Prediction of a roton-type feature in warm dense hydrogen

TL;DR

This paper predicts a roton-like feature in the dynamic structure factor of warm dense hydrogen, based on simulations and linear response theory, suggesting it could be observed experimentally under certain conditions.

Contribution

It introduces the prediction of a roton-type feature in warm dense hydrogen, extending previous electron gas results to a dense plasma context.

Findings

Non-monotonic dispersion relation predicted for warm dense hydrogen

Approximate density, temperature, and wave number ranges identified for observation

Potential for experimental detection of roton-like features in dense plasmas

Abstract

In a recent Letter [T. Dornheim \textit{et al.}, Phys. Rev. Lett. \textbf{121}, 255001 (2018)], it was predicted on the basis of \textit{ab initio} quantum Monte Carlo simulations that, in a uniform electron gas, the peak $\omega_0$ of the dynamic structure factor $S(q,\omega)$ exhibits an unusual non-monotonic wave number dependence, where $d\omega_0/dq < 0$, at intermediate $q$, under strong coupling conditions. This effect was subsequently explained by the pair alignment of electrons %at an intermediate range of wave numbers [T. Dornheim \textit{et al.}, Comm. Phys. \textbf{5}, 304 (2022)]. Here we predict that this non-monotonic dispersion resembling the roton-type behavior known from superfluids should be observable in a dense, partially ionized hydrogen plasma. Based on a combination of path integral Monte Carlo simulations and linear response results for the density response function, we present the approximate range of densities, temperatures and wave numbers and make predictions for possible experimental observations.