Electrical and magnetic properties of Pb and In nanofilaments in asbestos near the superconducting transition

Bulk nanocomposites based on superconducting metals Pb and In embedded into matrices of natural chrysotile asbestos with the nanotube internal diameter $d\sim$ 6 nm have been fabricated and studied. The low-temperature electrical and magnetic properties of the nanocomposites demonstrate the superconducting transition with the transition critical temperature $T _c\approx$ (7.18 $\pm$ 0.02) K for the Pb–asbestos nanocomposite (this temperature is close to $T^{\operatorname{bulk}}_{c}$ = 7.196 K for bulk Pb). The electrical measurements show that In nanofilaments in asbestos have $T_c\sim$ 3.5–3.6 K that is higher than $T_{c\,{\operatorname{bulk}}}$ = 3.41 K for bulk In. It is shown that the temperature smearing of the superconducting transition in the temperature dependences of the resistance $R(T)\Delta T\approx$ 0.06 K for the Pb–asbestos and $\Delta T\approx$ 1.8 K for the In–asbestos are adequately described by the fluctuation Aslamazov–Larkin and Langer–Ambegaokar theories. The resistive measurements show that the critical magnetic fields of the nanofilaments extrapolated to $T$ = 0 K are $H_ c(0)\sim$ 47 kOe for Pb in asbestos and $H_c (0)\sim$ 1.5 kOe for In in asbestos; these values are significantly higher than the values for the bulk materials ($H_c^{\operatorname{bulk}}$ = 803 Oe for Pb and $H_c^{\operatorname{bulk}}$ = 285 Oe for In). The results of the electrical measurements for Pb–asbestos and In–asbestos agree with the data for the magnetic-field dependences of the magnetic moment in these nanocomposites.