Seebeck coefficient in a nickelate superconductor: electronic dispersion in the strange metal phase

Superconducting nickelates are a new family of materials that combine strongly-correlated magnetism with unconventional superconductivity. While comparisons with the superconducting cuprates are natural, very little is known about the metallic state of the nickelates, making these comparisons difficult. We probe the electronic dispersion of thin-film superconducting 5-layer ($n=5$) and metallic 3-layer ($n=3$) nickelates by measuring the Seebeck coefficient, $S$. We find a temperature independent and negative $S/T$ for both the $n=5$ nickelate, with strange metal resistivity, and the $n=3$ compound, with more conventional Fermi liquid resistivity. These results are in stark contrast with the strongly temperature-dependent $S/T$ measured at similar electron filling in the cuprate La$_{1.36}$Nd$_{0.4}$Sr$_{0.24}$CuO$_4$. We reproduce the temperature dependence, sign, and amplitude of $S/T$ in the nickelates using Boltzmann transport theory combined with the electronic structure calculated from density functional theory. This demonstrates that the electronic structure obtained from first-principles calculations is a good starting point for calculating the transport properties of superconducting nickelates, and suggests that, despite indications of strong electronic correlations, there are well-defined quasiparticles in the metallic state of this family of materials. Finally, we explain the differences in the Seebeck coefficient between nickelates and cuprates as originating in strong dissimilarities in impurity concentrations. Beyond establishing a baseline understanding of how the electronic structure relates to transport coefficients in these new materials, this work demonstrates the power of the semi-classical approach to quantitatively describe transport measurements, even in the strange-metallic state.