We report on the growth of Nd_2-xCe_xCuO_4-δ single crystals (0<x<0.2) from Cu₂O flux. Free separated crystals with maximum size of 5x8x0.15 nm³Â have been obtained. Magnetic AC susceptibility measurements show a sharp superconducting transition at temperatures up to 23 K. The temperature dependence of the lattice parameters has been measured by means of X-ray powder diffraction between 10 K (a=3.9413(3) Å, c=12.0290(18) Å) and 290 K (a=3.9482(3) Å, c=12.0590(18) Å). Room temperature Raman spectra reveal a new band at 320 cm^-1 which is not observed in Nd₂CuO₄. Raman spectra of crystals withT_c ranging from 7 to 22 K show a systematic intensity change of the broad band at 590 cm^-1.

Electron spin resonance of a high-T_c Bi superconductor sample is reported. The d.c. susceptibility, d.c. resistivity and a.c. susceptibility show two superconducting transitions at 105 K and 75 K. The ESR spectra show a main resonance line whose temperature dependence is studied in detail. The g value shows a maximum of 2.24 at 230 K and decreases to 2.12 at 100 K. The line width also shows a maximum of 520 G at the same temperature and drops to 200 G at 100 K. An unusual behaviour is observed in the decrease of the integrated intensity from a maximum at 230 K to below noise at level 100 K. It is possible that the origin of this signal is due to impurity phases. However, the unusual behaviour of its intensity (disappearance of the signal below T_c) may indicate that it arises from pair formation much above T_c.

We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu²⁺ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.

The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of Gd_yY_1−yBa₂Cu₃O_6−x has been measured. In the insulating compound, with x ~ 0.1, and the superdconducting materials with 30 K < T_c < 80 K, the measurements were performed at X-band, 9.3 GHz, and K_α-band, 36 GHz, over a large temperature range above T_c. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual T_c of 70 K. The crystal field parameters are D = 3B⁰₂ = 1307 MHz, B⁰₄ = 3.014 MHz and B⁴₄ = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 ± 0.005. These values change only slightly in the superconducting crystals.