To clarify the superconducting gap structure of the spin-triplet superconductor Sr2RuO4, the in-plane thermal conductivity has been measured as a function of relative orientations of the thermal flow, the crystal axes, and a magnetic field rotating within the 2D RuO2 planes. The in-plane variation of the thermal conductivity is incompatible with any model with line nodes vertical to the 2D planes and indicates the existence of horizontal nodes. These results place strong constraints on models that attempt to explain the mechanism of the triplet superconductivity.

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Magnetic superlattice peaks are observed in single-crystal neutron-diffraction measurements on orthorhombic La 1.88 Sr 0.12 CuO 4 at reciprocal points of ͑1/2Ϯ⑀,1/2,0͒ and ͑1/2,1/2Ϯ⑀,0͒ in the tetragonal notation where ⑀ϭ0.126Ϯ0.003. The La NMR measurement reveals a broadening of the field-swept spectrum below ϳ45 K corresponding to the existence of magnetic order. The remarkable softening of longitudinal sound waves along ͓110͔ is observed in the same crystal. The features observed in the neutron diffraction, NMR, and ultrasonic measurements suggest that the dynamical incommensurate spin correlation is pinned by a lattice instability toward the low-temperature tetragonal phase. ͓S0163-1829͑98͒50706-0͔ RAPID COMMUNICATIONS R323057 T. SUZUKI et al.

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