  Nematic phase intertwines closely with high-Tc superconductivity in
iron-based superconductors. Its mechanism, which is closely related to the
pairing mechanism of superconductivity, still remains controversial.
Comprehensive characterization of how the electronic state reconstructs in the
nematic phase is thus crucial. However, most experiments focus only on the
reconstruction of band dispersions. Another important characteristic of
electronic state, the spectral weight, has not been studied in details so far.
Here, we studied the spectral weight transfer in the nematic phase of
FeSe$_{0.9}$S$_{0.1}$ using angle-resolved photoemission spectroscopy and
in-situ detwinning technique. There are two elliptical electron pockets
overlapping with each other orthogonally at the Brillouin zone corner. We found
that, upon cooling, one electron pocket loses spectral weight and fades away,
while the other electron pocket gains spectral weight and becomes pronounced.
Our results show that the symmetry breaking of electronic state is manifested
by not only the anisotropic band dispersion but also the band-selective
modulation of spectral weight. Our observation completes our understanding of
the nematic electronic state, and put strong constraints on the theoretical
models. It further provide crucial clues to understand the gap anisotropy and
orbital-selective pairing in iron-selenide superconductors.
