Temperature dependence of S(Q,ω) in liquidHe4under pressure

The temperature dependence of the dynamic form factor, S(Q,ω), of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at p=20 bars has been determined by inelastic neutron scattering measurements. Two wave vectors, Q=1.13 A${̊}^{\mathrm{-}1}$ and Q=2.03 A${̊}^{\mathrm{-}1}$, corresponding to the maxon and roton regions of the phonon-roton dispersion curve, were studied over a wide range of energy transfer, ħω. Based on previous data at SVP, Woods and Svensson proposed that S(Q,ω) could be represented as a sum of two components, one proportional to the superfluid density, ${\rho }_S$(T), and one proportional to the normal density ${\rho }_N$(T). The component proportional to ${\rho }_S$(T) contained the sharp one-phonon peak which vanished at T=$T_{\lambda }$. The aim here is, firstly, to present data on the temperature dependence of S(Q,ω) at 20 bars and, secondly, to explore whether or not the Woods-Svensson decomposition of S(Q,ω) holds at higher pressure. At 20 bars and for the Q values investigated here, we find that the sharp peak of S(Q,ω) does indeed decrease rapidly in intensity as T increases and the corresponding excitation either vanishes or changes abruptly in character at $T_{\lambda }$.