Invited Article: High-pressure techniques for condensed matter physics at low temperature
Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent Delta P/P per unit area of +/- 1.8 %/(10^(4) µm^(2)) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021 +/- 0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium.
Additional Information© 2010 American Institute of Physics. Received 12 October 2009; accepted 27 March 2010; published online 20 April 2010. We acknowledge discussions and technical help from V. Prakapenka and P. Dera, and the use of the helium loading facility at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source, Argonne National Laboratory. We are also grateful to I. Steele for the ion microprobe characterization of the Alfa Aesar ruby specimen, and D. Heinz for discussion of the Birch equations. In addition, we thank the staff of Sector 4 at the Advanced Photon Source, specifically J. Lang and Z. Islam for their help with x-ray diffraction, and D. Haskel and N. Souza-Neto for their help with the ruby spectrometer. GSE-CARS was supported by the National Science Foundation-Earth Sciences (Grant No. EAR-0622171) and U.S. Department of Energy-Geosciences (Grant No. DE-FG02-94ER14466). The work at the University of Chicago was supported by the National Science Foundation-Division of Materials Research (Grant No. DMR-0907025). Use of the Advanced Photon Source was supported by the U.S. DOE-BES under Contract No. DE-AC02-06CH11357.
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