Negative Charge Transfer and Reduced on-site Coulomb Energy in Layered Topological Metals

The layered 3d transition-metal dichalcogenides CoTe₂ and NiTe₂ are known to be Dirac semi-metals. We study the electronic structure of CoTe₂ to quantify the electronic structure parameters and compare it with the strongly correlated oxide CoO. Using Co L-edge 2p-3d resonant valence band photoemission spectroscopy(PES), we determine the Co on-site Coulomb energy (U_dd) to be U_dd = 3.0 eV for CoTe₂, from measurements of the single particle 3d partial density of states and the two-hole correlation satellite. Charge transfer(CT) cluster model calculations of core level spectra validate the U_dd value and also provide the charge transfer energy(Δ) between metal d-ligand p-states, and the d-p hybridization strength(T_eg). It is found that CoTe₂ is a negative charge-transfer material with Δ = -2.0 eV. In comparison, CoO is confirmed to be positive charge transfer insulator with U_dd = 5.0 eV and Δ = 4.0 eV.  The d-p hybridization strength T_eg for CoTe₂ < CoO, and indicates that the reduced U_dd in CoTe₂ is not due to T_eg. Similarly we obtain a negative Δ and reduced U_dd in NiTe₂ compared to NiO. However, in-spite of the reduced U_dd values, CoTe₂ and NiTe₂ become topological metals only because U_dd > |Δ| with p-p type lowest energy excitations which facilitate the formation of Dirac bands.

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