Magnetic-field-induced stabilization of nonequilibrium superconductivity in a normal-metal/insulator/superconductor junction

Abstract

A small magnetic field is found to enhance relaxation processes in a superconductor, thus stabilizing superconductivity in nonequilibrium conditions. In a normal-metal (N)/insulator/superconductor (S) tunnel junction, applying a field of the order of 100 μT leads to significantly improved cooling of the N island by quasiparticle (QP) tunneling. These findings are attributed to faster QP relaxation within the S electrodes as a result of enhanced QP drain through regions with a locally suppressed energy gap due to magnetic vortices in the S leads at some distance from the junction. Introduction. In this Rapid Communication, we report an observation that appears counterintuitive at first: A small magnetic field is found to stabilize superconductivity under quasiparticle (QP) injection. In our experiment, the cooling power of normal-metal (N)/insulator (I)/superconductor (S) tunnel structures is enhanced in perpendicular magnetic fields B ⊥ 100 μT = 1 G. A measured maximum temperature drop δT relative to a starting bath temperature T 0 = 285 mK exhibiting this behavior is shown in Fig. 1(a). The improvement is unexpected, as in general the effect of a magnetic field is to suppress superconductivity. Electronic cooling in NIS junctions in the presence of magnetic fields in both perpendicular and parallel orientations has been studied before, 1 but only in higher fields where the cooling power was already reduced due to a diminishing superconducting energy gap . On the other hand, the creation of magnetic vortices 2 has been shown to enhance QP relaxation in superconducting aluminum, as the QPs become trapped and thermalize in the regions of a reduced energy gap. 3 Here we demonstrate that the additional relaxation channel due to an enhanced QP drain through regions occupied by magnetic vortices enhances the superconducting performance of S leads and improves the electronic cooling in NIS junctions. This can be of relevance in superconducting qubits, 4-7 resonators, 8 and in hybrid SINIS turnstiles 9 in reducing the effects from nonequilibrium and residual QPs arising due to drive and microwave radiation from the environment. Moreover, improved relaxation caused by vortex creation in the S leads can partially explain the "reentrant superconductivity" observed in Zn and Al nanowires. 10,11 In the present case, as sketched in Fig. 1(a), vortex formation in the S electrodes away from the NIS junction improves relaxation of the injected QPs and leads to enhanced cooling of the N island. In higher fields, vortices move closer to the junction, deteriorating the cooling power.In a NIS junction, acts as an energy filter for the tunneling QP. [12][13][14][15] At low temperatures k B T and for bias voltages eV across the junction, the electrons in the N electrode cool considerably below the phonon temperature by hot QP extraction. The effect can be made to be more pronounced in a symmetric double-junction SINIS structure with a small N

Measurement and control of quasiparticle dynamics in a superconducting qubit

“…Electron-phonon-mediated pair recombination has been established as the canonical mechanism of QP decay 22 . Single-QP loss mechanisms in the presence of QP 'traps', such as normal metal contacts 7,16,17,23,24 , engineered gap inhomogeneity 8,25,26 , Andreev bound states 27 or magnetic field penetration 28,29 , have also been studied.…”

Electron cooling in diffusive normal metal–superconductor tunnel junctions with a spin-valve ferromagnetic interlayer

“…Recently, it was also shown that a small magnetic field enhances relaxation processes in a superconductor and leads to significant improvement of the cooling power in NIS junctions. 12 Improved cooling performance can be also achieved by proper tuning of the tunneling resistances of the individual NIS tunnel junctions in a double junction SINIS cooling device. 13 Another important limitation for NIS microcoolers arises from the intrinsic multi-particle nature of current transport in NIS junctions which is governed not only by single-particle tunneling but also by two-particle processes due to the Andreev reflection.…”

Interplay of the Inverse Proximity Effect and Magnetic Field in Out-of-Equilibrium Single-Electron Devices

“…The problem of heat evacuation in these devices is usually solved by introducing QP traps based on normal metal inclusions [15][16][17], on the local order parameter suppression by an external magnetic field [3,[11][12][13] or by using an alternative device design immune to QP overheating [18]. The traps arising from the inverse proximity phenomenon can be effective, of course, only for the relatively low resistive devices since the noticeable gap reduction at the SN interface requires quite transparent interfaces.…”

“…Reducing the superconducting gap the inverse proximity effect provides a physical mechanism responsible for the formation of the traps for the nonequilibrium quasiparticles (QPs) known to affect the performance of many superconducting devices such as X-ray detectors [8,9], single photon detectors [10], refrigerators based on normal metal (N) -insulator (I) -superconductor (S) junctions [11], superconducting resonators [12], superconducting qubits [13], and single-electronic hybrid turnstiles [3]. The problem of heat evacuation in these devices is usually solved by introducing QP traps based on normal metal inclusions [15][16][17], on the local order parameter suppression by an external magnetic field [3,[11][12][13] or by using an alternative device design immune to QP overheating [18].…”

“…The H-dependence of the terms in brackets is taken into account through the geometrical factor R H which varies from R 0 35 to R H 20 at H = 10 mT [21] due to the vortex penetration into superconducting leads [11].…”