Constraints on the superconducting order parameter in Sr2RuO4 from oxygen-17 nuclear magnetic resonance

A. Pustogow (), Yongkang Luo (), A. Chronister, Y.-S. Su, D. A. Sokolov, F. Jerzembeck, A. P. Mackenzie, C. W. Hicks, N. Kikugawa, S. Raghu, E. D. Bauer and S. E. Brown ()
Additional contact information
A. Pustogow: University of California Los Angeles
Yongkang Luo: University of California Los Angeles
A. Chronister: University of California Los Angeles
Y.-S. Su: University of California Los Angeles
D. A. Sokolov: Max Planck Institute for Chemical Physics of Solids
F. Jerzembeck: Max Planck Institute for Chemical Physics of Solids
A. P. Mackenzie: Max Planck Institute for Chemical Physics of Solids
C. W. Hicks: Max Planck Institute for Chemical Physics of Solids
N. Kikugawa: National Institute for Materials Science
S. Raghu: Stanford University
E. D. Bauer: Los Alamos National Laboratory
S. E. Brown: University of California Los Angeles

Nature, 2019, vol. 574, issue 7776, 72-75

Abstract: Abstract 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 3He-A phase1,2, 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 observed3,4. 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 shift5 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.

Date: 2019
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DOI: 10.1038/s41586-019-1596-2

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