Detection and manipulation of few-vortex states in mesoscopic layered superconductors


University of Tsukuba, Akinobu Kanda

In a type-II superconductor under magnetic fields, the vortex state appears, in which a bunch of magnetic flux quanta, called vortices, are piercing the superconductor. Each vortex consists of quantized flux Φ0=h/2e generated by circular supercurrent. While in bulk superconductors vortices form a triangular lattice, in mesoscopic superconductors with sizes comparable to superconducting characteristic lengths such as the magnetic penetration depth, the vortex configuration is strongly affected by geometry of the superconductor. For example, when two vortices are confined in a mesoscopic square, the vortices are situated on a diagonal, as shown in Fig. (a).

We are trying to manipulate these mesoscopic vortex states quantum-mechanically. Previously we used aluminum deposited films as superconductors, and its surface roughness caused a problem on controllability of vortex states. Instead, recently we use exfoliated thin films of layered superconductor NbSe2. Figure (b) shows the vortex penetration to a superconducting NbSe2 square detected by the small tunnel junction method. Each voltage jump in the saw-tooth wave pattern corresponds to penetration of a single flux quantum.