Hole doping in Tl2Ba2CuO6+δ (Tc=0–80 K) investigated by 205Ti nuclear magnetic resonance

Abstract Single phase samples of Tl 2 Ba 2 CuO 6+δ with a T c between 80 K and less than 4 K were investigated by 205 Tl NMR experiments indicate that the oxidation state of the thallium ions is predominantly 3+. The resonance line shapes, line positions and spin-lattice relaxation times as well as their temperature dependences are comparable for all samples of Tl 2 Ba 2 CuO 6+δ , irrespective of their T c . Redistribution of holes between TlO layers and CuO 2 planes can hence be disregarded. The hole concentration in Tl 2 Ba 2 CuO 6+δ is dominated by the amount of excess oxygen in the TlO layers. The change in T c with the relative change in oxygen content suggests that the phase diagram of Tl 2 Ba 2 CuO 6+δ is comparable to that of La 2− x Sr x CuO 4 , YBa 2 Cu 3 O 6+ x , Bi 2 Sr 2 CuO 6+δ and Bi 2 Sr 2 CaCu 2 O 8+δ . This leads to the conclusion that nominal Tl 2 Ba 2 CuO 6+δ samples may be regarded as being overdoped with holes.