Dichotomy in short superconducting nanowires: Thermal phase slippage vs. Coulomb blockade (original) (raw)

2006, Europhysics Letters (EPL)

Quasi-one-dimensional superconductors or nanowires exhibit a transition into a nonsuperconducting regime, as their diameter shrinks. We present measurements on ultrashort nanowires (∼40-190 nm long) in the vicinity of this quantum transition. Properties of all wires in the superconducting phase, even those close to the transition, can be explained in terms of thermally activated phase slips. The behavior of nanowires in the nonsuperconducting phase agrees with the theories of the Coulomb blockade of coherent transport through mesoscopic normal metal conductors. Thus it is concluded that the quantum transition occurs between two phases: a "true superconducting phase" and an "insulating phase". No intermediate, "metallic" phase was found. Under certain conditions, usually associated with a critical resistance per square [1,2], critical total resistance [3-6], or a characteristic diameter [7,8], a wire made of a superconducting metal looses its superconductivity and acquires two signatures of insulating behavior: i) The resistance increasing with cooling and ii) a zero-bias resistance peak [3, 4]. There are many models that capture certain features of the SIT in 1D wires. Some rely on the "fermionic" mechanism, in which disorder combined with electron-electron repulsion suppresses the critical temperature, T c , to zero [9]. In other, "bosonic", models the order parameter remains nonzero in the "insulating" phase while the coherence is destroyed by proliferating quantum phase slips (QPS) [10-15]. Existing theoretical models frequently predict a quantum superconductor-insulator transition (SIT) in thin wires [11, 13, 16, 17], driven, in many cases, by the interaction of the fluctuating phase with the Caldeira-Leggett environment [18]. Conditions that make QPS experimentally observable and the relation of the QPS to the SIT are still being actively researched [1-8, 19-22]. Here we present a quantitative analysis of the transport properties of ultrashort nanowires in each of the two phases-the insulating phase and the superconducting phase. We show that the insulating phase is characterized by the normal-electron transport and governed by the Coulomb blockade physics [23, 24]. The wires in the superconducting phase exhibit good agreement with the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips (TAPS) [25-27], without any QPS contribution. The TAPS physics is dominant, even in the vicinity of the SIT. Thus we conclude that the observed transition