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Henrik Jeldtoft Jensen

Publications and source records attributed to Henrik Jeldtoft Jensen.

104 records · Page 6Linked to original sources
arXiv2001

Creep of superconducting vortices in the limit of vanishing temperature: A fingerprint of off-equilibrium dynamics

We theoretically study the creep of vortex matter in superconductors. The low temperatures experimental phenomenology, previously interpreted in terms of ``quantum tunnelling'' of vortices, is reproduced by Monte Carlo simulations of a purely ``classical'' vortex model. We demonstrate that a non-zero creep rate in the limit of vanishing temperature is to be expected in systems with slow relaxations as a consequence of their off-equilibrium evolution in a complex free energy landscape.

cond-mat↗
arXiv2001

A solvable non-conservative model of Self-Organized Criticality

We present the first solvable non-conservative sandpile-like critical model of Self-Organized Criticality (SOC), and thereby substantiate the suggestion by Vespignani and Zapperi [A. Vespignani and S. Zapperi, Phys. Rev. E 57, 6345 (1998)] that a lack of conservation in the microscopic dynamics of an SOC-model can be compensated by introducing an external drive and thereby re-establishing criticality. The model shown is critical for all values of the conservation parameter. The analytical derivation follows the lines of Broeker and Grassberger [H.-M. Broeker and P. Grassberger, Phys. Rev. E 56, 3944 (1997)] and is supported by numerical simulation. In the limit of vanishing conservation the Random Neighbor Forest Fire Model (R-FFM) is recovered.

cond-mat.stat-mech↗
arXiv2001

Memory effects in response functions of driven vortex matter

Vortex flow in driven type II superconductors shows strong memory and history dependent effects. Here, we study a schematic microscopic model of driven vortices to propose a scenario for a broad set of these kind of phenomena ranging from ``rejuvenation'' and ``stiffening'' of the system response, to ``memory'' and ``irreversibility'' in I-V characteristics.

cond-mat↗
arXiv2000

Vortices Clustering: The Origin of the Second Peak in the Magnetisation Loops of High Temperature Superconductors

We study vortex clustering in type II Superconductors. We demonstrate that the ``second peak'' observed in magnetisation loops may be a dynamical effect associated with a density driven instability of the vortex system. At the microscopic level the instability shows up as the clustering of individual vortices at (rare) preferential regions of the pinning potential. In the limit of quasi-static ramping the instability is related to a phase transition in the equilibrium vortex system.

cond-mat.supr-con↗
arXiv2000

Off equilibrium magnetic properties in a model for vortices in superconductors

We study the properties of a simple lattice model of repulsive particles diffusing in a pinning landscape. The behaviour of the model is very similar to the observed physics of vortices in superconductors. We compare and discuss the equilibrium phase diagram, creep dynamics, the Bean state profiles, hysteresis of magnetisation loops (including the second peak feature), and, in particular, ``off equilibrium'' relaxations. The model is analytically tractable in replica mean field theory and numerically via Monte Carlo simulations. It offers a comprehensive schematic framework of the observed phenomenology.

cond-mat↗
arXiv1999

Forest-fire models as a bridge between different paradigms in Self-Organized Criticality

We turn the stochastic critical forest-fire model introduced by Drossel and Schwabl (PRL 69, 1629, 1992) into a deterministic threshold model. This new model has many features in common with sandpile and earthquake models of Self-Organized Criticality. Nevertheless our deterministic forest-fire model exhibits in detail the same macroscopic statistical properties as the original Drossel and Schwabl model. We use the deterministic model and a related semi-deterministic version of the model to elaborate on the relation between forest-fire, sandpile and earthquake models.

cond-mat.stat-mech↗
arXiv1998

Nonequilibrium roughening transition in a simple model of fungal growth in 1+1 dimensions

We introduce a simple model of yeast-like growth of fungi colonies, which exhibits a continuous roughening transition far from equilibrium from a smooth ($α= 0$) to rough phase ($α= 1/2$) in 1+1 dimensions. At the transition some scaling exponents are calculated by mapping the problem onto a directed percolation process. The model reproduces the roughening transition observed in some experiments of fungal growth

cond-mat.stat-mech↗
arXiv1997

Gravitational dependence of contact angle hysteresis for a solid defect pinning a liquid-vapor interface

We study the case of contact angle hysteresis, when a solid defect is quasi-statically pulled out and pushed into a liquid, in the presence of gravity. We solve the problem analytically in an approximation which, in the case of a truncated parabolic pinning force becomes exact in the the limit of slow variations in the height of the liquid vapour interface. The case of a Gaussian pinning force is solved by a geometrical construction. We show that the extension of the defect, and not the capillary length, is the determining length scale, when the extension of the defect is much smaller than the capillary length.

cond-mat↗
arXiv1997

Disorder driven destruction of a phase transition in a superconductor

We investigate the effects of disorder on a layered superconductor. The clean system is known to have a first order phase transition which is clearly identified by a sharp peak in the specific heat. The peak is lost abruptly as the strength of the disorder is increased. Hence, for strong disorder there is no phase transition as a function of temperature but merely a crossover which is still detectable in the IV characteristic.

cond-mat.supr-con↗
arXiv1997

Decoupling and melting in a layered superconductor

We report results for a 3D simulation of a layered superconductor. There are two significant temperatures. The first corresponds to melting of the vortex lattice in the a-b plane (T_ab) and the second to decoupling of the layers (T_dc) with T_ab \le T_ dc. The decoupling is found to be a first order transition with an associated entropy of ~0.25-0.4 k_B/pancake. The melting has no obvious thermodynamic signature, and could be a crossover. The width of the intermediate regime T_ab<T<T_dc, decreases with increasing anisotropy such that for the more anisotropic system we cannot distinguish the melting and decoupling temperatures.

cond-mat.supr-con↗
arXiv1997

Phenomenological theory of friction in the quasistatic limit : collective pinning and memory effects

When a elastic body is moved quasistatically back and forth over a surface, the friction of the interface is experimentally observed to circulate through a hysteretic loop. The asymptotic behaviour of the hysteresis loop is approached exponentially. We describe how this behaviour is connected to the collective properties of the elastic instabilities suffered by the elastic body as it is displaced quasistatically. We express the length scale of the exponential in terms of the elasticity of the surface and the properties of the rough substrate. The predicted scalings are confirmed numerically.

cond-mat.stat-mech↗
arXiv1996

Monte Carlo calculation of the current-voltage characteristics of a two dimensional lattice Coulomb gas

We have studied the nonlinear current-voltage characteristic of a two dimensional lattice Coulomb gas by Monte Carlo simulation. We present three different determinations of the power-law exponent $a(T)$ of the nonlinear current-voltage characteristic, $V \sim I^{a(T)+1}$. The determinations rely on both equilibrium and non-equilibrium simulations. We find good agreement between the different determinations, and our results also agree closely with experimental results for Hg-Xe thin film superconductors and for certain single crystal thin-film high temperature superconductors.

cond-mat↗
arXiv1995

Transitions in non-conserving models of Self-Organized Criticality

We investigate a random--neighbours version of the two dimensional non-conserving earthquake model of Olami, Feder and Christensen [Phys. Rev. Lett. {\bf 68}, 1244 (1992)]. We show both analytically and numerically that criticality can be expected even in the presence of dissipation. As the critical level of conservation, $α_c$, is approached, the cut--off of the avalanche size distribution scales as $ξ\sim(α_c-α)^{-3/2}$. The transition from non-SOC to SOC behaviour is controlled by the average branching ratio $σ$ of an avalanche, which can thus be regarded as an order parameter of the system. The relevance of the results are discussed in connection to the nearest-neighbours OFC model (in particular we analyse the relevance of synchronization in the latter).

cond-mat↗