Ryo Toda

Simultaneous measurement of torsional oscillations and NMR of very dilute ^{3}He in solid ^{4}He

We have investigated the NMR properties of dilute 3He impurities in solid 4He contained in a torsional oscillator (TO) by the simultaneous measurement of the NMR and the torsional oscillator response of the so-called supersolid 4He. From measurements on samples with one hundred to a few hundred ppm of 3He, we have found three different states of 3He. The first is the homogeneously distributed isolated 3He atom in a solid matrix of 4He. The second is the 3He cluster in a homogeneous 4He matrix, which appears below the phase separation temperature of a solid mixture. The third is the 3He cluster in some nonuniform part of a 4He crystal. We find that 3He atoms contained in the third component remain in a nearby location even above the phase separation temperature. Based on the fact that even a ppm of 3He affects the supersolid response in a TO below and above the phase separation temperature, we propose that the nonuniform part of a crystal that holds the third type of 3He and thus has a higher local concentration of 3He plays an important role in the supersolid phenomenon in a TO.

Amorphous solid like heat capacity of 4He fluid films adsorbed on pores

In the nanopores of HMM-2 whose mean pore diameter is 2.7 nm, adsorbed 4He forms layers up to about two atomic layers. At low temperatures, the 4He films show properties of the Bose quantum fluid and superfluidity above about 1.4 layers. Below the coverage or above the superfluid transition temperature, 4He film is in the normal fluid state. The heat capacity of the normal fluid shows linear temperature (T) dependence. The T-linear coefficient of the heat capacity is about 5 μJ/(m2-K2). According to Andreevs idea originally developed for bulk helium liquids, the 4He fluid can be regarded as an amorphous solid with T-linear dependence of the heat capacity. Its coefficient is described as π2D0κB2/6, where D0 is a density of states and κB is the Boltzmann constant. The density of states is estimated from the density (coverage) dependence of the isosteric heat of sorption qst which was obtained from the vapor pressure data of adsorption. The estimated density of state well explains the observed magnitude of the T-linear coefficient of heat capacity.

Phase diagrams of 4He bose fluids formed in one-and three-dimensional nanopores

In the one-dimensional (1D) nanopores of FSM-16 (2.8 nm in diameter) and the 3D pores of HMM-2 (2.7 nm, and 5.5 nm in 3D period), the 4He films adsorbed on these nanopore walls show the properties of the Bose quantum fluid and superfluidity above 1.4 atomic layers at low temperatures. The boundary of the Bose fluid region was determined from the kink (peak) temperature TC of the heat capacity at each coverage n. The phase diagrams obviously show dependence on the 1D and 3D pore connections, respectively. In the 3D nanopores, the coverage (density) dependence of TC is well reproduced by the Bose-Einstein condensation temperature of the 3D ideal gas which is proportional to (n - nc)2/3, where nc is the onset coverage. In the case of the 1D pores (channels), TC is proportional to (n - nc) at 0.1 < TC < 1K. This TC is likely to correspond to the crossover temperature of the 2D ideal gas heat capacity from the constant heat capacity of the Boltzmann gas to the linear in T, with decreasing temperature. At TC, the thermal de Broglie wavelength of the free particle is still shorter than the pore diameter, while the thermal phonon wavelength is estimated to become longer than the diameter below TC, which indicates the 1D phonon state.