For optical spectroscopy measurements at very high hydrostatic pressures up to 25 GPa (1 GPa= 10.000 bar) we employ a gasketed diamond anvil cell (DAC) of Syassen/Holzapfel type and a liquid pressure transmitting medium (typically a 4:1 methanol/ethanol mixture). High pressure experiments can be carried out also at low temperatures down to 4 K using a specially designed He-bath cryostat (Konti-IT from Cryovac), which can allocate the diamond anvil cell (DAC) in its cold bore while granting optical access. Two steel rods designed to fit the threaded rods of the DAC inside the cryostat allow pressure tuning without removing the DAC from the cryostat. When using the DAC at low temperatures the best (hydrostatic) pressure medium is He. The loading of the cell then proceeds inside the cryostat by immersing it in fluid He, which is pumped to lower its equilibrium temperature till the transition to the superfluid state at 2.17 K occurs. Under this condition, He easily enters into the cell, filling it completely and the DAC is remotely closed. Electrical transport measurements under high hydrostatic pressures between 1 and 2 GPa are also possible by using a liquid piston-cylinder cell combined with the corresponding hydraulic press, including the whole electronics like lock-in amplifiers, multimeters, etc., the computer hardware and the control software (developed by us).
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