Electronic structure studies on single crystalline Nd$_2$PdSi$_3$, an exotic Nd-based intermetallic: Evidence for Nd 4$f$ hybridization

In the series R$_2$PdSi$_3$, Nd$_2$PdSi$_3$ is an anomalous compound in the sense that it exhibits ferromagnetic order unlike other members in this family. The magnetic ordering temperature is also unusually high compared to the expected value for a Nd-based system, assuming 4$f$ localization. Here, we have studied the electronic structure of single crystalline Nd$_2$PdSi$_3$ employing high resolution photoemission spectroscopy and {\it ab initio} band structure calculations. Theoretical results obtained for the effective on-site Coulomb energy of 6 eV corroborate well with the experimental valence band spectra. While there is significant Pd 4$d$-Nd 4$f$ hybridization, the states near the Fermi level are found to be dominated by hybridized Nd 4$f$-Si 3$p$ states, which is possibly responsible for the ferromagnetism in Nd compound. Nd 3$d$ core level spectrum exhibits multiple features manifesting strong final state effects due to electron correlation, charge transfer and collective excitations. These results serve as one of the rare demonstrations of hybridization of Nd 4$f$ states with the conduction electrons possibly responsible for the exoticity of this compound.