101Ru knight shift measurement of superconducting Sr2RuO4 under small magnetic fields parallel to the RuO2 plane

We measured Ru-101 Knight shift (K-101) in Sr2RuO4 superconductor under low magnetic fields parallel to the RuO2 plane (ab direction) in order to determine the direction of superconducting d vector in zero magnetic field. K-ab 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 Kab is invariant with respect to temperature on passing through T, Our results indicate that the spins of the superconducting pair are in the RuO2 plane (with the superconducting d vector pointing in the c direction) when fields greater than 550 Oe are applied to the RuO2 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 Sr2RuO4 even if there exists a significant spin-orbit interaction originating from the Ru-4d orbitals and additional interactions between the chiral Cooper-pair electrons.