Superconductors with high transition temperature (high- T c SCs) are long-sought targets in the condensed matter physics and materials communities because of significant scientific and application values. Since the discovery of superconductivity in mercury more than one hundred years ago, only a handful of systems show T c higher than 30 K.
Scientists at the ShanghaiTech University and their collaborators at Renmin University of China discovered superconductivity up to 32 K in MoB 2 under pressure, which is the highest T c in transition-metal borides up to now. Their findings, published online Feb. 14 in the journal National Science Review , shed light on the exploration of high- T c superconductors in transition metal borides.
Molybdenum diboride (MoB 2 ) is unique among the MB 2 family since it is the only material that has two structural forms, α -MoB 2 phase (AlB 2 -type) and β -MoB 2 phase (CaSi 2 -type). Synchrotron X-ray diffraction (XRD) measurements indicate that β -MoB 2 transforms to α -MoB 2 at around 65 GPa, which possesses the same crystal structure as MgB 2 . “A question arises naturally: is it possible to achieve superconductivity in MoB 2 under high pressure?” said Dr. Yanpeng Qi, an assistant professor of School of Physical Science and Technology at ShanghaiTech University and one of the corresponding authors. “So, we carried out the in-situ high pressure electrical transport measurements. Superconductivity is observed at 21.7 GPa and T c increases with pressure. Beyond the critical pressure ( P c = 70 GPa) where the structural phase transition, the growth of T c slows down and the maximum T c of 32.4 K is attained at P = 109.7 GPa, which is the highest pressure we exert on the sample.”.
Further theoretical calculations suggest that although the compressed MoB 2 has a same structure and comparable T c with MgB 2 , the superconducting mechanism of the former is completely different from that of the latter: the d -electrons and phonon modes of transition metal Mo atoms play utterly important roles in the emergence of superconductivity in contrast to the dominance of p -electrons and phonon modes of B atoms in the superconductivity of MgB 2 . The results shown in this work highlight the roles of transitional metals in the superconductivity in borides, which is underestimated in borides before. Their discovery of superconductivity of MoB 2 with rather high T c could also sheds light on the exploration of high- T c superconductivity in system with light elements considering the synergic effects of light element with high frequency phonon modes and strongly correlated electron in transition metals.
This work got great support from National Key R&D Program of China, the National Natural Science Foundation of China, the Beijing Natural Science Foundation, the CAS Interdisciplinary Innovation Team, the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China.
National Science Review