  Despite having the simplest atomic structure, bulk FeSe has an observed
electronic structure with the largest deviation from the band theory
predictions among all Fe-based superconductors and exhibits a low temperature
nematic electronic state without intervening magnetic order. We show that the
Fe-Fe interatomic Coulomb repulsion $V$ offers a natural explanation for the
puzzling electron correlation effects in FeSe superconductors. It produces a
strongly renormalized low-energy band structure where the van Hove singularity
sits remarkably close to Fermi level in the high-temperature electron liquid
phase as observed experimentally. This proximity enables the quantum
fluctuations in $V$ to induce a rotational symmetry breaking electronic bond
order in the $d$-wave channel. We argue that this emergent low-temperature
$d$-wave bond nematic state, different from the commonly discussed
ferro-orbital order and spin-nematicity, has been observed recently by several
angle resolved photoemission experiments detecting the lifting of the band
degeneracies at high symmetry points in the Brillouin zone. We present a
symmetry analysis of the space group and identify the hidden antiunitary
$T$-symmetry that protects the band degeneracy and the electronic
order/interaction that can break the symmetry and lift the degeneracy. We show
that the $d$-wave nematic bond order, together with the spin-orbit coupling,
provide a unique explanation of the temperature dependence, momentum space
anisotropy, and domain effects observed experimentally. We discuss the
implications of our findings on the structural transition, the absence of
magnetic order, and the intricate competition between nematicity and
superconductivity in FeSe superconductors.
