Direct Experimental Evidence of Longitudinal and Transverse Mode Hybridization and Anticrossing in Simple Model Fluids

Abstract

A significant number of key properties of condensed matter are determined by the spectra of elementary excitations and, in particular-collective vibrations. However, behaviour and description of collective modes in disordered media (e.g. liquids and glasses) remains a challenging area of modern condensed matter science. Recently, anticrossing between longitudinal and transverse modes was predicted theoretically and observed in molecular dynamics simulations, but this fundamental phenomenon has never been observed experimentally. Here, we demonstrate the mode anticrossing in a simple Yukawa fluid constructed from charged microparticles in weakly-ionized gas. Theory, simulations, and experiments show clear evidence of mode anticrossing that is accompanied by mode hybridization and strong redistribution of the excitation spectra. Our results provide significant advance in understanding excitations of fluids, opening new prospectives for studies of dynamics, thermodynamics, and transport phenomena in a wide variety of systems from noble gas fluids and metallic melts to strongly coupled plasmas, molecular, and complex fluids.

Universal Effect of Excitation Dispersion on the Heat Capacity and Gapped States in Fluids

“…Experimentally, excitation spectra in fluids can be obtained with inelastic neutron and x-ray scattering [9][10][11][12][13][14][15][16][17]. Yet another approach is to use complex plasmas -the model systems of charged and strongly-interacting particles levitating in a weakly-ionized gas, whose interactions can be tuned, and the motions can be visualized [18][19][20][21]. Molecular dynamics (MD) simulations are also widely used [17,[22][23][24][25][26][27][28][29][30][31][32][33][34] to study collective dynamics in systems with a range of interatomic interactions and, crucially, allow access to a broad range of parameters, including the ones that may not be accessible experimentally.…”

“…The value of q-gap (characterizing transverse excitations at small q) has been assumed [30,41,42] to influence the heat capacity of fluids, but this has never been tested directly in either experiments or MD simulations. Another generic phenomenon recently studied with MD simulations, theoretically, and experimentally with the model fluids [21,43] is anticrossing of longitudinal and transverse excitations. It has been demonstrated that structural disorder and ahnarmonicity induce mutual mixing of the modes at large q.…”

“…S1 in SM [52]). However, the difference becomes apparent at qn −1/3 /π > 1 corresponding to the region of mode anticrossing [21,43] and is a consequence of formation of hybridized low-and high-frequency modes (red circles) instead of the crossed dispersion curves (grey triangles). The high-frequency mode, associated with longitudinal (sound) excitations at small q, behaves as ω h (q) ∝ √ T q at large q [9,32,43,54] and, beginning from some temperature, the high-frequency dispersion curve transforms from oscillating form (with sign-changing group velocity v g = ∂ω h /∂q, inherent for solids) to gas-like dispersion with v g > 0 at all q.…”

From soft- to hard-sphere fluids: Crossover evidenced by high-frequency elastic moduli

“…2(a) and 2(d) behave similarly to those reported in Refs. [31,[38][39][40]. In the long-wavelength limit, as usual, these branches are attributed to the longitudinal and transverse collective modes [31,40].…”

“…The excitation spectra were obtained using the procedures described in Refs. [31,[37][38][39][40]. First, we calculated the velocity current spectra [31,41]:…”