Field-induced quasiparticle excitations in novel type II superconductors

TL;DR

This paper discusses how muon spin rotation measurements of magnetic penetration depth reveal quasiparticle excitations caused by anisotropic superconducting order parameters in novel type II superconductors, highlighting the field dependence as a key indicator.

Contribution

It reviews recent experimental results showing how the field dependence of penetration depth indicates anisotropic order parameters in unconventional superconductors.

Findings

Field dependence of $\lambda$ reveals quasiparticle excitations.

Non-zero slope of $\lambda$ indicates anisotropic order parameter.

Muon spin rotation effectively probes these excitations.

Abstract

We show that the field dependence of the magnetic penetration depth ($\lambda$), for which muon spin rotation ($\mu$SR) is an excellent microscopic probe, provides useful information on the degree of anisotropy for the superconducting order parameter. In type II superconductors associated with anisotropic order parameters, $\lambda$ is sensitive to the quasiparticle excitation induced by the Doppler shift due to a supercurrent around magnetic vortices. The presence of such low-energy excitations manifests itself in the non-zero slope of $\lambda$ against an external magnetic field. We review recent results on the field dependence of $\lambda$ obtained from the application of $\mu$SR to novel superconductors that exhibit unconventional characters associated with the anisotropic order parameter.