Terahertz spectroscopy: a powerful tool to explore scaling in condensed matter

Summary form only given. The universal laws of complex systems close to phase transitions can often be expressed by scaling relations. We measured quantum phase transition metal-insulator-transition (MIT) in yttrium hydride. Yttrium hydride YH/sub x/ shows a remarkable transitions of its electronic and optical properties upon change of hydrogen concentration. Also we demonstrate that terahertz spectroscopy is a perfect tool to explore the frequency-temperature scaling in condensed matter.