Edoardo Baldini

The realization of magnetic materials in the two-dimensional limit has uncovered a regime where reduced dimensionality, symmetry, topology, and relativistic interactions fundamentally reshape magnetic order and collective excitations. In this setting, strong fluctuations and intertwined degrees of freedom often render key aspects of magnetic order and dynamics hidden to conventional probes, motivating the development of approaches capable of resolving both equilibrium order and dynamical responses in atomically thin systems.

In this talk, I will show how precision optical spectroscopy applied in and out of equilibrium reveals emergent magnetic states and collective modes that are otherwise inaccessible in van der Waals magnets. I will focus on two case studies. First, I will discuss monolayer NiPS3, where equilibrium nonlinear optical micropolarimetry exposes a transition from three-dimensional magnetic behavior to a genuinely two-dimensional regime consistent with a Berezinskii-Kosterlitz-Thouless (BKT) state, followed at lower temperatures by an instability toward a pinned magnetic phase with true long-range order [1]. These results uncover previously unrecognized states of matter that emerge uniquely in the monolayer limit.

Second, I will turn to the atomically thin multiferroic NiI2, where nonequilibrium pump–probe spectroscopy enables the coherent excitation and detection of low-energy collective modes, allowing the microscopic character of exotic magnon modes to be resolved [2]. These measurements reveal a giant dynamical magnetoelectric coupling arising from chiral spin textures and relativistic spin-orbit interactions, leading to exceptionally large natural optical activity at terahertz frequencies.

Together, these results demonstrate that precision spectroscopy across equilibrium and ultrafast regimes enables direct access to unconventional magnetic phases and dynamics in two-dimensional materials, and establishes a framework for coherently manipulating order parameters and collective excitations in the low-dimensional limit.

[1] F. Y. Gao*, D. Kim* et al., Nature Materials (in press)
[2] F. Y. Gao*, X. Peng* et al., Nature 632, 273-279 (2024)