Eur. Phys. J. B (2013) 86: 508
https://doi.org/10.1140/epjb/e2013-40337-3

Regular Article

Superconductor-insulator transition tuned by annealing in Bi-film on top of Co-clusters

¹ Centro Brasileiro de Pesquisas Físicas, Rua Dr. Xavier Sigaud 150, 22290-180, Rio de Janeiro, Brazil
² Instituto de Física, Universidade Federal Fluminense, 24210-346, Niteroi, Brazil

^a e-mail: william@cbpf.br

Received: 8 July 2013
Received in final form: 28 October 2013
Published online: 16 December 2013

Abstract

We deposited amorphous Bi films with a thickness between 3 and 6.5 nm at 4.2 K on top of previously deposited Co clusters having a mean size of ~4.5 nm. The Co cluster layers thickness was between 2.3 and 5 nm. In-situ electrical transport measurements were performed between 2 and 100 K. Measurements on as-prepared samples having a Bi layer thickness of 3.0 nm show hopping (tunneling) conductivity as σ(T) = σ₀ exp[−(T₀/T)^1/2] above the superconducting transition temperature T_C and re-entrance behavior again with hopping (tunneling) conductivity below T_C. Annealing of films having a Bi layer thickness of 5.5 nm results in a decrease of resistivity, with variable-range hopping conduction behavior as σ(T) = σ₀ exp[−(T₀/T)^1/3 ]. Quite different are the findings for films having a Bi layer thickness of 6.5 nm: annealing of these films results in a power-law behavior as σ(T) = σ₀T^α with α = 2/3, indicating that these films are close to a quantum critical point separating superconducting and insulating phases. A phase diagram including all experimental observations is proposed.