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, su perfluidity occurs at substantially higher pressures than in bulk He-4 . Ultrasonic measurements show that the amount of superfluid in the po res does not decrease until freezing begins at a pressure of about 36 bar. At slightly higher pressures partial freezing occurs with complet e freezing above about 36.3 bar. Because of the hysteresis between fre ezing and melting superfluidity sometimes occurs during cooling but no t during warming.