Strain-Induced Half-Metallicity and Giant Wiedemann-Franz Violation in Monolayer NiI$_2$

Abstract

Reversible control of spin-dependent thermoelectricity via mechanical strain provides a platform for next-generation energy harvesting and thermal logic circuits. Using first-principles and Boltzmann transport calculations, we demonstrate that monolayer NiI$_2$ undergoes a strain-driven semiconductor-to-half-metal transition, enabled by the selective closure of its spin-down band gap while preserving a robust ferromagnetic ground state. Remarkably, this transition is accompanied by a giant, non-monotonic violation of the Wiedemann-Franz law, with the Lorenz number enhanced up to $7.17\,L_0$. This anomaly arises from a strain-sensitive hybridization between Ni-$d$ and I-$p$ orbitals, leading to spin-polarized transport channels and decoupling of heat and charge currents. These properties make NiI$_2$ a promising candidate for mechanically gated spin-caloritronic devices and thermal logic elements, where reversible control of heat and spin flow is essential. Our findings position NiI$_2$ as a model system for exploring non-Fermi-liquid transport and for realizing strain-tunable, energy-efficient functionalities in low-dimensional platforms.