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Register a new userElectronic coolers based on superconducting tunnel junctions: fundamentals and applications
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Authors
Herve Courtois
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Thermo-electric transport at the nano-scale is a rapidly developing topic, in particular in superconductor-based hybrid devices. In this review paper, we first discuss the fundamental principles of electronic cooling in mesoscopic superconducting hybrid structures, the related limitations and applications. We review recent work performed in Grenoble on the effects of Andreev reflection, photonic heat transport, phonon cooling, as well as on an innovative fabrication technique for powerful coolers.
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The specific property of a planar tunnel junction with thin-film diffusive plates and long enough leads is an essential enhancement of its transmission coefficient compared to the bare transparency of the tunnel barrier [1,2]. In voltage-biased junctions, this creates favourable conditions for strong nonequilibrium of quasiparticles in the junction plates and leads, produced by multiparticle tunneling. We study theoretically the interplay between the nonequilibrium and relaxation processes in such junctions and found that nonequilibrium in the leads noticeably modifies the current-voltage characteristic at $eV > 2Delta$, especially the excess current, whereas strong diffusive relaxation restores the result of the classical tunnel model. At $eV leq 2Delta$, the diffusive relaxation decreases the peaks of the multiparticle currents. The inelastic relaxation in the junction plates essentially suppresses the $n$-particle currents ($n>2$) by the factor $n$ for odd and $n/2$ for even $n$. The results may be important for the problem of decoherence in Josephson-junction based superconducting qubits.
Micro-refrigerators that operate in the sub-kelvin regime are a key device in quantum technology. A well-studied candidate, an electronic cooler using Normal metal - Insulator - Superconductor (NIS) tunnel junctions offers substantial performance and power. However, its superconducting electrodes are severely overheated due to exponential suppression of their thermal conductance towards low temperatures, and the cooler performs unsatisfactorily - especially in powerful devices needed for practical applications. We employ a second NIS cooling stage to thermalize the hot superconductor at the backside of the main NIS cooler. Not only providing a lower bath temperature, the second stage cooler actively evacuates quasiparticles out of the hot superconductor, especially in the low temperature limit. The NIS cooler approaches its ideal theoretical expectations without compromising cooling power. This cascade design can also be employed to manage excess heat in other cryo-electronic devices.
When biased at a voltage just below a superconductors energy gap, a tunnel junction between this superconductor and a normal metal cools the latter. While the study of such devices has long been focussed to structures of submicron size and consequently cooling power in the picoWatt range, we have led a thorough study of devices with a large cooling power up to the nanoWatt range. Here we describe how their performance can be optimized by using a quasi-particle drain and tuning the cooling junctions tunnel barrier.
Magnetic flux quantization in superconductors allows the implementation of fast and energy-efficient digital superconducting circuits. However, the information representation in magnetic flux severely limits their functional density presenting a long-standing problem. Here we introduce a concept of superconducting digital circuits that do not utilize magnetic flux and have no inductors. We argue that neither the use of geometrical nor kinetic inductance is promising for the deep scaling of superconducting circuits. The key idea of our approach is the utilization of bistable Josephson junctions allowing the representation of information in their Josephson energy. Since the proposed circuits are composed of Josephson junctions only, they can be called all-Josephson junction (all-JJ) circuits. We present a methodology for the design of the circuits consisting of conventional and bistable junctions. We analyze the principles of the circuit functioning, ranging from simple logic cells and ending with an 8-bit parallel adder. The utilization of bistable junctions in the all-JJ circuits is promising in the aspects of simplification of schematics and the decrease of the JJ count leading to space-efficiency.
We consider a planar SIS-type Josephson junction between diffusive superconductors (S) through an insulating tunnel interface (I). We construct fully self-consistent perturbation theory with respect to the interface conductance. As a result, we find correction to the first Josephson harmonic and calculate the second Josephson harmonic. At arbitrary temperatures, we correct previous results for the nonsinusoidal current-phase relation in Josephson tunnel junctions, which were obtained with the help of conjectured form of solution. Our perturbation theory also describes the difference between the phases of the order parameter and of the anomalous Green functions.
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