CU SPIN DYNAMICS AND MAGNETIC QUASI-PARTICLES IN LA2-XSRXCUO4 - A LA-139 NQR INVESTIGATION

La-139 NQR spectra and relaxation measurements in the La2-xSrxCuO4 (LS CO) system are presented for x < 0.02. In this compound the introducti on of electronic holes in the planar antiferromagnet (AF) La2CuO4 by S r doping and/or excess oxygen drastically reduces the temperature of o rdering TN whereby for x greater-than-or-equal-to 0.02 the long-range AF order is totally suppressed. For 0.02 < x < 0.08, LSCO systems disp lay 2D short-range AF correlations in regions of mesoscopic size leadi ng to an unconventional spin glass phase at low temperature (T < 15 K) . For x < 0.02 the system is in a doped Neel state, and at low tempera ture critical effects are observed, probably involving magnetic quasi- particles introduced by holes. Furthermore we find for T > 200 K a rel axation contribution due to excess O, and this phenomenon is related t o the phase separation which leads to a superconducting phase for appr opriate O doping of La2CuO4.