Motoo Shinohara

Two-stage nuclear demagnetization refrigerator reaching 27 µK

A two-stage nuclear demagnetization refrigerator with large cooling capacity has been constructed and successfully operated. The first stage consists of 11 moles of the Van Vleck paramagnet PrNi5 in an initial field of 6 T, and the second stage consists effectively of 19 moles of copper in an initial field of 7.6 T. The minimum temperature as measured with pulsed platinum NMR is 27 µK, the lowest ever obtained in a low-field experimental region.

Two-stage nuclear demagnetization refrigerator

Two-stage nuclear demagnetization has been performed using PrCu6 and Cu as coolants. The Cu nuclear stage reached temperatures as low as 10 ΜK with a rate of temperature rise of less than a few ΜK/h. The corresponding conduction electron temperature is estimated to be less than 50 ΜK. A nuclear orientation thermometer of Al54Mn has been successfully used to measure temperatures down to 10 ΜK.

Thermometry using 18 W carbon resistors in a temperature region around 10 mK

Abstract The resistance-temperature characteristics of 1 8 W carbon resistors of grade ERC-18SG, manufactured by Matsushita, with the nominal values of 48, 82, 100, 220, and 330 Ω have been measured in the region 4.2 K-25 mK and their application as thermometers in this region is confirmed. For the 82 Ω resistor, measurements were taken at temperatures below 10 mK. The temperature dependence of the resistance was found to be linear on the log-log plot over a wide range below 50 mK. The sensitivity remains finite even at 6 mK, but below 10 mK rapid measurements were prevented by a considerable increase in the thermal relaxation time. Measurement of the characteristics of several 100 Ω resistors from two different sets showed that resistors from the same set separate into two groups with different characteristics. This becomes appreciable at temperatures below 4.2 K, so it is difficult to predict the behaviour of Matsushita resistors below 4.2 K from the characteristics at higher temperatures.

The Mössbauer Effect of Fe(HCOO)2·2H2O between 4.2 and 0.027 K

Mossbauer effect measurements of powdered Fe(HCOO) 2 ·2H 2 O have been made at temperatures between 4.2 and 1.6 K and at the temperature of 0.027 K in average. A unit cell of this salt contains two each of inequivalent iron ions at A and B sites. Values of Mossbauer parameters at these two types of sites obtained at 4.2 K agree with those obtained at 6 K by other authors within experimental error. It is found that the internal magnetic at A sites is practically saturated at 1.6 K, while that at B sites remains to be zero even at 0.027 K; this suggests that spins at B sites are quenched at low temperatures. Mossbauer parameters for the A site are determined at 1.6 K.

Transport properties of amorphous Si1−xAux: Metal-insulator transition and

Abstract The metal-insulator transition in a-Si:Au films was studied in magnetic fields 0 to 5 T by measuring the electrical conductivity σ(T,H). σ(T,H) of thin samples show two-dimensional behaviour at low temperatures. How the superconductivity of a-Si:Au behaves near the metal-insulator transition is discussed.

Improved Demagnetization Cryostat for Mössbauer Effect Experiments

Some improvements in a previously constructed demagnetization cryostat for use in Mossbauer effect experiments have been made. In the new design, a mechanical heat switch previously used for establishing thermal contact between the paramagnetic salt and the liquid helium bath was replaced with helium exchange gas; the use of vacuum-tight beryllium windows has made it possible. CrK alum or MnNH4 Tutton salt weighing 60 g was used as a cooling salt. The lowest temperature reached by using CrK alum was 0.025°K and temperature of the alum arrived at 0.05°K in 6.5 hr. The lowest temperature reached by using MnNH4 Tutton salt was 0.15°K and temperature of it was practically constant as long as liquid helium stayed in the bath. The magnetic hyperfine structure of K3Fe(CN)6 (TN=0.13°K) has been observed by employing this cryostat, and the internal magnetic field at the position of an iron nucleus in K3Fe(CN)6 was found to be 193 kOe.

Mössbauer Study of FeCl2·4H2O in the Temperature Range 4.2° to 0.025°K

The Mossbauer spectrum of 57Fe in FeCl2·4H2O has been examined in the temperature range 4.2° to 0.025°K. The temperature dependence of the internal magnetic field indicates that the spin ordering starts at 1.3°K. The spectrum below the Neel temperature is well‐explained with the parameters H = 266 kOe, e2qQ/2 = 3.10 mm/sec, η= −0.2 and ∠Hq = 25°, where H, e2qQ/2, and η have the conventional meanings and ∠Hq is the angle between the internal magnetic field and the electric field gradient at the iron nucleus. The spectrum for a single crystal with gamma rays being parallel to the monoclinic crystal axis b determines ∠Hb = 15°. Thus, below the Neel temperature the directions of the internal magnetic field at the two sites of iron nuclei in this material make an angle of 30° with each other.

Dielectrc Constant of Both Amorphous and Crystalline PbTiO3 at Low Temperatures

The dielectric constant of PbTiO 3 in both amorphous and crystalline states has been measured down to 18 mK at several frequencies in the range between 0.3 and 100 kHz. In each state, the dielectric constant around 100 mK has a frequency-dependent minimum, which is characteristic of vitreous state.

TheT*-T relation for chrominum-potassium alum

TheT*-T relation for powdered chromium-potassium alum was determined by calibration with a CMN thermometer. The result was considerably different from the hitherto reported ones but was highly reproducible. The obtained relation is expressed in the region ofT=600−30 mK as a linear equationT=1.01T*+1.01(4π/3−ε)fC−47.0 (mK), where ε is the demagnetizing factor,f the filling factor, andC the Curie constant. The effect of the crystalline field on this relation is discussed.

Demagnetization Cryostat for Mössbauer Effect Experiments

A demagnetization cryostat for use in Mossbauer effect experiments is described. Thermal contact of the paramagnetic salt with the liquid helium bath was established with a mechanical heat switch. 50 g of CrK alum was used as the refrigerant. Temperature of the alum reached 0.08°K after demagnetization and took 3 hours to rise to 0.3°K, the liquid helium bath being held at 1.8°K. The magnetic hyperfine structure of FeCl24H2O (TN1°K) has been observed by employing this cryostat.