title: Discovery of Superconductivity in Hard Hexagonal ε-NbN.
abstract: Since the discovery of superconductivity in boron-doped diamond with a critical temperature (TC) near 4 K, great interest has been attracted in hard superconductors such as transition-metal nitrides and carbides. Here we report the new discovery of superconductivity in polycrystalline hexagonal ε-NbN synthesized at high pressure and high temperature. Direct magnetization and electrical resistivity measurements demonstrate that the superconductivity in bulk polycrystalline hexagonal ε-NbN is below ∼11.6 K, which is significantly higher than that for boron-doped diamond. The nature of superconductivity in hexagonal ε-NbN and the physical mechanism for the relatively lower TC have been addressed by the weaker bonding in the Nb-N network, the co-planarity of Nb-N layer as well as its relatively weaker electron-phonon coupling, as compared with the cubic δ-NbN counterpart. Moreover, the newly discovered ε-NbN superconductor remains stable at pressures up to ∼20 GPa and is significantly harder than cubic δ-NbN; it is as hard as sapphire, ultra-incompressible and has a high shear rigidity of 201 GPa to rival hard/superhard material γ-B (∼227 GPa). This exploration opens a new class of highly desirable materials combining the outstanding mechanical/elastic properties with superconductivity, which may be particularly attractive for its technological and engineering applications in extreme environments.
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, China.|Mineral Physics Institute, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: Yongtao
authors.initials: Y
authors.lastname: Zou
authors.affiliation: Department of Geosciences, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: Xintong
authors.initials: X
authors.lastname: Qi
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, N.Y. 11973, United States.
authors.forename: Cheng
authors.initials: C
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, China.
authors.forename: Shuailing
authors.initials: S
authors.lastname: Ma
authors.affiliation: School of Science, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
authors.affiliation: Mineral Physics Institute, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: Ying
authors.initials: Y
authors.lastname: Li
authors.affiliation: Department of Geosciences, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: Ting
authors.initials: T
authors.lastname: Chen
authors.affiliation: Department of Geosciences, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: Xuebing
authors.initials: X
authors.lastname: Wang
authors.affiliation: Mineral Physics Institute, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: Zhiqiang
authors.initials: Z
authors.lastname: Chen
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, N.Y. 11973, United States.|Department of Materials Science and Engineering, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: David
authors.initials: D
authors.lastname: Welch
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, China.
authors.forename: Pinwen
authors.initials: P
authors.lastname: Zhu
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, China.
authors.forename: Bingbing
authors.initials: B
authors.lastname: Liu
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, N.Y. 11973, United States.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Li
authors.affiliation: State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, China.
authors.forename: Tian
authors.initials: T
authors.lastname: Cui
authors.affiliation: Mineral Physics Institute, State University of New York, Stony Brook, N.Y. 11794, United States.
authors.forename: Baosheng
authors.initials: B
authors.lastname: Li