Dynamical ordering induced by preferential transitions in planar arrays of superheated superconducting granules

Abstract

We perform simulations of planar arrays of superheated superconducting granules (PASS) under an external magnetic field, analyzing transitions undergone by the system when the external field is slowly increased from zero. We observe, for high concentrations, the existence of an interval of external fields for which no transitions are induced. This effect is analogous to a "hot border zone" identified in the response of Superheated Superconducting Granule detectors. We explain such behaviour as produced by a geometrical ordering dynamically induced in the system by transitions in preferential sites due to diamagnetic interactions. Superheated superconducting granules systems are being developed as detectors in areas such as dark matter, neutrino, neutron, x-ray and transition radiation [1]. An ensemble of spherical granules of a Type I superconductor material is maintained in a metastable superheated state by adequate conditions of temperature and external magnetic field. An increasing of the applied field or the deposition of energy in a microgranule by radiation can produce a transition to the normal state. The change of magnetic flux inherent to the loss of Meissner effect can be sensed by a sensitive magnetometer which provides information about the incident radiation.An early detector was proposed as a disordered colloidal suspension of microgranules in a suitable medium such as paraffin wax [2]. The state of each grain of this suspension in the phase diagram depends, in addition to the external field and temperature, on its metallurgical defects and the diamagnetic interactions of the other grains of the suspension. Metallurgical defects can increase the local magnetic field of the grain and can precipitate the transition to normal state. Diamagnetic interactions depend on the environment of each grain, producing an additional dispersion in the surface magnetic field values. As a consequence of these combined effects, the spread of transitions fields of the suspension reaches values of about 20% that can reduce the resolution of the detector [3].In a previous work we showed that this spreading in transition fields is effectively reduced following an increase in the applied field. We obtained that the successive transitions induced by the external field are a strong ordering mechanism which produce a more homogeneus distribution of surface magnetic fields of the granules. Consequently, by using this effect, the uncertainty could be reduced in these devices [4,5].Experimentally, a variant of the colloidal device has been developed in response to the spread problem [6]. The microgranules are arrayed on thin planar substrates. These Planar Arrays of Superconducting Spheres (PASS) have yielded differential superheating curves in which the spreading is reduced by an order of magnitude.Although the technique of fabrication of this device can only produce arrays of relative small size, it has been shown that the PASS has both good energy and position sensitivity. On the other hand, the avalanche effect, demonstrate...

Hot-border effects, ordering, and avalanches in planar arrays of superheated superconducting granules

“…In this sense, this effect corresponds to a hot border [6]. This behaviour is apparently similar to that observed in systems immersed in a perpendicular applied magnetic field [4], but here the plateau appears for a higher f value.…”

“…Results from the perpendicular case were presented in Ref. [4], showing a reduction of the spread in transition fields, analogous to that experimentally observed. In the same work, and in agreement with other authors [5], the existence of a gap or interval of external field values for which no transitions occur was shown for concentrated systems.…”