Coupled commensurate charge density wave and lattice distortion in Na_2Ti_2Pn_2O (Pn = As,Sb) determined by x-ray diffraction and angle-resolved photoemission spectroscopy

摘要

We report single-crystal x-ray-diffraction measurements on Na_2Ti_2Pn_2O (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-0 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_2Ti_2Pn_2O and fully characterize a proximate ordered phase that could compete with superconductivity in doped BaTi_2Sb_2O.