Valence Bond Glass Ground State and Gapped Itinerant Excitations in a Frustrated Quantum Magnet

The experimental observation of the quantum spin liquid (QSL) has been challenging and contentious. This is largely due to an overly simplistic classification of real-world magnetic systems as either conventionally long-range ordered (LRO)—with minimal spin entanglement—or as fully entangled QSLs. We contend that this binary categorization is insufficient.

Our measurements of specific heat and thermal conductivity on NaYb_xLu_1-xSe₂, a magnetic-site-dilution of the QSL candidate NaYbSe₂, suggest that the spin ground state of NaYbSe₂ is in fact a valence bond glass (VBG). This state is spatially inhomogeneous and exhibits a diverse range of entanglement length scales. The VBG model successfully resolves a long-standing paradox: while specific heat and inelastic neutron scattering (INS) measurements reveal dispersed gapless excitations in various materials, thermal conductivities show gapped itinerant magnetic excitations universally. By tracking phonon thermal conductivities of NaYb_xLu_1-xSe₂ across a broad temperature range, we propose that spin-induced quenched lattice randomness is responsible for the spatial inhomogeneity that ultimately leads to the VBG ground state [Lyu 2025].