Deconstruction of the anisotropic magnetic interactions from spin-entangled optical excitations in van der Waals antiferromagnets

TL;DR

This work investigates how the antiferromagnetic order in two-dimensional van der Waals magnets MnPS3 and NiPS3 couples to sub-bandgap optical transitions. It combines self-consistent many-body perturbation theory (QS$GW$ and QS$G\hat{W}$) with exact-diagonalization DMFT to resolve spin-allowed versus spin-flip excitations and to determine the fundamental bandgaps from atom- and orbital-resolved contributions. The study identifies the X feature as a spin-entangled, on-site spin-flip dd transition localized on Mn or Ni, and maps the broader exciton landscape including intersite dd and dp/charge-transfer excitons, consistent with low-temperature PL/PLE measurements. By analyzing magneto-optical field dependences, the authors extract magnetic interaction parameters (g-factors, exchange J, and anisotropy D) and show distinct anisotropy-driven field responses in MnPS3 (uniaxial) and NiPS3 (biaxial). These insights provide a parameter-free framework for all-optical probing and potential manipulation of antiferromagnetic order in two-dimensional vdW magnets.

Abstract

Magneto-optical excitations in antiferromagnetic d systems can originate from a multiplicity of light-spin and spin-spin interactions, as the light and spin degrees of freedom can be entangled. This is exemplified in van der Waals systems with attendant strong anisotropy between in-plane and out-of-plane directions, such as MnPS3 and NiPS3 films studied here. The rich interplay between the magnetic ordering and sub-bandgap optical transitions poses a challenge to resolve the mechanisms driving spin-entangled optical transitions, as well as the single-particle bandgap itself. Here we employ a high-fidelity ab initio theory to find a realistic estimation of the bandgap by elucidating the atom- and orbital-resolved contributions to the fundamental sub-bands. We further demonstrate that the spin-entangled excitations, observable as photoluminescence and absorption resonances, originate from an on-site spin-flip transition confined to a magnetic atom (Mn or Ni). The evolution of the spin-flip transition

Figures (12)

• Figure 1: Schematic crystallographic and magnetic structure of (a) MnPS$_3$ and (b) NiPS$_3$, highlighting the spin orientations in the antiferromagnetic phase. Blue/gray/green/yellow spheres represent manganese/nickel/sulfur/phosphorus atoms. These figures are created using the VESTA software package Momma2011. The electronic band structure and atom-projected density of states for the bulk (c) MnPS$_3$ and (d) NiPS$_3$ calculated with the and QS${G\hat{W}}$ method. The Fermi level is set to zero.
• Figure 2: Low-temperature (5 K) PL and the PL excitation spectra (PLE) corresponding to the dominant PL feature of (a) MnPS$_3$ and (b) NiPS$_3$. The computed optical absorption spectra by $\mathrm{QSG\hat{W}}$ and DMFT methods are also shown in the corresponding plot.
• Figure 3: Low temperature (5 K) PL and transmission (T) spectra of (a) MnPS$_3$ and (b) NiPS$_3$ shown in a narrow energy range near the spin-flip resonance, labeled as X. The high-energy satellite peaks X$_\alpha$ and X$_\beta$ correspond to the phonon replica of X-transition, while X$_1$ peak corresponds to the exciton-magnon continuum coupled state. The schematic spin arrangements of the ground state, onsite spin-allowed, and spin-flip excited states of (c) MnPS$_3$ and (d) NiPS$_3$. No onsite spin-allowed transition is feasible in MnPS$_3$ due to half-filled spin arrangements.
• Figure 4: False color map of transmission of bulk MnPS$_3$ as a function of the (a) out-of-plane ($B \parallel c$ -axis) and (b) in-plane ($B \perp c$ -axis) magnetic field. False color map of PL of bulk NiPS$_3$ as a function of the (c) out-of-plane ($B \parallel c$ -axis) and (d) in-plane ($B \perp c$ -axis) magnetic field. Representative spectra at 5 T intervals are shown in the plot. The Green dashed line corresponds to the simulation of the splitting of the X-transition with low field approximation Dipankar2023 ($J \gg g\mu _BB$) while the white dashed line corresponds to the simulation in the high field limit following Eq. \ref{['eq:1']}. Black arrows show the schematic spin alignment of the two spin sub-lattices for the external magnetic field applied along the vertical axis (B(T) axis).
• Figure :