O.M. Vyaselev

Tl NMR study of Tl2Ba2CuOx single crystals with various Tc

Abstract A number of Tl 2 Ba 2 CuO x single crystals with different T c s from 32 to 115 K have been investigated by means of 205 Tl 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, T 1 obeys T 1 T = a + bT with a increasing and b decreasing as T c is lowered. As in other HTCSs a gap between values of T 1 and K S points to the presence of strong antiferromagnetic correlations. With lowering temperature, T 1 T 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 c ⩾100 K both K S and T 1 exhibit a gap-like behaviour with a typical Δ/T c ≈3. In crystals with lower T c s a residual K S ( T →0) has been found which is most likely related to nuclei lying inside vortex cores.

Inhomogeneous 205Tl NMR line broadening in Tl2Ba2CuOx single crystals

Abstract Temperature and angular dependences of 205Tl line width, Δ 1 2 , and transverse relaxation time, τ, have been measured in two crystals of Tl2Ba2CuOx with Tc = 32 and 115 K, respectively. In the normal state both Δ 1 2 and τ have been found to be temperature-independent and not to vary with varying Tc, implying that the size of inhomogeneous broadening is also invariable. Analysis of the line width angular dependence shows that the inhomogeneous broadening originates, most probably, from local distortions associated with Cu substitution for Tl. A conclusion is that variations in Tc are not related to this substitution.

Superconductivity of Calcium C60 Intercalation Compound Synthesized by Shock-Wave Pressure

The C60 fullerite has been Ca-intercalated under high pressure up to 240 kbar induced by a pressure shock wave. Magnetometric measurements have revealed superconductivity of the as-prepared sample at 9.9 K. The sample relaxed under normal conditions in the helium atmosphere has exhibited traces of superconductivity with the onset at T = 30 K.

Superconductivity of Al/Al2O3 interface formed under shock-wave conditions

A mixture of powdered Al and Al2O3 has been subjected to a shock-wave pressure of ≈ 170 kbar, followed by vacuum-encapsulating and quenching of the product to liquid nitrogen. The ac magnetic susceptibility measurements of the samples have revealed metastable superconductivity with Tc ≈ 37 K, characterized by glassy dynamics of the shielding currents below Tc. Comparison of the ac susceptibility and the dc magnetization measurements infers that the superconductivity arises within the interfacial granular layer formed between metallic Al and its oxide due to the shock-wave treatment.

Influence of oxygen doping on the sign of the slope of Tc vs pressure curves in Tl2Ba2CuO6+x

Abstract We report on resistive measurements of the critical temperature of Tl 2 Ba 2 CuO 6+ x single crystals with x ≈0 (“optimally doped”, T c =89 K) and x >0 (“overdoped”, T c =40 K) under the applied quasihydrostatic pressure up to 1.1 GPa. The pressure has been found to shift T c upwards for the optimally doped sample and downwards for the overdoped one. We discuss the reasons leading to the change of the sign of d T c /d P upon overdoping, and the ramifications of our data for the existing models put forward to explain the influence of oxygen doping and pressure on T c .

Transition from strong to weak coupling regime with lowering Tc in Tl2Ba2CuO6+x

Abstract Using 205 Tl NMR, the changes in electronic properties in the normal and superconducting state of the Tl 2 Ba 2 CuO 6+x compound associated with variation of T c , are studied in samples with T c =30, 100, and 115K. Decreasing T c with oxygen doping is shown to be associated with the growth of the static electron spin susceptibility and with lowering the Δ / T c ratio (Δ is superconductinh gap). This effect indicates to the corresponding weakening of coupling regime.