SPring-8, the large synchrotron radiation facility

Release Date

04 Mar, 2009

Researchers at Hiroshima Institute of Technology (Kazuhiro Mori, President) were the first in the world to observe a transverse acoustic wave in a simple monatomic liquid in an experiment using high-energy resolution X-ray inelastic scattering at SPring-8 in collaboration with Hiroshima University (Toshimasa Asahara, President), Kyoto University (Hiroshi Matsumoto, President), University of Marburg in Germany (Volker Nienhaus, President), Deutsches Elektronen-Synchrotron (DESY) (Albrecht Wagner, Chairman), University of Valladolid in Spain (Evaristo J. Abril Domingo, Chancellor), the Japan Synchrotron Radiation Research Institute (Akira Kira, Director General), and RIKEN (Ryoji Noyori, President).

In physics textbooks for high school students, it is clearly stated that "a transverse acoustic wave cannot travel in a liquid because no restoring force is generated there."  However, it has been predicted from theoretical calculations that a transverse acoustic wave can exist for about a picosecond (one trillionth of a second) in a nanometer space (approximately the distance between atoms; one billionth of a meter), even in a simple liquid.  Unfortunately, the prediction had never been demonstrated.

In this research, by carrying out X-ray inelastic scattering measurements on liquid gallium, a monatomic liquid metal, the researchers succeeded in clearly detecting the signal of a transverse acoustic wave from the scattering spectrum.  They also clarified that the transverse acoustic wave travels 0.5 nm at 1050 m/s, thus existing for about 0.5 ps.  The space and time scales of such a transverse acoustic wave correspond to the size and life of a nanometer-size solid cagelike atomic cluster, which had been conventionally considered to exist in a liquid.

This discovery has not only expanded basic studies by experimentally demonstrating the theoretical prediction of the existence of a transverse acoustic wave in a liquid, but it can also significantly contribute to the basic understanding of crystal growth from a liquid to a solid and its advancement to practical application because we can determine the elastic characteristics of the solid cagelike atomic cluster in a liquid.

This achievement is the result of joint research by the group of Shinya Hosokawa, associate professor of Hiroshima Institute of Technology, and his colleagues.  It was reported online in the American scientific journal Physics Review Letters on 13 March 2009.

Publication:
"Transverse Acoustic Excitations in Liquid Ga"
S. Hosokawa, M. Inui, Y. Kajihara, K. Matsuda, T. Ichitsubo, W.-C. Pilgrim, H. Sinn, L. E. González, D. J. González, S. Tsutsui, and A. Q. R. Baron
Physical Review Letters 102, 105502 (2009), published online 13 March 2009

fig1. Schematics of an atomic cluster resembling a solid cage conventionally believed to exist in a liquid and the X-ray wave used to detect it.
An atom is almost immobilized because of the surrounding atoms.

fig2. X-ray inelastic scattering spectrum of liquid gallium and the fitting using a theoretical function (damped harmonic oscillator model).
The abscissa indicates the change in energy.  The upper lines (2DHO) show spectra with respect to Q when a transverse acoustic wave is considered, and the lower lines (1DHO) show that without consideration.  The lower solid lines in each case are obtained by doubling the difference between the experimental result and theoretical value.  In the case of 1DHO, there is no agreement at the part indicated by the arrow.  The dotted line (...) indicates a quasi-elastic scattering component, the dashed line (---) indicates a longitudinal acoustic wave component, and the dashed-dotted line (-.-.) indicates a component appearing to be of a transverse acoustic wave.

fig3. Dispersion relationship between the transverse and longitudinal acoustic waves in liquid gallium.
The solid line indicates the result of the calculation by first principle molecular dynamics for a transverse acoustic wave.  The slope of the line from the origin to each point, ω/Q, indicates the microscopic sound velocity.