Study of Novel Quantum Phenomena in Helium Confined in Nano-Structures

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This research project was planned to study and search for various novel quantum phenomena in helium (^4He) confined in nanoporous structures. The project involves mainly five subjects. The outcome are as follows:1. ^4He in nano-porous glass : (A) The liquid-solid phase boundary was determined by isochoric pressure measurements. We have discovered a peak in heat capacity of confined ^4He at much higher temperatures than the superfluid transition temperature. The heat capacity peak definitely shows the formation of localized Bose-Einstein condensates in nanopores. (B) We developed a new simultaneous measurement technique of torsional oscillator and ultrasound. By this technique we have found that the superfluid transition temperature is independent of frequency, but has a strong porous sample dependence. (C) We have succeeded to control the pore size of the nanoporous glass by preplating a few atomic layers of Krypton atoms. The superfluid transition temperature decreases after preplatin … More g, indicating that the decrease of the effective pore size suppress superfluidity.2. Search for quantum effects in hydrogen (H_2) in nanoporous glasses : Aiming at searching for BEC and superfluidity in molecular hydrogen, we have studied the pore size dependence of supercooling of liquid H_2 by isochoric pressure measurements. We have found that the majority of supercooled H_2 solidify at around 9 K. However, a small amount of H_2, probably confined in narrowest pores, remains liquid down to about 7 K in 2.5-nm porous glass. This indicates that BEC can be observed by confining H_2 in uniform pores narrower than 2.5 nm.3. Superfluidity of ^4He in one-dimensional nanopores: Torsional oscillator studies for ^4He in 1D nanopores in FSM-16 revealed that superfluidity is dramatically suppressed by pressurizing liquid, with Tc reaching 0 K at around 2 Mpa. This is second example of quantum superfluid transition of confined ^4He.4. ^4He in regular 3D pore structure : Torsional oscillator and pressure experiments are underway.5. Supersolid studies of ^4He : We have conducted torsional oscillator experiments for bulk solid ^4He and solid in nano porous glasses. The former study revealed strong dependence of supersolid behaviors on sample history. Less