Heavy-fermion strange metal and quantum spin liquid in a 4d-electron trimer lattice*

Strange metals, heavy fermion metals, and quantum spin liquids are among the most intriguing and yet intellectually challenging topics in condensed matter physics. Here we report the experimental discovery of a perpetual heavy Fermi surface consisting of charge-neutral spinons that underpins both a heavy-fermion strange metal and a quantum spin liquid, which occur in an unlikely place, a 4d-electron trimer lattice Ba₄Nb_1-xRu_3+xO₁₂. Both states exhibit a universally large storage of entropy at the milli-Kelvin regime and a disassociation of charges and spins as such the heavy-fermion strange metal grossly violates the Wiedemann-Franz law, and the quantum spin liquid is a much better thermal conductor than the heavy-fermion strange metal. The novel phenomenology offers an unprecedented paradigm of correlated quantum matter.