One of the frontiers of quantum condensed matter physics seeks to analyze and classify scenarios of the superconductor-insulator quantum phase transition (SIT). Fermionic scenario [1] rules that disorder, when strong enough, breaks down Cooper pairs thus transforming a superconductor into a metal. The further cranking up disorder strength localizes quasiparticles turning the metal into an insulator. According to Bosonic scenario [2,3] disorder localizes Cooper pairs which survive on the insulating side of the SIT and provide an insulating gap. In the Fermionic scenario, the disorder-driven SIT is a two-stage transition through the intermediate state that exhibits finite resistance R_□ and is ordinarily referred to as quantum metal. In Bosonic scenario, the SIT this intermediate state shrinks into a single point in which the resistance assumes the universal quantum resistance per square R_c = 6.45 kΩ/□ [3]. The disorder-driven SIT was reported in films of InO_x [4, 5], Be [6], TiN [7]. However, the resistance R_c that separates superconducting and insulating states in these films is not universal. The access and detailed study of the phases in the critical vicinity of the SIT in different materials remains one of the major challenges.

            Here we observe the direct disorder-driven superconductor-insulator transition in NbTiN films with R_c = 2.7 kΩ/□ at room temperature. We show that the increasing the film's resistance suppresses the superconducting critical temperature T_c in accord with the Fermion model. We find that incrementally increasing R_□ suppresses the Berezinskii-Kosterlitz-Thouless temperature down to zero, while the critical temperature T_c remains finite, which complies with the Bosonic model. Upon further increase of R_□, the ground state of system becomes insulating. Finally, we demonstrate that the temperature dependence of the resistance of insulating films follows the Arrhenius law.

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M. V. Burdastyh, S. V. Postolova, T. I. Baturina, T. Proslier, V. M. Vinokur,

A.Yu. Mironov

JETP Letters 106 (11) (2017)