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.