The dynamic structure factor S(Q, E), where Q and E are momentum and energy transfer, respectively, has been measured for liquid Sb, using inelastic x-ray scattering. A modified damped harmonic oscillator model function was applied to analyse S(Q, E) of liquid Sb and also to that of liquid Bi by Inui et al (2015 Phys. Rev. B 92, 054206). The obtained excitation energy was in fairly good agreement with that predicted by ab initio molecular dynamics simulations on these liquid semi-metals. The excitation energy of the longitudinal acoustic mode in liquid Sb and liquid Bi exhibits flat-topped Q dependence whereas the lower excitation energy below the longitudinal acoustic excitation shows Q-gap behaviour. From the viscosity estimated from the Q-gap experimentally obtained, it is inferred that the lower energy excitation arises from the transverse acoustic excitation in the liquids.
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