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Measurement of electron-phonon interaction time of Niobium using heating effect in SIS tunnel junction

Boon-Kok Tan, G. Yassin, Phichet Kittara, Jamie Leech

Published Apr 1, 2009• 2 citations
OpenAlex

Abstract

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.

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Heating effects in Superconductor- Insulator- Superconductor (SIS) tunnel junctions are studied by comparing SIS junctions equipped with niobium and aluminum electrodes. A temperature rise of several K is observed with niobium, which is reduced by an order of magnitude with aluminum. A simple model satisfactorily explains this observation and predicts an increase in the noise temperature of up to 30% at the operating voltage for heterodyne mixing due to this effect.

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Physics of the Solid State•Jan 1, 2022•0 citations
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.
We describe the performance of a superconductor- insulator-superconductor (SIS) mixer operating in the frequency range of 780-950 GHz. Unlike most SIS mixers, the tunnel junction employs two different superconductors, a niobium nitride top and a niobium bottom electrode sandwiching an aluminum nitride barrier layer, fabricated on a niobium titanium nitride ground plane. The mixer was tested in a pulse tube cryostat, with all the optical components, in the signal path, mounted inside the vacuum environment to avoid attenuation of the RF signal as it propagates from the hot/cold loads to the mixer. With this setup, we have measured an RF-corrected noise temperature of ~220 K. In this article, we focus on investigating the influence of local oscillator (LO) power heating on the performance of the terahertz mixer. The increase in the junction's physical temperature can be observed experimentally by noting the suppression of the gap voltage in the pumped current-voltage (I-V ) curve as the LO pumping level is increased. Similar observation has already been reported, and attempts were made to estimate the effective temperature of the device using equations of heat transfer between the mixer chip layers. Here, we present an experimental method of quantifying this effect by recovering the effective temperature of the junction through comparing the pumped I-V curves at different pumping levels and fixed bath temperature, with the unpumped I-V curves obtained at varying bath temperatures. We also estimate, for the first time, the effect of heating on the noise temperature as a function of bath temperature and frequency. We show that for typical experimental parameters, the LO heating can increase the double-sideband receiver noise temperature by as much as 20%, and that in the frequency range of the measurements, the effective temperature of the junction at fixed LO power increases linearly with frequency at a rate of 0.5 K/100 GHz.
In this paper, we present a method to measure the effective physical temperature of the tunnel junction in a terahertz superconductor-insulator-superconductor (SIS) mixer when illuminated with a strong local oscillator (LO) power that results in local hot spot around the tunnel junction. We observed that the gap voltage of the pumped current-voltage (IV) curve is suppressed when the LO pumping level is increased, indicating that the junction physical temperature is increased beyond the mixer block temperature. We quantified this extra heating effect by recovering the effective junction temperature through comparing the gap voltage of the pumped IV curves measured at a fixed block temperature, with the unpumped IV curve measured at varying block temperatures. We found that the heat trapped in the tunnel junction can be as high as 1.7 K when the mixer stabilised at 3.3 K is pumped at only 21% of the gap current.
DC heating effects in superconductor–insulator–superconductor (SIS) tunnel junctions are studied by comparing junctions sandwiched between niobium or aluminum layers. With niobium a temperature rise of several Kelvin is observed, which is reduced by an order of magnitude by using aluminum. A simple model satisfactorily explains this observation and predicts a 30% increase in the subgap current due to the elevated temperature. At the operating voltage for heterodyne mixing the receiver noise temperature increases by only 2%.
The level of signal reflection at an intermediate frequency from an SIS-mixer was calculated and experimentally measured. The measurement was performed for the mixer in operation, namely, it was biased by voltage and the signal of the local oscillator was applied. It is shown that the power reflection coefficient can vary from -20 to -3 dB depending on the operating point. An experimental method for determining the impedance of the supply line of the SIS-mixer is proposed and tested. Keywords: superconductor--insulator--superconductor tunnel junction, intermediate frequency, submillimeter SIS-mixer.