Intra-unit-cell nematic charge order in the titanium-oxypnictide family   of superconductors
  Understanding the role played by broken symmetry states such as charge, spin, and orbital orders in the mechanism of emergent properties such as high-temperature superconductivity (HTSC) is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here we show that the recently discovered BaTi$_2$Sb$_2$O superconductor and its "parent" compound BaTi$_2$As$_2$O form a symmetry-breaking nematic ground state that can be naturally explained as an intra-unit-cell charge order with $d$-wave symmetry, pointing to the ubiquity of the phenomenon. These findings, together with the key structural features in these materials being intermediate between the cuprate and iron-pnictide HTSC materials, render the titanium oxypnictides an important new material system to understand the nature of nematic order and its relationship to superconductivity. 
Benjamin A. Frandsen, Emil S. Bozin, Hefei Hu, Yimei Zhu, Yasumasa   Nozaki, Hiroshi Kageyama, Yasutomo J. Uemura, Wei-Guo Y...
cond-mat.supr-con cond-mat.str-el