101Ru Knight Shift Measurement of Superconducting Sr2RuO4 under Small Magnetic Fields Parallel to the RuO2 Plane

We measured 101 Ru Knight shift ( 101 K ) in Sr 2 RuO 4 superconductor under low magnetic fields parallel to the RuO 2 plane ( a b direction) in order to determine the direction of superconducting d vector in zero magnetic field. K a b was measured in a small field down to 550 Oe and in a temperature range down to 97 mK, by using nuclear-quadrupole-resonance (NQR) spectra. We found that K a b is invariant with respect to temperature on passing through T c . Our results indicate that the spins of the superconducting pair are in the RuO 2 plane (with the superconducting d vector pointing in the c direction) when fields greater than 550 Oe are applied to the RuO 2 plane. Taking into account the previous results that 101 K is unchanged in the superconducting state when fields greater than 200 Oe are applied along the c axis. We suggest that the effect to fix the spins of the superconducting pairs in a certain crystal direction is weak in Sr 2 RuO 4 even if there exists a significant spin–orbit interaction originating from the Ru-4 d orbitals and additional interactions between the chiral Cooper-pair electrons.