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The ground-state orbital occupancy of the Ru$^{4+}$ ion inCa$_{2-x}$La$_x$RuO$_4$ [x=0, 0.05(1), 0.07(1) and 0.12(1)] was investigated byperforming X-ray absorption spectroscopy (XAS) in the vicinity of the O K edgeas a function of angle between the incident beam and the surface of thecrystals. A minimal model of the hybridization between the O 2p states probedat the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowingthe ratio of hole occupancies $n_{xy}/n_{yz,zx}$ to be determined as a functionof doping and temperature. For the samples displaying a low-temperatureinsulating ground-state ($x\leq0.07$), $n_{xy}/n_{yz,zx}$ is found to increasesignificantly with increasing doping. For x=0.12, which has a metallicground-state, the XAS spectra are found to be independent of temperature, andnot to be describable by the minimal hybridization model. To understand theorigin of the evolution of the electronic structure across the phase diagram,we have performed theoretical calculations based on a model Hamiltonian,comprising electron-electron correlations, crystal field ($\Delta$) andspin-orbit coupling ($\lambda$), of a Ru-O-Ru cluster. Our calculations of theRu hole occupancy as a function of $\Delta/\lambda$ establish that theenhancement of $n_{xy}/n_{yz,zx}$ is driven by significant changes to thecrystal field as the tetragonal distortion of the RuO$_6$ octahedral changesfrom compressive to tensile with La doping. It also shows that the holeoccupancy of the O 2p and Ru 4d orbitals display the same trend as a functionof $\Delta/\lambda$, thus validating the minimal hybridization model. Inessence, our results suggest that the predominant mechanism driving theemergence of the low-temperature metallic phase in La doped Ca$_2$RuO$_4$ isthe structurally induced redistribution of holes within the t2g orbitals,rather that the injection of free carriers.

Tuning of the Ru$^{\mathbf{4+}}$ ground-state orbital population in the $\mathbf{4d^4}$ Mott insulator Ca$_2$RuO$_4$ achieved by La doping