Xinwei Li

Breakdown of selection rules and emergence of optical activity in Raman spectroscopy typically signal symmetry-breaking events essential for understanding novel phases of solids. Here, we apply our recently developed rotational anisotropy Raman spectroscopy technique to single-crystal ruthenium dioxide—a compound of recent interest as a candidate workhorse material for altermagnetism yet whose magnetic ground state remains under intense debate. Our data reveal a salient breakdown of selection rules for zone-center phonons, accompanied by clear splitting in the angle-dependent polarimetry plots for the two opposite helicity-nonconserving circularly polarized scattering channels. These observations provide definitive evidence of a previously overlooked symmetry-breaking event, involving either spatial or time-reversal symmetry. In either scenario, the underlying mechanism is likely a bulk hidden order parameter, potentially involving multipole moments of Ruthenium(IV) set by the local crystal field environment. Our work demonstrates how subtle symmetry-breaking events can manifest through lattice dynamics and calls for renewed experimental and theoretical efforts to map the phase diagram of ruthenium dioxide as a system poised near quantum phase transition boundaries.

Second-harmonic generation (SHG) is a sensitive optical probe of symmetry breaking because the allowed nonlinear susceptibility components are directly constrained by crystal and magnetic symmetry. In magnetic materials, the onset of spin order commonly activates additional SHG channels or redistributes intensity among polarization geometries, often producing an enhanced nonlinear response. Here we demonstrate the opposite phenomenology in the van der Waals antiferromagnet CrOCl: SHG is strongly quenched across the incommensurate and commensurate magnetic transitions. The suppression occurs broadly across polarization angles, in both co- and cross-polarized detection channels, and in all fitted magnetic-dipole tensor elements. By ruling out structural distortion, classical optical effects, and optical-resonance shifts as dominant origins, we assign the anomalous quench to a bulk magnetic-dipole SHG process whose radiative matrix element is reduced by reconstruction of the Cr d-orbital wave function assisted by spin–orbit coupling. This mechanism can be understood as magnetic-order-induced rotation of orbital pseudospin within the even-parity d manifold, revealing a novel route to nonlinear optical control that is distinct from conventional symmetry activation.

Raman spectroscopy stands as a cornerstone technique for probing collective excitations and quantum phases in solids. While polarization‐resolved Raman scattering has been widely used to extract symmetry information of eigenmodes, its conventional geometry suffers from significant limitations: it accesses only a subset of Raman tensor elements, enforces ‐periodic intensity patterns that obscure intrinsic crystalline symmetries, and lacks sensitivity to wavevector‐dependent anisotropy. To overcome these constraints, here we introduce rotational‐anisotropy Raman spectroscopy (RA‐Raman). By measuring scattering intensity during full azimuthal rotation of the optical scattering plane at oblique incidence, this geometry enables reconstruction of the relative ratios of all Raman tensor elements and reveals rotational anisotropy patterns essential for accessing subtle symmetry information elusive to conventional methods. We developed a prototype instrument to validate this approach experimentally. For centrosymmetric crystals, RA‐Raman unambiguously identifies phonon symmetry representations and determines crystallographic axes. In noncentrosymmetric crystals, it resolves directional anisotropy of both intensity and frequency of phonon‐polaritons, enabling quantitative determination of the Faust‐Henry coefficient and the separation of deformation‐potential and electro‐optic scattering contributions. By demonstrating unprecedented symmetry‐resolving power in benchmark crystals, we establish RA‐Raman as a powerful tool with far‐reaching potential to discover and characterize symmetry‐breaking phases and topological excitations in quantum materials.

Long-lived dark excitons in monolayer WSe ₂ present promising candidates for carrying spin and valley information, but their optical access and spin manipulation have conventionally required the use of strong external magnetic fields. Here, using a ferroelectric hybrid perovskite heterostructure, we leverage the ferroelectric proximity effect to break the WSe ₂ ’s in-plane rotational symmetry and brighten the spin-forbidden dark excitons under zero magnetic field conditions. Furthermore, we show that the twist angle between the WSe ₂ and perovskite crystals controls the ferroelectric coupling strength and valley-contrasting polarization. Our proposed mechanism, supported by a four-band tight-binding model, suggests that the ferroelectric proximity effect induces an asymmetric intersublattice interaction, generating an effective in-plane spin-orbit coupling (SOC) field that rotates spin/valley polarization and brightens dark excitons. Our work establishes ferroelectric proximity coupling as a symmetry-tunable, magnetic-field-free strategy for spin exciton control in two-dimensional semiconductors.

Two-level atoms ultrastrongly coupled with single-mode cavity photons are predicted to exhibit a quantum phase transition, entering a phase in which both the atomic polarization and the photonic field are finite even without external driving. However, this phenomenon, the superradiant phase transition (SRPT), is forbidden by a no-go theorem due to the existence of the diamagnetic term. Here, we present spectroscopic evidence for a magnonic SRPT in ErFeO ₃ , where the role of the photonic mode (two-level atoms) in the photonic SRPT is played by an Fe ³⁺ magnon mode (Er ³⁺ spins). The absence of the diamagnetic term in the Fe ³⁺ -Er ³⁺ exchange coupling ensures that the no-go theorem does not apply. Ultrabroadband terahertz and gigahertz magnetospectroscopy experiments revealed the signatures of the SRPT in thermal equilibrium, a kink and a softening, respectively, of two spin-magnon hybridized modes at the critical point. Systems near this phase are expected to harbor large-scale squeezing, which will potentially provide a route to next-generation quantum technologies.

Two-dimensional Mott insulators host antiferromagnetic (AFM) correlations that are predicted to enhance the attractive interaction between empty (holons) and doubly occupied (doublons) sites, creating a novel pathway for exciton formation. However, experimental confirmation of this spin-mediated binding mechanism remains elusive. Leveraging the distinct magnetic critical properties of the Mott antiferromagnets Sr 2 IrO 4 and Sr 3 Ir 2 O 7 , we show using time-resolved THz spectroscopy that excitons exist only at temperatures below where short-range AFM correlation develops. The excitons remain stable up to photodoping densities approaching the predicted excitonic Mott insulator-to-metal transition, revealing a unique robustness against screening. Our results establish the viability of spin-bound excitons and introduce opportunities for excitonic control through magnetic degrees of freedom.

The Dicke model describes the cooperative interaction of an ensemble of two-level atoms with a single-mode photonic field and exhibits a quantum phase transition as a function of light–matter coupling strength. Extending this model by incorporating short-range atom–atom interactions makes the problem intractable but is expected to produce new physical phenomena and phases. Here, we simulate such an extended Dicke model using a crystal of ErFeO₃, where the role of atoms (photons) is played by Er³⁺ spins (Fe³⁺ magnons). Through terahertz spectroscopy and magnetocaloric effect measurements as a function of temperature and magnetic field, we demonstrated the existence of a novel atomically ordered phase in addition to the superradiant and normal phases that are expected from the standard Dicke model. Further, we elucidated the nature of the phase boundaries in the temperature–magnetic-field phase diagram, identifying both first-order and second-order phase transitions. These results lay the foundation for studying multiatomic quantum optics models using well-characterized many-body solid-state systems.