  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.
