Electronic structures and spin fluctuations in nickelate oxide NdNiO$_2$

We perform core-level X-ray photoemission spectroscopy (XPS) and Raman spectroscopy studies of electronic structures and spin fluctuations in the nickelate oxide NdNiO$_2$. According to the Ni 2$p$ and O 1$s$ XPS spectra, there is strong covalence between Ni $3d$ and O $2p$ in the reference samples NiO and NdNiO$_3$, due to the positive and negative charge transfer in NiO and NdNiO$_3$, respectively. In contrast, the Ni $3d$ and O $2p$ hybridization is significantly suppressed, indicating the Hubbard-like Mott insulator in the NdNiO$_2$. The three-peak structure in Nd 3$d$ core-level XPS implies the strong mixing state between 4$f^3$ and 4$f^4\underline{L}$ in NdNiO$_2$, giving rise to a heavy-fermion like system. The two-magnon peak in the Raman scattering provides direct evidence for the strong spin-fluctuation in NdNiO$_2$. The peak position determines the antiferromagnetic exchange $J=25$~meV. Our experimental results agree very well with our previous theoretical results.