Transition between Heavy-Fermion-Strange-Metal and Quantum Spin Liquid in a 4d-Electron Trimer Lattice

We have observed a heavy Fermi surface of charge-neutral spinons that, depending on Nb concentration, underpins both a heavy-fermion-strange-metal and adjacent quantum-spin-liquid phases in the 4d-electron trimer lattice Ba₄Nb_1-xRu_3+xO₁₂. The extraordinarily large entropy and linear electronic heat capacity of both phases extend into milli-Kelvin temperatures, which is best explained by charge and spin dissociation. The insulating spin liquid is a much better thermal conductor than the heavy-fermion strange metal that is separately observed to strongly violate the Wiedemann-Franz law. The control of such exotic ground states provided by variable Nb concentration offers a new paradigm for correlated quantum matter [1].