FREEZING, MELTING AND SUPERFLUIDITY OF HE-4 NEAR THE MELTING CURVE MINIMUM IN VYCOR

Since freezing is suppressed in small pores such as those of Vycor, superfluidity occurs at substantially higher pressures than in bulk He-4. Ultrasonic measurements show that the amount of superfluid in the pores does not decrease until freezing begins at a pressure of about 36 bar. At slightly higher pressures partial freezing occurs with complete freezing above about 36.3 bar. Because of the hysteresis between freezing and melting superfluidity sometimes occurs during cooling but not during warming.