Physicists in the US think they have solved a puzzle in superconductivity that has stumped scientists for forty years. It is well known that the resistance-free current in a superconductor disappears when the superconductor is heated above its critical temperature. But the current in some superconductors rises to a sharp peak just before it falls to zero. Now Shobo Bhattacharya of the NEC Research Institute in the US and co-workers believe that interplay between vortices in the current could produce the surprising surge (M Marchevsky et al 2001 Nature 409 591).
Bhattacharya and colleagues studied the superconductor niobium diselenide, which – like other so-called type-II superconductors – displays this anomalous current peak. They applied a magnetic field to a crystal of the material and used a technique called scanning Hall probe microscopy to measure the varying magnetic response all over its surface. The team converted these measurements – taken every 5 millikelvin as the sample was cooled through the superconducting transition – into a series of ‘magnetic maps’ of the niobium diselenide crystal. “Our work provides a direct visualization of the dynamics of the peak effect”, Marchevsky told PhysicsWeb.
Under the influence of a magnetic field, wandering vortices can develop in the superconducting current, hindering the flow of current. But the vortices are less disruptive if they are static. Bhattacharya and colleagues found that their sequence of maps revealed two distinct and changing regions. These regions corresponded to two different behaviours of these vortices – a stable phase and a mobile phase – which have different current densities. In the stable phase, the vortices are strongly ‘pinned’ to defects in the crystal and are mostly static. In the mobile phase, the vortices are weakly ‘pinned’ and can move more easily.
These two vortex phases compete for space and occupy distinct regions of the crystal as it is cooled through its transition. Bhattacharya’s team believes that this interaction is the key to understanding the anomalous peak in type-II superconductors. “The complex dynamics of the phase mixture appears to be the primary cause of many unexplained anomalies in the peak region over the years”, said Marchevsky. Their results may also shed light on the second peak observed in the current profile of many high-temperature copper oxide superconductors.
