NMR Study of the Tl-Based High-Tc Cuprate Tl(Ba,Sr)2(Y,Ca)Cu2O7 in a Wide Hole Concentration Range from the Antiferromag
Many studies on high-T c cuprates conducted in this decade have repeatedly emphasized universality in the electronic phase diagram. Undoped cuprates are, as is well known, insulators and are two-dimensional Heisenberg antiferromagnets. With doping of a sm
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T. Goto et al.
known 1/8-issue, which is the phenomenon that Te is unexpectedly depressed at the specific hole concentration where it might be expected to be highest. The behavior of the spin-gap in Y1248 is successfully explained by the theoretical electronic phase diagram, but is completely unexplained in other cuprates such as La214, LazCaCuZ06 (La2126) or the Bi-based ones [4,5]. These phenomena seem to break the universality. In order to test whether or not the universal picture is applicable for most cuprates, it is necessary to accumulate data for many kinds of cuprates. In this paper, we select three topics for review from our recent NMR work [6,7] on the Tl-based cuprate Tl(Ba,Sr) z(Y,Ca)Cuz07 (T11212) in a wide range of hole concentration from the antiferromagnetic to the over-doped region. The first topic is the antiferromagnetism in the end member T11212, revealed by NMR, supporting universality. By the study of the end member, we also expect the possibility of finding a hidden key parameter determining T e , which varies from 10 to 150 K for different materials. The objective is to investigate the magnetic character in the antiferromagnetic phase to reveal differences between the Tl-based system and other systems such as LaZCu04 and YBaZCu306. The second topic is the spin-gap in the Tl-based system; we have revealed that the spin-gap also exists in systems in the lightly doped region. This supports universality in high-Te cuprates. However, a gap-like behavior was also observed in the Knight shift and the relaxation rate for the slightly overdoped region. This seems to contradict the theory of a universal phase diagram, which predicts that the spin-gap must appear only in the lightly doped region. The third topic is the difference in T e between TB1212 and TS1212. Since both Ba and Sr are divalent, the nominal hole concentration is the same for the stoichiometric systems. However, the state of the as-sintered sample is quite different for TB1212 and TS1212 [8-10]. We have revealed by NMR that this difference comes from the partial atomic exchange between Tl and Ca-sites.
17.1
Experimental
Polycrystalline samples of TlBazCal-xY xCUZ07+8 (x = 0.95, 1) and TlSrz Y I-x Cax CU Z07+8 (x = 0.2, 0.5) have been prepared by the solid-state reaction method from Tlz0 3, Y Z0 3, CaO, BaOz and CuO with a purity of four nines [8]. The hole concentration of TB1212 is controlled by the oxygen content O. The as-sintered sample is in the slightly overdoped region with Te = 80 K. The optimally doped sample with Te = 100 K is obtained by heat treatment in flowing Ar-gas. The as-sintered sample of TS1212 with x = 0.2 and T e = 34 K belongs to the underdoped region and that with x = 0.5 and T e = 79 K belongs to the optimally doped region. The superconducting transition temperature T e was determined as the onset of the diamagnetic
17
NMR Study of the Tl(Ba,SrMY,Ca)Cu z 07
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Table 17.1. List of samples dealt in this paper composition
gas treatment
TlBaz YCU Z 07
as-sintered
TlBazY O.95 CaO.05Cu Z07 TlSrzY
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