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 the superconducting transition, 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, but spins could align antiparallel to each other (known as singlet pairing, as in BCS theory) or parallel to each other (known as triplet pairing). 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 (TRS) breaking, 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.
A collaboration between scientists from NIT Agartala, UPES Dehradun and RKMVERI Kolkata India, Ariel University, Helmholtz-Zentrum Berlin and ISIS has used muon spin relaxation and rotation measurements to investigate the compound CaPd2As2 and to discover if TRS is preserved or broken.
Time-reversal symmetry breaking in the superconducting state is proposed to occur in unconventional superconductors because of the presence of anisotropic or multi-band energy gap p- and d-wave pairing superconductivity. However, their previous experiments had shown evidence of TRS breaking in the conventional s-wave BCS superconductor, CaPd2Ge2.
“Finding TRS breaking in single-band isotropic s-wave superconductor CaPd2Ge2 motivated us to investigate the superconducting gap structure and time-reversal symmetry state of the related compound CaPd2As2," explained Devashibhai Adroja, ISIS.
In this investigation, published in Physical Review B, and highlighted as an editor's suggestion, they used both zero field and transverse field muon spectroscopy on the MuSR beamline at ISIS, and found that TRS breaking did occur in the superconducting state of CaPd2As2. They also found that the energy gap structure of CaPd2As2 is isotropic, confirming that it is a conventional BCS superconductor.
Their results suggest that the TRS breaking in CaPd2As2 cannot be understood by the existing theories related to unconventional or multi-band superconductors, or superconductors having multiorbital character of states at the Fermi level.
“The TRS breaking in a conventional electron-phonon mediated single-band superconductor CaPd2As2 with an isotropic gap structure and an s-wave singlet paring is very exciting as well as mystifying," adds Vivek Kumar Anand.
“Our findings should stimulate further experimental and theoretical works to understand the mechanism behind the TRS breaking in s-wave superconductors."
The full paper can be found at DOI: 10.1103/PhysRevB.110.144506
