Radio-Spectroscopic Studies of Magnetic Properties of High Temperature Superconductors

Abstract

We have performed 17 0 and 63 Cu NMR measurements in YBa 2 Cu 3 0 7 _ ä ((5% 0.15) oriented powder samples at Cu0 2 plane sites Cu(2) and 0(2, 3) in the temperature range 10-300 K. The temperature dependent Knight shift K(T) and spin lattice relaxation rate 1 -(T) of 0(2, 3) yield K 2 7"iT = const in accordance with the presence of free carriers at plane oxygen sites. A sharp decrease of T{~1 of Cu(2) below 120 K is associated with the opening of a gap in the spectrum of antiferromagnetic spin fluctuations of localized copper 3 d 9 electrons. The comparison of different temperature dependences of spin densities at the Cu and O sites shows the presence of two nearly independent spin systems. A close similarity of YBa 2 Cu 3 0 7 _ a with heavy fermion superconductors is discussed.