One‐Dimensional ⁴He and ³He Quantum Fluids Realized in Nanopores

We have studied ⁴He and ³He adsorbed onto FSM‐16, which has straight one‐dimensional (1D) nanopores 18 or 28 Å in diameter. Fluid film tubes are formed on the nanopore walls covered with about one atomic layer of inert (solid) helium. The low‐temperature heat capacity of ⁴He fluid tubes is linear in temperature. This is due to 1D phonons, whose contribution becomes observable at temperatures below 1/10 of the energy‐level separation between the ground state and the first excited state quantized in the azimuthal direction. The superfluidity of ⁴He adsorbed in the nanopores was studied by a torsional oscillator. When the nonopores are preplated with ⁴He layers, ³He adatoms at a low density show a large heat capacity characteristic of a fluid. At high temperatures, the heat capacity is as large as that of a Boltzmann gas. With decreasing temperature, the heat capacity shows a Schottky‐like peak followed by an asymptotic, linear dependence on temperature. A model calculation for a dilute ³He gas in an adsorption potential of a nanopore suggests a dimensional crossover from two dimensional to 1D around the Schottky‐like peak. The calculation also suggests a degenerate 1D state at sufficiently lower temperatures than the energy gap for the motion in the azimuthal direction.