应力预释放对单晶硅片的压痕位错滑移的影响

单晶硅片的压痕位错在一定温度下的滑移距离反映了硅片的机械强度. 压痕位错的滑移是受压痕的残余应力驱动的, 因此研究残余应力与位错滑移之间的关系具有重要的意义. 本文首先采用共聚焦显微拉曼术研究了单晶硅片压痕的残余应力经过300或500 ℃ 热处理后的预释放, 然后研究了上述应力预释放对压痕位错在后续较高温度(700–900 ℃)热处理过程中滑移的影响. 在未经应力预释放的情况下, 压痕位错在700–900 ℃热处理2 h后即可滑移至最大距离. 当经过上述预应力释放后, 位错在900 ℃热处理2 h后仍能达到上述最大距离, 但位错滑移速度明显降低; 而在700和800 ℃时热处理2 h后的滑移距离变小, 其减小幅度在预热处理温度为500 ℃时更为显著. 然而, 进一步的研究表明: 即使经过预应力释放, 只要足够地延长700和800 ℃ 的热处理时间, 位错滑移的最大距离几乎与未经预应力释放情形时的一样. 根据以上结果, 可以认为在压痕的残余应力大于位错在某一温度滑移所需临界应力的前提下, 压痕位错在某一温度滑移的最大距离与应力大小无关, 不过达到最大距离所需的时间随应力的减小而显著增长.     

Dislocation glasses: aging during relaxation and coarsening

The dynamics of dislocations is reported to exhibit a range of glassy properties. We study numerically various versions of 2D edge dislocation systems, in the absence of externally applied stress. Two types of glassy behavior are identified (i) dislocations gliding along randomly placed, but fixed, axes exhibit relaxation to their spatially disordered stable state; (ii) if both climb and annihilation are allowed, irregular cellular structures can form on a growing length scale before all dislocations annihilate. In all cases both the correlation function and the diffusion coefficient are found to exhibit aging. Relaxation in case (i) is a slow power law, furthermore, in the transient process (ii) the dynamical exponent z approximately 6, i.e., the cellular structure coarsens relatively slowly.     

Reversible to irreversible flow transition in periodically driven vortices

We show that periodically driven superconducting vortices in the presence of quenched disorder exhibit a transition from reversible to irreversible flow under increasing vortex density or cycle period. This type of behavior has recently been observed for periodically sheared colloidal suspensions and we demonstrate that driven vortex systems exhibit remarkably similar behavior. We also provide evidence that the onset of irreversible behavior is a dynamical phase transition.     

The Steady State Fluctuation Relation for the Dissipation Function

We give a proof of transient fluctuation relations for the entropy production (dissipation function) in nonequilibrium systems, which is valid for most time reversible dynamics. We then consider the conditions under which a transient fluctuation relation yields a steady state fluctuation relation for driven nonequilibrium systems whose transients relax, producing a unique nonequilibrium steady state. Although the necessary and sufficient conditions for the production of a unique nonequilibrium steady state are unknown, if such a steady state exists, the generation of the steady state fluctuation relation from the transient relation is shown to be very general. It is essentially a consequence of time reversibility and of a form of decay of correlations in the dissipation, which is needed also for, e.g., the existence of transport coefficients. Because of this generality the resulting steady state fluctuation relation has the same degree of robustness as do equilibrium thermodynamic equalities. The steady state fluctuation relation for the dissipation stands in contrast with the one for the phase space compression factor, whose convergence is problematic, for systems close to equilibrium. We examine some model dynamics that have been considered previously, and show how they are described in the context of this work.     

Direct experimental measurement of the speed-stress relation for dislocations in a plasma crystal

The speed-stress relation for gliding edge dislocations was experimentally measured for the first time. The experimental system used, a two-dimensional plasma crystal, allowed observation of individual dislocations at the "atomistic" level and in real time. At low applied stress dislocations moved subsonically, at higher stress their speed abruptly increased to 1.9 times the speed of shear waves, then slowly grew with stress. There is evidence that immediately after nucleation dislocations can move faster than pressure waves.     

Discovery,characterization and modelling of novel shape memory behaviour of fcc metal nanowires

Novel shape memory behaviour was discovered recently in single-crystalline fcc nanowires of Cu, Ni and Au with lateral dimensions below 5?nm. Under proper thermomechanical conditions, these wires can recover elongations up to 50%. This phenomenon only exists at the nanoscale and is associated with reversible lattice reorientations within the fcc lattice structure driven by surface stresses. Whereas the propagation of partial dislocations and twin planes specific to fcc metals are the required mechanism, only materials with higher propensities for twinning (e.g. Cu and Ni) show this behaviour and those with lower propensities for twinning (e.g. Al) do not. This paper provides an overview of this novel behaviour with a focus on the transformation mechanism, driving force, reversible strain, size and temperature effects and energy dissipation. A mechanism-based micromechanical continuum model for the tensile behaviour is developed. This model uses a decomposition of the lattice reorientation process into a reversible, smooth transition between a series of phase-equilibrium states and a superimposed irreversible, dissipative propagation of a twin boundary. The reversible part is associated with strain energy functions with multiple local minima and quantifies the energy conversion process between the twinning phases. The irreversible part is due to the ruggedness of the strain energy landscape, associated with dislocation nucleation, gliding and annihilation, and characterizes the dissipation during the transformation. This model captures all major characteristics of the behaviour, quantifies the size and temperature effects and yields results which are in excellent agreement with data from molecular dynamics simulations.     

Elastic energy of a straight dislocation and contribution from core tractions

We derive an expression of the core traction contribution to the dislocation elastic energy within linear anisotropic elasticity theory using the sextic formalism. With this contribution, the elastic energy is a state variable consistent with the work of the Peach–Koehler forces. This contribution needs also to be considered when extracting from atomic simulations core energies. The core energies thus obtained are real intrinsic dislocation properties: they do not depend on the presence and position of other defects. This is illustrated by calculating core energies of edge dislocation in bcc iron, where we show that dislocations gliding in {110} planes are more stable than those gliding in {112} planes.     

Motion of dislocations through ensembles of forest dislocations and point obstacles upon the simultaneous action of static and cyclic loads

Methods of computer simulation developed for hcp crystals were used to analyze the motion of gliding dislocations through composite ensembles of points obstacles and vibrating forest dislocations. It is shown that the possibility for forest dislocations to suffer forced vibrations increases the transparency of a composite ensemble. It was established that as the amplitude of dislocation vibrations reaches a certain limit depending on the strength of point obstacles, such obstacles in a composite ensemble almost completely lose their ability to hinder the motion of gliding dislocations.     

Slowly rocking symmetric, spatially periodic Hamiltonians: The role of escape and the emergence of giant transient directed transport

The nonintegrable Hamiltonian dynamics of particles placed in a symmetric, spatially periodic potential and subjected to a periodically varying field is explored. Such systems can exhibit a rich diversity of unusual transport features. In particular, depending on the setting of the initial phase of the drive, the possibility of a giant transient directed transport in a symmetric, space-periodic potential when driven with an adiabatically varying field arises. Here, we study the escape scenario and corresponding mean escape times of particles from a trapping region with the subsequent generation of a transient directed flow of an ensemble of particles. It is shown that for adiabatically slow inclination modulations the unidirectional flow proceeds over giant distances. The direction of escape and, hence, of the flow is entirely governed whether the periodic force, modulating the inclination of the potential, starts out initially positive or negative. In the phase space, this transient directed flow is associated with a long-lasting motion taking place within ballistic channels contained in the non-uniform chaotic layer. We demonstrate that for adiabatic modulations all escaping particles move ballistically into the same direction, leading to a giant directed current.     

Nondispersive wave packets as solitonic solutions of level dynamics

We discuss the analogy between nondispersive wave packets in periodically driven hydrogen atoms and solitons as solitary wave solutions of nonlinear dynamical systems. Whereas the wave packet eigenstates should not be considered as solitons propagating in real space, they are shown to display the essential features of solitonic solutions as they propagate through the irregular lattice of quasienergy levels evolving with a fictitious time like the strength or the frequency of the driving force.     

Symmetry-Dependent Kinetics of Dislocation Reaction

Reactions between dislocations are investigated in two-dimensional colloidal crystals. It is found that, because of the conservation of total Burgers vectors, the kinetics of the reaction is dependent on the the symmetry of the crystal lattice. Merging is possible only when the total Burgers vector of the reacting dislocations is in line with existing crystal lines. In non-merging reactions, the number of dislocations cannot be reduced but the interacting dislocations can exchange their Burgers vectors and migrate to different gliding lines. The changing of gliding lines promises additional annihilation in multi-dislocation reactions. The bonding of non-merging dislocations determines the configuration and the orientation of the grain boundaries. The findings in this study may shed new light on understanding of dislocations and have potential applications in fabrication of crystalline materials.     

Statistically locked-in transport through periodic potential landscapes

Classical particles driven through periodically modulated potential energy landscapes are predicted to follow a devil's staircase hierarchy of commensurate trajectories depending on the orientation of the driving force. Recent experiments on colloidal spheres flowing through arrays of optical traps do indeed reveal such a hierarchy, but not with the predicted structure. The microscopic trajectories, moreover, appear to be random, with commensurability emerging only in a statistical sense. We introduce an idealized model for periodically modulated transport in the presence of randomness that captures both the structure and statistics of such statistically locked-in states.     

Information Entropy to Probe Revivals in Dynamical Systems

It is shown that sum of information entropies in position and momentum space, quantifies the temporal information in wave packet dynamics of a dynamical system. Quantum fractional revivals are investigated on these bases in periodically driven Fermi-Ulam accelerator. It is observed that the entropic measure provides deeper insight of the wave packet dynamics for the long time evolution as compared with conventional autocorrelation function. It is shown that these revival times are not symmetric in driven situations and may lead to a random behavior.     

Cyclic Thermodynamic Processes and Entropy Production

We study the time evolution of a periodically driven quantum-mechanical system coupled to several reservoirs of free fermions at different temperatures. This is a paradigm of a cyclic thermodynamic process. We introduce the notion of a Floquet Liouvillean as the generator of the dynamics of the coupled system on an extended Hilbert space. We show that the time-periodic state which the state of the coupled system converges to after very many periods corresponds to a zero-energy resonance of the Floquet Liouvillean. We then show that the entropy production per cycle is (strictly) positive, a property that implies Carnot's formulation of the second law of thermodynamics.     

On the elastic fields produced by non-uniformly moving dislocations: a revisit

We investigate the non-uniform motion of straight dislocations in infinite media using the theory of incompatible elastodynamics. The equations of motion are derived for non-uniformly moving screw dislocations, gliding edge and climbing edge dislocations. The exact closed-form solutions of the elastic fields are calculated. The fields of the elastic velocity and elastic distortion surrounding the arbitrarily moving dislocations are given explicitly in the form of integral representations free of non-integrable singularities. The elastic fields describe the response in the form of non-uniformly moving elastic waves caused by the motion of the dislocation.     

Reversible destruction of dynamical localization

Dynamical localization is a localization phenomenon taking place, for example, in the quantum periodically driven kicked rotor. It is due to subtle quantum destructive interferences and is thus of intrinsic quantum origin. It has been shown that deviation from strict periodicity in the driving rapidly destroys dynamical localization. We report experimental results showing that this destruction is partially reversible when the deterministic perturbation that destroyed it is slowly reversed. We also provide an explanation for the partial character of the reversibility.     

Diffusion dynamics and first passage time in a two-coupled pendulum system

We present the numerical investigation of diffusion process and features of first passage time (FPT) and mean FPT (MFPT) in a two-coupled damped and periodically driven pendulum system. The effect of amplitude of the external periodic force and phase of the force on diffusion constant, distribution of FPT, P(tFPT), and MFPT is analyzed. Normal diffusion is found. Diffusion constant is found to show power-law variation near intermittency and sudden widening crises while linear variation is observed in the quasiperiodic region. In the intermittency crisis the divergence of diffusion constant is similar to the divergence of mean bursting length. P(tFPT) of critical distances of state variable exhibit periodic multiple peaks with decaying amplitude. MFPT of critical distances also follows power-law variation. Diffusion constant and MFPT are sensitive to the phase factor of the periodic force.     

Analogs of renormalization group transformations in random processes

We review the properties of a real-space renormalization group transformation of the free energy, including the existence of oscillatory terms multiplying the non-analytic part of the free energy. We then construct stochastic processes which incorporate into probability distributions the features of the free energy scaling equation. (The essential information is obtainable from the scaling equation and a direct solution for a probability is not necessary.) These random processes are shown to be generated directly from Cantor sets. In a spatial representation, the ensuing random process exhibits a transition between Gaussian and fractal behavior. In the fractal regime, the trajectories will, in an average sense, form self-similar clusters. In a temporal representation, the random process exhibits a transition between an asymptotically constant renewal rate and fractal behavior. The fractal regime represents a frozen state with only transient effects allowed and is related to charge transport in glasses.     

Annihilation of a dislocation dipole

The time dependence of annihilation and the life-time of an edge dislocation dipole at elevated temperatures is studied theoretically. The driving forces for the gradual approach of both dislocations composing the dipole are due to their elastic interaction. The two dislocations approach one another by climbing by the emission or absorption of vacancies; climbing is complemented by gliding.Part of this paper was presented at the International Conference on Electron Diffraction and Crystal Defects, Melbourne 1965.