Inelastic collective dynamics of liquid rubidium with increasing temperature by neutron scattering studies

Abstract

Although the collective dynamics of liquid alkali metals are rather well understood near the melting point, there is not much research on these dynamics at temperatures higher than the melting temperature. We carried out a set of neutron scattering experiments on liquid rubidium, an alkali metal, to study the effects of temperature on the collective dynamics. In particular, we determined the dispersion relations at different temperatures from the current correlation spectra functions and from the evolution in the momentum transfer, Q, of the longitudinal phase velocity. We also performed a set of molecular dynamics simulations for each temperature sampled in the experimental data. These numerical results confirm the neutron scattering findings. The most striking feature is a frequency change at a momentum transfer of about Q Ϸ 0.45 Å −1 with rising temperature. The amount of change exceeds predictions from generalized hydrodynamics based on global density changes. The frequency softening with increasing temperature might be related to local structural changes on length scales of this momentum transfer.

Collective High Frequency Motions in Liquid Deuterium Fluoride

“…However, the experimentally obtained second moment exceeds the calculated one. Simple liquids, like alkali metals, exhibit a good agreement [19] and we conclude an already much more emphasized inelastic response around Q ≈ 2.0 Å K1 in that associated liquid, which seems to be not covered by the second moment. Fig.…”

Experimental evidence for two different dynamical regimes in liquid rubidium

“…More experimental details can be found in [9,12,23]. Furthermore, inelastic experiments to follow the collective excitations at 320 K, 420 K, 500 K and 600 K have been performed and analysed [11]. More details about the extensive data analysis and comparison with MD-simulations can be found there.…”

“…Near the melting point the dynamics has been related to high frequency elastic moduli of liquid rubidium [30]. Previously we analysed the inelastic collective dynamics of liquid rubidium in detail and derived from spectra of the current correlation functions dispersion curves over a wide range of momentum vectors [11]. From these dispersion curves phase velocities c(Q) = ω/Q can be calculated for the acoustic-type excitations.…”

“…The c ∞ (Q) values from the measurement [11] have been averaged between Q = 0.45 − 0.55 Å −1 and then G ∞ has been calculated accordingly. Fig 3 shows the obtained shear modulus G ∞ with rising temperature.…”

“…However, recent studies demonstrated distinct changes in the rubidium dynamics on atomic length scales with increasing temperature from the melting point T melt = 312.65 K up to about twice the melting temperature [9][10][11][12]. The alkali metal rubidium has been studied for several decades and a lot of data, macroscopic and microscopic, are available over a wide temperature range from experiment and simulation (e.g.…”