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.

Substrate mediated nitridation of niobium into superconducting Nb2N thin films for phase slip study

“…the Nb 2 N phase. The wide transition in region-I mainly indicates the influence of inhomogeneity including the chemical impurities like the presence of other nitride phases as evident in the XRD spectra 12 . Further, region-I features the widest transition for the thinnest sample B3 and the width gets reduced with increasing the thickness.…”

“…However, no other member from this TMN family has been explored at the equal footing in the field of superconductivity. Recently, hard hexagonal ε-NbN has been shown to possess superconducting properties with T c ~ 11 K 12 . Among the several stable phases in the niobium-nitrogen based compounds, Nb 2 N has recently attracted a growing interests towards its epitaxial growth on SiC substrate 13 .…”

Room temperature deposition of superconducting niobium nitride films by ion beam assisted sputtering

“…Recently the superconductivity with high critical temperature in hard -NbN grown at high pressure and high temperatures was discovered. [6] While there exist pro- * Corresponding author: novosad@anl.gov cesses capable of achieving high-T c NbN films deposited at room temperature, such as incorporation of methane gas with RF diode sputtering [7], they lead to films with a granular or columnar void structure, resulting in normal state resistivity well above 10 4 µΩ · cm [2,7]. Use of substrate biasing during deposition procedure has also been demonstrated to be a viable method of room temperature deposition, but deposited films show presence of lower-T c tetragonal phase of NbN, which leads to suppression of overall film critical temperature.…”

Physical Properties and Electronic Structure of Cr2B Under Pressure

“…Superhard metals show excellent performance, including the high hardness, wear resistance, good thermodynamic stability and metallic behavior, compared with traditional superhard materials. [ 1–9 ] The superhard metals can be synthesized by inserting strong covalent bonds of light element (LE), such as carbon, boron, nitrogen, into transition metal (TM) with a large number of valence electrons. Based on the classical thought, to obtain a high hardness, a straightforward way was to incorporate more light atoms into the transition metal to form strong covalent bonds.…”