λ-Transition between normal and superfluid ⁴He in the high–speed rotating frame

Abstract

THE rotating helium cryostat (1-m diameter, 3,000 r.p.m.) at Southampton provides a novel way of studying the behaviour of liquid helium. The centrifugal acceleration fields (up to 5,000g at the periphery) enable pressure differences up to 20 bar to be generated in radial ducts containing liquid helium between the axis and the periphery of the rotor. We have recently rotated the cryostat at temperatures down to 1.8 K, well below the λ-transition to superfluid helium. The λ-transition temperature is suppressed by increasing pressure so that the rotor becomes a unique tool for the study of the phase transition boundary between normal helium He(I) and superfluid helium He(II). Here we report observations of a step-like temperature difference of several tens of millikelvin across the boundary. The temperature jump can be explained in terms of the positive value of the volume coefficient of expansion for He(I) over a finite temperature interval above the λ-line, and the presence of a non-convecting intermediate state of finite thickness separating the two phases.