Lattice distortion and charge density wave in Na₂Ti₂Sb₂O revealed by scanning tunnelling microscopy

M. Q. RenState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China

Y. J. YanState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China

J. JiangState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China

S. Y. TanState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China

J. MiaoState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China

C. ChenState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China

Y. SongDepartment of Physics and Astronomy, Rice University, Houston, TX, USA

C. L. ZhangDepartment of Physics and Astronomy, Rice University, Houston, TX, USA

P. C. DaiDepartment of Physics and Astronomy, Rice University, Houston, TX, USA

B. P. XieState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China

T. ZhangState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China;Collaborative Innovation Center of Advanced Microstructures, Nanjing, China

D. L. FengState Key Laboratory of Surface Physics, Department of Physics and Advanced Materials Laboratory, Fudan University, Shanghai, China;Collaborative Innovation Center of Advanced Microstructures, Nanjing, ChinaCorrespondence[email protected]

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Abstract

Na₂Ti₂Sb₂O is a parent compound of the titanium-based oxypnictide superconductors. It possesses a phase transition at T_s = 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 T_s, 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 E_F. 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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Lattice distortion and charge density wave in Na₂Ti₂Sb₂O revealed by scanning tunnelling microscopy

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Lattice distortion and charge density wave in Na₂Ti₂Sb₂O revealed by scanning tunnelling microscopy