The breakdown of superfluidity in liquid 4He: an experimental test of Landau’s theory

A single pulse time of flight technique has been used to determine the drift velocity F of the negative ions injected into liquid 4He from a field emission source. Measurements of v as a function of temperature T, pressure P and electric field E are presented within the range: 0.29 ≤ T ≤ 0.5 K; 21 x 105 ≤ P ≤ 25 x 105 Pa; 1 ≤ E ≤ 300kV m-1. The experimental results are in good agreement with Landau’s theory of superfluidity. The data are used to demonstrate the inapplicability of two theories of supercritical dissipation: by Takken, based on an assumption of coherent roton emission; and by Bowley & Sheard, based on the assumption of incoherent single-roton emission. The results are, however, shown to be in excellent agreement with Bowley & Sheard’s incoherent two-roton theory, and the data are used to derive a numerical value of the matrix element characterizing two-roton emission. The surprising absence of the single-roton emission process is discussed, and an upper bound is placed on the relevant matrix element.