Underdoped Single Crystals Research Articles

Superconductivity-induced effect on “Raman-forbidden” modes in underdopedYBa2Cu3O7−xsingle crystals

A remarkable resonance-Raman effect has been observed for some ‘‘defect oxygen’’ modes in underdoped YBa2Cu3O72x single crystals. In resonant conditions, we have studied the temperature dependence of frequencies for these originally Raman-forbidden modes as well as for the Raman-active ones. It has been found that, similar to the B1g-like mode, the yy -polarized ‘‘Raman-forbidden’’ modes soften below Tc , while the A1g-like modes dominating in the zz-polarized spectra do not. This difference in the phonon behaviors indicates a peculiarity of the interaction between the phonons and quasi-two-dimensional electron system in high-Tc superconductors. Above Tc , no clear phonon anomaly relevant to a pseudogap has been observed. @S0163-1829~98!50910-1#

63Cu NMR in the normal state of HgBa2Ca2Cu3O8+δ

We present a63Cu NMR study of HgBa2Ca2Cu3O8+δ underdoped single crystals with Tc ≃ 115 K. While the uniform spin susceptibility decreases below To ≃ 370 K, relaxation rate measurements demonstrate the opening of a spingap at Q = (π, π) below T* ≃ 230 K, the highest temperature reported so far. The characteristic energy of spin fluctuations is shown to be higher than in underdoped YBa2Cu3O7−δ, and the analysis of the quadrupole and hyperfine couplings suggests that the in-plane Cu-O hybridization is also stronger. The T-dependence of T1 is the same in the three CuO2 planes which seems hardly compatible with the pure interlayer spin-pairing picture.

Temperature dependence of the in-plane resistivity in underdoped Bi2Sr2CaCu2Oy single crystals.

A number of underdoped Bi2Sr2CaCu2Oy single crystals have been obtained by two different ways: (1) substitution of Y for Ca, and (2) removing oxygen from crystals by vacuum annealing. The in-plane resistivity (rho(ab)) measurements reveal different transp