Single-crystal growth and properties of substituted YBa2Cu4O8

Single crystals of YBa2Cu4O8 have been grown from a BaO-CuO flux under 600 bar of O2 at ∼1100 deg. C. ZrO2, BeO, Al2O3 and MgO materials were used for crucibles, resulting in Tc = 81, 76, 74 and 55 K, respectively. Good quality of crystals grown in ZrO2 crucibles was confirmed by single-crystal x-ray diffraction, magnetic and transport measurements. Angle-resolved photoemission showed the presence of the 'extended' Van Hove singularity close to EF along the Γ-Y direction. Optical conductivity, resistivity and thermopower measurements along the a, b and c directions indicate a change in the character of the transport along the c direction at ∼170 K. First Zn- and Ni-substituted crystals with sharp superconducting transitions at Tc = 81-20 K for Zn and Tc = 81-0 K for Ni have been grown to study the effect of magnetic and nonmagnetic impurities located in the CuO2 plane. Ratios of the starting flux compositions to the compositions substituted in crystals are close to 3.5 and 1 for Zn and Ni, respectively. Optical conductivity shows that the low-energy electronic states are at the verge of localization. The irreversibility line and critical currents were studied for pure and substituted crystals showing that intrinsic flux pinning is not significantly modified by Zn substitution on the CuO2 planes. (author)