A new technique has been developed which has allowed high-frequency ultrasonic measurements up to 323 MHz under a pressure of ~1.3 GPa (~ten thousand atmospheres) at very low temperatures (down to 1.5 K) to be performed for the first time. Installing a coaxial cable into a high pressure cell is a significant challenge, since the coaxial tube could act as vent to relieve the pressure. To overcome this technical difficulty, the authors installed a pair of one-side covered ultra-thin semi-rigid coaxial cables with a diameter of 0.33 mm into a hybrid piston-cylinder cell. The ‘ultrasonic method’ is one of the essential tools for investigating the electronic state via the electron-phonon interaction, while ‘pressure’ is an important parameter that allows the electronic correlations to be ‘tuned’ by controlling the lattice parameter of the crystals. By merging these technologies, the authors’ experiments revealed the exotic behavior of the 4f electrons in the ground state of SmOs₄Sb₁₂, which transform from being delocalized at ambient pressure to being localized at high pressures with a crossover pressure of approximately 0.7 GPa. Such a traditional but “nonstandard” technique not only provides special insight into this extraordinary compound, but also represents a new milestone in the development of ultrasonic measurement methods in solid-state physics.

For more information please see our paper: S. Mombetsu et al., Phys. Rev. B 94 (2016) 085142.

(also available on arXiv:1608.01066 [cond-mat.str-el])