[论文解读] Spontaneously polarized half-gapped superconductivity

Sheng Ran, Chris Eckberg+9 more|arXiv (Cornell University)|Nov 28, 2018

Rare-earth and actinide compounds被引用 88

一句话总结

该论文报道UTe2中的非单位自旋三重态超导性，特征是自旋极化超导态，有一半电子未被能隙化，极大且各向异性的 Hc2，以及通过 Knight shift 在 Tc 下面保持不变来证实的自旋三重态配对。

ABSTRACT

Nonunitary superconductivity is a rare and striking phenomenon in which spin up and spin down electrons segregate into two different quantum condensates. Because they support topological excitations, such superconductors are being seriously considered for potential quantum information applications. We report the discovery of nonunitary spin-triplet superconductivity in UTe2, featuring the high transition temperature of 1.6 K and a remarkably large and anisotropic upper critical field exceeding 40 T. In this unusual superconducting state, electrons with parallel spins pair, yet only half of the available electrons participate, yielding a spin-polarized condensate that coexists with a spin-polarized metal. The superconducting order parameter, which breaks both gauge and time reversal symmetries, arises from strong ferromagnetic fluctuations, placing UTe2 as the paramagnetic end member of the ferromagnetic superconductor series. This discovery yields a new platform for encoding information using topological excitations and for manipulation of spinpolarized currents.

研究动机与目标

• Motivate the search for nonunitary triplet superconductivity and spontaneous time-reversal symmetry breaking in paramagnetic systems.
• Demonstrate that UTe2 hosts a spin-polarized superconducting state with half of the electronic states ungapped.
• Characterize the superconducting and normal state properties to establish triplet pairing mediated by ferromagnetic fluctuations.

提出的方法

• Synthesize single-crystal UTe2 and characterize structure via X-ray and neutron diffraction.
• Measure electrical resistivity, magnetization, specific heat, and nuclear magnetic resonance (125Te Knight shift) to probe superconducting and normal-state properties.
• Analyze upper critical field Hc2 along three crystallographic axes and compare to orbital and paramagnetic limits.
• Utilize scaling analysis (Belitz-Kirkpatrick-Vojta theory) of magnetization to reveal quantum critical ferromagnetic fluctuations.
• Interpret data to infer a nonunitary two-component triplet order parameter with unequal gaps and intrinsic spin polarization.

实验结果

研究问题

• RQ1 Does UTe2 host spontaneous nonunitary spin-triplet superconductivity without external magnetic order?
• RQ2 Is the superconducting state characterized by two gaps of unequal magnitude (half-gapped) and intrinsic spin polarization?
• RQ3 Do ferromagnetic spin fluctuations mediate pairing, as suggested by Hc2 behavior and scaling analyses?
• RQ4 Does the Knight shift remain constant below Tc, consistent with spin-triplet pairing?
• RQ5 How do the normal-state properties (quantum critical ferromagnetism, heavy Fermi liquid behavior) relate to the superconducting state?

主要发现

• Tc = 1.6 K with a highly anisotropic upper critical field (Hc2) exceeding 40 T along the b-axis.
• Hc2 greatly exceeds both orbital and paramagnetic limits, especially along b, indicating spin-triplet pairing.
• Residual electronic density of states in the superconducting state is large (gamma0 = 55 mJ/mol-K^2), implying half the electrons remain ungapped.
• C(T) below Tc follows a power law (n ≈ 3.2), indicating point nodes compatible with nonunitary pairing.
• 125Te NMR Knight shift is temperature-independent through Tc, supporting spin-triplet pairing.
• Magnetization data show scaling consistent with Belitz-Kirkpatrick-Vojta quantum critical ferromagnetism, suggesting pairing mediated by ferromagnetic fluctuations.
• The simplest pairing symmetry compatible with observations is a nonunitary triplet state with unequal gaps in the two spin channels, yielding a spin-polarized superconducting condensate.