Spin-orbital liquid in Ba${}_3$CuSb${}_2$O${}_9$ stabilized by oxygen holes

Abstract

Both the Jahn-Teller distortion of Cu${}^{2+}$O${}_6$ octahedra and magnetic ordering are absent in hexagonal Ba${}_3$CuSb${}_2$O${}_9$ suggesting a Cu 3$d$ spin-orbital liquid state. Here, by means of resonant x-ray scattering and absorption experiment, we show that oxygen 2$p$ holes play crucial role in stabilizing this spin-orbital liquid state. These oxygen holes appear due to the "reaction" Sb${}^{5+}$$\to$Sb${}^{3+}$ $+$ two oxygen holes, with these holes being able to attach to Cu ions. The hexagonal phase with oxygen 2$p$ holes exhibits also a novel charge-orbital dynamics which is absent in the orthorhombic phase of Ba${}_3$CuSb${}_2$O${}_9$ with Jahn-Teller distortion and Cu 3$d$ orbital order. The present work opens up a new avenue towards spin-charge-orbital entangled liquid state in transition-metal oxides with small or negative charge transfer energy.

21 pages, 6 figures

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