Invited article: High-pressure techniques for condensed matter physics at low temperature.

Area of Science:

• High-pressure condensed matter physics
• Materials science
• Diamond anvil cell technology

Background:

• Accurate pressure scales and homogeneous pressure environments are vital for high-pressure experiments across various temperatures.
• Diamond anvil cell (DAC) technology faces challenges with pressure-transmitting media quality and low-temperature secondary pressure scale accuracy.

Purpose of the Study:

• To directly calibrate the ruby fluorescence R1 line shift with pressure at cryogenic temperatures (4.5 K).
• To assess pressure inhomogeneity and anisotropy in helium and methanol:ethanol (4:1) pressure media at low temperatures.
• To evaluate the suitability of pressure media for single-crystal X-ray diffraction at cryogenic temperatures.

Main Methods:

• High-resolution X-ray powder diffraction measurements of silver's lattice constant at 4.5 K up to 16 GPa.
• Direct calibration of the ruby R1 fluorescence line shift against pressure using silver's equation of state.
• Ruby fluorescence used to characterize pressure distribution and deviatoric stress in pressure media within DACs.

Main Results:

• A low-temperature ruby pressure scale was established, showing a 6% deviation from the room-temperature scale.
• Both helium and methanol:ethanol (4:1) exhibited similar pressure inhomogeneity (+/-1.8 %/(10^4) µm²) up to 20 GPa at 5 K.
• Helium showed constant deviatoric stress up to 16 GPa, while methanol:ethanol's anisotropy increased above 10 GPa.

Conclusions:

• The established low-temperature ruby scale provides a more accurate pressure measurement in cryogenic DAC experiments.
• Contrary to assumptions, helium and methanol:ethanol show comparable inhomogeneity, impacting experimental design.
• Sample chamber volume ratio is critical for maintaining sample quality in cryogenic high-pressure experiments, influencing pressure medium choice.