Section 3.1.5.2.13. High-temperature superconductors

It is useful to play Devil's Advocate and point out difficulties with the technique discussed, to indicate where caution might be exercised in its application. YBa₂Cu₃O_7−x (YBaCuO) provides such a case. As in the case of BaMnF₄ discussed in Section 3.1.5.2.7, there was strong evidence for a structural phase transition near 235 K, first from ultrasonic attenuation (Wang, 1987; Laegreid et al., 1987) and then from Raman studies (Zhang et al., 1988; Huang et al., 1987; Rebane et al., 1988). However, as years passed this was never verified via neutron or X-ray scattering. Researchers questioned (MacFarlane et al., 1987) whether indeed a phase transition exists at such a temperature in this important material. At present it is a controversial and occasionally contentious issue. A difficulty is that light scattering in metals probes only the surface. No information is obtained on the bulk. Ultrasonic attenuation and internal friction probe the bulk, but give scanty information on mechanisms or structure.

In the specific case of YBaCuO, the `extra' phonon line (Fig. 3.1.5.25) that emerges below 235 K is now known not to be from the superconducting YBa₂Cu₃O_7−x material; its frequency of 644 cm⁻¹ is higher than that of any bulk phonons in that material. However, this frequency closely matches that of the highest LO (longitudinal optical) phonon in the semiconducting YBa₂Cu₃O_6+x material, suggesting that the supposed phase transition at 235 K may be not a structural transition but instead a chemical transition in which oxygen is lost or gained at the surface with temperature cycling.

Figure 3.1.5.25 | top | pdf | Raman spectra of YBa2Cu3O7−x below an apparent phase transition at ca. 235 K (Zhang et al., 1988).