  We present experimental evidence that a heavy Fermi surface consisting of
itinerant, charge-neutral spinons underpins both heavy-fermion-strange-metal
(without f electrons) and quantum-spin-liquid states in the 4d-electron trimer
lattice, Ba4Nb1-xRu3+xO12 (|x| < 0.20). These two exotic states both exhibit an
extraordinarily large entropy, a linear heat capacity extending into the
milli-Kelvin regime, a linear thermal conductivity at low temperatures, and
separation of charges and spins. Furthermore, the insulating spin liquid is a
much better thermal conductor than the heavy-fermion-strange-metal that
separately is observed to strongly violate the Wiedemann-Franz law. We propose
that at the heart of this 4d system is a universal, heavy spinon Fermi surface
that provides a unified framework for explaining the exotic phenomena observed
throughout the entire series. The control of such exotic ground states provided
by variable Nb concentration offers a new paradigm for studies of correlated
quantum matter.
