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Register a new userUltra-efficient superconducting Dayem bridge field-effect transistor
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Superconducting field-effect transitor (SuFET) and Josephson field-effect transistor (JoFET) technologies take advantage of electric field induced control of charge carrier concentration in order to modulate the channel superconducting properties. Despite field-effect is believed to be unaffective for superconducting metals, recent experiments showed electric field dependent modulation of the critical current (IC) in a fully metallic transistor. Yet, the grounding mechanism of this phenomenon is not completely understood. Here, we show the experimental realization of Ti-based Dayem bridge field-effect transistors (DB-FETs) able to control IC of the superconducting channel. Our easy fabrication process DB-FETs show symmetric full suppression of IC for an applied critical gate voltage as low as VCG~+-8V at temperatures reaching about the 85% of the record critical temperature TC~550mK for titanium. The gate-independent TC and normal state resistance (RN) coupled with the increase of resistance in the supercoducting state (RS) for gate voltages close to the critical value (VCG) suggest the creation of field-effect induced metallic puddles in the superconducting sea. Our devices show extremely high values of transconductance (gMAXm~15uA/V at VG~+-6.5V) and variations of Josephson kinetic inductance (LK) with VG of two orders of magnitude. Therefore, the DB-FET appears as an ideal candidate for the realization of superconducting electronics, superconducting qubits, tunable interferometers as well as photon detectors.
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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.
Superconductors are known to be excellent thermal insulators at low temperature owing to the presence of the energy gap in their density of states (DOS). In this context, the superconducting textit{proximity effect} allows to tune the local DOS of a metallic wire by controlling the phase bias ($varphi$) imposed across it. As a result, the wire thermal conductance can be tuned over several orders of magnitude by phase manipulation. Despite strong implications in nanoscale heat management, experimental proofs of phase-driven control of thermal transport in superconducting proximitized nanostructures are still very limited. Here, we report the experimental demonstration of efficient heat current control by phase tuning the superconducting proximity effect. This is achieved by exploiting the magnetic flux-driven manipulation of the DOS of a quasi one-dimensional aluminum nanowire forming a weal-link embedded in a superconducting ring. Our thermal superconducting quantum interference transistor (T-SQUIPT) shows temperature modulations up to $sim 16$ mK yielding a temperature-to-flux transfer function as large as $sim 60$ mK/$Phi_0$. Yet, phase-slip transitions occurring in the nanowire Josephson junction induce a hysteretic dependence of its local DOS on the direction of the applied magnetic field. Thus, we also prove the operation of the T-SQUIPT as a phase-tunable textit{thermal memory}, where the information is encoded in the temperature of the metallic mesoscopic island. Besides their relevance in quantum physics, our results are pivotal for the design of innovative coherent caloritronics devices such as heat valves and temperature amplifiers suitable for thermal logic architectures.
Despite metals are believed to be insensitive to field-effect and conventional Bardeen-Cooper-Schrieffer (BCS) theories predict the electric field to be ineffective on conventional superconductors, a number of gating experiments showed the possibility of modulating the conductivity of metallic thin films and the critical temperature of conventional superconductors. All these experimental features have been explained by simple charge accumulation/depletion. In 2018, electric field control of supercurrent in conventional metallic superconductors has been demonstrated in a range of electric fields where the induced variation of charge carrier concentration in metals is negligibly small. In fact, no changes of normal state resistance and superconducting critical temperature were reported. Here, we review the experimental results obtained in the realization of field-effect metallic superconducting devices exploiting this unexplained phenomenon. We will start by presenting the seminal results on superconducting BCS wires and nano-constriction Josephson junctions (Dayem bridges) made of different materials, such as titanium, aluminum and vanadium. Then, we show the mastering of the Josephson supercurrent in superconductor-normal metal-superconductor proximity transistors suggesting that the presence of induced superconducting correlations are enough to see this unconventional field-effect. Later, we present the control of the interference pattern in a superconducting quantum interference device indicating the coupling of the electric field with thesuperconducting phase. Among the possible applications of the presented phenomenology, we conclude this review by proposing some devices that may represent a breakthrough in superconducting quantum and classical computation.
We present a linear-response theory for the thermopower of a single-electron transistor consisting of a superconducting island weakly coupled to two normal-conducting leads (NSN SET). The thermopower shows oscillations with the same periodicity as the conductance and is rather sensitive to the size of the superconducting gap. In particular, the previously studied sawtooth-like shape of the thermopower for a normal-conducting single-electron device is qualitatively changed even for small gap energies.
We have directly measured the quantum noise of a superconducting single-electron transistor (S-SET) embedded in a microwave resonator consisting of a superconducting LC tank circuit. Using an effective bath description, we find that the S-SET provides damping of the resonator modes proportional to its differential conductance and has an effective temperature that depends strongly on the S-SET bias conditions. In the vicinity of a double Cooper pair resonance, when both resonances are red detuned the S-SET effective temperature can be well below both the ambient temperature and the energy scale of the bias voltage. When blue detuned, the S-SET shows negative differential conductivity,
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