Исследование тепловой структуры гелия II с помощью рассеяния холодных нейтронов

Abstract

In the basis for current descriptions of the behavior of helium II lies the idea of the existence of two fundamental types of quantum thermal excitations, called phonons and rotons. A series of investigations in which the scattering of cold neutrons in liquid helium is interpreted in terms of scattering events in which a phonon or roton is created and takes up the recoil momentum is surveyed. Correlation of the neutron's energy and momentum changes occurring in the event yields a dispersion curve relating the energy and momentum of the thermal excitations. The experimental techniques and the analysis and interpretation of data are outlined. The results reproduce quite well a segment of the dispersion curve predicted by Landau. The high-momentum end of the curve (beyond the range of existing data) and its interpretation in terms of various possible forms of the behavior of unstable rotons is discussed. The possibility of the direct determination of the roton's chirality from the forward scattering cross section is discussed. Finally the temperature dependence of a series of thermodynamic functions predicted using an experimental dispersion curve is presented. The resulting curves agree remarkably well with direct measurements, except for a low-temperature discrepancy in the velocitymore » of second sound. (TTT)« less