Out-of-equilibrium phonons in gated superconducting switches

Abstract

Recent experiments have suggested that superconductivity in metallic nanowires can be suppressed by the application of modest gate voltages. The source of this gate action has been debated and either attributed to an electric-field effect or to small leakage currents. Here we show that the suppression of superconductivity in titanium nitride nanowires on silicon substrates does not depend on the presence or absence of an electric field at the nanowire, but requires a current of high-energy electrons. The suppression is most efficient when electrons are injected into the nanowire, but similar results are obtained when electrons are passed between two remote electrodes. This is explained by the decay of high-energy electrons into phonons, which propagate through the substrate and affect superconductivity in the nanowire by generating quasiparticles. By studying the switching probability distribution of the nanowire, we also show that high-energy electron emission leads to a much broader phonon energy distribution compared with the case where superconductivity is suppressed by Joule heating near the nanowire.

Electrostatic Field-Driven Supercurrent Suppression in Ionic-Gated Metallic Superconducting Nanotransistors

“…Indeed, lateral solid gates tend to produce localized and strongly anisotropic electric fields, , while electrolytes tend to generate rather isotropic electric field profiles on the nanotransistor channel surface. Since ionic gating avoids any leakage current, we conclude that the observed supercurrent suppression is exclusively driven by the electrostatic field, therefore ruling out any quasiparticle injection-related mechanism at the origin of the effect. − Moreover, the observed bipolarity and the invariance of R N and T C also exclude significant modulations of the superconductor chemical potential due to charge accumulation/depletion (due to a conventional field-effect or electrochemistry) which, differently, were reported in ionic-gated microscale metals in the normal and superconducting , states. This difference might stem from the nanoscale channel of our ISFETs in comparison to the microscale channel in other works on Nb thin films or from the superconducting film thickness .…”

“…Yet, subsequent works showed gate-induced critical current (I C ) suppression in metallic superconductor nanostructures explained in terms of overheating due to quasiparticle injection into the transistor channel, 13,14 cold electron field emission from either the gate electrode or the channel, 15,16 and nonequilibrium phononmediated interaction. 17 Differently, a recent experiment attributed the supercurrent suppression to current injection from the gate electrode inducing large energy fluctuations without any overheating. 18 Despite the fact that quasiparticle injection can account for the critical current suppression, all these works have been clearly unable to reconcile all previous experimental evidence together.…”

“…Differently from charge accumulation/depletion, this gating effect also seems to influence the phase of the Cooper pair condensate − without apparent impact on T C and R N , thereby suggesting a nontrivial interaction between electrostatic fields and superconductivity. Yet, subsequent works showed gate-induced critical current ( I C ) suppression in metallic superconductor nanostructures explained in terms of overheating due to quasiparticle injection into the transistor channel, , cold electron field emission from either the gate electrode or the channel, , and nonequilibrium phonon-mediated interaction . Differently, a recent experiment attributed the supercurrent suppression to current injection from the gate electrode inducing large energy fluctuations without any overheating .…”

Electrostatic field-driven supercurrent suppression in ionic-gated metallic Josephson nanotransistors

“…Differently from charge accumulation/depletion, this gating effect seems to influence as well the phase of the Cooper pairs condensate [8][9][10] without apparent impact on T C and R N [11], thereby suggesting a non-trivial interaction between electrostatic fields and superconductivity. Yet, subsequent works attempted to explain the critical current (I C ) suppression in terms of overheating due to quasiparticle injection into the transistor channel [12,13], cold electron field-emission either from the gate electrode or the channel [14,15] and non-equilibrium phonon-mediated interaction [16], although with clear inability to reconcile together all previous experimental evidences. Moreover, quasiparticle overheating by itself is not able to account for the ∼ 30% enhancement of the critical current observed in NbN nanobridges subject to conventional gating [17].…”

“…Indeed, lateral solid gates tend to produce localized and strongly anisotropic electric fields [5,6], while electrolytes tend to generate rather isotropic electric field profiles on the nanotransistors channel surface. Since ionic gating avoids any leakage current, we conclude that the observed supercurrent suppression is exclusively driven by the electrostatic field therefore ruling out any quasiparticle injection-related mechanism at the origin of the effect [12][13][14][15][16]. Moreover, the observed bipolarity together with the invariance of R N and T C also exclude significant modulations of the superconductor chemical potential due to charge accumulation/depletion which, differently, were reported in ionic-gated micro-scale metals in the normal [18] and superconducting [19,20] state.…”