Next-nearest-neighbor frustration and bond-dependent exchange in low-dimensional quantum magnets

Antiferromagnetic next-nearest-neighbor (NN) exchange (J₂) and bond-dependent anisotropic interactions are key ingredients in modern models of quantum spin liquids (QSLs). In this talk, we present inelastic neutron scattering (INS) studies on two low-dimensional (pseudo-)spin-1/2 magnets–CuGeO₃ and YbBi₂IO₄–that offer new insights into how these interactions reshape spin-liquid behavior and how they can be realized in real materials. In the prototypical quantum spin-Peierls chain compound CuGeO₃, we identify strong next-NN frustration by quantitatively comparing high-resolution INS spectra with state-of-the-art DMRG+TEBD simulations. We also describe how this frustration modifies fractionalization and confinement beyond conventional Bethe ansatz expectations in 1D. In the square-lattice Yb³⁺ compound YbBi₂IO₄, INS analysis reveals both a large J₂/J₁ ratio (crucial for enhancing quantum fluctuations) and strong bond-dependent exchange anisotropy (which has not been widely demonstrated in square-lattice compounds). Supported by theoretical analysis, we attribute the emergence of these interactions to the compound’s unique edge-sharing cubic ligand environment. This stabilizes an Yb³⁺ ground-state doublet that is distinct from those found in typical octahedral geometries, potentially offering insight into how to promote sizable J₂ or bond-dependent anisotropic exchange in Yb³⁺-based quantum magnets.