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

TL;DR

This work addresses how mechanical strain can reversibly control spin-dependent thermoelectric transport in a 2D magnetic semiconductor. Using first-principles density functional theory plus Boltzmann transport calculations, the authors show that biaxial strain drives a semiconductor-to-half-metal transition in ferromagnetic NiI2, mediated by spin-selective closure of the spin-down gap and robust FM order. A striking result is the giant, non-monotonic violation of the Wiedemann-Franz law, with the Lorenz number reaching up to $7.17 L_0$ near electronic crossover regions, attributable to strain-sensitive Ni-d and I-p hybridization and spin-polarized channels. These findings highlight NiI2 as a versatile platform for mechanically gated spin-caloritronic devices and non-Fermi-liquid transport phenomena in low-dimensional systems, enabling decoupled control of heat and charge flow for energy-efficient technologies.

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.

Figures (7)

• Figure 1: Schematic representation of monolayer NiI$_2$ in the ferromagnetic configuration. The rectangular supercell has lattice parameters $a = 6.88\,\text{\AA}$ and $b = 3.97\,\text{\AA}$, and contains four iodine atoms and two nickel atoms. The nearest-neighbor Ni--Ni distance is 3.9 Å, and the layer thickness, defined by the vertical I--I separation, is 3.1 Å.
• Figure 2: Electronic band structures (left panels), Seebeck coefficients $S$ (middle panels), and electrical conductivities $\sigma/\tau$ (right panels) for the ferromagnetic (top row) and antiferromagnetic (bottom row) configurations of monolayer NiI$_2$. Energies are referenced to the Fermi level, set at 0 eV. Shaded regions indicate the occupied states. Blue and red curves represent the spin-up and spin-down channels, respectively. In the right panels, the gray line denotes the total electrical conductivity, while the colored lines correspond to the spin-resolved contributions.
• Figure 3: Spin-projected local density of states (DOS) for NiI$_2$ in both ferromagnetic (top panel) and antiferromagnetic (bottom panel) configurations under different strain conditions ($-7.5\%$, $0\%$, $+7.5\%$). The plots display the partial contributions from Ni (blue) and I (orange) atoms, alongside the total density of states (black), with the energy axis referenced to the Fermi level at 0 eV. Positive DOS values represent the spin-up projection, while negative values correspond to spin-down.
• Figure 4: Relative energies of the ferromagnetic (black lines) and antiferromagnetic (orange lines) configurations of monolayer NiI$_2$ as a function of biaxial strain (top panel), referenced to the ferromagnetic ground-state energy at 0% strain. The bottom panel shows the evolution of the electronic band gap for both magnetic phases under strain. The inset displays the spin-resolved band gap components for the FM configuration, with blue and red lines indicating the spin-up and spin-down channels, respectively.
• Figure 5: Seebeck coefficient $S$ (top panel) and electrical conductivity $\sigma/\tau$ (bottom panel) as functions of energy relative to the Fermi level, $E - E_F$, for the ferromagnetic configuration of monolayer NiI$_2$ under various biaxial strain values ranging from $-7.5\%$ to $+7.5\%$, calculated at a fixed temperature of $300\,$K.