H. Jeldtoft Jensen

Fracturing Described by a Spring-Block Model

A spring-block model containing only one parameter, the ratio between the threshold for block slips and the threshold for springs to break, is introduced to study general features of the statics and dynamics of fracturing. We find that a domain growth of positive and negative components of the stress field before cracking sets in is crucial for the pattern formations of the cracks. The domain growth obeys an algebraic growth law. The total number of cracks and the size distribution for crack events have been calculated as a function of the ratio between the threshold for spring breaking and the threshold for block slips, and as a function of time.

A solvable non-conservative model of Self-Organised Criticality

We present the first solvable non-conservative sandpile-like critical model of Self-Organised Criticality (SOC), and thereby substantiate the suggestion by Vespignani and Zapperi (Vespignani A. and Zapperi S., Phys. Rev. E, 57 (1998) 6345) 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 Broker and Grassberger (Broker H.-M. and Grassberger P., Phys. Rev. E, 56 (1997) 3944) and is supported by numerical simulation. In the limit of vanishing conservation the Random Neighbour Drossel Schwabl Forest Fire Model (R-DS-FFM) is recovered.

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 this kind of phenomena ranging from rejuvenation and stiffening of the system response, to memory and irreversibility in I-V characteristics.

Decoupling and melting in a layered superconductor

We report results for a 3D simulation of a layered superconductor. The low-temperature phase corresponds to a triangular lattice of vortex lines. On increasing the temperature order is lost in the a-b plane, Tab with no detectable thermodynamic signature. On further increasing the temperature the vortex lines decouple completely at a first-order phase transition, Tdc. The entropy associated with the transition is dependent on the anisotropy chosen for the system and has values of 0.25 kB and 0.4 kB for the two anisotropies studied. The width Tdc − Tab is anisotropy dependent and is too narrow to be measured for the more anisotropic system studied, corresponding to sublimation of the vortices as reported in a recent experiment by Fuchs et al.