Reversibility Of Superconducting Nb Weak Links Driven By The Proximity Effect In A Quantum Interference Device
Résumé
We demonstrate the role of the proximity effect in the thermal hysteresis of superconducting constrictions. From the analysis of successive thermal instabilities in the transport characteristics of micron-size superconducting quantum interference devices with a well-controlled geometry, we obtain a complete picture of the different thermal regimes. These determine whether or not the junctions are hysteretic. Below the superconductor critical temperature, the critical current switches from a classical weak-link behavior to one driven by the proximity effect. The associated small amplitude of the critical current makes it robust with respect to the heat generation by phase slips, leading to a nonhysteretic behavior. Micron-size superconducting quantum interference devices (μ-SQUID), based on superconducting (SC) weak links (WLs), have been of interest for probing magnetism at small scales [1–8]. A major obstacle of a μ-SQUID proper operation is its hysteretic current-voltage characteristic (IVC). During current ramp-up, the WL switches to a dissipative state at the critical current I c , and during current ramp-down, it comes back to a zero-voltage state at the retrapping current I r < I c. In conventional tunnel-barrier-type Josephson junctions, the hysteresis arises from large junction capacitance [9]. In WLs with negligible capaci-tance, hysteresis is found at low temperatures below a crossover temperature T h < T c [10], with T c as the SC critical temperature. Although an effective capacitance can arise from the recovery time of the SC order parameter [11], it is now understood that hysteresis in WLs is of thermal origin [12–14], similar to that observed in SNS WLs [15]. A recent report on high-T c-SC based μ-SQUID shows nonhysteretic IVCs over a wide temperature range [16]. Thermal hysteresis in WLs and its effect on IVCs has been modeled by local thermal balance dictating the position of a normal metal-superconductor (N-S) interface [12–14]. In the case of poor heat evacuation, phase fluctuations can trigger a thermal runaway giving a resistive hot spot. This topic is of great practical importance, in particular, for SC-magnet wires and cables, helium level sensors, bolometers [17], μ-SQUIDs, and other nanoscale SC structures [18]. A systematic understanding of various thermal phases which a WL device exhibits can help in designing nonhysteretic devices. In this Letter, we report on the transport characteristics of Nb-film based μ-SQUIDs with a well-controlled geometry and describe a complete picture of different thermal regimes. The IVCs show a critical current and two retrapping currents that we describe using a thermal instability model in SC leads. The critical current I c follows the theoretical expectation at low temperatures but changes its behavior while crossing the smaller retrapping current. In this hysteresis-free regime, the WLs super-conduct, despite being slightly heated by individual phase slips, thanks to the proximity effect of the adjacent SC. We fabricated [19] μ-SQUIDs from Nb films using common techniques [2,20,21]. The transport measurements were carried out down to 4.2 K in a homemade cryostat with built-in copper-powder filters [10]. We have studied six devices with similar behavior, but here we report on two devices, μS1 and μS2. For all devices, the patterned SQUID-loop area is 1 μm 2 and the width of its arms is 0.3 μm. The designed WL length is 150 nm, while the WL width is 70 and 50 nm in μS1 and μS2, respectively. Figure 1(a) shows the SEM image of μS1. Four different parts of the pattern contribute to the electrical characteristics, namely, (1) the two WLs, each of normal resistance R WL , (2) the SQUID loop with normal resistance as R L , including the WLs, (3) the narrow leads of width 0.3 μm and length 1.7 μm on either side of the SQUID loop, each with a resistance R 1 , and (4) the wide leads of width 2 μm, length 27.5 μm, and normal resistance R 2. From the geometry, the total normal-state resistance between the voltage leads is R N ¼ R L þ 2R 1 þ 2R 2 ¼ 40.3R □ þ 0.5R WL. Here, R □ is the film's square resistance. Figures 1(b) and 1(c) show the resistance R versus temperature for μS1. Multiple SC transitions are observed. The resistance jumps from its residual value of 128 Ω down to about 40 Ω at T c2 ¼ 8.7 K, jumps further down from 38 to 8 Ω at T c1 ¼ 8.35 K, and finally decreases smoothly to zero. We attribute the transition at T c2 to the wide leads and that at T c1 to both the narrow leads and the SQUID loop.
Domaines
Physique Générale [physics.gen-ph]
Origine : Fichiers produits par l'(les) auteur(s)
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