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

TL;DR

This study reveals that monolayer NiI$_2$ can switch from a semiconductor to a half-metal under strain, causing a giant violation of the Wiedemann-Franz law and enabling advanced spin-caloritronic applications.

Contribution

We demonstrate a strain-driven semiconductor-to-half-metal transition in monolayer NiI$_2$, accompanied by a significant Wiedemann-Franz law violation, advancing the understanding of strain-tunable spin-dependent thermoelectricity.

Findings

Strain induces a semiconductor-to-half-metal transition in NiI$_2$.

Giant, non-monotonic violation of the Wiedemann-Franz law observed.

Strain-sensitive hybridization leads to spin-polarized transport channels.

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.