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Anomalous excitons and screenings unveiling strong electronic correlations in SrTi$_{1-x}$Nb$_x$O$_3$, 0$\leq$x$\leq$0.005

Electron-electron (e-e) and electron-hole (e-h) interactions are often associated with many exotic phenomena in correlated electron systems. Here, we report an observation of anomalous excitons at 3.75 , 4.67 and 6.11 eV at 4.2 K in bulk-SrTiO$_3$. Fully supported by ab initio GW Bethe-Salpeter equation calculations, these excitons are due to surprisingly strong e-h and e-e interactions with different characters: 4.67 and 6.11 eV are resonant excitons and 3.75 eV is a bound Wannier-like exciton with an unexpectedly higher level of delocalization. Measurements and calculations on SrTi$_{1-x}$Nb$_x$O$_3$ for 0.0001$\leq$x$\leq$0.005 further show that energy and spectral-weight of the excitonic peaks vary as a function of electron doping (x) and temperature, which are attributed to screening effects. Our results show the importance of e-h and e-e interactions yielding to anomalous excitons and thus bring out a new fundamental perspective in SrTiO$_3$.