By Tian Shang, Paul Scherrer Institut
Based on article published in Physical Review Letters
The study of unconventional superconductors is of great interest to the physics community. Complex superconducting ground states can form the basis of novel quantum-field theories and have the potential for applications ranging from particle accelerators to quantum computers.
The ongoing interest in non-centrosymmetric superconductors (NCSCs) initially stemmed from the possibility to attain singlet-triplet mixing, in turn arising from antisymmetric spin-orbit coupling. Such mixing can lead to exotic phenomena, as e.g., nodes in the excitation spectrum, critical fields exceeding the Pauli limit or, as recently proposed, topological superconductivity. Interestingly, several NCSCs which are considered to be weakly correlated systems have been shown to have superconducting ground states with broken time-reversal symmetry (TRS). A notable feature of such NCSCs is the presence of fully developed superconducting gaps. This, however, not only contrasts with the general expectations of a strong singlet-triplet mixing, but also sets these systems apart from the strongly-correlated superconductors with broken TRS (as, e.g., Sr2RuO4 and UPt3), where the unconventional pairing mechanism is more unambiguously determined. Hence, the broken TRS in weakly correlated systems, otherwise appearing to behave as conventional superconductors, has led to various proposals to account for this behaviour by conventional pairing mechanisms.
To date, there are no known examples of NCSCs which clearly exhibit both broken TRS and nodal-gap superconductivity. Our work illustrates unambiguously that CaPtAs represents indeed such a rare case, which comprises both these unconventional phenomena. More specifically, we use zero-field muon-spin rotation/relaxation (mSR) to detect the TRS breaking and magnetic penetration measurements (via mSR and tunnel-diode oscillator methods) and thermodynamic measurements, to show that there are two-gap superconducting features, with one of the gaps exhibiting nodes. This corresponds to the expected behaviour for strong singlet-triplet mixing. Our finding is expected to stimulate further studies of unconventional superconductors and the role played by the different types of symmetries in NCSCs.
Figure: CaPtAs is a rare noncentrosymmetric superconductor (with Tc = 1.5 K) to encompass two unconventional phenomena: time-reversal symmetry breaking and nodal-gap superconductivity. The figure shows the two superconducting pairing functions of CaPtAs: fully-gapped (top) and nodal (bottom).