Coumou PCJJ, Driessen EFC, Bueno J, Chapelier C, Klapwijk TM. Electrodynamic response and local tunneling spectroscopy of strongly disordered superconducting TiN films. Phys. Rev. B. 2013;88(18):180505 (1 to 5).
Abstract: We have studied the electrodynamic response of strongly disordered superconducting TiN films using microwave resonators, where the disordered superconductor is the resonating element in a high-quality superconducting environment of NbTiN. We describe the response assuming an effective pair-breaking mechanism modifying the density of states and compare this to local tunneling spectra obtained using scanning tunneling spectroscopy. For the least disordered film (kFl=8.7, Rs=13Ω), we find good agreement, whereas for the most disordered film (kFl=0.82, Rs=4.3kΩ), there is a strong discrepancy, which signals the breakdown of a model based on uniform properties.
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Shcherbatenko M, Tretyakov I, Lobanov Y, Maslennikov SN, Kaurova N, Finkel M, et al. Nonequilibrium interpretation of DC properties of NbN superconducting hot electron bolometers. Appl. Phys. Lett.. 2016;109(13):132602.
Abstract: We present a physically consistent interpretation of the dc electrical properties of niobiumnitride (NbN)-based superconducting hot-electron bolometer mixers, using concepts of nonequilibrium superconductivity. Through this, we clarify what physical information can be extracted from the resistive transition and the dc current-voltage characteristics, measured at suitably chosen temperatures, and relevant for device characterization and optimization. We point out that the intrinsic spatial variation of the electronic properties of disordered superconductors, such as NbN, leads to a variation from device to device.
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Klapwijk TM, Semenov AV. Engineering physics of superconducting hot-electron bolometer mixers. IEEE Trans THz Sci Technol. 2017;7(6):627–48.
Abstract: Superconducting hot-electron bolometers are presently the best performing mixing devices for the frequency range beyond 1.2 THz, where good-quality superconductor-insulator-superconductor devices do not exist. Their physical appearance is very simple: an antenna consisting of a normal metal, sometimes a normal-metal-superconductor bilayer, connected to a thin film of a narrow short superconductor with a high resistivity in the normal state. The device is brought into an optimal operating regime by applying a dc current and a certain amount of local-oscillator power. Despite this technological simplicity, its operation has found to be controlled by many different aspects of superconductivity, all occurring simultaneously. A core ingredient is the understanding that there are two sources of resistance in a superconductor: a charge-conversion resistance occurring at a normal-metal-superconductor interface and a resistance due to time-dependent changes of the superconducting phase. The latter is responsible for the actual mixing process in a nonuniform superconducting environment set up by the bias conditions and the geometry. The present understanding indicates that further improvement needs to be found in the use of other materials with a faster energy relaxation rate. Meanwhile, several empirical parameters have become physically meaningful indicators of the devices, which will facilitate the technological developments.
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Titova N, Kardakova AI, Tovpeko N, Ryabchun S, Mandal S, Morozov D, et al. Slow electron–phonon cooling in superconducting diamond films. IEEE Trans Appl Supercond. 2017;27(4):1–4.
Abstract: We have measured the electron-phonon energy-relaxation time, τ eph , in superconducting boron-doped diamond films grown on silicon substrate by chemical vapor deposition. The observed electron-phonon cooling times vary from 160 ns at 2.70 K to 410 ns at 1.8 K following a T -2-dependence. The data are consistent with the values of τ eph previously reported for single-crystal boron-doped diamond films epitaxially grown on diamond substrate. Such a noticeable slow electron-phonon relaxation in boron-doped diamond, in combination with a high normal-state resistivity, confirms a potential of superconducting diamond for ultrasensitive superconducting bolometers.
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Hajenius M, Yang ZQ, Gao JR, Baselmans JJA, Klapwijk TM, Voronov B, et al. Optimized sensitivity of NbN hot electron bolometer mixers by annealing. IEEE Trans Appl Supercond. 2007;17(2):399–402.
Abstract: We report that the heterodyne sensitivity of superconducting hot-electron bolometers (HEBs) increases by 25-30% after annealing at 85degC in high vacuum. The devices studied are twin-slot antenna coupled mixers with a small area NbN bridge of 1 mum times 0.15 mum, above which there is a SiO 2 passivation layer. The mixer noise temperature, gain, and resistance versus temperature curve of a HEB before and after annealing are compared and analysed. We show that the annealing reduces the intrinsic noise of the mixer by 37% and makes the superconducting transition of the bridge and the contacts sharper. We argue that the reduction ofthe noise is mainly due to the improvement of the transparency of the contact/film interface. The lowest receiver noise temperature of 700 K is measured at a local oscillator frequency of 1.63 THz and at a bath temperature of 4.2 K.
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