Coupled commensurate charge density wave and lattice distortion inNa2Ti2Pn2O(Pn=As,

Abstract

We report single crystal X-ray diffraction measurements on Na2Ti2P n2O (P n = 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, q1 = (1/2, 0, l) and q2 = (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 (ARPES) measurements, which show band folding and back bending consistent with a density wave with the same wave vectors q1 and q2 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 phononassisted charge density wave order in Na2Ti2P n2O and fully characterise a proximate ordered phase that could compete with superconductivity in doped BaTi2Sb2O.

Investigation into Structural Phase Transitions in Layered Titanium-Oxypnictides by a Computational Phonon Analysis

“…We therefore took these for further lattice relaxations from the original I 4/ mmm structure along the mode displacements to get the C 2/ m (monoclinic, No. 12) superstructure (Figures a and a), being consistent with the experiments . The results of geometry optimizations for the undistorted ( I 4/ mmm ) and distorted ( C 2/ m ) structures are shown in Tables and .…”

Theory for doping trends in titanium oxypnictide superconductors

“…S3 for the other 3d weights). The BaTi 2 Sb 2 O Fermi surface is in excellent agreement with angle resolved photoemission (ARPES) experiments [39,40] (see Ref. 32, Fig.…”

Emergence of a Pressure-Driven Superconducting Phase in Ba0.77Na0.23Ti2Sb2O

“…Thus, the phase diagrams of T c – x for Ba 1– x Na x Ti 2 Sb 2 O, Ba 1– x K x Ti 2 Sb 2 O, and Ba 1– x Rb x Ti 2 Sb 2 O are similar; these materials at ambient pressure assumed the same type of crystal structure. − The appearance of superconductivity coexisted with the CDW/SDW ordered state may be characteristic of unconventional superconductors, such as heavy fermions, ion pnictides, and two-dimensional layered chalcogenides . In addition, the CDW transition was confirmed for Na 2 Ti 2 Sb 2 O using scanning tunneling microscopy and X-ray diffraction because the structural transition was unambiguously observed. Moreover, only one paper suggested the CDW ordered state for Ba 0.95 Na 0.05 Ti 2 Sb 2 O, but it was not confirmed for other titanium pnictide oxides; that is, the term “CDW/SDW transition’’ may be still valid for titanium pnictide oxides including Ba 1– x Na x Ti 2 Sb 2 O.…”