in each group the leader decides first on his or her investment in the public bad.After being informed about the leader s decision, the four followers in each group make their investment decision.Two treatments of the leadership game are played with each group.In the same-leader-costs treatment, all subjects are confronted with the same costs, while in the no-leader-costs treatment the leader faces no direct costs of acting socially.It is found that followers invest significantly less in the public bad when there is a leader compared with a situation when there is no leader.Comparing the two treatments, we find, moreover, that the leadership effect is somewhat stronger when leaders face the same costs as followers compared with the situation in which leaders bear no costs.Randomly chosen leaders set an example by investing less than average players in a standard public bad game, and leader investments are lowest when the costs of leading are low.investment;public goods;experiment;leadership

Analysis of temperature effects near mode I cracks in glassy polymers

A previous isothermal study has shown that the toughness of glassy polymers is governed by the competition between shear yielding and crazing. The present work aims at investigating loading rates for which thermal effects need to be accounted for. The influence of the heat coming from the viscoplastic shear yielding and from crazing on their competition and on the toughness is examined. Crazing is shown to be the dominant heat source, and the dependence of the craze properties on temperature appears to be key in controlling the toughness of the material.