High-pressure infrared spectroscopy: Tuning of the low-energy excitations in correlated electron systems

We have extended the range of the high-pressure optical spectroscopy to the far-infrared region keeping the accuracy of ambient-pressure experiments. The developed method offers a powerful tool for the study of pressure-induced phase transitions and electronic-structural changes in correlated electron systems as the optical pressure cell, equipped with large free-aperture diamond window, allows the measurement of optical reflectivity down to omega approximate to 20-30 cm(-1) for hydrostatic pressures up to p approximate to 26 kbar. The efficiency of the technique is demonstrated by the investigation of the two-dimensional charge-density-wave 1T-TaS2 whose electronic structure shows high sensitivity to external pressure. The room-temperature semimetallic phase of 1T-TaS2 is effectively extended by application of pressure and stabilized as the ground state above p=14 kbar. The corresponding fully incoherent low-energy optical conductivity is almost temperature independent below T=300 K. For intermediate pressures, the onset of the low-temperature insulating phase is reflected by the sudden drop of the reflectivity and by the emergence of sharp phonon resonances.