7 1.0 0.5 FWHM (nm)  Fluorinert FC70/77  Daphne7373 T   = 77 K 0.5 10 5 0  Pressure (GPa)  NaCl  Silicon oil  Vaselin 0.5 0 10 5  Pressure (GPa)  Nitrogen  Argon  Helium 1.0 0.5 FWHM (nm)  1:1 n-pentane and  isopentane   4:1 methanol and ethanol  Petroleum ether T   = 77 K 1.0 0.5 0 FWHM (nm) 10 5  Pressure (GPa)  2-propanol  Glycerin  Daphne7474 T   = 77 K 0 1.0 0 1.0  Group I  Group II  Group III FIG. 7: (Color online)Pressure dependence of ΔFWHM (= FWHM( P )-FWHM(0)) of the ruby R$_{1}$ line for fourteen pressure- media at 77 K. Here, FWHM( P ) and FWHM(0) are the values of the fitted function for the FWHM under high pressure and ambient pressure, respectively. Error bars indicate averaged deviations of the experimental data of the FWHM from the fitted line at each pressure. IV. DISCUSSION AND CONCLUSION a sample even if the helium medium is used [55]. fect of the ruby R 1 line in the nitrogen, argon and helium media are significantly small, suggesting that the media are more appropriate for cryogenic experiments with the DAC. Finally, we mention some notes for interpretation of the present results from different points of views. In this section, the relative strength of the nonhy- drostatic effects in the media is discussed from the line widths of the ruby R 1 line. For the sake of simplicity, the individual data points are omitted and only the guide (fitted) lines for the FWHMs at 77 K in Figures 2 - 6 are compared. To discuss the pressure effect clearly, the am- bient pressure width FWHM(0) is subtracted from the guide line: ΔFWHM = FWHM( P ) - FWHM(0). Here FWHM(0) was determined for each of the media. Figure 7 shows the pressure dependences of ΔFWHMs for all the media. (1) The present results are applicable in high-pressure studies using opposed anvil type high-pressure cells (DAC and Bridgman cells) but not simply in studies us- ing multi-anvil type high-pressure cells such as a cubic anvil cell or Kawai-type (6-8) multi anvil high-pressure cell where a sample with a medium is pressed by six or eight anvils[56–58]. Pressurizing a sample highly sym- metrically generally reduces the nonhydrostaticity of the pressure caused by a solid pressure medium and therefore achieves a good hydrostatic pressure. (2) Ruby crystals are relatively stiff material with a bulk modulus of 253 GPa [16]. If the material to be studied is strain sensitive or has mechanical strength smaller than the ruby crystal, the nonhydrostatic effects could influence the physical properties of the material, even though the broadening of the ruby R 1 line would be small or absent. There are usually discrepancies be- tween hydrostatic-limit pressures determined using dif- ferent methods for one pressure-medium. An extreme example is the nonhydrostatic effects on elastically soft quartz (SiO 2 ) with a bulk modulus of 37.1 GPa [11]. (3) Some of the organic media in Groups II and III could chemically react with organic materials. There is a possibility that small argon and helium atoms could pen- The ΔFWHMs for the media in Group I show the large value of above 0.3 nm at 5.0 GPa, indicating the strong nonhydrostatic effects of the media. Although some of the media in the group have advantages for use in high-pressure experiments, they are not suitable me- dia in cryogenic experiments with the DAC at the higher- pressure region. The nonhydrostatic effects of the media in Group II are smaller than those of the media in Group I below 5 GPa. However, the effects become stronger above the pressure and are comparable with those of the media in the former group around 10 GPa. The effects of the media in Group III are smaller than those of the media in the other groups, even in the low temperature region, up to 10 GPa. In particular, the broadening ef-