Anomalous suppression of superfluidity in 4He confined in a nanoporous glass

We explore the superfluidity of 4He confined in a porous glass, which has nanopores of 2.5 nm in diameter, at pressures up to 5 MPa. With increasing pressure, the superfluidity is drastically suppressed, and the superfluid transition temperature approaches 0 K at some critical pressure, Pc approximately 3.4 MPa. The feature suggests that the extreme confinement of 4He into the nanopores induces a quantum phase transition from a superfluid to a nonsuperfluid at 0 K and at Pc.