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Abstract The quantum dimer magnet, with antiferromagnetic intradimer and interdimer Heisenberg exchange between spin-1/2 moments, is known to host an$$(\left|\uparrow \downarrow \right\rangle -\left|\downarrow \uparrow \right\rangle )/\sqrt{2}$$ $\left(\left(\uparrow \downarrow \right)-\left(\downarrow \uparrow \right)\right)/\sqrt{2}$singlet ground state when the intradimer exchange is dominant. Rare-earth-based quantum dimer systems with strong spin-orbit coupling offer the opportunity for tuning their magnetic properties by using magnetic anisotropy as a control knob. Here, we present bulk characterization and neutron scattering measurements of the quantum dimer magnet Yb₂Be₂SiO₇. We find that the Yb³⁺ions can be described by an effective spin-1/2 model at low temperatures and the system does not show signs of magnetic order down to 50 mK. The magnetization, heat capacity, and neutron spectroscopy data can be well-described by an isolated dimer model with highly anisotropic exchange that stabilizes a singlet ground state with a wavefunction$$(\left|\uparrow \uparrow \right\rangle -\left|\downarrow \downarrow \right\rangle )/\sqrt{2}$$ $\left(\left(\uparrow \uparrow \right)-\left(\downarrow \downarrow \right)\right)/\sqrt{2}$or$$(\left|\uparrow \uparrow \right\rangle+\left|\downarrow \downarrow \right\rangle )/\sqrt{2}$$ $\left(\left(\uparrow \uparrow \right)+\left(\downarrow \downarrow \right)\right)/\sqrt{2}$. Our results show that strong spin-orbit coupling can induce unusual entangled states of matter in quantum dimer magnets. 

The advent of layered materials has unveiled new opportunities for tailoring electromagnetic waves at the subwavelength scale, particularly through the study of polaritons, a hybrid light–matter excitation. In this context, twist-optics, which investigates the optical properties of twisted stacks of van der Waals (vdW) layered specimens, has emerged as a powerful tool. Here, we explore the tunability of phonon polaritons in α-V₂O₅via interlayer twisting using scanning nano-infrared (IR) imaging. We show that the polaritonic response can be finely adjusted by varying their interlayer electromagnetic coupling, allowing for precise control over the propagation direction and phase transition from open unidirectional iso-frequency contours to closed elliptic geometries. Our experimental results, in conjugate with theoretical modeling, reveal the mechanisms underpinning this tunability, highlighting the role of twist-induced nano-light modifications for advanced nanophotonic control at the nanoscale. 

We constructed the magnetic field-temperature phase diagrams of new quasi-two-dimensional isosceles triangular lattice antiferromagnets (TLAF) Ca 3 MNb 2 O 9 (M=Co, Ni) from dc and ac magnetic susceptibilities, specific heat, dielectric constant, and electric polarization measurements on single crystalline samples. Ca 3 CoNb 2 O 9 with effective spin-1/2 Co 2+ ions undergoes a two-step antiferromagnetic phase transition at T N1 = 1.3 K and T N2 = 1.5 K and enters a stripe ordered state at zero magnetic field. With increasing field, successive magnetic phase transitions, reminiscent of the up-up-down ( uud ) and the oblique phases, are observed. The dielectric constant of Ca 3 CoNb 2 O 9 shows anomalies related to the magnetic phase transitions, but clear evidence of ferroelectricity is absent. Meanwhile, Ca 3 NiNb 2 O 9 with spin-1 Ni 2+ ions also shows a two-step antiferromagnetic transition at T N1 = 3.8 K and T N2 = 4.2 K at zero field. For Ca 3 NiNb 2 O 9 , the electric polarization in the magnetic ordered phases was clearly observed from the pyroelectric current measurements, which indicates its coexistence of magnetic ordering and ferroelectricity.