High field optical response of η-Mo₄O₁₁

We report the polarized optical reflectance of quasi-two-dimensional η-Mo₄O₁₁ in a perpendicular magnetic field up to 30 T at 4.2 K. The in-field measurements are done on the conducting bc plane covering the 6000-26000 cm^-1 energy range. The absolute reflectance shows a free carrier response in the near infrared and two electronic transitions in the optical regime: an intraband transition (d→d) near 12000 cm^-1 and a charge transfer (oxygen p→molybdenum d) near 30000 cm^-1. In the magnetic field, no change in the normalized reflectance is observed in the intraband transition range through the 20 T Landau transition, whereas a notable deviation from unity is observed on the low-energy side of the charge transfer transition along both b and c polarizations. We discuss the results in terms of the effect of the magnetic field on the electronic structure and quantum limit response of η-MO₄O₁₁.