Optical absorption in ytterbium monochalcogenides under pressure

The pressure dependence of the lowest-lying $4f\to 5d$ absorption band in crystalline films of YbTe, YbSe, and YbS has been studied. The absorption edge which occurs at about 1.8 eV in YbTe, 1.5 eV in YbSe, and 1.1 eV in YbS at zero pressure is shown to close at a rate of 11.0, 9.8, and 6.5 meV/kbar, respectively, for pressures up to 8 kbar. The measurements predict gap collapses in all the three chalcogenides at pressures of the order of 175 kbar, in good agreement with pressure-volumes studies and qualitative changes in the reflectivity at high pressures.