Yu-ichi Miura

The Structural, Dielectric, Raman-Spectral and Low-Temperature Properties of Amorphous PbTiO3

Amorphous ribbons of pure PbTiO3 were obtained by the rapid solidification technique and were confirmed to be in the amorphous state by several different experimental techniques. Structural studies revealed clusters with radii up to about 150 A, larger than those so far found in other materials such as metals or semiconductors in the amorphous state. Dielectric and Raman spectral studies confirmed that the material remains in the amorphous state indefinitely. Low-temperature studies showed that not only the amorphous state but also the crystalline state of PbTiO3 has a dielectric constant minimum in the mK region, a characteristic of the amorphous state.

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.

A Re-Entrant Glasslike Phase in Rb1-x(NH4)xH2PO4

The mixed crystal Rb 1- x (NH 4 ) x H 2 PO 4 shows ferroelectricity for small x and antiferroelectricity for large x . The frequency dispersion of the dielectric constant was studied in the concentration range 0.5≤ x . The low-frequency dispersion was found for 0.5≤ x ≤0.8 below 40 K. From the results, the phase diagram was obtained. Around x =0.75, a re-entrant glasslike phase with the dispersion of the dielectric constant was found.

Search for superfluidity of3He in3He-4He solution

We have tried to cool a3He-4He solution down to the microkelvin temperature region to search for a superfluid transition of the3He component in the solution. The contact surface area between the solution and a sintered powder has been increased enormously by the use of a fine platinum powder, to reduce the effect of the residual heat leak directly entering into the solution. Although the heat leak was found to be time dependent, the ultimate ratio of the heat leak to the surface area is about 0.046 pW/m2, improved very much from those by the other groups. But the lowest temperature of the solution is still around 0.2 mK, and no evidence of the transition has been observed yet.

The decay rate of critical fluctuations in3He-4He mixtures near the gas-liquid critical point

We have measured the critical light scattering intensity and the Rayleigh line shape for3He and for3He-4He mixtures with compositionX(3He)=0.95, 0.79, and 0.63 along their respective critical isochores near the plait point. The experimental linewidth of3He is compared with the calculated one from heat conductivity and equation of state measurements, and satisfactory agreement is obtained. For mixtures, gravity effects in our cell of finite height prevent us from reaching the critical point along a path at strictly constant composition and density. HenceTc cannot be determined directly. Using the prediction that the scattered light intensity in the mixtures has the same diverging behavior as for the pure fluid, we determine the reduced temperaturet≡[T − Tc(X)]/Tc from the intensity. The measured Rayleigh line shape can be expressed by a single decay rate Γ as a function oft for a given scattering angle of the light beam. Our experiments show that Γ in the mixtures is only weakly dependent on composition. Our analysis leads to the determination of the mass diffusion coefficientD, which is found to be nearly independent of composition and nearly equal to the thermal diffusivityDT measured for3He. The results are discussed in the light of the predictions from mode coupling theory.

Dielectric Dispersion of Rb1-x(NH4)xH2PO4

The frequency dispersion of the dielectric constants of mixed crystals of Rb1-x(NH4)xH2PO4 was investigated over the full range of the concentration. For 0.25≤x≤0.8, a remarkable frequency-dispersion was found, which supports the existence of a glasslike phase. For 0≤x≤0.2, two kinds of dispersions, which are distinct from those observed for 0.25≤x≤0.8, were found.

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.

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.

Two Level Tunneling Systems in the Glasslike Phase of Rb1-x(NH4)xH2PO4 at Low Temperatures

The dielectric constants of the mixed crystals of ferroelectric RbH2PO4 and antiferroelectric NH4H2PO4 have been measured down to 20 mK. The temperature dependence of the dielectric constant for x=0.9, in the antiferroelectric phase, shows no anomaly. The results in the glasslike phase, however, show the frequency dependent minimum, which Is the universal property in amorphous solids, explained by the existence of two level tunneling systems (TLS). Our results show that the origin of TLS In the glasslike phase is not the structural disorder but the random configuration of the electric dipoles.

Dielectric Constant of Rb0.25(NH4)0.75H2PO4 below 1 K

The dielectric constant e of a mixed crystal of hydrogenbonded ferroelectric RbH2PO4 and antiferroelectric NH4H2PO4 has been measured along the a-axis at temperatures between 0.027 and 1.0 K. The results show, for the first time, the existence of two-level tunneling systems in these mixed crystals.