  Structure with orbital degeneracy is unstable toward spontaneous distortion.
Such orbital correlation usually has a much higher energy scale than spins, and
therefore, magnetic transition takes place at a much lower temperature, almost
independently from orbital ordering. However, when the energy scales of
orbitals and spins meet, there is a possibility of spin-orbital entanglement
that would stabilize novel ground state such as spin-orbital liquid and random
singlet state. Here we review on such a novel spin-orbital magnetism found in
the hexagonal perovskite oxide Ba$_3$CuSb$_2$O$_9$, which hosts a
self-organized honeycomblike short-range order of a strong Jahn-Teller ion
Cu$^{2+}$. Comprehensive structural and magnetic measurements have revealed
that the system has neither magnetic nor Jahn-Teller transition down to the
lowest temperatures, and Cu spins and orbitals retain the hexagonal symmetry
and paramagnetic state. Various macroscopic and microscopic measurements all
indicate that spins and orbitals remain fluctuating down to low temperatures
without freezing, forming a spin-orbital entangled liquid state.
