Optical reflectivity measurements with E k c on YBa 2 ͑Cu 12x Zn x ͒ 3 O 6.6 reveal a dramatic Znsubstitution effect on the anomaly of the in-plane oxygen bending mode and the broad absorption band around 450 cm 21 which are observed at low temperatures for x 0, whereas the low-v suppression of the optical conductivity remains unchanged. This indicates a close relation between the phonon anomaly and the absorption band, while the conductivity suppression is an independent phenomenon. The former suggests a coupling of the phonons to spin fluctuations at ͑p, p͒, leading to anomalous spectral behavior when a spin gap opens. [S0031-9007(96)01777-2] PACS numbers: 74.25. Gz, 63.20.Kr, 74.72.Bk, 78.30.Er Many of the anomalous properties in the normal state of high-T c superconductors (HTSC) have been discussed in connection with a strong electron correlation and/ or spin fluctuations. Particularly, the so-called "spin gap" phenomenon, observed in the spin-lattice relaxation rate T 21

No abstract

No abstract

The single-crystal susceptibility and magnetization hysteresis ofYBa 2 Cu 3 0 7 and YBa 2 Cu 3 0 6 _ 6 with Tc's of90K and 58K, respectively, were measured on the twin-free samples. The detwnning and retwinning processes are described for overdoped and underdoped regimes. The critical current in the untwinned overdoped YBa 2 Cu 3 0 7 is dramatically lowered relative to both oxygen-deficient and twinned crystals.

Submillimetre-wave ESR measurements for single crystal have been performed for the first time in the frequency region from 60 to 383 GHz using a pulsed magnetic field up to 16 T. AFMR modes below have been analysed by a model proposed previously, which is essentially the molecular-field theory with the assumption of an S = 1 dimer of two spins. The anisotropies of the system are determined by the analysis, and they are discussed together with the results of our magnetization measurements.