Local Jahn-Teller distortions and orbital ordering inBa3Cu1+xSb2−xO9investigated by neutron scattering

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A spin-orbital quantum liquid state is theoretically proposed in the honeycomb lattice of ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$, enabled by dynamic short-range correlations between the spin and orbital degrees of freedom. Using neutron diffraction, the local atomic structure of ${\mathrm{Ba}}_{3}{\mathrm{Cu}}_{1+x}{\mathrm{Sb}}_{2\ensuremath{-}x}{\mathrm{O}}_{9}$ at $x=0$ and 0.1 is obtained via the pair density function analysis. The results indicate that both compositions exhibit local Jahn-Teller (JT) distortions with the elongated ${\mathrm{CuO}}_{3}$ octahedral configuration. In ${\mathrm{Ba}}_{3}{\mathrm{Cu}}_{1.1}{\mathrm{Sb}}_{1.9}{\mathrm{O}}_{9}$, JT ordering of the distorted ${\mathrm{CuO}}_{3}$ octahedra gives rise to the orthorhombic symmetry with ferro-orbital order. On the other hand, in ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$, even though the ${\mathrm{CuO}}_{3}$ octahedra are JT distorted, there is no long-range ordering hence the symmetry is hexagonal. Furthermore, the local singlet excitation at 5.8 meV observed in ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$ below 50 K is absent in ${\mathrm{Ba}}_{3}{\mathrm{Cu}}_{1.1}{\mathrm{Sb}}_{1.9}{\mathrm{O}}_{9}$. Instead, an excitation at 2.5 meV is observed in the latter, which is likely associated with short-range spin order.