A substantial and continuous volume expansion from liquid to rarefied vapor occurs by the change of temperature and pressure surrounding the liquid-vapor critical point without crossing the saturated vapor pressure curve. Many investigations have been carried out with molecular fluids including supercritical fluids, and several variations of the equation of state have been advanced since the famous work by van der Waals. In the expansion process the mean interatomic distance increases by up to 10 times that found under standard conditions. However reconstitution of the molecular units or other major structure changes are rare in these fluids. On the other hand, in the expansion process of metallic or semiconducting liquids, physical properties can drastically change. Liquid Hg, well-known as a prototype of liquid metals, undergoes a metal-insulator transition when the density is reduced to about 9 g/cm3 near the critical point.Elemental Se, which units in its liquid form is a semiconductor with a twofold-coordinated chain structure in which atoms are covalently bonded, also experiences a semiconductor-metal-insulator transition around the critical point (critical temperature and pressure data of Se: Tc = 1615°C, Pc = 385 bar, pc = 1.85 g/cm3).The first indication of the semiconductormetal (SC-M) transition in fluid Se was found in electrical conductivity data by Hoshino and co-workers. Figure 1 shows the contours of constant dc conductivity plotted on the pressure-temperature (P-T) plane. The high-conductivity region appears in the immediate vicinity of the critical point. Another indication of the SC-M transition was obtained from the measurement of the optical reflectivity by Seyer et al. Drude-like energy dependence of the reflectivity spectra was observed at elevated pressures and temperatures.
Quantum critical local spin dynamics near the Mott metal-insulator transition in infinite dimensions