Maxon and roton measurements in nanoconfined $^4$He

TL;DR

This study examines how collective excitations of adsorbed $^4$He in nanoconfined pores change with filling, revealing a monotonic maxon transition and complex roton behavior that differ from bulk liquid properties.

Contribution

It provides detailed inelastic scattering measurements of $^4$He in nanoconfined pores, highlighting how excitations evolve with filling and differ from bulk helium.

Findings

Maxon excitation increases monotonically with filling.

Roton minimum shows non-monotonic behavior with filling.

Pore scattering resembles bulk liquid with additional layer modes.

Abstract

We investigate the behavior of the collective excitations of adsorbed $^4$He in an ordered hexagonal mesopore, examining the crossover from a thin film to a confined fluid. Here we present the inelastic scattering results as a function of filling at constant temperature. We find a monotonic transition of the maxon excitation as a function of filling. This has been interpreted as corresponding to an increasing density of the adsorbed helium, which approaches the bulk value as filling increases. The roton minimum exhibits a more complicated behavior that does not monotonically approach bulk values as filling increases. The full pore scattering resembles the bulk liquid accompanied by a layer mode. The maxon and roton scattering, taken together, at intermediate fillings does not correspond to a single bulk liquid dispersion at negative, low, or high pressure.