Pressure is a clean tuning parameter which can be used to tune the lattice parameters, band structures, orbital overlaps, electron-lattice coupling, and magnetic interactions. Hydrostatic pressure is applied by placing a sample inside a pressure cell which is typically a piston-cylinder assembly that confines a hydrostatic pressure medium, the sample, and the leads. Two common types of pressure cells are Diamond anvil cells and clamp cells.
Diamond Anvil Cell
The main components of a diamond anvil cell (DAC) are two diamond anvils which are pressed against each other in a piston-cylinder mechanism. The pressure is exerted on a metallic disc, named “gasket” and placed between the two anvils. The sample (red spot) is contained inside a hole which is drilled at the centre of the gasket and filled with the hydrostatic pressure medium (yellow fill). Diamond anvil cells work in the ultra-high pressure regime up to 50 GPa. Due to the extreme conditions inside the cell, the design and the operation of DAC requires a lot of experience. My PhD thesis was devoted to set up the DAC for transport measurements up to 20 GPa. Examples of real cells are shown in the picture below. DAC allows for optical access to the sample. In the top left corner of the picture, you see a picture of FeCrAs sample at 17 GPa with the four contacts for the resistivity measurement. Next to it, you a picture of the gasket before pressurization with the sample inside the central hole and connected to the wires. Typical sample dimensions are ~ 200×100×20 μm and putting the four contacts on them is quite an art (top right panel).
Working at lower pressures is much easier with clamp cells. The sample is typically mounted on a feed-through and placed inside a Teflon cap filled with the hydrostatic pressure medium as shown schematically below. The maximum pressure attainable with a clamp cell is ~ 3 GPa. Compared to a diamond anvil cell, the maximum pressure is lower in a clamp cell but it can be tuned in much finer steps. The sample space in a clamp cell is much larger than DAC and sample dimensions are typically in the millimetre range. Due to the large volume of the sample area (Teflon cap) in a clamp cell, the pressure is more hydrostatic. Typical pressure media are Daphne oil 7474 and 7373, Pentane mixture, Ethanol/Methanol mixture, and Fluorinert mixture. Since the sample area is much smaller in DAC, we sometimes use liquid Argon or even liquid Helium as the pressure medium to ensure hydrostatic conditions.