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. We report evidence of unconventional superconductivity emerging from a spin-3/2 quasi-particle 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 Γ₈ 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 Δ 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 k ⋅ 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 nontrivial 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 series of topological superconductors.

中文翻译：

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超越三重态：自旋 3/2 拓扑半金属中的非常规超导性。


在所有已知的费米子超流体中，库珀对由自旋 1/2 准粒子组成，这些准粒子配对形成自旋单重态或自旋三重态束缚态。然而，布洛赫电子的“自旋”是由晶体的对称性和它所源自的原子轨道所固定的，并且在某些情况下，它的行为就像一个自旋 3/2 粒子一样。这种系统的超导状态允许超越自旋三重态的配对，更高自旋的准粒子组合形成五重态或七重态对。我们报告了半赫斯勒半金属 YPtBi 中自旋 3/2 准粒子电子结构出现非常规超导性的证据，YPtBi 是一种低载流子密度非中心对称立方材料，具有高对称性，保留了p状j = 3/2在存在强自旋轨道耦合的情况下，Bi 基 Γ ₈能带中的流形。由于伦敦穿透深度具有显着的线性温度依赖性，超导有序参数 Δ 中线节点的存在可以通过具有高总角动量的混合宇称库珀配对模型直接解释，与高自旋费米子超流体状态一致。我们提出了j = 3/2 费米子的k ⋅ p模型，以解释占主导地位的J = 3 七重配对状态如何成为在混合偶奇奇偶校验间隙中自然产生节点的最简单解决方案。与潜在的拓扑非平凡能带结构一起，该系统中的非常规配对代表了一种真正新颖的超流形式，具有引领一系列新拓扑超导体发展的强大潜力。