X-ray diffraction study of thin-film samples of the Dirac semimetal SrMnSb₂

Dirac semimetals have electronic bands that cross at a node, called the Dirac point. Some Dirac semimetals, such as SrMnSb₂, have an energy gap at the node. It is desirable to be able to rapidly and reversibly open and close this gap. Theory predicts that coherent vibration of the crystal lattice could modify the 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. To learn whether these films had a bulk-like crystal structure, we used X-ray diffraction, which measures the atomic positions. Although the substrate is expected to provide compressive strain, surprisingly one sample has regions that are “over-compressed” (smaller lattice constant than the substrate) and other “over-relaxed” regions, larger even than bulk crystals. Another sample is mostly bulk-like. Both have unit cells doubled along the growth direction. The bulk-like properties of the second sample make such films suitable for future ultrafast studies like the phonon’s dynamics and its effect on the material’s Dirac electrons.