Electronic structure of the titanium-based oxypnictide superconductor Ba$_{\mathrm{0.95}}$Na$_{\mathrm{0.05}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O and direct observation of its charge density wave order

The unconventional superconducting ground state usually emerges in proximity to a spin or charge ordering state, such as that in cuprates, iron-based superconductors and layered chalcogenides. This unique character offers a platform for searching unconventional superconductivity in analogous layered compounds. Recently, superconductivity has been achieved in Ba$_{\mathrm{1x}}$Na$_{\mathrm{x}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O with maximum Tc at 5.5 K, which makes this material more interesting. Here we perform high resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy studies on the titanium-based oxypnictide superconductor Ba$_{\mathrm{0.95}}$Na$_{\mathrm{0.05}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O. The electronic structure shows both multi-orbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surface is well nested along the ($\pi $,$\pi )$ direction, which might probably be the driving force of the CDW transition. This is further proved by the scanning tunneling microscopy result, which directly observed a CDW wave vector at ($\pi $,$\pi )$ direction. However, due to the weak CDW coupling, we didn't observe the CDW gap here. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in Ba$_{\mathrm{0.95}}$Na$_{\mathrm{0.05}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O