Calcium polyhydrides are discovered experimentally superconductivity above 210K

Metallic hydrogen is predicated to be room temperature superconductor simply based on conventional BCS theory. However the pressure required for reaching pure metallic hydrogen is too high to achieve at the moment. Hence scientists turned to polyhydrides wherein the chemical precompression provided by additional elements offers the chance to metallize a hydrogen framework at moderate or relatively low pressure. Since experimental discovery of superconductivity in sulfur hydrides, several polyhydrides such as LaH₁₀ are experimentally reported to be superconductors. Although alkali earth superhydrides are theoretically predicted to be superconductivity, but until now there is no experimental observations of superconductivity. "Alkaline earth elements, as the neighboring group where hydrogen is located on the periodic table, offer one of best recipes for forming superconducting polyhydrides that to a great extent resemble to metallic hydrogen. They are chemically similar in terms of valence electron configurations." commented Changqing Jin.

It is a big challenging in terms of materials process at extreme conditions. One needs to fabricate calcium polyhydrides at very high pressure high temperature first before measuring their superconductivity at high pressures. The advanced techniques to integrate in situ high pressure high temperature synthesis together with extreme condition characterizations developed by Jin team play important role in the discovery. The team synthesized the materials at 160~190 GPa and ~2000 K using diamond anvil cell combined with in situ laser heating technique. The superconductivity was evidenced through in situ high pressure electric conductance measurements in an applied magnetic field for the sample quenched from high temperature while maintained at high pressures. The calcium polyhydrides became superconductive with critical transition temperature above 210K at 160 GPa. It is the third polyhydride superconductor with Tc above 200K, in addition to sulfur hydride & lanthanum hydride. The upper critical field Hc2 was estimated to be ~268 T while the Ginzburg Landau coherent length was calculated to be ~11 Å. Both high Tc & high Hc2 indicate the prospects of the calcium polyhydrides superconductors for potential applications. The in situ synchrotron x ray diffraction measurements suggested that the synthesized calcium hydrides are composed of CaH₆. "But there may also exist other superconducting calcium hydrides with different hydrogen contents in the synthesized samples as indicated from the multistep superconducting transitions” commented coauthor Xin He at IOPCAS. "We are trying to stabilize the high T_c phases at lower pressure or near ambient condition by introducing chemical pressure.”commented coauthor Xiancheng Wang who is a professor at IOPCAS.

The study entitled "Superconductivity above 200 K discovered in superhydrides of calcium" was published on Nature Communications 13, 2863(2022). The study was supported by the National Science Foundation & the Ministry of Science and Technology of China, as well as Chinese Academy of Sciences.

Address:

Institute of Physics, Chinese Academy of Sciences(IOPCAS)

Changqing JIN (Email: Jin@iphy.ac.cn)

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