Phonon dispersions and Fermi surfaces nesting explaining the variety of charge ordering in titanium-oxypnictides superconductors

TL;DR

This paper explains the variety of charge ordering in titanium-oxypnictide superconductors through phonon dispersions and Fermi surface nesting, resolving previous experimental-theoretical discrepancies without unconventional mechanisms.

Contribution

It provides a new theoretical framework linking phonon dispersions and Fermi surface nesting to charge order structures, clarifying experimental observations.

Findings

Different superlattice structures explained by nesting vectors.

Orthorhombic $2 imes 2 imes 1$ structure for BaTi$_2$As$_2$O identified.

Electronic structure analysis aligns with experimental CDW observations.

Abstract

There has been a puzzle between experiments and theoretical predictions on the charge ordering of layered titanium-oxypnictides superconductors. Unconventional mechanisms to explain this discrepancy have been argued so far, even affecting the understanding of superconductivity on the compound. We provide a new theoretical prediction, by which the discrepancy itself is resolved without any complicated unconventional explanation. Phonon dispersions and changes of nesting vectors in Fermi surfaces are clarified to lead to the variety of superlattice structures even for the common crystal structures when without CDW, including orthorhombic $2 \times 2 \times 1$ one for BaTi$_2$As$_2$O, which has not yet been explained successfully so far, being different from tetragonal $\sqrt{2} \times \sqrt{2} \times 1$ for BaTi$_2$Sb$_2$O and BaTi$_2$Bi$_2$O. The electronic structure analysis can naturally explain experimental observations about CDW including most latest ones without any cramped unconventional mechanisms.