Homes, Christopher2018-05-252018-05-252018-05https://hdl.handle.net/11299/197543Multiband materials display a wide variety of interesting transport phenomena, often with striking optical effects. Two examples are the colossal magnetoresistance in the semimetal WTe_2, and the emergence of superconductivity in the iron-based material FeTe_0.55Se_0.45 with a critical temperature (T_c) of ~14 K. The complex optical properties are determined from a Kramers-Kronig analysis of the reflectance, which is measured in the transition metal-chalcogenide (a-b) planes over a wide energy range. A poor metal at room temperature, WTe_2 undergoes a Lifschitz transition to become a perfectly compensated semimetal at low temperature, leading to the formation of a striking plasma edge in the far-infrared reflectance. By considering Drude components for the electron and hole pockets, and by examining both the real and imaginary parts of the optical conductivity, it can be demonstrated that one of the scattering rates collapses at low temperature [1]. Dirac and Weyl semimetals display very small scattering rates, and WTe_2 is thought to be a type-II Weyl semimetal. FeTe_0.55Se_0.45 is also a poor metal at room temperature, with a flat and almost frequency-independent optical conductivity. Just above T_c, a narrow Drude response emerges, superimposed on a broad, temperature-independent Drude component. Below T_c, dramatic changes in the in-plane reflectance signal the formation of multiple superconducting energy gaps which may be determined from the real part of the optical conductivity to be 2Δ_1 = 5.6 and 2Δ_2 = 11.2 meV on the broad and narrow bands, respectively. Interestingly, this material is simultaneously in both the clean and dirty limit [2]. *This work done in collaboration with A. Akrap, R. J. Cava, Y. M. Dai, and G. D. Gu. Supported by the Office of Science, U.S. Department of Energy, under Contract No. DE-SC0012704. [1] C. C. Homes, M. N. Ali, and R. J. Cava, Phys. Rev. B 92, 161109(R) (2015). [2] C. C. Homes, Y. M. Dai, J. S. Wen, Z. J. Xu, and G. D. Gu, Phys. Rev. B 91, 144503 (2015).enFTPICESND 2018Presentation