  Excitons, quasiparticles of electrons and holes bound by Coulombic
attraction, are created transiently by light and play an important role in
optoelectronics, photovoltaics and photosynthesis. While they are also
predicted to form spontaneously in a small gap semiconductor or a semimetal,
leading to a Bose-Einstein condensate at low temperature, their material
realization has been elusive without any direct evidence. Here we detect the
direct photoemission signal from spontaneously formed excitons in a debated
excitonic insulator candidate Ta2NiSe5. Our symmetry-selective angle-resolved
photoemission spectroscopy reveals a characteristic excitonic feature above the
transition temperature, which provides detailed properties of excitons such as
anisotropic Bohr radius. The present result evidences so called preformed
excitons and guarantees the excitonic insulator nature of Ta2NiSe5 at low
temperature. Direct photoemission can be an important tool to characterize
steady-state excitons.
