Anticrossing of Longitudinal and Transverse Modes in Simple Fluids

Abstract

If interacting modes of the same symmetry cross, they repel from each other and become hybridized. This phenomenon is called anticrossing and is well-known for mechanical oscillations, electromagnetic circuits, waveguides, metamaterials, polaritons, and phonons in crystals, but it still remains poorly understood in simple fluids. Here, we show that structural disorder and anharmonicity, governing properties of fluids, lead to the anticrossing of longitudinal and transverse modes, which is accompanied by their hybridization and strong redistribution of excitation spectra. We combined theory and simulations for noble gases to prove the reliability of mode anticrossing in simple fluids, studied here for the first time. Our results open novel prospects in understanding collective dynamics, thermodynamics, and transport phenomena in various fluids, spanning from noble gas fluids and metallic melts to strongly coupled plasmas and molecular and complex fluids.

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

“…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.…”

“…The excitation spectra were calculated with the methods developed in Refs. [32,43]. At the first step, we calculated particle current spectra C L,T (q, ω) [38]:…”

“…Because of simple fluids are isotropic, the spectra depend only on q = |q| and we used this to enhance the accuracy of C L,T (q, ω) calculation. Then, C L,T (q, ω) data were analyzed with the separate mode analysis and the two-oscillator model [32,43] to obtain dispersion curves ω l,h (q) (for low-and high-frequency branches) and q-gap width. Using the two-oscillator model, the frequencies and damping rates of excitations are obtained by fitting the total current C(q, ω) = C L (q, ω) + 2C T (q, ω) by the sums of Lorentzians attributed to the high-and the lowfrequency oscillators [32,43] (see details in Supplementary Materials (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. Simultaneously, the separate modes (triangles in Fig.…”

“…Discussion and Conclusions. -With MD simulations and methods for excitation analysis we recently developed [32,43], we studied liquified noble gases, liquid iron, liquid mercury, and model fluids of particles interacting via inverse-power-law potentials, to understand the roles of repulsion softness, attractive and many-body interactions in dynamics and thermodynamics. Remarkably, we found that the heat capacity c V depends linearly on the q-gap width in a broad range of temperatures, but has a clear bilinear behaviour with a common crossover point, whereas the functions c V and q g themselves are smooth and essentially nonlinear in temperature.…”