Chiharu Urano

Extremely low temperature coefficient of resistance in antiperovskite Mn3Ag1−xCuxN

Electrical resistivity is systematically investigated in Mn3AgN and related compounds with an antiperovskite structure. Despite its overall metallic character, Mn3AgN features a broad maximum in the temperature-resistivity curve in the paramagnetic state and the temperature coefficient of resistance (TCR) is negative at higher temperatures. The resistivity-peak temperature was tuned to just room temperature by the partial substitution of Cu for Ag, and a TCR as low as 10−6 K−1 was achieved over a wide temperature window including room temperature. These peculiar behaviors are possibly due to collapse of coherent quasiparticle states by strong magnetic scattering.

Operation of a Josephson arbitrary waveform synthesizer with optical data input

In order to establish a quantum ac voltage standard at NMIJ/AIST, we have developed a Josephson arbitrary waveform synthesizer based on overdamped Josephson junction arrays (JJAs) using optoelectronics and a liquid helium free compact cryocooler. We have succeeded in the generation of sine waves at 60 Hz and 152.6 kHz. To verify the performance of the system, two different types of JJAs with different characteristic frequencies have been developed. An application to a quantized voltage noise source for Johnson noise thermometry is also discussed, with experimental results.

Interplay between magnetism and charge transport in antiperovskite manganese nitrides: Extremely low temperature coefficient of resistance due to strong magnetic scattering

The recently discovered low temperature coefficient of resistance (low TCR) in Mn3Ag1-xCuxN exemplifies the peculiar magnetic and electronic states of this class of manganites. Despite its overall metallic character, a broad maximum appears in the temperature–resistivity curve in the paramagnetic region, and extremely low TCR is achieved over a temperature window that includes room temperature. The peak temperature can be tuned via Cu content x. It is apparently related to the magnetic transition temperature. These peculiar behaviors are possibly a result of the collapse of coherent quasiparticle states by strong magnetic scattering. We discuss the interplay between magnetism and charge transport in terms of magnetoresistance.

Standard-resistor compounds with adjustable operating temperature

The broad peak of the temperature-resistivity curve ρ(T) in antiperovskite Mn3AgN can potentially be used to develop materials with temperature coefficients of resistance as low as that of the widely used standard-resistor material Manganin. By partially replacing Ag by In or Mn by Fe, we suppressed the peak height of ρ(T) in Mn3AgN to levels that in some cases were lower than that of Manganin, and by co-doping with Cu and In or Fe, the peak temperature could be controlled over a temperature width of at least 50 K around room temperature while retaining the broadness of ρ(T) curve.

Single-Chip 10-V Programmable Josephson Voltage Standard System Based on a Refrigerator and Its Precision Evaluation

We precisely evaluated a single-chip 10-V programmable Josephson voltage standard (PJVS) system based on a closed-cycle refrigerator by directly comparing it with a conventional Josephson voltage standard system. The PJVS chip fabricated using NbN/TiNx/NbN junction technology was used. The result of the measurement showed a good agreement with a combined standard uncertainty of 3.1 parts in 1010 at 10 V.

Observation of Quantized Voltage Steps Using a Josephson Junction Array Driven by Optoelectronically Generated Pulses

In this paper, quantized voltage steps were observed by driving an overdamped Josephson junction array (JJA) with current pulse trains. The current pulse trains were generated by triggering a metal-semiconductor-metal photodetector installed close to JJA at 4.2 K with a mode-locked fiber laser. The intervals of the constant voltage steps were 10 muV, which agrees with the repetition frequency of the light source (49.7 MHz) and the number of Josephson junctions (100) in the JJA. This is the first step toward establishing an ac voltage standard at the National Institute of Advanced Industrial Science and Technology

Integrated quantum voltage noise source for Johnson noise thermometry

We have been developing a new implementation of quantum voltage noise source (QVNS), an integrated QVNS (IQVNS), for Johnson noise thermometry (JNT) in thermal metrology. We designed an IQVNS chip based on rapid single flux quantum (RSFQ) technology. A general formula for the output power spectrum has been derived by using a Fourier analysis method, which shows that the spectrum is calculable with sufficiently small errors for practical JNT measurements.

Generation of AC waveforms using a NbN-based programmable Josephson voltage standard system with a 10-K cooler

We are developing a system for precise ac waveform synthesis in a frequency range below kilo-hertz using a programmable Josephson voltage standard (PJVS). Our PJVS circuit is composed of 65 536 NbN-based over-damped Josephson junctions and operated at a temperature of 9–10 K with a mechanical cooler. We succeeded in generating stepwise-approximated sinusoidal waveforms with peek voltage amplitude of 2 V using a 10-bit PJVS circuit and a bias source with twelve 12-bit digital-to-analog converters. Rough estimations suggest the transient time between each step in the obtained waveforms is less than 5 µs, sufficient for sampling measurements.

Generation of 10 Vrms waveforms using AC-programmable Josephson voltage standard system with 10 K cooler

We have succeeded in synthesizing AC waveforms with the rms voltage of greater than 10 V using an AC-programmable Josephson voltage standard (AC-PJVS) system. The system has a NbN-based junction array, which is cooled with a compact cryocooler. The array is composed of total 524 288 junctions, enabling to generate arbitrary signals with the maximum output level of about 17 V for 15.9 GHz microwave input. In this experiment, we used a modified bias circuit with 6 parts of floating channels for eliminating interferences and also saturation in the output level. We will also discuss about heating effect of the 10 Vrms chip in the situation of using a mechanical cooler.

A direct comparison of a 10 V Josephson voltage standard between a refrigerator-based multi-chip programmable system and a conventional system

A multi-chip 10 V programmable Josephson voltage standard (PJVS) system was demonstrated using a closed-cycle refrigerator. We precisely measured the PJVS by a direct comparison measurement with a conventional Josephson voltage standard. The result agreed within a combined standard uncertainty of 3.9 × 10−10 at 10 V.