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Krasnosvobodtsev, S. I., Shabanova, N., P., Ekimov, E. V., Nozdrin, V. S., & Pechen, E., V. (1995). Critical magnetic field of NbC: new data on clean superconductor films. Zh. Eks. Teor.Fiz., , 534–537.
Abstract: The temperature dependence of the upper critical magnetic fields of exceptionally low-defect-density films of the superconducting compound NbC has been investigated, and previously unknown parameters of this clean superconductor and its electronic characteristics have been evaluated. An electron density of states at the Fermi level equal to 1.3 states/ eV. Nb atom, a Fermi velocity equal to 2.2X lo7 cmls, a plasma frequency equal to 3.6 eV, and a coherence length to 24 nm have been obtained with an electron mean free path exceeding 40 nm. A vortex-free state existing over the entire temperature range below T, which causes a many-fold increase in the critical magnetic field of the films when the field is aligned parallel to their surface, has been discovered in very thin films of superconducting niobium carbide.
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Гольцман, Г. Н., Веревкин, А. А., Гершензон, Е. М., Птицина, Н. Г., Смирнов, К. В., & Чулкова, Г. М. (1995). Исследования процессов неупругой релаксации и примесная спектроскопия-релаксометрия в двумерном электронном газе в полупроводниковых структурах с квантовыми ямами.
Abstract: В гетероструктурах GaAs/AlGaAs впервые прямым методом измерена температурная зависимость вpемени энеpгетической pелаксации двумерного электронного газа te(T) в широком диапазоне температур Т=1,5 – 50 К в квазиравновесных условиях. Для измерений использовался высокочувствительный спектрометр миллиметрового диапазона волн с высоким временным разрешением, который позволял измерять релаксационные времена до 150 пс с погрешностью не более 20%. Верхний предел температуры определялся временным разрешением спектрометра. Исследования проводились на высококачественных образцах с поверхностной концентрацией носителей ns = 4,2 1011 см-2 и подвижностью m = 7 105 см2В-1с-1 (при Т = 4,2К). В квазиравновесных условиях из температурной зависимости tе определен предел подвижности при низких температурах (T<4.2 K), связанный с рассеянием на пьезоакустическом потенциале, получено время неупругой релаксации, связанное с рассеянием на деформационном потенциале (15 K25 K), получено характерное время испускания оптического фонона (tLO>4,5пс), которое существенно превышает время сронтанного излучения оптического фонона (примерно в 30 раз), что связано с большой ролью процессов перепоглащения фононов электронами.При низких температурах проведены измерения tе в условиях сильного разогрева. Полученные значения tе и зависимость tе от температуры электронов Те совпадают с tе(Т) в квазиравновесных условиях при Т=Те. Из полученных значений tе(Те) построена зависимость мощности энергетических потерь от Те, которая хорошо согласуется с литературными данными.Начаты измерения в магнитном поле, которые показывают переспективность использованного нами метода измерений как в области слабых магнитных полей при факторе заполнения >10, так и в области сильных магнитных полей при факторе заполнения >1-2.
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Gol'tsman, G. N., Karasik, B. S., Okunev, O. V., Dzardanov, A. L., Gershenzon, E. M., Ekstrom, H., et al. (1995). NbN hot electron superconducting mixers for 100 GHz operation. IEEE Trans. Appl. Supercond., 5(2), 3065–3068.
Abstract: NbN is a promising superconducting material for hot-electron superconducting mixers with an IF bandwidth larger than 1 GHz. In the 1OO GHz frequency range, the following parameters were obtained for 50 /spl Aring/ thick NbN films at 4.2 K: receiver noise temperature (DSB) /spl sim/1000 K; conversion loss /spl sim/10 dB; IF bandwidth /spl sim/1 GHz; and local oscillator power /spl sim/1 /spl mu/W. An increase of the critical current of the NbN film, increased working temperature, and a better mixer matching may allow a broader IF bandwidth up to 2 GHz, reduced conversion losses down to 3-5 dB and a receiver noise temperature (DSB) down to 200-300 K.
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Hans Ekstrom, Karasik, B. S., Kollberg, E. L., & Sigfrid Yngvesson. (1995). Conversion gain and noise of niobium superconducting hot–electron–mixers. IEEE Trans. Appl. Supercond., 43(4), 938–947.
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Zorin, M., Gol'tsman, G. N., Karasik, B. S., Elantev, A. I., Gershenzon, E. M., Lindgren, M., et al. (1995). Optical mixing in thin YBa2Cu3O7-x films. IEEE Trans. Appl. Supercond., 5(2), 2431–2434.
Abstract: High quality, j/sub c/ (77 K)>10/sup 6/ A/cm/sup 2/, epitaxial YBa2Cu3O7-x films of 50 nm thickness were patterned into ten parallel 1 /spl mu/m wide strips. The film structure was coupled to a single-mode fiber. Mixer response was obtained at 0.78 /spl mu/m using laser frequency modulation and an optical delay line. Using two semiconductor lasers at 1.55 /spl mu/m wavelength the beating signal was used to measure the photoresponse up to 18 GHz. Nonequilibrium photoresponse in the resistive state of the superconductor was observed. Bolometric response dominates up to 3 GHz, after which the nonequilibrium response is constant up to the frequency limit of our registration system. Using an electron heating model the influence of different thermal processes on the conversion loss has been analyzed. Ways of increasing the sensitivity are also discussed.
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Karasik, B. S., Milostnaya, I. I., Zorin, M. A., Elantev, A. I., Gol'tsman, G. N., & Gershenzon, E. M. (1995). High speed current switching of homogeneous YBaCuO film between superconducting and resistive states. IEEE Trans. Appl. Supercond., 5(2), 3042–3045.
Abstract: Transitions of thin structured YBaCuO films from superconducting (S) to normal (N) state and back induced by a supercritical current pulse has been studied. A subnanosecond stage in the film resistance dynamic has been observed. A more gradual (nanosecond) ramp in the time dependence of the resistance follows the fast stage. The fraction of the film resistance which is attained during the fast S-N stage rises with the current amplitude. Subnanosecond N-S switching is more pronounced for smaller amplitudes of driving current and for shorter pulses. The phenomena observed are viewed within the framework of an electron heating model. The expected switching time and repetition rate of an optimized current controlling device are estimated to be 1-2 ps and 80 GHz respectively.
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Gol'tsman, G., Kouminov, P., Goghidze, I., & Gershenzon, E. (1995). Nonequilibrium kinetic inductive response of YBCO thin films to low power laser pulses. IEEE Trans. Appl. Supercond., 5(2), 2591–2594.
Abstract: We have discovered a transient nonequilibrium kinetic inductive voltage response of YBCO thin films to 20 ps pulses of YAG:Nd laser radiation with 0.63 /spl mu/m and 1.54 /spl mu/m wavelength. By increasing the sensitivity of the read-out system with 100 ps resolution time and diminishing the light intensity (fluence 0.1-2 /spl mu/J/cm/sup 2/) and transport current (density /spl les/10/sup 5/ A/cm/sup 2/) we were able to observe a peculiar bipolar signal form with nearly equal amplitudes for each sign. The integration of the kinetic inductive response over time gives the result which is qualitatively, of the same form as the response in the resistive and normal states: the nonequilibrium picosecond scale component is followed by the bolometric nanosecond component. The nonequilibrium response is interpreted as suppression of the order parameter by excess quasiparticles followed by a change both in resistance (for the resistive state) and in kinetic inductance (for the superconducting state).
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Karasik, B. S., Gol'tsman, G. N., Voronov, B. M., Svechnikov, S. I., Gershenzon, E. M., Ekstrom, H., et al. (1995). Hot electron quasioptical NbN superconducting mixer. IEEE Trans. Appl. Supercond., 5(2), 2232–2235.
Abstract: Hot electron superconductor mixer devices made of thin NbN films on SiO/sub 2/-Si/sub 3/N/sub 4/-Si membrane have been fabricated for 300-350 GHz operation. The device consists of 5-10 parallel strips each 5 /spl mu/m long by 1 /spl mu/m wide which are coupled to a tapered slot-line antenna. The I-V characteristics and position of optimum bias point were studied in the temperature range 4.5-8 K. The performance of the mixer at higher temperatures is closer to that predicted by theory for uniform electron heating. The intermediate frequency bandwidth versus bias has also been investigated. At the operating temperature 4.2 K a bandwidth as wide as 0.8 GHz has been measured for a mixer made of 6 nm thick film. The bandwidth tends to increase with operating temperature. The performance of the NbN mixer is expected to be better for higher frequencies where the absorption of radiation should be more uniform.
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Kerr, A. R. (1995). Some fundamental and practical limits on broadband matching tocapacitive devices, and the implications for SIS mixer design. IEEE Trans. Microw. Theory Techn., 43(1), 2–13.
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Ekstrom H., Karasik B. S., Kollberg E.L., & Yngvesson K.S. (1995). Conversion Gain and Noise of Niobium Superconducting Hot-Electron-Mixers. IEEE Trans. Microw. Theory Techn., 43, 938–947.
Abstract: A study has been done of microwave mixing at 20 GHz using the nonlinear (power dependent) resistance of thin niobium strips in the resistive state. Our experiments give evidence that electron-heating is the main cause of the nonlinear phenomenon. Also a detailed phenomenological theory for the determination of conversion properties is presented. This theory is capable of predicting the frequency-conversion loss rather accurately for arbitrary bias by examining the I-V-characteristic. Knowing the electron temperature relaxation time, and using parameters derived from the I-V-characteristic also allows us to predict the -3-dB IF bandwidth. Experimental results are in excellent agreement with the theoretical predictions. The require ments on the mode of operation and on the film parameters for minimizing the conversion loss (and even achieving conversion gain) are discussed in some detail. Our measurements demon-strate an intrinsic conversion loss as low as 1 dB. The maximum IF frequency defined for -3-dB drop in conversion gain, is about 80 MHz. Noise measurements indicate a device output noise temperature of about 50 K and SSB mixer noise temperature below 250 K. This type of mixer is considered very promising for use in low-noise heterodyne receivers at THz frequencies.
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