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O.M. Vyaselev, N.N. Kolesnikov, M.P. Kulakov and I.F. Schegolev.
Tl NMR Study of Tl₂Ba₂CuO_x single crystals with various T_c.

Abstract:
A number of Tl₂Ba₂CuO_x single crystals with different T_cs from 32 to 115 K have been investigated by means of  ²⁰⁵Tl NMR in order to elucidate the possible role of electron spin fluctuations in high-T_c superconductivity. In the normal state, the spin part of the Knight shift, K^s, has been found to be temperature independent whereas the relaxation time, T₁ obeys T₁T=a+bT with a increasing and b decreasing as T_c is lowered. As in other HTCSs a gap between values of T₁ and K^s points to the presence of strong antiferromagnetic correlations. With lowering temperature, T ₁T reaches a minimum value, which is nearly the same for all crystals, at temperatures 1.3 to 1.5 times higher than T_c. Below the superconducting transition, in crystals with T ³100 K both K^s and T₁ exhibit a gap-like behaviour with a typical D /T_c@ 3. In crystals with lower T_cs a residual K^s (T-->0) has been found which is most likely related to nuclei lying inside vortex cores.