  Nematicity, defined as broken rotational symmetry, has recently been observed
in competing phases proximate to the superconducting phase in the cuprate high
temperature superconductors. Similarly, the new iron-based high temperature
superconductors exhibit a tetragonal to orthorhombic structural transition
(i.e. a broken C4 symmetry) that either precedes or is coincident with a
collinear spin density wave (SDW) transition in undoped parent compounds, and
superconductivity arises when both transitions are suppressed via doping.
Evidence for strong in-plane anisotropy in the SDW state in this family of
compounds has been reported by neutron scattering, scanning tunneling
microscopy, and transport measurements. Here we present an angle resolved
photoemission spectroscopy study of detwinned single crystals of a
representative family of electron-doped iron-arsenide superconductors,
Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via
application of in-plane uniaxial stress, enabling measurements of single domain
electronic structure in the orthorhombic state. At low temperatures, our
results clearly demonstrate an in-plane electronic anisotropy characterized by
a large energy splitting of two orthogonal bands with dominant dxz and dyz
character, which is consistent with anisotropy observed by other probes. For
compositions x>0, for which the structural transition (TS) precedes the
magnetic transition (TSDW), an anisotropic splitting is observed to develop
above TSDW, indicating that it is specifically associated with TS. For
unstressed crystals, the band splitting is observed close to TS, whereas for
stressed crystals the splitting is observed to considerably higher
temperatures, revealing the presence of a surprisingly large in-plane nematic
susceptibility in the electronic structure.
