Low temperature anomalies and pressure effects on the structure and Tc of the superconductor YBa 2Cu 4O 8 ( Tc=80 K)

Abstract

The structure of the 80 K superconductor YBa 2Cu 4O 8 has been measured as a function of temperature and pressure between 5 K and 165 K by high resolution neutron powder diffraction. The oxygen bridging CuO planes to double CuO chains moves away from the chains, toward the planes as T c increases with increasing pressure, indicating charge transfer to the planes. A similar effect occurs when YBa 2Cu 3O 6+ x is oxidised to YBa 2Cu 3O 7, again increasing T c. The a- and c-axes of YBa 2Cu 4O 8 increase smoothly with temperature, but the b-axis, parallel to the direction of the CuO chains, appears to have a minimum near T c. This results in a slight inflexion in the orthorhombic distortion near T c, similar to that observed in YBa 2Cu 4O 7. The oxygens within these chains appear disordered even at low temperature, again as in YBa 2Cu 4O 7. However, in YBa 2Cu 4O 8 oxygens from the CuO-planes also have relatively large out-of-plane amplitudes. High resolution electron microscopy and diffraction show that YBa 2Cu 4O 8, unlike YBa 2Cu 3O 7, does not have planar structural defects; there are no twins or superstructures.