¹³⁹La NQR Study in Superconducting La_2-ˣCuO_4-y

Nuclear quadrupole resonance (NQR) measurements were performed on superconducting La_2-ˣCuO_4-y (T_c~30 K) synthesized by high-temperature and high-pressure oxygen enrichment. At liquid ⁴He temperature, the existence of a weak and broad line has been found around 6.35 MHz which is the resonance frequency expected in stoichiometric La₂CuO₄ when an internal magnetic field is absent. The spin-echo intensity of this new signal increases in weak magnetic field (~100 Oe) which is characteristic of the resonance in the mixed state of the superconductors. On the other hand, the dominant lines corresponding to the magnetic phase do not show any significant changes in contrast with the case of the Sr-doped samples. These results indicate that superconductivity occurs in the regions where antiferromagnetic ordering is suppressed sufficiently by the carrier doping.