Dynamical spin-orbital correlation in the frustrated magnet Ba3CuSb2O9

At low temperatures, atomic magnetic moments usually exhibit some order, for example ferromagnetic order. An exception is frustrated magnets, in which the symmetry impedes the minimization of energy by pairwise magnetic interactions. In such frustrated magnets, new quantum phases, such as spin liquids, are expected. Theoretically, a quantum liquid based on the orbital degree of freedom has also been considered possible when spin and orbital degrees of freedom are entangled. However, to date, experimental observation of such a dynamic spin-orbital state has been a challenge. Here we report an X-ray scattering study of a dynamic spin-orbital state in the frustrated magnet Ba3CuSb2O9. Orbital dynamical motion and increasing short-range orbital correlation with cooling are observed. The most significant feature is that the temperature variation of the orbital correlation is clearly affected by the magnetic interaction. This finding strongly supports a new quantum state in which spins and orbitals are entangled.