Ian Jasper Agulo

Optimization of electron cooling by SIN tunnel junctions

We report on the optimization of electron cooling by SIN tunnel junctions due to the advanced geometry of superconducting electrodes and very effective normal metal traps for more efficien tr emoval of quasiparticles at temperatures from 25 to 500 mK. The maximum decrease in electron temperature of about 200 mK has been observed at bath temperatures 300–350 mK. We used four-junction geometry with Al–AlOx –Cr/Cu tunnel junctions and Au traps. Efficient electron cooling was realized due to the improved geometry of the cooling tunnel junctions (quadrant shape of the superconducting electrode) and optimized Au traps just near the junctions (≈0.5 µm) to reduce reabsorption of quasiparticles after removing them from normal metal. The maximum cooling effect wa si ncreased from a temperature drop of dT =− 56 mK (ordinary cross geometry) to −130 mK (improve dg eometry of superconducting electrodes) and to dT =− 200 mK (improve dg eometry of superconducting electrodes and effective Au traps). The heating peak (instead of cooling) near the zero voltage across cooling junctions has been observed in practice for all samples at temperatures below 150 mK. For higher cooling voltages close to the superconducting gap, the heating was converted to cooling with decreased amplitude. The leakage resistance of the tunnel junctions gives a reasonable explanation of the heating peak. The phonon reabsorption due to the recombination of quasiparticles in superconducting electrodes gives an additional improvement in the theoretical fitting but could not explain the heating peak. An anomalous zero-bias resistance peak has been observed for all tested structures. The peak is explained by Coulomb blockade of tunnelling in transistor-type structures with relatively small tunnel junctions. The work on electron cooling is devoted to the development o fac old-e lectron bolometer (CEB) with capacitive coupling by SIN tunnel junctions to the antenna for sensitive detection in the terahertz region. Direct electron cooling of an absorber plays a crucial role in supersensitive detection in the presence of a realistic background power load. (Some figures in this article are in colour only in the electronic version)

An array of SIN tunnel junctions as a sensitive thermometer

We have fabricated and measured an array of superconductor-insulator-normal metal (SIN) tunnel junctions for the purpose of using it as a sensitive thermometer. An increase in the temperature responsivity of dV/dT similar to 5 mu V mK(-1) for ten junctions is observed from dV/dT similar to 1 mu V mK(-1) for a single junction. We then used such an array thermometer to measure the temperature stability of the Heliox AC-V cryogen-free refrigerator. We have been able to measure a temperature stability of +/- 250 mu K over a period of 8 h with a temperature resolution of +/- 100 mu K.

Terahertz transmission spectroscopy by Josephson oscillator and cold-electron bolometer

For sensitive wideband spectroscopy at TeraHertz frequencies one needs a wide-range electrically tunable THz source and a sensitive detector. In this paper a superconducting normal metal cold electron bolometer (CEB) was used as a broadband sensor. Bolometers were integrated with broadband log-periodic antenna designed for 0.2-2 THz frequency range and double-dipole antennas designed for 300 and 600 GHz central frequency. A Josephson junction was used as a wide band electrically tuned terahertz cryogenic oscillator. Bicrystal YBaCuO Josephson junctions demonstrated a characteristic voltage IcRn of over 4 mV that corresponds to characteristic frequency about 2 THz. The bolometer chip is attached to a Si substrate lens at 260 mK and the oscillator chip is attached to the sapphire substrate lens at 1.8 K, with lenses facing each other at the distance of few centimeters. High signal was measured in the whole frequency range up to 1.7 THz by simple changing the bias voltage of Josephson junction from zero to 3.5 mV. A voltage response of the bolometer up to 4*108 V/W corresponds to an amplifier-limited technical noise equivalent power of the bolometer NEP=1.25*10-17 W/Hz1/2. Combining a Terahertz band Josephson junction, a high-sensitive hot electron bolometer, and a sample under test in between, makes it possible to develop a cryogenic compact Terahertz-band transmission spectrometer with a resolution below 1 GHz corresponding to the linewidth of Josephson oscillations. For frequencies below 600 GHz a conventional Nb shunted SIS junction can be used as Josephson oscillator.

Effective electron microrefrigeration by superconductor–insulator–normal metal tunnel junctions with advanced geometry of electrodes and normal metal traps

We demonstrate effective electron cooling of the normal metal strip by superconductor–insulator–normal metal (SIN) tunnel junctions. The improvement was achieved by two methods: first, by using an advanced geometry of the superconducting electrodes for more effective removal of the quasiparticles; and second, by adding a normal metal trap just near the cooling junctions. With simple cross geometry and without normal metal traps, the decrease in electron temperature is 56 mK. With the advanced geometry of the superconducting electrodes, the decrease in electron temperature is 129 mK. With the addition of the normal metal traps, the decrease in electron temperature is 197 mK. (Some figures in this article are in colour only in the electronic version)

FTIR and UV-Visible Absorbance Studies of Hydrothermally Grown ZnO Coated with Polyvinylpyrrolidone

Hydrothermally grown hexagonal wurtzite ZnO microrods coated with polyvinylpyrrolidone (PVP) via an ex-situ method, was successfully synthesized without using complex procedure or experimental setup. FTIR results confirm the presence of different functional groups of PVP at the ZnO surface and the chemical interaction of ZnO with the C=O ligand of the PVP molecule. The presence of PVP molecules prevents the absorption of atmospheric CO2 by the Zn2+ ions since PVP chemically interacts with ZnO by attaching to the exposed cations. The coating concentration doesn’t induce a frequency shift in the vibrations of the PVP functional groups. The ZnO microrods possess good optical quality as indicated by the high UV absorption and pronounced excitonic peak at room temperature, even after coating with higher PVP concentrations.