X-ray diffraction of MBE films of the Dirac semimetal SrMnSb₂

SrMnSb₂ is a Dirac semimetal with a structural distortion that opens a gap at the Dirac point. Theory predicts that coherent vibration of the crystal lattice could open and close this gap in less than a picosecond, but experiments to explore this effect require thin-film samples that are larger and flatter than bulk crystals. We grew films of SrMnSb₂ on InAs by MBE, and measured their lattice constants by X-ray diffraction. Although the substrate provides 2% compressive strain, our measurements show that a few-layer film has regions with smaller lattice constant even than the substrate, and other regions larger even than bulk. A 30-nm-thick film, however, is mostly bulk-like. Both have unit cells doubled along the growth direction, indicative of the structural distortion. The bulk-like properties of the thicker sample make such films suitable for future ultrafast studies of phonons’ dynamics and their effect on the material’s Dirac electrons.