The Joint Condensed Matter Seminar (JCMS) series is organised by KTH Royal Insitute of Technology, Nordita, and Stockholm University.
On Monday, September 26th, 2022 from 11 am to 12 am we will host a seminar by Albert Samoilenka from KTH.
Boundary States and Enhanced Superconductivity
The standard textbook picture describes the onset of superconductivity in a BCS model as a single phase transition. The picture relies on the microscopic derivation of the boundary conditions between a BCS superconductor and vacuum by De Gennes and later by Abrikosov. They concluded that the normal derivative of the order parameter is zero. Hence the superconducting order parameter should vanish near the boundary at the same temperature as in the bulk. I will show that there are additional boundary states that were missed in previous works. They are characterized by an increased superconducting gap near boundaries, which asymptotically decays in the bulk. Moreover, the gap survives near the sample boundaries at higher temperatures than in the bulk. Therefore, BCS superconductors have multiple critical points associated with separate phase transitions at the boundary and in the bulk. We will revise the microscopic derivation of superconductor-insulator boundary conditions for the Ginzburg-Landau model. These are shown to follow from considering the order parameter reflected in the boundary. We will also study how boundary states behave in the presence of pair-density-wave states or magnetic fields.