Spin-orbital separation in the anisotropic ladder system CaCu$_2$O$_3$

Recently, resonant inelastic X-ray scattering (RIXS) on the 1D spin system Sr$_2$CuO$_3$ has revealed an unprecedented dispersion of orbital excitations [Nature 485, 82 (2012)]. This result has been interpreted as the fractionalization of spin and orbital degree of freedom from the elementary electron, hallmark of one dimensional physics as the previously observed spin-charge separation [Nature Phys. 2, 397 (2006)]. How these phenomena carry over into higher dimensions remains currently unclear. To clarify this point, we studied the spin and orbital excitations of the anisotropic ladder CaCu$_2$O$_3$, which realizes a first step towards 2D correlated electron systems. Combining high-resolution RIXS experiments with theoretical model calculations we show that spin-orbital fractionalization indeed occurs in CaCu$_2$O$_3$ and prevails beyond the strict 1D limit [arXiv:1310.8346]. We also establish that such a fractionalization is far more robust than the spin-charge separation. The main reasons behind this are the intrinsic 1D orbital dynamics and the fact that the spinons are faster than the orbitons but slower than the holons.