Bulklike excitations in nanoconfined liquid helium

The effects of confinement, disorder, and reduced dimensionality upon quantum fluids have been studied by the adsorption of liquid helium in porous media. The effects of extreme, nanoscale confinement upon its microscopic excitations are not presently understood. Several previous experiments have suggested that at sufficiently low temperature, the roton mean free path is set by the restricted geometry. Here we show that the lifetime of the roton excitation is unaffected when superfluid helium is confined within cylindrical pores only a few nanometers in diameter. The temperature dependence of its lifetime are found to be identical to the bulk fluid, implying that the lifetime is not set by the scale of the confinement. Our results demonstrate that the rotons in the pore center propagate without being modified by the confining media, unlike the collective excitations of classical fluids.

Figure 1

A cut of the dynamic structure factor $S(Q,E)$ along the roton minimum $Q_R$ at 0.34 K. The inset shows the dispersion for both the bulklike mode and the layer mode. The low-intensity layer mode is plotted with wide bins to increase statistics, while the strongly scattering bulklike mode is plotted with narrow bins to resolve the width. The total fit (black dotted line) is comprised of a linear background, a Gaussian layer mode (red dotted line), and a damped harmonic-oscillator bulklike mode (green dotted line). Error bars throughout the text represent one standard deviation, derived from the square root of detector counts and binned using standard data-reduction software [30, 31].

Figure 2

Measured values of the shift in the energy gap from base value as a function of temperature, ${\mathrm{\Delta }}_0-\mathrm{\Delta }\left(T\right)$. BPZ theory: dashed line; FSM-16 current measurement: red circles; FSM-16 43.0 mmol/g Cold Neutron Chopper Spectrometer (CNCS): maroon triangles [19]; FSM-16 46.4 mmol/g CNCS: open maroon triangles [19]; Vycor: orange circles [10]; MCM-41: green diamonds [20]; bulk liquid: black squares [15].

Figure 3

Reported linewidths (full width at half maximum) for ${}^4\mathrm{He}$ confined in porous media as a function of temperature. Points used here are identical to the ones used in Fig. 2.

Figure 4

A schematic depiction of rotons confined in FSM-16. The silica walls are shown in blue, while the solid, dense liquid, and bulklike helium layers are shown in red, green, and yellow, respectively. The rotons are depicted as moving ballistically through the pore space, undergoing elastic collisions with the dense liquid layer. Four roton processes that are responsible for limiting the excitation lifetime are also shown.