Mitsunori Hieda

Ultrasensitive quartz crystal microbalance with porous gold electrodes

The quartz crystal microbalance is a sensitive thickness monitor used in a variety of technological applications. It is also a versatile research tool for chemical sensor, nanotribology, wetting transition, and superfluid transition studies. By replacing the conventional gold electrodes with porous gold electrodes, the effective adsorbing surface area and hence sensitivity was enhanced by a factor of 40 with no sacrifice of the mechanical quality factor or the stability in the resonant frequency.

Observation of superfluidity in two- and one-dimensions

Even though there is no long-range-ordered state of a superfluid in dimensions lower than the three-dimension (3D) such as bulk 4He liquid, superfluidity has been observed for flat 4He films in 2D and recently for nanotubes of 4He in 1D by the torsional oscillator method. In the 2D state, in addition to the superfluid below the 2D Kosterlitz–Thouless transition temperature TKT, superfluidity is also observed in a normal fluid state above TKT, which depends strongly on the measurement frequency and the system size. In the 1D state of the nanotubes, superfluidity is directly observed as a frequency shift in the torsional oscillator experiment. Some calculations suggest a superfluidity of a 1D Bose fluid with a finite length, where thermal excitations of 2π–phase winding play the main role for superfluid onset of each tube. Dynamics of the 1D superfluidity is also suggested by observing the dissipation in the torsional oscillator experiment.

Slippage of 4He Films Adsorbed on Grafoil

We applied the quartz-crystal microbalance (QCM) technique to 4He films adsorbed on grafoil. It was found that both of nonsuperfluid 4He films and the inert layers underneath the superfluid film slip under a certain condition at low temperatures, and that the temperature at which these films start to slip depends on the 4He areal density. In addition, this slippage also depended strongly on the amplitude of the quartz crystal.

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.

Quantum State of 4He Confined in Nanocages of Na-Y Zeolite

We have studied the heat capacity of 4He adsorbed in Na-Y zeolite down to 70 mK. The Y-type zeolite has void cages 1.3 nm in diameter connected through narrow apertures 0.8 nm in diameter. After the first solid layer of adsorbed 4He is completed at 10.3 atoms/cage (about 37% of full pore), the pore apertures are considered to be so narrow that 4He adatoms in the second layer are confined in each cage at low temperatures, and form a cluster of several atoms. After the first layer completion, the low-temperature heat capacity isotherm has two peaks around 13.7 and 17.0 atoms/cage (about 50 and 63%), between which quantum-statistical differences from those of 3He appear. From the peaks in the heat capacity isotherm, the quantum region for 4He adatoms in Na-Y zeolite was determined.

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.

Vapor Pressure Measurement for 4He Films Adsorbed on 2D Mesoporous Hectorite

The vapor‐pressure measurement for adsorbed films is equivalent to the measurement of the chemical potential. By the measurement of 4He films adsorbed on two‐dimensional (2D) mesoporous Hectorite, we obtained the 2D isothermal compressibility, the isosteric heat, and the effective thickness deduced from the FHH model, mainly above a coverage of 15 μmol/m2. The compressibility shows two dips at n1 = 17.5±0.5 μmol/m2 and n2 = 22.7±0.5 μmol/m2 which correspond to the first layer completion and appearance of the quantum‐Bose‐fluid layer, respectively. For the quantum‐Bose‐fluid layer, the phonon velocity deduced from the compressibility is on the order of 100 m/s, and it reasonably agrees with that obtained from the 2D phonon heat capacity.

3He Fluid Formed in One‐Dimensional 1.8 nm Pores Preplated with 4He Layer

In one‐dimensional (1D) nanopores 1.8 nm in diameter preplated with 1.2 atomic layers of 4He, we have made 3He film tubes whose diameter is estimated to be about 1.0 nm. At a low 3He density, the heat capacity shows that of a 2D Boltzmann gas at high temperatures followed by a Schottky‐like peak at about 300 mK. This result can be understood in terms of dimensional crossover from 2D to 1D with decreasing temperature. The peak appears at a temperature about 0.3 times the energy gap between the ground state and the first excited states of the azimuthal motion in the 3He fluid tubes.

Slippage of Nonsuperfluid 4He Films on Ag

We measured the mechanical response of nonsuperfluid 4He films adsorbed on a Ag substrate using the quartz-crystal microbalance technique. In 4He coverages from 0.06 to 0.1 atoms/Å2, we observed that the resonance frequency increases at low temperatures accompanied by a decease in Q-value. This behavior suggests that nonsuperfluid 4He films undergo partial slipping relative to the Ag substrate.

Decoupling of 3He Films Adsorbed on Two-Dimensional Mesoporous Hectorite

The mechanical responses of physisorbed films to the vibration of a substrate were investigated by measuring the sound velocity and attenuation of a two-dimensional mesoporous hectorite. The sound velocity of a pellet with adsorbed3He films increased drastically at low temperatures accompanied by the attenuation. The temperature dependence of the sound velocity and attenuation can be described by the thermally activated relaxation process, and the increase in sound velocity was found to be proportional to the amount of adsorbed3He. The most plausible explanation for this behavior is that3He films decouple from the vibrating substrate at low temperatures.