NQR and NMR of139La in Antiferromagnetic La2CuO4-δ

Abstract

Rather complex Nuclear Quadrupole Resonance (NQR) spectrum of ¹³⁹ La in La ₂ CuO _4-δ , observed at low temperatures is successfully interpreted as a signal from a single La site which suffers a combined effect of a quadrupole coupling (coupling constant ν _Q =6.38±0.02 MHz with asymmetry parameter η=0.01±0.01) and a small Zeeman term where the internal field ( H _N =0.997±0.010 kOe) is nearly perpendicular to the direction of the maximum field gradient. The spin structure of Cu at low temperatures is suggested to be a simple antiferromagnet with sublattice moments parallel to the a -axis. The temperature dependences of ν _Q and H _N have been measured in detail. The fractional decrease of H _N which is proportional to the Cu sublattice moments is found to be in general accord with the prediction of spin fluctuations theory for weak itinerant antiferromagnets.