Optimization of electron cooling by SIN tunnel junctions

Abstract

We report on the optimization of electron cooling by SIN tunnel junctions due tothe advanced geometry of superconducting electrodes and very effective normalmetal traps for more efficient removal of quasiparticles at temperatures from 25 to500 mK. The maximum decrease in electron temperature of about 200 mK has beenobserved at bath temperatures 300–350 mK. We used four-junction geometry withAl–AlOx–Cr/Cu tunnel junctions and Au traps. Efficient electron cooling was realized due to the improvedgeometry of the cooling tunnel junctions (quadrant shape of the superconducting electrode)and optimized Au traps just near the junctions () to reduce reabsorption of quasiparticles after removing them from normalmetal. The maximum cooling effect was increased from a temperature drop ofd T = −56 mK (ordinarycross geometry) to −130 mK (improved geometry of superconducting electrodes) and tod T = −200 mK (improved geometry of superconducting electrodes and effective Au traps).The heating peak (instead of cooling) near the zero voltage across cooling junctions hasbeen observed in practice for all samples at temperatures below 150 mK. For higher coolingvoltages close to the superconducting gap, the heating was converted to coolingwith decreased amplitude. The leakage resistance of the tunnel junctions gives areasonable explanation of the heating peak. The phonon reabsorption due to therecombination of quasiparticles in superconducting electrodes gives an additionalimprovement in the theoretical fitting but could not explain the heating peak.An anomalous zero-bias resistance peak has been observed for all tested structures. Thepeak is explained by Coulomb blockade of tunnelling in transistor-type structures withrelatively small tunnel junctions.The work on electron cooling is devoted to the development of a cold-electron bolometer(CEB) with capacitive coupling by SIN tunnel junctions to the antenna for sensitivedetection in the terahertz region. Direct electron cooling of an absorber plays a crucial rolein supersensitive detection in the presence of a realistic background power load.