Infrared study of semimetal LaSb: coexistence of ultrahigh- and moderate-mobility carriers

Abstract We performed optical reflectivity measurements on the cubic pnictide semimetal LaSb. The optical conductivity, calculated from the reflectivity, shows that two Drude carriers with drastically different scattering rates coexist: one carrier (D1) with moderate scattering rate γ1 (= 130 cm−1) and the other (D2) with very low scattering rate γ2 (= 0.116 cm−1). The ratio between these two scattering rates is as large as γ1/γ2 = 1120, which stands in sharp contrast to other semimetals such as Bismuth (Bi), where γ1/γ2 = 2.9. The coexistence of two carriers, D1 and D2, with such drastically different scattering rates demonstrates that LaSb is an unconventional semimetal. To investigate the origins of the two carriers, D1 and D2, we calculate their optical spectral weights and compare them with those of the existing carriers in LaSb, namely, bulk electron, bulk hole, linear-dispersing band electron, and Dirac-like surface carrier. This quantitative analysis suggests that D1 arises from the semimetal band electron, while D2 originates from the semimetal band hole, respectively. The coexistence of the ultra-high mobility and moderate mobility carriers provides new insights on the electrical transport of this material including, importantly, the mechanism behind the extreme magneto resistance effect of LaSb.