Albert Samoilenka: Boundary states and enhanced superconductivity

Abstract

The standard textbook picture describes the onset of superconductivity in a Bardeen–Cooper–Schrieffer (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. I present the numerical study of boundary states and revise the microscopic derivation of superconductor-insulator boundary conditions for the Ginzburg-Landau model.

The talk is mostly based on the following works:

1. Albert Samoilenka and Egor Babaev. Microscopic derivation of superconductor-insulator boundary conditions for Ginzburg-Landau theory revisited: Enhanced superconductivity at boundaries with and without magnetic field., Physical review B 103.22 (2021): 224516.
2. Albert Samoilenka and Egor Babaev. Boundary states with elevated critical temperatures in Bardeen-Cooper-Schrieffer superconductors., Physical review B 101.13 (2020): 134512.