Coupled commensurate charge density wave and lattice distortion in Na 2 Ti 2 P n 2 O ( P n = As , Sb ) determined by x-ray diffraction and angle-resolved photoemission spectroscopy

We report single-crystal x-ray-diffraction measurements on Na(2)Ti(2)Pn(2)O (Pn = As,Sb) which reveal a charge superstructure that appears below the density wave transitions previously observed in bulk data. From symmetry-constrained structure refinements we establish that the associated distortion mode can be described by two propagation vectors q(1) = (1/2,0,l) and q(2) = (0,1/2,l) with l = 0 (Sb) or l = 1/2 (As) and primarily involves in-plane displacements of the Ti atoms perpendicular to the Ti-O bonds. We also present angle-resolved photoemission spectroscopy measurements, which show band folding and backbending consistent with a density wave with the same wave-vectors q(1) and q(2) associated with Fermi-surface nesting, and muon-spin relaxation data, which show no indication of spin density wave order. The results provide direct evidence for phonon-assisted charge density wave order in Na(2)Ti(2)Pn(2)O and fully characterize a proximate ordered phase that could compete with superconductivity in doped BaTi2Sb2O.