High Pressure Technology

In HPSTAR, the following techniques have been studied.

High pressure optical spectroscopy
A broad range of optical spectroscopy techniques are used by scientists in HPSTAR. These techniques mainly include high pressure Raman spectroscopy, Brillouin scattering, infrared spectroscopy, and optical reflectivity.
Raman spectroscopy is one of the most informative probes for studies of material properties under high pressure. It probes elementary excitations in materials by utilizing inelastic scattering processes of a laser (wavelength ranges from near ultraviolet to near infrared). The elastic, vibrational, electronic, and magnetic subsystems can be characterized through the observations of the corresponding elementary excitations. The information on materials phase, crystal structure can also be obtained through Raman spectroscopy. Brillouin scattering allows the determination of acoustic velocities and adiabatic elastic moduli in materials. Infrared absorption spectroscopy is a powerful tool to investigate the molecular vibrations/rotations, and molecular structure for polar molecules and organic materials. Optical reflectivity provides information about optical conductivity and band structure of materials.

High pressure resistivity measurements
Electrical resistivity of materials under high pressure can be obtained by four probe electrical measurements in a diamond anvil cell. This technique is critical for condensed matter physics research, e.g. high pressure superconductivity and pressure-induced metallization.
The measurements are challenging because the electrical contacts attached to the sample have to survive to extreme stress conditions. The experiments are generally limited to solid (non-hydrostatic) pressure transmitting media. Very recently, resistivity measurements under quasi-hydrostatic pressure using inert gas as a pressure transmitting medium are realized by utilizing focused ion beam ultrathin lithography.

High pressure magnetic measurements
Magnetic measurements under pressure yield useful information about magnetic moments, magnetic structures and interactions in condensed matter, especially in itinerant electron systems. A special designed non-magnetic high pressure cell is used for the measurements. The magnetic measurements are usually combined with low temperature techniques (cryostats) in the high pressure research.

High pressure LT/HT techniques
In addition to pressure, temperature is an important thermodynamic parameter to tune materials properties. Therefore extreme temperatures (low/high temperature) are frequently employed in high pressure research.
Low temperatures down to 4.2K or 10K can be achieved in a diamond anvil cell placed in a cryostat, depending on the type of cryostats used. High temperature is generated by external resistive heating or laser heating. External resistive heating is usually used for temperatures below 1200K. It can introduce a stable and uniform temperature distribution in the sample chamber. Laser heating uses an infrared laser to generate high temperatures up to several thousands K, with relatively larger temperature gradients in the sample.