C-axis anomaly

Fig. 66. High-resolution neutron difiraction at r = 5K of BAO samples (sect. 3.2.2.4). (a) With increasing oxygen content the dimpling increases. The dimpling anomaly corresponds to the c-axis anomaly (fig. 29f) at x = 6.93, earlier found using X-rays by Rusiecki et al. (1990), cf fig. 28a. (b) The apical bond shows only a very small anomaly. After Conder et al. (1994a) and Hewat et al. (1991).

Historically, the first indication from materials research for the existence of an overdoped regime came with the finding of a maximum of T (Graf et al. 1990, Rusiecki et al. 1990) and with the c-axis anomalies discussed in sect. 3.2.2.4 (Rusiecki et al. 1990). More data were presented later by Claus et al. (1992a,b). However, in the earlier literature there is a series of indications for several anomalies of the oxygen-rich samples. 

Structure Unit-cell volume and c-axis anomalies (XRD, fig. 30). 

Figure 2. Applied-field dependence of T determined from the anomaly in magnetization. Inset is the magnetization observed at 0.05 T (1) B // a-axis, (2) B // c-axis.

Another anomaly in the ultrasonic attenuation of c-axis shear waves has been detected at 95.5 K by the same group (Lee et al. 1975). Applied fields along both a- and c-directions shifted the peak to higher temperatures. Volkov et al. (1974) found an extremum in the a-axis susceptibility (but not in c) together with an anomaly in the Hall resistivity at 93 K. The origin of these effects is not clearly understood, but it is presumably reflected in the magnetization curves (fig. 6.7) as the disappearance, between 95.2 K and 100.2 K, of the transition at Additional neutron diffraction studies extending Koehler s examination beyond 75 K would be helpful here. 

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