Magnetotransport Experiments on Fully Metallic Superconducting Dayem-Bridge Field-Effect Transistors

Abstract

In the last 60 years conventional solid and electrolyte gating allowed sizable modulations of the surface carrier concentration in metallic superconductors resulting in tuning their conductivity and changing their critical temperature. Recent conventional gating experiments on superconducting metal nano-structures showed full suppression of the critical current without variations of the normal state resistance and the critical temperature. These results still miss a microscopic explanation. In this article, we show a complete set of gating experiments on Ti-based superconducting Dayem bridges and a suggested classical thermodynamic model which seems to account for several of our experimental findings. In particular, zero-bias resistance and critical current IC measurements highlight the following: the suppression of IC with both polarities of gate voltage, the surface nature of the effect, the critical temperature independence from the electric field and the gate-induced growth of a sub-gap dissipative component. In addition, the temperature dependence of the Josephson critical current seems to show the transition from the ballistic Kulik-Omelyanchuck behavior to the Ambegaokar-Baratoff tunnel-like characteristic by increasing the electric field. Furthermore, the IC suppression persists in the presence of sizeable perpendicular-to-plane magnetic fields. We propose a classical thermodynamic model able to describe some of the experimental observations of the present and previous works. Above all, the model grabs the bipolar electric field induced suppression of IC and the emergence of a sub-gap dissipative component near full suppression of the supercurrent. Finally, applications employing the discussed effect are proposed.

Field-Effect Controllable Metallic Josephson Interferometer

“…Importantly, field-effect is almost symmetric in the polarity of V L (see Fig. 2-c), as reported for the reduction of I S in metallic wires [1] and Dayem-bridge JJs [3,4]. As a consequence, any charge accumulation or depletion seem to be excluded since they would provide an opposite effect on the magnitude of the switching current.…”

“…The monolithic dc-SQUID is realized in the form of a 30-nm-thick Ti superconducting loop interrupted by two 150-nm-long and 150nm-wide nano-constrictions (Dayem-bridges), that we label as L for left and R for right. Corresponding to each constriction a side electrode (V L and V R , at a distance of about 30 nm and 50 nm, respectively) allows to independently control the switching current of the JJ (I L and I R ) via an electrostatic field [1,3,4]. The details of the simple single-step fabrication process can be found in the Methods.We start the basic investigation of our interferometer by measuring its resistance R versus temperature T characteristic shown in Fig.…”

“…On the other hand, the disappearance of a multivalued I S (Φ) is related to the decrease of the screening parameter [22]. The latter, together with a lower impact of the electric field on the supercurrent [1, 3,4], is also reflected in the decrease of the phase shift caused by V L = 9 V at higher temperatures (see…”

Josephson Field-Effect Transistors Based on All-Metallic Al/Cu/Al Proximity Nanojunctions

“…This latter behavior resembles the results obtained on Ti and Al superconducting FETs. [28][29][30] These findings seem to indicate a direct link between the electric field applied to the Cu wire and the ob-served I s suppression, with apparently no obvious relation with the electric field experienced by the superconducting Al leads. This observation suggests that the presence of superconducting correlation is enough for the manifestation of electrostatic field control of the supercurrent.…”

“…31 Moreover, under the action of the electrostatic field, all-metallic supercurrent transistors exhibit a behavior which cannot be assimilated to conventional field-effect super-conducting devices as the device charge density and normal-state resistance remains totally unchanged. On the technological side, these discoveries promise the realization of all-metallic field-effect superconducting devices such as, e.g., metallic gatemons [4][5][6] or the so-called EF-Trons, i.e., field-effect n-Trons, 30 that could take advantage, differently from the high-T c superconductors or their semiconducting counterparts, of a monolithic structure requiring a simple single-step fabrication process based on abundant and easy-to-manage materials like aluminum or titanium. On the fundamental physics side, this phenomenology, which still does not have a microscopic description, indicate an obscure zone in our understanding of the nature of the BCS state, and demand for a profound theoretical and experimental re-investigation of superconductivity.…”