Koichi Nara

Superconducting, electronic and magnetic anomalies due to structural transition around x=0.12 in La2−xBaxCuO4

Abstract From anomalies of the heat capacity and the thermal expansion, the structural phase boundary of the LTT-LTO transition is determined in the narrow range around x=0.12 of La 2−x Ba x CuO 4 , where superconductivity is suppressed remarkably. The widely-spread CuNQR spectra and the decay of the asymmetry parameter of μSR are indicative of the magnetic order of Cu moments in the LTT phase where the induced charge transfer between Cu and O atoms causes the localization of the electronic state and the increase of magnetic moments at the Cu site.

Laser interferometric dilatometer at low temperatures: application to fused silica SRM 739

Abstract An optical heterodyne interferometer has been combined with a helium flow cryostat to measure the linear thermal expansion coefficient of solids at cryogenic temperatures. The absolute accuracy in length measurement is within a few nanometres. Measurement results on a specimen of fused silica (SRM 739; by US NIST) in the temperature range 6–273 K are presented and compared with some literature data.

Some Anomalies Due to the Low-Temperature Structural Transition around x=0.12 in La2-xBaxCuO4

Anomalies of heat capacity and thermal expansion have been observed near x =0.12 in La 2- x Ba x CuO 4 . The anomalies are ascribed to the low-temperature structural transition. We determine accurately the phase baundary as a function of x . Widely broadened Cu-NQR spectra for x <0.14 show that the local charged states of the CuO 2 plane are largely distributed in the low-temperature phase. The phase diagram and its relation with the suppression of superconductivity around x =0.12 are briefly discussed.

Measurements of linear thermal expansion coefficients of copper SRM 736 and some commercially available coppers in the temperature range 20-300 K by means of an absolute interferometric dilatometer

Abstract The thermal expansivities of several grades of copper were measured in the temperature range 20–300 K by means of an absolute laser interferometric dilatometer. The present results for copper standard reference material (NIST; SRM 736) and three commercially available kinds of copper, such as a high-purity copper (99.999% purity), an oxygen-free high-conductivity copper and a tough-pitch copper, show good agreement with the NIST certified values and the CODATA recommended values within 0.09 × 10 −6 K −1 over the whole temperature range.

Magneto-resistance of a highly stable industrial-grade platinum resistance thermometer between 20 and 240 K

Abstract The stability and magneto-resistance of a newly introduced Japanese Industrial Standard (JIS) platinum resistance thermometer were studied. The stability was estimated from the change in resistance at the triple point of water after thermal cycles between room temperature and liquid nitrogen temperature. The variation among the resistance values at 20, 50 and 100 cycles was B of up to 8 T. The relative resistance increase, ϵ, was found to obey the equation ϵ = D ( T ) B 2 / [1 + E ( T ) B ]. The fitting error in the field dependence was D ( T ) and E ( T ) are tabulated for the new JIS thermometer and a standard rhodium-iron thermometer.

Thermal expansion of some advanced ceramics applicable as specimen holders of high Tc superconductors

Abstract The thermal expansion of some advanced ceramics and well orientated Bi 2 Sr 2 -CaCu 2 O 8+ x were measured from 10 to 300 K to find suitable substrates with similar levels of thermal expansion to high T c superconductors. The length measurements were based on readings from an interferometric dilatometer with a precision of 5 nm. The thermal expansion and coefficient of thermal expansion of each material are presented in this paper, along with a brief guide to specimen holders.

Design of platinum resistance thermometer with small magnetic field correction

Abstract A new design for the platinum resistance thermometer is developed to reduce its magnetic field induced errors. The reduction in the magnetoresistance is realized by placing four sensor elements so as to allow their magnetoresistances to cancel each other. To evaluate its effectiveness, prototype sensors with the present design are made from commercial thin film sensors. Their performance is studied from 16 to 140 K under magnetic fields of up to 8 T. The optimum result shows a reduction in the magnetic field induced error by a factor of 20 at 19 K.

Instrumentation for highly sensitive measurement of magnetocaloric effect: application to high Tc superconductors

Abstract Instrumentation for precise calorimetry under magnetic fields of up to 7 T in the temperature range 40 – 120 K is reported. A new type of resistance bridge with a self-compensating function for the magnetoresistance of platinum resistance thermometers has been developed for this purpose. This thermometric system suppresses the measuring error due to a magnetic field of 7 T to within 40 mK and has a resolution of 1 mK. It is also applied to observing the small magnetocaloric effect of a high Tc superconductor, YBa2Cu3O7 − x, in adiabatic conditions.

Possible design for a thin wire resistance thermometer with isotropic magnetoresistance

Abstract A design for a thin wire thermometer with isotropic magnetoresistance is proposed. The isotropic character is achieved by adjusting the pitch angle, θ p , in the winding of the sensing wire to a magic angle, arctan ( 1 2 1 2 ). A prototype of the sensor was made, based on the new Japanese industrial standard platinum resistance thermometer. The magneto-resistance was measured and the anisotropy was found to be reduced by a factor of 10 in the best case.

Derivation of optimized calibration procedures for practical thermometers

Abstract An algorithm is developed to derive optimized calibration procedures for practical thermometers. The algorithm, utilizing a set of calibration data for one kind of practical thermometer, analyses the characteristics of the sensors so as to obtain the optimum locations of the calibration temperatures and relevant deviation functions. Following the procedure, one can estimate the calibration accuracy for a given number of calibration points with their best allocation. As an example, the algorithm is applied to industrial grade platinum resistance thermometers. It is shown that five calibration points are enough to realize a calibration accuracy better than 10 mK for a 20–273.16 K temperature range.