Uncovering Exotic Topological Quantum States in Pure and Magnetically-Doped Pyrite OsS₂

Discovering topological quantum materials that combine nontrivial topology with large bandgaps and experimentally accessible signatures remains a central pursuit in condensed matter physics. We demonstrate, through first-principles calculations and tight-binding modeling, that pyrite-type osmium disulfide (OsS₂) is a fragile topological insulator (FTI) characterized by an exceptionally large direct bandgap of 602 meV, placing it among the highest-gap FTIs reported. Contrary to typical fragile phases that lack prominent boundary states, OsS₂ features distinct, symmetry-protected gapless surface states across multiple cleavage planes, enabling direct experimental verification via angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). Additionally, the presence of van Hove singularities in the electronic structure further distinguishes OsS₂ as a unique 3D quantum material. By systematically varying the magnetic moment from 0 to 4 µB through substitutional doping with Pd, Fe, Ni, and Co, we map out a rich topological phase diagram. This diagram reveals transitions from the FTI phase to a strong topological insulator (STI), a topological semimetal, a three-dimensional quantum anomalous Hall (3D-QAH) insulator, and ultimately a trivial magnetic semiconductor, each phase corresponding to a specific magnetic moment. The structural similarity and near-identical lattice constants of PdS₂, FeS₂, NiS₂, and CoS₂ with OsS₂, all of which crystallize in the cubic pyrite phase, suggest the practical realization of these phases. Collectively, our findings establish OsS₂ as a promising platform for exploring fragile topology, tunable quantum phase transitions, and novel boundary phenomena within a single material system.