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Abstract
Na2Ti2Sb2O is a parent compound of the titanium-based oxypnictide superconductors. It possesses a phase transition at Ts = 115 K whose nature is still unclear. Here we report a scanning tunnelling microscopy/spectroscopy (STM/STS) study on a Na2Ti2Sb2O single crystal. We observe a lattice distortion on the Na-terminated surface below Ts, which displays a 2 × 2 modulation (superstructure). The modulation vectors q = (±0.5, 0) and (0, ±0.5) match well with the Fermi surface nesting vectors observed by angle resolved photoemission spectroscopy (ARPES), and also agree with recent X-ray diffraction. A partially opened gap-like feature is present in STS near EF. This evidences that the lattice distortion is driven by commensurate charge density wave order. Our results suggest the electron–phonon interaction is strong in Na2Ti2Sb2O and could be responsible for the superconductivity in titanium-based oxypnictides.
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