1.2 Even Oxygen Becomes Superconducting at Very High Pressure

Fig. 1-3 Enlarged photograph of metallic oxygen sample (S) and platinum electrodes (C+, C-, V+ and V-) for electrical resistance measurement
The sample is set on an insulated gasket in a pressure cell at a pressure of 120 GPa. The measuring current is applied through platinum electrodes of C+ to C- and the electrical resistance is obtained from the voltage between V+ and V-. Metallic reflection from the oxygen sample can be compared with that from the platinum electrodes. The color of the image comes from the yellow synthetic diamond window, through which the photograph was taken.

Fig. 1-4 Temperature dependence of the electrical
resistance of metallic oxygen
The applied pressure is 120 GPa, His the external magnetic field, and the electrical resistance is normalized to its value at 1 K. The inset shows the critical magnetic field, which destroys the superconductivity, as a function of temperature.

It is well known that diatomic gas molecules such as nitrogen and oxygen transform into insulating solids under high pressure and, as the pressure is further increased, such molecular solids become metallic, conducting electric current. Among these diatomic molecules, oxygen is of particular interest because it shows magnetism at low temperatures. After having metallized oxygen at pressures exceeding 100 GPa (about 1 Mbar), we lowered the temperature to near absolute zero (-273 degrees) and examined whether metallic oxygen becomes superconducting or not. At last we discovered that, at pressures of around 100 GPa and at temperatures below 0.6 K, oxygen becomes superconducting.
Figure 1-3 shows an enlarged photograph of the metallic oxygen sample and the platinum electrodes for the electrical resistance measurement at very high pressure, which was taken through a diamond window. Figure 1-4 shows the resistance ratio (R/R1K) of oxygen as a function of temperature with the external magnetic field Has a parameter. If we take the case of H=0 as an example, the resistance ratio decreases abruptly at a temperature of 0.6 K, indicating that the transformation to the superconducting state has occurred. The resistance ratio below 0.6 K is from a part that is nonsuperconducting. This part is due to some pressure distribution in the sample, which is inevitable under the present extreme environment. The inset shows the critical magnetic field versus transition temperature, which characterizes the superconductivity. In addition, we confirmed the superconductivity by observing the Meissner effect of expelling magnetic flux lines from the sample.
We have already found that sulfur becomes superconducting at very high pressure. Both oxygen and sulfur belong to the VIb series in the periodic table, however, they are very different in superconducting transition temperature: oxygen and sulfur become superconducting at 0.6 K and 15 K, respectively. It is conceivable that, in the case of oxygen, magnetism suppresses the appearance of superconductivity. These results are the latest frontiers in the field of very high pressures and extremely low temperatures.

Reference
K. Shimizu et al., Superconductivity of Oxygen, Nature, 393 (6687), 767 (1998).

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