Helium in nanoporous media has attracted much interest as a model Bose system with disorder and confinement. Here we have examined how a change in porous structure by preplating a monolayer of krypton affects the superfluid properties of $^4$He adsor
bed or confined in a nanoporous Gelsil glass, which has a three-dimensional interconnected network of nanopores of 5.8 nm in diameter. Isotherms of adsorption and desorption of nitrogen show that monolayer preplating of Kr decreases the effective pore diameter to 4.7 nm and broadens the pore size distribution by about eight times from the sharp distribution of the bare Gelsil sample. The superfluid properties were studied by a torsional oscillator for adsorbed film states and pressurized liquid states, both before and after the monolayer Kr preplating. In the film states, both the superfluid transition temperature $T_{mathrm c}$ and the superfluid density decrease about 10 percent by Kr preplating. The suppression of film superfluidity is attributed to the quantum localization of $^4$He atoms by the randomness in the substrate potential, which is caused by the preplating--induced broadening of the pore size distribution. In the pressurized liquid states, the superfluid density $rho_{mathrm s}$ is found to increase by 10 percent by Kr preplating, whereas $T_{mathrm c}$ is decreased by 2 percent at all pressures. The unexpected enhancement of $rho_{mathrm s}$ might indicate the existence of an unknown disorder effect for confined $^4$He.
The superfluid transition in liquid 4He filled in Gelsil glass observed in recent experiments is discussed in the framework of quantum critical phenomena. We show that quantum fluctuations of phase are indeed important at the experimentally studied t
emperature range owing to the small pore size of Gelsil, in contrast to 4He filled in previously studied porous media such as Vycor glass. As a consequence of an effective particle-hole symmetry, the quantum critical phenomena of the system are described by the 4D XY universality class, except at very low temperatures. The simple scaling agrees with the experimental data remarkably well.
We have examined superfluid properties of $^4$He confined to a nano-porous Gelsil glass that has nanopores 2.5 nm in diameter. The pressure-temperature phase diagram was determined by torsional oscillator, heat capacity and pressure studies. The supe
rfluid transition temperature $T_{mathrm c}$ approaches zero at 3.4 MPa, indicating a novel quantum superfluid transition. By heat capacity measurements, the nonsuperfluid phase adjacent to the superfluid and solid phases is identified to be a nanometer-scale, localized Bose condensation state, in which global phase coherence is destroyed. At high pressures, the superfluid density has a $T$-linear term, and $T_{mathrm c}$ is proportional to the zero-temperature superfluid density. These results strongly suggest that phase fluctuations in the superfluid order parameter play a dominant role on the phase diagram and superfluid properties.
We report the measurements of the heat capacity of ^4He confined in nanoporous Gelsil glass that has nanopores of 2.5-nm diameter at pressures up to 5.3 MPa. The heat capacity has a broad peak at a temperature much higher than the superfluid transiti
on temperature obtained using the torsional oscillator technique. The peak provides a definite thermodynamic evidence for the formation of localized Bose-Einstein condensates (LBECs) on nanometer length scales. The temperature dependence of heat capacity is well described by the excitations of phonons and rotons, supporting the existence of LBEC.
We have studied the liquid - solid (L-S) phase transition of ^4He confined in nanoporous glass, which has interconnected nanopores of 2.5 nm in diameter. The L-S boundary is determined by the measurements of pressure and thermal response during slow
cooling and warming. Below 1 K, the freezing pressure is elevated to 1.2 MPa from the bulk freezing pressure, and appears to be independent of temperature. The T-independent L-S boundary implies the existence of a localized Bose-Einstein condensation state, in which long-range superfluid coherence is destroyed by narrowness of the nanopores and random potential.
