LAUSR

Translational symmetry breaking is antagonistic to static fluidity but can be realized in superconductors, which host a quantum-mechanical coherent fluid formed by electron pairs. A peculiar example of such a state is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, induced by a time-reversal symmetry–breaking magnetic field applied to spin-singlet superconductors. This state is intrinsically accompanied by the superconducting spin smecticity, spin density–modulated fluidity with spontaneous translational-symmetry breaking. Detection of such spin smecticity provides unambiguous evidence for the FFLO state, but its observation has been challenging. Here, we report the characteristic “double-horn” nuclear magnetic resonance spectrum in the layered superconductor Sr2RuO4 near its upper critical field, indicating the spatial sinusoidal modulation of spin density that is consistent with superconducting spin smecticity. Our work reveals that Sr2RuO4 provides a versatile platform for studying FFLO physics. Description Double horn points to an exotic state The so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, a particular kind of superconducting order in which the order parameter oscillates in space, has fascinated physicists because it is tricky to stabilize. Kinjo et al. used nuclear magnetic resonance (NMR) measurements to observe a signature of the FFLO state in the material strontium ruthenate (see the Perspective by Pavarini). The modulation in the superconducting order parameter caused a corresponding modulation of spin density, which resulted in a peculiar, double horn–shaped structure of NMR intensity for a certain range of temperatures and applied magnetic field. —JS A double-horn-shaped structure in nuclear magnetic resonance data indicates a particular inhomogeneous superconducting state.