Investigating the origins of unconventional superconductivity
16 May 2022
- Rosie de Laune



Muons are ideal probes for studying unconventional superconducting materials, demonstrated by two recent publications studying the origins of time reversal symmetry breaking.

A magnet floating over a superconductor due to the Meisner effect



​A superconducting material excludes magnetic fields and has no electrical resistance, meaning that a current can pass through it without losing any energy. However, they only behave this way below a certain temperature, known as Tc, which is typically very low.

Superconductivity in a material is caused by condensation of Cooper pairs formed of two electrons with equal and opposite momentum. Then, depending on the nature of the Cooper pairs, a small spontaneous magnetic field can arise in the superconducting ground state.

This phenomenon is known as time-reversal symmetry breaking (TRSB), as the process of time reversal would cause the direction of the magnetic field to change. Muon spectroscopy is an ideal tool for probing these small spontaneous magnetic fields and, in two recent studies, researchers have used the MuSR instrument at ISIS to study the TRSB properties of superconducting materials.

In a recent study, published in Phys Rev B, a group of researchers from the Indian Institute of Science Research and Education, Bhopal, and the Indian Institute of Science, Bangalore, studied the origins of a time-reversal symmetry breaking state in Re2Hf. By combining muon spectroscopy with computational modelling, the researchers were able to identify the cause of the TRSB as being the geometric frustration present between Fermi surface sheets within the structure.

In another study, published recently in Phys Rev Research, the researchers studied the isostructural silicide superconductors TaOsSi and NbOsSi. These materials are special because, in addition to showing spontaneous time-reversal symmetry breaking in their superconducting state, they are rare examples of symmetry protected Dirac semimetals in three dimensions. As a result, these materials provide unique material platforms to investigate the interplay of topology and superconductivity, with the aim of creating topological superconductivity. Topological superconductors are crucial for the development of fault-tolerant quantum computers.

Using the MuSR instrument, the group studied the materials above and below their Tc values. They found evidence of spontaneous magnetic fields in the superconducting state, but also low temperature thermodynamic properties similar to a conventional superconductor. This combination proves that this class of materials is a promising candidate for quantum computing applications, and one that can be investigated in further experiments.

ISIS muon group leader, Adrian Hiller, adds; “These studies truly exploit the incredible sensitivity of muon spin relaxation to very weak magnetic signals and, by combining these results with other techniques, enable a more complete understanding of these materials and what they can offer to potential applications."

Contact: de Laune, Rosie (STFC,RAL,ISIS)