Enhanced Topological Surface State Mediated Transport Elevates Thermoelectric Performance in SnSb₂Te₄ Quantum Material

The discovery of topological quantum materials harbouring Dirac-like massless surface states with high charge carrier mobility presents an exciting opportunity for unlocking superior thermoelectric (TE) performance, contingent upon accessing the unique properties of non-trivial topological surface states (TSS). However, harnessing these exotic TSS properties necessitates precise positioning of the Fermi level (E_F) within the insulating bulk band gap. Unfortunately, inherent bulk defects often result in the E_F being submerged deep into the bulk bands, rendering the contribution of TSS to electronic transport negligible. Herein, to address this challenge, we have devised a novel strategy to augment TSS-mediated electronic transport in a topological insulator, SnSb₂Te₄, by fine-tuning the E_F within the valence band through iodine (I) doping. Through extensive investigation of low-temperature electronic and magneto-transport, we have successfully demonstrated the systematic access to TSS upon I doping, unveiling fascinating quantum diffusive transport phenomena. Our findings reveal a gradual enhancement in the phase coherence length originating from TSS-mediated weak anti-localization, accompanied by a simultaneous reduction in bulk-state-dominated electron-electron interactions upon I doping, significantly elevating carrier mobility. Moreover, while aliovalent doping of I^- at the Te^2-site orchestrates an optimized <i>p</i>-type carrier concentration, leading to an amplified Seebeck coefficient, the introduction of I doping-induced point defects disrupts phonon propagation in the inherently low thermally conductive SnSb₂Te₄. As a result, we achieved a promising thermoelectric figure of merit zT~0.55 in I-doped SnSb₂Te₄.