2 Ar laser $^{λ }$= 532 nm Ruby chips Pressure medium Ruby fluorescence Intensity (a.u.) 700 695 Wavelength (nm) 10.1 GPa 9.6 Glycerin Argon 4:1 methanol-ethanol NaCl 10.6 9.5 77 K R$_{1}$ R$_{2}$ FIG. 1: (Color online) Schematic illustration of the DAC set up and the fluorescence spectra of the ruby R lines with repre- sentative media of NaCl, glycerin, 4:1 M-E mixture and argon at 77 K and around 10 GPa strength of the nonhydrostatic effects in the media was evaluated by the ruby (Al 2 O 3 : Cr $^{3+}$) fluorescence method using a clamp-type diamond anvil cell originally designed by Dunstan and Spain [16–18]. Figure 1 shows a schematic illustration of the DAC setup and the fluo- rescence spectra of the ruby R lines for the representative media of NaCl, glycerin, 4:1 M-E mixture and argon at 77 K and around 10 GPa. The culet of the diamond was 800 µ m. A sample chamber of 400 µ m in diame- ter was prepared in a 304 stainless steel gasket. Small ruby chips, containing 0.5 wt% of Cr 2 O 3 , were uniformly placed in the chamber filled with a pressure-transmitting medium. The diameter of the chips was less than 10 µ m. It was confirmed by X-ray diffraction analysis that the chips were single crystals. The ruby fluorescence was measured using a charged coupled device (CCD) spectrometer. More details on the optical system are given in Reference 19. A diode pumped solid-state (DPSS) green laser light (532 nm), introduced into the sample chamber through fiber op- tics, excited the ruby chips in the chamber. The diam- eter of the beam line was 600 µ m, larger than that of the sample chamber. The present work reveals spatially averaged information on the nonhydrostatic effects in the whole sample chamber. When the pressure becomes nonhydrostatic, the two R 1 and R 2 lines broaden. The pressure shift of the ruby R 1 line is sensitive to the uniaxial stress [20–22]. In the present study, a number of small ruby single crystals were uniformly placed inside the chamber with the directions of the single crystals being random. The broadening of the ruby R 1 line reflects both the inhomogeneous pres- sure distribution and uniaxial stress pressure. The peak positions and line widths of the ruby R 1 and R 2 fluorescence lines were determined by deconvoluting measured R -line spectra into a pair of pseudo-Voigt func- tions that represent the contribution of the ruby R 1 and R 2 lines with a linear background. The pressure was determined using the hydrostatic ruby pressure scale by Zha, Mao and Hemley [23, 24]. Here we note differences between the present work and the previous studies on pressure-media from a technical point of view. The references 10 and 13 reported the inhomogeneous pressure distribution inside the sample chamber at room temperature [10, 13]. The previous studies in references 11, 12, 14 and 15 revealed spatially local information on the nonhydrostatic effects around the center of the chamber as the size of the “sensors” used to detect the effects, for example the ruby chips, Cu 2 O or NaCl, was very small compared with the sam- ple space [11, 12, 14, 15]. This work shows spatially aver- aged information on the nonhydrostatic effects inside the sample chamber. The observed broadening effect of the ruby R 1 line reflects both the inhomogeneous pressure distribution and uniaxial stress pressure. The purpose of the present study is to clarify the relative strength of the nonhydrostatic effects of the media in the low tempera- ture region. As shown in Figure 1, the broadening effect on the R 1 line depends on the media at 77 K and around 10 GPa. A clear broadening effect was observed in the ruby R 1 lines for NaCl and glycerin. However, the lines for the 4:1 M-E mixture and argon are comparably sharper. Therefore, it is possible to discuss the relative strength of the nonhydrostatic effects of the media from the width of the ruby R 1 line. The pressure-transmitting medium was loaded into the sample chamber at room temperature except for nitro- gen, argon and helium done using a purpose built cryo- genic device at 77 or 1.4 K [17, 18]. The pressure was applied and changed at room temperature. The ruby flu- orescence spectra were measured after the pressure and the width of the ruby R 1 line had been stabilized. It took several hours for the quantities to be stabilized when a medium was in a solid state under high-pressure. The DAC was slowly cooled down to 77 K and 4.2 K using liq- uid nitrogen and helium, respectively. It should be noted here that the pressure did not significantly change dur- ing the cooling process with the present DAC where load was maintained using springs. The difference in pres- sure between at 4.2 K and 300 K was in the order of a few % when organic and argon media were used. The pressure-change with the nitrogen and helium media will be discussed later. In this section, we show the pressure dependence of the full-width at half maximum (FWHM) of the ruby R 1 line III. RESULTS