Revealing multiple density wave orders in nonsuperconducting titanium oxypnictide Na2 Ti2 As2O

We report an optical spectroscopy study on single crystals of Na2Ti2As2O, a sister compound of the superconductor BaTi2Sb2O. The study unexpectedly reveals two density wave phase transitions. The first transition, at 320 K, results in the formation of a large energy gap and removes the majority of the Fermi surfaces. But the compound remains metallic with residual itinerant carriers. Below 42 K, another density wave phase transition with a smaller energy gap scale occurs and drives the compound into the semiconducting ground state. These experiments thus enable us to shed light on the complex electronic structure in titanium oxypnictides.