Comment on ‘‘Universal relationship betweenTcand the hole content inp-type cuprate superconductors’’

Abstract

Comment on "Universal Relationship between T c and the Hole Content in />-Type Cuprate Superconductors"In Ref.[1], Zhang and Sato have recently shown a universal relation to hold between T c and the hole content in high-r c cuprates. It is characterized by a plateau, with sharp bends at both sides. The relation is primarily based on the experimental results [2][3][4][5][6][7][8] and is depicted in Fig. 1 It is evident from Fig. 1 of Ref.[1] that it is only the La(2:l:4) system that truly follows the entire curve in both ascending and descending parts of the plateau. Rare-earth-based (1:2:3) and Bi-based 2122 compounds do not follow the curve in the range where p^ > 0.25 which is clear from the corresponding references, from where the pertinent data are taken [3,5]. In fact there are no experimental points available to substantiate the descending part of the plateau.In the case of Tl-based cuprates the above authors have considered the Tl 1212 where the data points have been depicted on both sides of the plateau. The hole concentration in this case is estimated by iodometric titration on the assumption of a fixed valence state of Tl 3+ . This we feel is incorrect and needs some discussion. In fact using the data of Ref.[9] the authors could well have similarly filled data points on both sides of the plateau for all Bibased cuprate systems also. We again feel this to be unjustified as the estimation of Cu valence in Ref.[9] is based on the iodometric titration on the assumption of a fixed 3 + valence state of Bi.Recently, there has been a weight of evidence that the valence states of Bi and Tl are less than 3+ [10-13] and can undergo variation, which has also been theoretically substantiated [14][15][16]. As a consequence of the simultaneous presence of two mixed valence cations, viz., Tl and Cu (or Bi and Cu), the task of estimating the copper valence, and hence the hole concentration, in both Tl-and Bi-based cuprates is seemingly an unsolved problem through the iodometric technique and, thus, the results yielded by the latter are not free from ambiguity. Moreover the T c of these systems is not only dependent on the number of holes in Cu, but is also controlled by the valence state of Bi and Tl [10-13].In the above background, the so-called universal relation presented in Ref.[1] is strictly valid, over the whole range of the plateau curve, only for the La(2:l:4) system [17], and the data presented for other systems remain far from conclusive in this respect. Clearly the problem of high T c in these materials is far more complicated. In cuprate systems with relatively large unit cells, such as those of Bi and Tl compounds formed with several stacks of cationic oxygen layers, the T c is controlled by interlayer coupling effects rather than by the carrier concentration alone, and as such it seems futile for us to expect any such universal relation existing between T c and carrier concentration.

Anomalous thermoelectric power of overdoped Bi2Sr2CaCu2O8 superconductor

“…However, the same is not directly applicable in case of Bi, Tl, and Hg-based compounds due to fluctuating valence of Bi, Tl, and Hg. 28 The thermoelectric power S͑T͒ behavior of the studied Bi2212-MQ, Bi2212-N 2 , and Bi2212-O 2 samples is given in main panel in Fig. 3.…”