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

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 × 2 × 1 one for BaTi2As2O, which has not yet been explained successfully so far, being different from tetragonal for BaTi2Sb2O and BaTi2Bi2O. The electronic structure analysis can naturally explain experimental observations about CDW including most latest ones without any cramped unconventional mechanisms.

Titanium Pnictide Oxide Superconductors

“…The T c of BaTi 2 Sb 2 O has been estimated by Subedi and Nakano et al using the Allen-Dynes formula, and found to be 2.7 K and 2.30 K, respectively [43,47]. The estimated T c is almost consistent with the experimental value (T c = 1.2 K), suggesting BaTi 2 Sb 2 O is a conventional BCS-type superconductor.…”

“…The three-dimensional hole sheet located around the R point is derived from the mixture of three d orbitals, where the sheet compensates other two electron sheets reflecting that an even electron count in BaTi2Sb2O. Consistent results for the FS of BaTi2Sb2O were also obtained in other theoretical studies [41][42][43]. The electronic structure for BaTi2Bi2O have been obtained by Suetin et al, [44].…”

“…However, several phenomena have not been addressed: the mechanism of superconductivity in BaTi2Bi2O, large MR observed below TDW, and the enhancement of magnetic fluctuations at around TDW observed in 121/123 Sb-NQR measurements. Nakaoka et al recently predicted the formation of orbital ordering with q = (0, 0, 0), i.e., intra-unitcell orbital order, which may account the magnetic fluctuation around TDW [43].…”

“…In theoretical studies, both SDW and CDW instability have been suggested: the SDW instability is predicted by Singh and Wang et al while CDW instability is predicted by Subedi and Nakano et al [40,41,43,47]. Singh found the Fermi surface nesting which lead to peaks in susceptibility, giving rise to SDW instability.…”

Ab Initio Search of Polymer Crystals with High Thermal Conductivity

“…[8,9] The predictions were, however, found to be seriously depending on the choice of empirical force fields [10]. Ab initio approaches [11,12] are expected to exclude such ambiguity, and are getting to be feasible realized by the implementations of phonon analysis especially those beyond the harmonic approximation [13]. Such approaches have been applied to inorganic crystalline materials such as semiconductors, [13][14][15] achieving the predictions fairly coinciding with experiments over the range, 1-10 2 W/mK.…”