Study of high-temperature superconductivity in metal polyhydrides (CV)The search for high-temperature superconductivity is one of the most challenging tasks of condensed matter physics and materials science [Ref. 1]. An appealing idea dating back to the early 1960s was that metallic hydrogen should be a high-temperature superconductor [Ref. 2]. Due to the small mass of hydrogen, phonon frequencies in metallic hydrogen would be very high, of the order of several thousand kelvins (K) [Ref. 3], and the critical temperature of superconductivity (TC) can reach 300-400 K. However, extremely high pressures are needed for the transition of hydrogen from the molecular insulating phase into the metallic state [Ref. 4]. For this reason, the researchers have recently started to explore the possibility of inducing the superconducting state by adding other elements to hydrogen resulting in formation of new chemical compounds – polyhydrides, and strong reduction of the metallization pressure while maintaining high TC.Since 2018, Dmitrii Semenok has been actively involved in experimental and theoretical studies of the structure and properties of metal polyhydrides in collaboration with Prof. Artem Oganov, Dr. Ivan Troyan and Dr. Xiaoli Huang (Jilin University, China). In 2018, Dmitrii published 3 theoretical studies on the formation and superconducting properties of the thorium ThH10 (Appl. Mater. Interfaces 2018, ), actinium AcH10, AcH12, AcH16 (JPCL 2018, ), and iron FeH5 (JPCC 2018, ) polyhydrides. This computational project was the prologue for the subsequent experimental study of Th-H system (Materials Today 2020, ), which revealed a unique high-temperature superconductivity in the two radioactive polyhydrides: cubic ThH10 and hexagonal ThH9. The investigation of superconductivity in actinium hydrides made it possible to establish the distribution of superconductivity in all metal polyhydrides on the basis of knowledge of their electronic structure and position in the Periodic Table (COSSMS 2020, ).During 2019-2020, Dmitrii, together with Chinese and Russian colleagues, carried out large-scale studies of metal superhydrides in the Th-H, Ba-H, Pr-H, Nd-H and Eu-H systems. In the praseodymium-hydrogen system, cubic and hexagonal superhydrides-polymorphs PrH9 were found (Science Advances 2020, ). In the neodymium-hydrogen and europium-hydrogen systems we discovered hexagonal NdH9 (JACS 2020, ) and EuH9 (JPCL 2020, ), probably, the first examples of magnetic superhydrides. In the Ba-H system, Dmitrii and colleagues from Jilin University, found the most hydrogen-rich polyhydride BaH12 (Nature Communications 2020, ) studied so far, the hydrogen content of which reaches 92 at. %. This compound serves as a link between molecular hydrogen and superconducting metal hydrides with an atomic H-sublattice. It simultaneously contains molecular hydrogen and is a superconducting metal with a TC of about 20 K.In 2020-2021, Dmitrii Semenok studied cerium hydrides CeH9, CeH10 (PRL 2021, ). The purpose of this project was to search for stable superconducting hydrides in diamond cells at the lowest pressure with the highest critical temperature. As a result of experiments, it was found that CeH9 goes into a superconducting state below 70 K at a pressure of about 80-85 GPa. Theoretical studies in 2018-2019 demonstrated [Ref. 5] that yttrium hexahydride (YH6) should be a room superconductor with TC > 270 K. However, it was not possible to synthesize this compound for a long time. In the work of 2021 (Advanced Materials 2021, ), Dmitrii and his Russian colleagues, were able to synthesize this compound and showed that its critical temperature is significantly lower (~ 220 K) than the predicted one. This result had a serious impact on the prediction of new superconductors, forcing theorists more carefully estimate the TC in polyhydrides. As the hopes for an increase in the TС in binary metal polyhydrides were fading away, researchers turned their attention to ternary systems. And here the most promising were ternary lanthanum-yttrium polyhydrides. Dmitrii’s study of this system (Materials Today 2021, ) was the first work on ternary metal superhydrides in chemistry of compressed hydrides. In addition to the very high critical temperature (253 K) in (La,Y)H10, it was figured out that the introduction of Y does not change the structure of LaH10, but leads to the formation of a solid solution in the mixed La-Y sublattice. Ternary polyhydrides are promising for achieving the critical temperature close to the room temperature due to extremely strong electron-phonon coupling in the hydrogen sublattice. In the near future, these compounds can be used to design low-current superconducting electronics on diamond: SQUID magnetometers, magnetic memory elements, quantum qubits and single-photon detectors.The total number of Dmitrii’s publications is 21, which were cited more than 680 times in 2018-2021.
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