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Applied Physics Letters

Geometric heat trapping in niobium superconductor–insulator– superconductor mixers due to niobium titanium nitride leads

Bruno Leone, Brian D. Jackson, J. R. Gao, T. M. Klapwijk

Published Feb 7, 2000• 27 citations
Publisher

Abstract

We analyze the current–voltage characteristics of a Nb superconductor–insulator–superconductor mixer with NbTiN leads to identify the heating processes in this device. We argue that the electron–electron interaction is much faster than the electron–phonon interaction, and show that the heat flow to the bath temperature is limited by the electron–phonon interaction time in the junction. A solution is suggested to considerably reduce the heating.

DOI
10.1063/1.125893

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We analyze the current–voltage characteristics of a Nb superconductor–insulator–superconductor mixer with NbTiN leads to identify the heating processes in this device. We argue that the electron–electron interaction is much faster than the electron–phonon interaction, and show that the heat flow to the bath temperature is limited by the electron–phonon interaction time in the junction. A solution is suggested to considerably reduce the heating.
In this paper we demonstrate the reduction of heating in a niobium superconductor-insulator-superconductor (SIS) junction with aluminum-oxide tunnel barrier embedded in a niobium-titanium-nitride circuit. Nonequilibrium quasiparticles which are created due to the Andreev trap at the interface between the niobium and the niobium-titanium-nitride layers are relaxed by inserting a normal-metal conductor of gold between these two layers. In an earlier work we explained the observed relaxation of nonequilibrium quasiparticles due to the geometrically assisted cooling effect. In this paper we investigate this cooling effect in dependence of the normal-metal layer shape and size. We expect that an adapted normal-metal layer is necessary for implementation in practical terahertz SIS heterodyne mixer circuits. We observe in DC-measurements of a large number of devices a clear relation between the volume of the gold layer and the effective electron temperature in the device. Our central finding is that the shape of the gold layer does not influence the cooling provided that the volume is sufficient.
We analyse both the direct current and submillimeter pumped current-voltage characteristics of a hybrid superconductor-insulator-superconductor terahertz mixer consisting of a Nb tunnel junction integrated with NbTiN tuning striplines. We find that the presence of the Nb/NbTiN interface gives rise to a hot electron effect. The assumption of an equilibrium electron temperature, as distinct from the phonon temperature, allows a complete understanding of the processes involved. The impact of the hot electron effect on superconducting terahertz detectors is discussed.
The heating of SIS tunnel junctions by local oscil- lator (LO) power and bias voltage is well known and has been reported previously. In this paper, we present a novel method for recovering the heating parameters from the experimental pumped I-V curves of an SIS device at 700 GHz, together with the coupled LO power and the embedding impedance. Since this is obtained without assuming a particular power law between LO power and junction temperature, we will be able to find ?eph, the electron-phonon interaction time of the superconducting material at various bath temperatures. We would deduce a power law that describes the dependence of the heat flow equation on temperature. Index Terms—Superconductor-insulator-superconductor de- vices, electron-phonon interaction time, heat flow, power law.
This work investigates the relationship between self-heating and the characteristic features observed in the current–voltage characteristics (IVCs) of superconductor–insulator–superconductor (SIS) junctions. Finite-element analysis is employed to evaluate the steady-state temperature distribution around SIS junctions, explicitly accounting for the temperature dependence of the thermal conductivities of the constituent materials. This approach enables flexible estimation of self-heating under various practical conditions, such as different substrate materials, interfacial thermal resistances, and geometric layouts. A heating coefficient is extracted from the simulations and used as an input parameter for IVC modeling. Incorporating self-heating through temperature-dependent gap energy and gap broadening, the simulated IVCs reproduce bending features near the energy gap that agree with measured characteristics. Furthermore, when an unintended weak link, such as a fabrication-induced defect near a via hole, is present near an SIS junction, its critical current can be significantly reduced by junction heating, producing unexpected bends at the linear branch of measured IVCs. Conversely, such bends may serve as indicators that the junction temperature approaches the superconducting transition temperature.
In this paper, we demonstrate experimentally the implementation of a niobium-trilayer junction with an aluminum-oxide tunnel barrier, embedded in a high-gap superconducting niobium-titanium-nitride circuit. Previously reported heating by quasiparticle trapping is removed by inserting a normal metal layer of gold between the niobium junction and the niobium-titanium-nitride layer. We analyze in dc-characterization measurements the cooling of the nonequilibrium quasiparticles in various device geometries having different gold layer thickness and shape. Our work is concluded with remarks for future heterodyne mixer experiments using our device technology. (C) 2013 AIP Publishing LLC.