Constraints on the superconducting order parameter in Sr2RuO4 from oxygen-17 nuclear magnetic resonance
Constraints on the superconducting order parameter in Sr2RuO4 from oxygen-17 nuclear magnetic resonance
复制标题
DOI：
10.1038/s41586-019-1596-2
复制
发表时间：
2019-10-03
期刊：
影响因子：
64.8
通讯作者：
Brown, S. E.
中科院分区：
文献类型：
--
作者：
Pustogow, A.;Luo, Yongkang;Brown, S. E.
物质的相通常通过自发的对称性破缺来识别，特别是关于非常规超导电性及其产生的相互作用。在此背景下，准二维强关联钙钛矿型Sr2RuO4超导态被认为是超流体He-3-A相(1，2)的唯一固态类似物，其奇宇称序参数在所有电子动量的自旋空间中是单向的，打破了时间反转对称性。最近，在平面内单轴压力下寻找在Sr2RuO4晶体中预期的‘分裂’转变，这一特征受到了质疑，但没有找到任何这样的证据；相反，观察到了单一转变温度的急剧上升和峰值(3，4)。在这里，我们利用氧-17的核磁共振谱，通过超精细耦合到电子自旋自由度，直接对序参数敏感，来探索Sr2RuO4超导电性的本质及其在应变下的演化。在所有应变值和低于临界温度的温度下，观察到奈特移位的减小，这与超导状态下的自旋极化的下降一致。在无应变的样品中，我们的结果与之前的一系列核磁共振工作相矛盾，这些工作报告了奈特位移没有变化，以及最普遍的序参数理论解释为手性p波态。Sr2RuO4是一种非常纯净的层状钙钛矿，它的超导电性来自强关联的费米液体，我们的工作对这个原型系统的序参数对称性施加了严格的约束。
Phases of matter are usually identified through spontaneous symmetry breaking, especially regarding unconventional superconductivity and the interactions from which it originates. In that context, the superconducting state of the quasi-two-dimensional and strongly correlated perovskite Sr2RuO4 is considered to be the only solid-state analogue to the superfluid He-3-A phase(1,2), with an odd-parity order parameter that is unidirectional in spin space for all electron momenta and breaks time-reversal symmetry. This characterization was recently called into question by a search for an expected 'split' transition in a Sr2RuO4 crystal under in-plane uniaxial pressure, which failed to find any such evidence; instead, a dramatic rise and a peak in a single-transition temperature were observed(3,4). Here we use nuclear magnetic resonance (NMR) spectroscopy of oxygen-17, which is directly sensitive to the order parameter via hyperfine coupling to the electronic spin degrees of freedom, to probe the nature of superconductivity in Sr2RuO4 and its evolution under strain. A reduction of the Knight shift is observed for all strain values and at temperatures below the critical temperature, consistent with a drop in spin polarization in the superconducting state. In unstrained samples, our results contradict a body of previous NMR work reporting no change in the Knight shift' and the most prevalent theoretical interpretation of the order parameter as a chiral p-wave state. Sr2RuO4 is an extremely clean layered perovskite and its superconductivity emerges from a strongly correlated Fermi liquid, and our work imposes tight constraints on the order parameter symmetry of this archetypal system.