Temperature-dependent spectroscopic ellipsometry study of exciton-phonon coupling in bulk ReSe2

초록

We investigate the in-plane optical anisotropy and exciton-phonon interactions in bulk ReSe2 using temperature-dependent spectroscopic ellipsometry from 10 K to 302 K. Optical conductivity spectra measured at selected polarization angles relative to the Re-Re chain reveal strongly anisotropic excitonic absorption. The dominant X2 and X4 excitons exhibit pronounced Fano lineshapes, indicating interference between the discrete excitons and a sub-gap continuum. The temperature evolution of the exciton energies follows a Bose-Einstein model with an average phonon energy of about 15 meV, consistent with the in-plane optical phonons of ReSe2. The below-gap absorption shows an exponential, Urbach-like tail, but its anomalously large characteristic energy of about 550 meV and weak temperature dependence are incompatible with a phonon-induced Urbach tail. Band-gap extraction from the absorption tail of ReSe2 therefore requires particular caution. Together, these results reveal how strong exciton-phonon coupling shapes the excitonic optical response of this quasi-one-dimensional van der Waals semiconductor.

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