Satoshi Kohjiro

Superconducting properties and normal‐state resistivity of single‐crystal NbN films prepared by a reactive rf‐magnetron sputtering method

In this letter, we report some superconducting properties and normal‐state resistivities of single‐crystal NbN films prepared by a reactive rf‐magnetron sputtering method. It was found that the prepared single‐crystal NbN films have Tc’s above 16 K, normal‐state resistivities in the range from 12.2 to 14.3 μΩ cm (at 20 K), and magnetic penetration depths in the range from 93 to 100 nm (at 4.2 K). From a temperature dependence of the upper critical magnetic field for a single‐crystal NbN film, the zero‐temperature Ginzburg–Landau coherence length ξGL(0) for the film was calculated to be 6.9 nm.

A 0.2–0.5THz single-band heterodyne receiver based on a photonic local oscillator and a superconductor-insulator-superconductor mixer

We have demonstrated that a superconductor-insulator-superconductor (SIS) mixer pumped by a photonic local oscillator (LO) covers the whole frequency range of 0.2–0.5THz. In the bandwidth of 74% of the center frequency, this single-band receiver exhibits noise temperature of TRX⩽20hf∕kB, where h is Planck’s constant, f is the frequency, and kB is Boltzmann’s constant. Resultant TRX is almost equal to TRX of the identical SIS mixer pumped by three conventional frequency-multiplier-based LOs which share the 0.2–0.5THz band. This technique will contribute to simple, wide-band, and low-noise heterodyne receivers in the terahertz region.

Surface resistance of epitaxial and polycrystalline NbCN films in submillimeter wave region

Surface resistances, R/sub s/s, of epitaxial and polycrystalline NbCN films in submillimeter-wave region were evaluated from measurements of Fiske-resonant modes in Josephson tunnel junctions. It is found that epitaxial NbCN films have about one order of magnitude smaller R/sub s/ values than those for polycrystalline NbCN films. It is also found that the frequency dependence of R/sub s/ of epitaxial and polycrystalline NbCN films can be explained by the two-fluid model.<<ETX>>

Preparation of NbC/sub x/N/sub 1-x/ Josephson junctions with TiN/sub x/ barriers

This paper presents the preparation process and the characteristics of titanium nitride films and nonhysteretic NbC_xN_1-x,/TiN_x/NbC_xN_1-x SNS junctions using TiN_x as the barrier material. Structural studies show that TiN_x films prepared by rf sputtering at room temperature have a preferential orientation of TiN(111) phase, while those grown at higher substrate temperatures are (200)-oriented. Resistivilies of the TiN_x films range from 4SμΩ·cm to 1lOμΩ·cm, depending on the values of N₂ partial pressure. Polycrystalline NbC_xN<sub1-x</sub> films with high T_c have been grown on TiN_x films by rf sputtering. NbC<sub>N_1-x/TiN_x/NbC_xN_l-x trilayer junctions with various dimensions and TiN_x thicknesses have been fabricated. Current-voltage characteristics of the junction with a 130nm-thick TIN_x barrier show nonhysteiretic behavior and strong critical current modulatilon with applied magnetic field, suggesting the characteristics of a Josephson SNS large junction. The I_cR_N product and the critical current density J_c of a 5μm x 5μm junction at 4.2K are estimated to be 30μV and 36kA/cm², respectively.

Kinetic Inductance of Superconducting Coplanar Waveguides

An analytical expression for the kinetic inductance of the superconducting coplanar waveguide is given using the conformal mapping technique. This expression is shown to be in good agreement with experimental results obtained from the temperature dependence of the resonant frequency of the NbN coplanar waveguide resonators, especially in the case of a film thickness smaller than the magnetic penetration depth. It is also shown that the magnetic penetration depth of the superconducting thin films can be evaluated by this method.

Absorption Spectra of Smoke Emitted from Heated Nylon Fabric Measured with a Continuous-Wave Sub-Terahertz Spectrometer

The absorption spectra of smoke emitted from heated nylon fabric are studied in the sub-terahertz band. The transmission loss of 1.5-µm-wavelength light in the smoke is 50 dB/m or more. Even in such dense smoke, the loss for waves with the frequency ranging from 200 to 500 GHz is so small that absorption lines of gas molecules in the smoke can be detected in this frequency range. Spectral analysis indicates that the smoke contains molecules of toxic gas, such as HCN and CH3CN, generated due to incomplete combustion of the nylon fabric when heated to 500 °C with air.

Preparation of overdamped NbTiN Josephson junctions with bilayered Ti–TiN barriers

Overdamped NbTiN Josephson junctions with Ti–TiN bilayered normal-metal barrier are proposed for metrological applications. Binary arrays consisting of 32 768 NbTiN/Ti–TiN/NbTiN junctions were fabricated for quantum voltage standards, and a Shapiro step at 1 V with the step height of about 0.5 mA was generated at around 8 K. The Ti layer was added to protect the base NbTiN electrode from nitrogen plasma during the deposition of the TiN layer. While the critical current Ic and the junction resistance Rn are mainly dominated by the thickness of the TiN barrier, Ic can be also adjusted by changing the thickness of the Ti barrier to maximize the operating margin for voltage standard circuits. An optimization of the NbTiN film in terms of film stress is also described for fabrication of circuits containing such a large number of junctions.

High-sweeping-speed optically synchronized dual-channel terahertz-signal generator for driving a superconducting tunneling mixer and its application to active gas sensing

We propose a high-sweeping-speed optically synchronized dual-channel terahertz (THz) signal generator for an active gas-sensing system with a superconductor-insulator-superconductor (SIS) mixer. The generator can sweep a frequency range from 200 to 500 GHz at a speed of 375 GHz/s and a frequency resolution of 500 MHz. With the developed gas-sensing system, a gas-absorption-line measurement was successfully carried out with N(2)O gas in that frequency range.

An Octave Bandwidth SIS Mixer for Accurate and Compact Terahertz Spectrometers

A SIS mixer with RF bandwidth of 1 octave is designed, fabricated, and characterized. The mixer chip consists of 8 Nb/AlOx/Nb junctions connected in parallel through Nb/SiO2/Nb microstrip lines (MSL), a twin-slot antenna, and a 3-stage MSL transformer. Experimentally, the 2.5 dB RF band is found to be 230-440 GHz, which agrees quantitatively with theoretical calculation. Receiver noise temperature Trx is measured as 260-680 K in the band. After correction for IF-amplifler noise and beam-splitter reflectivity, the noise is 170-320 K, i.e., 13-20 times of the quantum noise. The sensitivity flatness in the band can be improved to 1.9 dB by further optimization of the transformer.

White noise of Nb-based microwave superconducting quantum interference device multiplexers with NbN coplanar resonators for readout of transition edge sensors

White noise of dissipationless microwave radio frequency superconducting quantum interference device (RF-SQUID) multiplexers has been experimentally studied to evaluate their readout performance for transition edge sensor (TES) photon counters ranging from near infrared to gamma ray. The characterization has been carried out at 4 K, first to avoid the low-frequency fluctuations present at around 0.1 K, and second, for a feasibility study of readout operation at 4 K for extended applications. To increase the resonant Q at 4 K and maintain low noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1 × 104 ≤ Q ≤ 2 × 104 and the square root of spectral density of current noise referred to the SQUID input √SI = 31 pA/√Hz. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Two-directional...