Searching of spin-triplet superconductivity

Abstract

Conventional superconductors consist of spin-singlet Cooper pairs with opposing spins. In contrast, triplet superconductors are formed from Cooper pairs with parallel spins. In both types of superconductivity, the total integer spin (0 for singlets, 1 for triplets) enables Bose-Einstein condensation, allowing Cooper pairs to condense around the Fermi level. Regardless of whether a system involves electrons or holes, its total wavefunction must remain antisymmetric. Therefore, in spin-singlet superconductivity, the spin wavefunction is antisymmetric and the orbital-wavefunction is symmetric, as seen in s-wave or d-wave superconductors. For spin triplet superconductivity, however, the spin and orbital wavefunctions are formed oppositely, symmetric and antisymmetric, such as the p-wave configuration.
The pursuit of triplet superconductors has been a fascinating area of research since the development of BCS theory. Approaches to achieving triplet superconductivity fall into two categories: bulk materials and interface-induced effects. Notable bulk triplet superconductors include the organic compound (TMTSF)₂PF₆, discovered in 1980, and UTe₂, discovered in 2019, both of which exhibit superconductivity around 1 K. Interface-induced triplet superconductivity occurs at superconductor/ferromagnet interfaces. The first breakthrough in this area was observed in 2006 in low-Tc superconductor/metallic-ferromagnet systems and later in 2022 in high-Tc superconductor/ferromagnetic-oxide systems, with effects manifesting at several tens of Kelvin. However, the existence and properties of triplet superconductivity are still debated and not fully understood.
Our study focuses on the YBCO/LSMO superlattice system, where we have observed several intriguing phenomena. We found that the formation of triplet superconductivity requires interfacial contact along the b-axis of YBCO. We also observed exotic collective behaviors, such as enhanced superconducting critical current in the system and increased magnetization of the LSMO layers under an applied magnetic field—an opposite response compared to spin-singlet superconductivity. When spin-triplet pairs form within the LSMO layer, they modify the electronic structure of the layer and open a gap at O-2p band. In this talk, we will provide a detailed overview of the current status, challenges, and future prospects in this field.