Macroscopic Transport Signatures of Alkali Metal Surface Doping in Quantum Materials

Carrier injection by surface deposition of alkali metals has become a staple technique in the investigation of doping effects in many materials. Owing to their extremely low ionization potentials, alkali metals act readily as electron donors and since the resulting free carriers are localized to the material surface this technique has traditionally been used in conjunction with surface sensitive probes of the electronic structure, namely ARPES and STM. Here we present the use of macroscopic electrical transport measurements, in conjunction with potassium surface dosing, to examine doping effects in bulk single crystals of Ba(Fe_1-xCo_x)₂As₂ and MoS₂ as well as thin film FeSe. This technique allows measurement of the effects of doping on macroscopic electrical properties while maintaining a high level of crystalline order in the doped structure. Additionally, it provides a promising route to studying the evolution of electronic properties with carrier concentration in materials less amenable to electrostatic gating techniques, namely superconductors with a metallic parent state.