In the low doping limit, a high Tc cuprate preserves a two band structure. O2p electrons are itinerant, Cu3d electrons are localized. Therefore the two component model is suitable to describe nuclear spin relaxation at copper sites. In addition to the Korringa process, the hyperfine interaction between nuclear spins and local electron spins is considered, which gives rise to the anomalous relaxation rate 1/T1 = a + bT. The decrease of the susceptibility near Tc, as shown by the Knight shift measurements, can be attributed to the ordering of local spins and the pairing of the uncompensated spins created by holes at the oxygen sites.
Knight Shift and Nuclear Spin Relaxation Rate in a Charge-Ordered State of the One-Dimensional Extended Hubbard Model at Quarter Filling