Effective temperature in driven vortex lattices with random pinning

We study numerically correlation and response functions in nonequilibrium driven vortex lattices with random pinning. From a generalized fluctuation-dissipation relation, we calculate an effective transverse temperature in the fluid moving phase. We find that the effective temperature decreases with increasing driving force and becomes equal to the equilibrium melting temperature when the dynamic transverse freezing occurs. We also discuss how the effective temperature can be measured experimentally from a generalized Kubo formula.     

Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current reminiscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV/dI.     

Regular reflection of a strong shock wave by the surface of a wedge

Experimental investigations [1] show that when allowance is made for the real properties of a gas it is possible to have a regime of regular reflection of a shock wave from the surface of a wedge in which the reflected shock wave is attached to the tip of the wedge. In the present paper, which uses perturbation theory in a form close to a modified small-parameter method [2], an approximate analytic solution is constructed to the problem of the interaction of a strong shock wave with the surface of a wedge for such a regime. In contrast to the problem considered by the same authors in an earlier paper [3], the half-angle at the tip of the wedge is not assumed to be small.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 92–96, March–April, 1983.     

Irregular reflection of a strong shock wave from a thin wedge

The problem of irregular reflection of a strong shock wave from a rigid wall has been studied [1–3] mainly within the framework of the linear theory. It has been found that near the front of a shock wave there exist a region of large gradients of gasdynamic parameters in which the linear theory is no longer valid [4]. In the present paper we consider the nonlinear problem of Mach reflection when there is interaction between a shock wave of high intensity and a thin wedge. The solution of the problem is constructed on the assumption that the ratio of densities along the front of the impinging shock wave is small [5, 6].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 55–61, September–October, 1974.In conclusion, the authors wish to express their gratitude to A. A. Grib for his interest in the subject and to L. A. Rumyantsev for his help in carrying out the calculations.     

Mode locking in ac-driven vortex lattices with random pinning

We find mode-locking steps in simulated current-voltage characteristics of ac-driven vortex lattices with random pinning. For low frequencies there is mode locking above a finite ac force amplitude, while for large frequencies there is mode locking for any small ac force. This is correlated with the nature of temporal order in the different regimes in the absence of ac drive. The mode-locked state is a frozen solid pinned in the moving reference of frame, and the depinning from the step shows plastic flow and hysteresis.     

On peculiarities of the application of Lagrangian variables in problems of time-dependent supersonic flow past bodies

The paper considers particularities in applying Lagrangian variables in problems of hypersonic flow past bodies. It is pointed out that, in problems with intense shock waves, it is advisable to introduce Lagrangian variables as the values of parameters that characterize a particle not on surface t = t ₀ (t ₀ = const) but on surface t = σ, where σ is the time instant at which the particle meets the surface of discontinuity. Considering examples of two-dimensional flow past two-dimensional and axisymmetric bodies moving with high time-dependent velocity, we show that the passage to Lagrangian variables enables us to obtain a system of equations describing the gas flow behind the front of an intense shock wave, which is suitable for the application of the thin-shock-layer flow method. A solution is constructed in the form of series in powers of a small parameter which characterizes the ratio of the gas densities at the front of the leading shock wave. We remark that all nonlinear effects of the problem are concentrated in the equation to determine the law of motion of a particle in zero-order approximation. The cases in which this equation can be integrated are pointed out. For the remaining unknowns, the solution is taken in quadratures. We study the rearrangement of the gas flow in the shock layer when the motion of the body is changed. The flow rearrangement zone is distinguished. Also, a condition to determine the life time of that region (the time of establishment of the new flow regime) is obtained. In a specific case of passing from a steady motion of a wedge to a uniformly accelerated motion, the time of establishment of the uniformly accelerated motion is determined from a quadratic equation.     

Crossed-ratchet effects for magnetic domain wall motion

We study both experimentally and theoretically the driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes. We find two crossed-ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. By solving numerically a simple phi(4) model we show that the essential physical ingredients for this effect are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials.     

Nonequilibrium relaxation of an elastic string in a random potential

We study the nonequilibrium motion of an elastic string in a two dimensional pinning landscape using Langevin dynamics simulations. The relaxation of a line, initially flat, is characterized by a growing length L(t) separating the equilibrated short length scales from the flat long distance geometry that keeps a memory of the initial condition. We show that, in the long time limit, L(t) has a nonalgebraic growth with a universal distribution function. The distribution function of waiting times is also calculated, and related to the previous distribution. The barrier distribution is narrow enough to justify arguments based on scaling of the typical barrier.     

On the transient regime in nonstationary supersonic flow past a body

The transient regime in gas flow past a stepwise accelerated body is analyzed by the method of singular perturbations at small Froude numbers. A marked effect of the initial conditions on the shock layer gas flow is demonstrated. The problem of flow past a wedge is solved in quadratures. St.Petersburg. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 111–121, January–February, 1997.     

Creep motion of an elastic string in a random potential

We study the creep motion of an elastic string in a two-dimensional pinning landscape by means of Langevin dynamics simulations. We find that the velocity-force characteristics are well described by the creep formula predicted from phenomenological scaling arguments. We analyze the creep exponent mu and the roughness exponent zeta. Two regimes are identified: when the temperature is larger than the strength of the disorder, we find mu approximately 1/4 and zeta approximately 2/3, in agreement with the quasi-equilibrium-nucleation picture of creep motion; on the contrary, when lowering the temperature enough, the values of mu and zeta increase, showing a strong violation of the latter picture.