Identification of Bosonic Quantum States in a Finite Momentum Superconductor

In most superconductors Cooper pairs have zero momentum, but it is possible in a quasi two dimensional material to create Cooper pairs that have finite momentum. When subjected to a small magnetic field, finite momentum Cooper pairs, which are bosons, can form a novel type of Landau level that consists of bosons rather than fermions.¹ We have studied highly two dimensional organic superconductors in a magnetic field oriented very close to parallel to the conducting planes. In this orientation, the material falls into an inhomogeneous superconducting state, caused by finite momentum Cooper pairs, called the FFLO state, named after the authors (Fulde, Ferrell, Larkin, Ovchinnikov) who predicted it. By tilting the sample to small angles from perfectly parallel, a small magnetic field is produced perpendicular to the layers while preserving the superconducting state, creating Landau levels. As the transverse field increases, the highest occupied bosonic Landau level decreases until all the bosons (Cooper pairs) are in the n = 0 state, which corresponds to the common Abrikosov vortex lattice. Bosonic quantum systems with n > 0 are rare. By accurately rotating our samples in constant magnetic fields between 8 and 25 tesla, and measuring the penetration depth with a tunnel diode oscillator, we have identified transitions between bosonic Landau levels. We will compare our measurements to calculations where we numerically solved for the phase diagrams of these materials.

1. Devarakonda et al., Nature 599, 51 (2021)