"Broken translational symmetry at edges of high-temperature superconductors"
(Open access)

Superconductivity is a collective state of many electrons when they all bind into electron-electron pairs and move in unison with a single quantum mechanical phase. We call this state "coherent" and we know such states in other systems, the familiar example being a laser. The question is what happens to these electrons near an edge of a physical sample, especially when the superconducting states under consideration are non-trivial, that we know appear many different materials.

In collaboration with theoretical group at Chalmers university, Sweden, we looked at surfaces of superconductors with transition temperatures, Tc, of around 100 Kelvin. We found that below about 0.2 Tc something interesting happens. A new superconducting phase appears, that is manifested through a spontaneously generated currents in a form of circulating loops. These currents are driven by periodic modulation of the phase of electronic pairs, that gives rise to superflow patterns depicted in the figures. The spontaneous superflow field has a structure that one did not anticipate: it shows a periodic array of 'sources' and 'sinks' at the boundaries of the sample (sources are blue triangles, sinks are red triangles). These 'field defects' at the boundaries come with another set of defects inside the sample (red and blue circles) that together satisfy the Poincare-Hopf theorem and relate the geometrical properties of these defects to the topological Euler characteristic of the sample.

The origin of these formations and the ways we can detect and use them are currently under investigation.