behave as those for conventional spin-singlet
superconductors; the Knight shift at the peak
decreases owing to the reduced electron spin
susceptibility of the pairs, and the linewidth
increases because of the distribution of the
local magnetic field in the vortex state. Be-
tween these regions, unusual behavior occurs:
An additional NMR peak appears starting from
1.2 T and grows with increasingH, resulting
in a“double-horn”structure (two peaks having
equal intensities) at 1.3 T. A modulation of
similar magnitude (~40% of the spin part of
the Knight shift) was also observed at the O(1)||
site (Fig. 2C). Comparison between O(2) and
O(1)||NMR spectra may reveal the orbital de-
pendence of the FFLO state because they hy-bridize with different sets of Ru-4dorbitals,
as discussed in ( 27 ) (fig. S6). Further,Kof the
right-side peak is larger than that of the normal-
state peak, as shown in Fig. 2D, indicating the
realization of a large spin-dense region in the
sample. This two-peak spectrum cannot be
ascribed to a simple phase separation between
the SC and normal states ( 27 ).398 22 APRIL 2022•VOL 376 ISSUE 6591 science.orgSCIENCE
Fig. 1. Schematic comparison among the normal state, homogeneous SC
state, and FFLO SC state.(Left) In the normal state, conduction electrons
having spin 1/2 are polarizable and have a finite spin susceptibilitycspin=cnormal.
Thiscspinis detected as a single NMR peak centered atKnormal.(Middle) In
an ordinary spin-singlet superconductor with homogeneous SC order parameter
jjY, electrons uniformly form spin-less pairs, and thus spin-density is homoge-
neously reduced fromcnormal.(Right) By contrast, in the FFLO state the spatial
modulation ofjjYbreaks the translational symmetry, with nodes appearing
periodically. Because the adjacent antinodes have opposite order-parameter signs
as depicted by the colors on the schematic at top right, quasiparticles induced
around the nodes are more spin-polarized than that in the normal state. This
results in SC spin smecticity with nontrivial local spin enhancement. The spin
smecticity is evidenced by the double-horn NMR spectra, with one peak having a
Knight shift larger thanKnormal.A B CDFig. 2. Double-horn NMR evidence for the FFLO state.(A) Crystal structure
of Sr 2 RuO 4. The blue plane indicates the RuO 2 conduction layer. (B)^17 O-NMR
spectrum at the O(2) site measured at 70 mK and various fields. The spectra in
red exhibit the two-peak structure that indicates the FFLO state. The spectra
are offset for clarity. (C) Comparison of the spectra for the O(1)||and O(2) sites.
Both the O(2) site (red circle) and the O(1)||site (green circle), whose Ru-O
bonding is parallel to the applied magnetic field, exhibit the characteristic
double-horn structure. (D) Comparison of O(2)-site spectra at 1.2 K (normal
state) and 0.07 K (FFLO state) at 1.3 T. The intensity is normalized with
the maximum at each temperature. The right-side peak has a larger Knight
shift than that of the normal state, indicating the emergence of a nontrivial
spin-dense region.RESEARCH | REPORTS