Probing The Electronic Dispersion of Superconducting Nickelates

The origin of unconventional superconductivity in the cuprates remains a mystery partly due to the complex interplay of several competing states and relatively strong disorder. Superconducting nickelates are a new family of materials [1,2] that also 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.<br/>To investigate the electronic structure of the nickelates, we measured the Seebeck coefficient <i>S</i> of a superconducting 5-layer nickelate Nd₆Ni₅O₁₂ (<i>n</i> = 5 nickelate), whose resistivity is predominantly linear—indicative of strange metal physics—as well as a more-overdoped, non-superconducting, 3-layer nickelate Nd₄Ni₃O₈ (<i>n</i> = 3 nickelate) for comparison. We find that both the <i>n</i> = 5 and <i>n</i> = 3 nickelate share a similar temperature-independent, negative <i>S/T</i>, but dramatically differs from the positive Seebeck effect measured on cuprates at similar doping [3].<br/>To interpret our measurements, we used Boltzmann transport theory to show that the electronic dispersion obtained from first-principles calculations accounts for both the magnitude and sign of the temperature-independent Seebeck coefficient in the nickelates. 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 strange metallic state of this family of materials.<br/>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 the superconducting nickelates, this work [4] demonstrates the power of the semi-classical approach to quantitatively describe transport measurements in strongly correlated electron systems, even in the strange-metallic state.<br/><br/>[1] Li et al. Nature 572, 624 (2019)<br/>[2] Pan et al. Nature Materials 21, 160 (2022)<br/>[3] Gourgout et al. Physical Review X 12, 011037 (2022).<br/>[4] Grissonnanche et al. arXiv : 2210.10987