¹⁰¹Ru knight shift measurement of superconducting Sr ₂RuO₄ under small magnetic fields parallel to the RuO ₂ plane

We measured ¹⁰¹Ru Knight shift (¹⁰¹K) in Sr ₂RuO₄ 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 K_ab 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 RuO₂ plane. Taking into account the previous results that ¹⁰¹K 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 ₂RuO₄ even if there exists a significant spin-orbit interaction originating from the Ru-4d orbitals and additional interactions between the chiral Cooper-pair electrons.