Structural distortion and suppression of superconductivity in stoichiometricB1−MoNepitaxial thin films

Abstract

Molybdenum nitride films with the $\mathrm{NaCl}$ structure $(B1\text{\ensuremath{-}}\mathrm{MoN})$ were epitaxially grown on $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(001)$ and $\mathrm{MgO}(001)$ substrates at $973\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by pulsed laser deposition (PLD) under nitrogen radical irradiation. The highly crystalline epitaxial films enabled us to determine the three-dimensional cell parameters, which was motivated by theoretical calculations that $B1\text{\ensuremath{-}}\mathrm{MoN}$, a predicted superconductor, is elastically unstable against tetragonal and trigonal distortions. On $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(001)$, the $B1\text{\ensuremath{-}}\mathrm{MoN}$ phase (composition, ${\mathrm{Mo}}_{1}{\mathrm{N}}_{0.98}$) was grown with its (111) planes parallel to the substrate surface. X-ray diffraction analysis with a multiaxes diffractometer detected only a small trigonal lattice distortion [$a=0.4219(3)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $\ensuremath{\alpha}=89.28(5)\ifmmode^\circ\else\textdegree\fi{}$] with an expansion along the [111] direction perpendicular to the substrate surface. The film grown on $\mathrm{MgO}(001)$ had the ${\mathrm{MoN}}_{1.03}$ composition and showed a slight tetragonal distortion ($a=0.4213$ and $c=0.424\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) due to fitting to the $\mathrm{MgO}$ substrate lattice $(a=0.4213\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. These two stoichiometric films showed no superconductivity above $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A lower nitrogen content $({\mathrm{MoN}}_{0.86})$ film was obtained on $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(001)$ using a higher deposition rate. The corresponding film had a much smaller lattice constant $[a=0.4184(3)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}]$, and a similar distortion $[\ensuremath{\alpha}=89.41(5)\ifmmode^\circ\else\textdegree\fi{}]$. The $B1\text{\ensuremath{-}}{\mathrm{MoN}}_{0.86}$ film showed superconductivity with a transition temperature ${T}_{c}=4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The suppression of the superconductivity of the former stoichiometric phase can be interpreted in terms of the lattice expansion.