Beyond Triplet: Unconventional Superconductivity in a Spin-3/2 Topological Semimetal

TL;DR

This paper reports evidence of unconventional superconductivity in a spin-3/2 topological semimetal, revealing high-spin pairing states with line nodes, which could lead to new topological superconductor platforms.

Contribution

It introduces the first experimental evidence of high-spin (septet) pairing in a spin-3/2 fermionic system within a topological semimetal, expanding understanding of unconventional superconductivity.

Findings

Linear temperature dependence of London penetration depth

Presence of line nodes in the superconducting gap

Dominant septet pairing state explained by a k·p model

Abstract

In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The "spin" of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived, and in some cases can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or septet pairs. Here, we report evidence of unconventional superconductivity emerging from a spin-3/2 quasiparticle electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the $p$-like $j=3/2$ manifold in the Bi-based $\Gamma_8$ band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter $\Delta$ is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic superfluid state. We propose a $\mathbf{k\cdot p}$ model of the $j=3/2$ fermions to explain how a dominant $J$=3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically non-trivial band structure, the unconventional pairing in this system represents a truly novel form of superfluidity that has strong potential for leading the development of a new generation of topological superconductors.