Je-Geun Park

Driving specific lattice modes with ultrafast stimuli offers a dynamic route for tailoring material properties out of equilibrium. However, achieving dynamic coherent control of the nonequilibrium phases via resonant excitation of lattice coherences remains largely unexplored. In this study, we demonstrate coherent phononic control of a newly discovered, light-induced metastable magnetization in the van der Waals antiferromagnet FePS ₃ . By using a sequence of terahertz (THz) pulses, we modulate the magnetization amplitude at the frequencies of phonon coherences, whose infrared-active nature and symmetries are further revealed by polarization- and field-strength-dependent measurements. Furthermore, our two-dimensional THz spectroscopy, in tandem with first-principles numerical simulations, shows that these phonons nonlinearly displace a Raman-active phonon, which induces the metastable net magnetization. These findings not only clarify the microscopic mechanism underlying the metastable state in FePS ₃ but also establish vibrational coherences in solids as a powerful tool for ultrafast quantum phase control, enabling manipulation of material functionalities far from equilibrium.

Has attracted considerable attention, yet the origin of its intriguing magnetic behaviors remains elusive due to limited insight into its low‐energy anisotropy. Here, high‐field torque magnetometry measurements are performed, revealing a previously unrecognized anisotropic spin response that reflects interaction terms beyond the standard XY or XXZ models. In combination with relativistic density functional theory calculations and magnetic force response theory, an effective low‐energy spin Hamiltonian is constructed, elucidating not only the observed anomalous magnetic behavior but also other recent experiments. Remarkably, the magnetic dipole interaction and the associated spin‐flop transition play a crucial role in driving the deviation from two‐dimensional spin alignment, providing direct insight into the underlying anisotropic energy hierarchy. The generalized Hamiltonian can serve as the important fundamental ground for future study, and our approach adopted here will hopefully be relevant to many other related material systems.

We report the intrinsic thermal and magnetic properties of the low‐dimensional van der Waals (vdW) antiferromagnet NiPS ₃ and explore its emergent magnetic phases by controlling crystallographic twinning. Using nearly twin‐free crystals, we resolve intrinsic properties that are typically obscured by multidomain effects in bulk samples. Magnetization results reveal a highly anisotropic, sharp spin‐flop transition, confirming the high domain purity of our crystals. Furthermore, high‐precision thermodynamic and transport data reveal a broad fluctuation regime around the Néel temperature ( T _N = 157.5 K), with a heat capacity anomaly and a concurrent suppression of thermal conductivity. Field‐dependent thermal transport shows a small but distinct contribution from spin‐lattice coupling, as evidenced by the dip at the spin‐flop transition. We develop a theoretical model to explain these properties reported in this paper, with good agreement between experiment and theory. Our work establishes a definitive baseline for bulk properties of NiPS ₃ and demonstrates the feasibility of resolving intrinsic anisotropies by addressing crystallographic twinning in vdW magnets.

Two ultranarrow absorption peaks around 1.5 eV, which are widely believed to originate from a transition from a spin-orbital entangled triplet to a singlet state, in the two-dimensional van der Waals crystal NiPS_{3}, have attracted considerable attention because of their pronounced spin-dependent character. An interesting question is whether ultrahigh magnetic fields modify an interaction-driven hybridization between those two peaks. In this Letter we perform systematic magneto-optical measurements of NiPS_{3} in pulsed magnetic fields of up to 178 T and observe a pronounced mutual repulsion between the two sharp exciton peaks accompanied by a redistribution of oscillator strength, while the band edge shows no detectable field-induced shift within our experimental resolution. We construct a minimal two-level interaction model and compare it semiquantitatively with the experimental data. Our results reveal a magnetic-field-tunable exciton-exciton coupling as the dominant high-field response of NiPS_{3}, and clarify this material as a new experimental platform for exploring strongly correlated exciton physics in magnetic van der Waals insulators.

An intercalated transition‐metal dichalcogenide hosts a triple‐ Q (3 Q ) noncoplanar antiferromagnetic state that co‐exists with electronic nematicity, indicating broken threefold rotational symmetry. This nematicity exhibits versatile field‐ and strain‐tunability, making it promising for spintronics applications. However, its microscopic connection to magnetism has remained unclear. Here, we report rotational hysteresis observed in both magnetoresistance and magnetic torque, revealing strongly pinned in‐plane weak ferromagnetic moments in the triple‐ Q phase and the magnetism‐driven nature of the co‐existing nematicity. In particular, when fields are rotated within a narrow angular range to restrict magnetization reversal, we observe additional hysteresis loops. This hysteresis can be well explained by domain repopulation, further supported by our theoretical simulations based on the spin Hamiltonian of . These results demonstrate that the weak in‐plane ferromagnetic moment offers an additional handle on the spin‐driven electronic nematicity, providing a microscopic picture of the field‐tunable electronic responses in the 3 Q state of .