In the realm of energy materials research, a study led by I. V. Leonov from the Moscow Institute of Physics and Technology has shed new light on the electronic structure and magnetic correlations of a promising superconducting material, La$_3$Ni$_2$O$_7$ (LNO). This research, published in the journal Physical Review Letters, explores how electronic correlations and doping affect the normal state electronic structure of this double-layer nickelate superconductor under pressure.
The study employs a sophisticated computational approach, combining Density Functional Theory (DFT) with dynamical mean-field theory (DMFT), to investigate the effects of electronic correlations and doping on LNO. The researchers found significant orbital-dependent quasiparticle renormalizations, particularly in the Ni $x^2-y^2$ and $3z^2-r^2$ bands. This means that the effective mass of electrons in these orbitals is significantly different from their bare mass, indicating strong electronic correlations. Notably, the $3z^2-r^2$ states exhibit incoherent behavior, characteristic of a ‘bad metal,’ due to their proximity to localization.
One of the most intriguing findings is the non-monotonic dependence of the quasiparticle renormalization on doping. Specifically, there’s a remarkable 20% increase in the effective mass of the Ni $x^2-y^2$ orbitals upon electron doping at a level of approximately 0.2 electrons per Ni ion. This suggests a significant enhancement of orbital-dependent correlations with oxygen deficiency in LNO.
The researchers also observed a reconstruction of the low-energy electronic structure of LNO upon doping above certain thresholds. This reconstruction is associated with a Lifshitz transition, a phenomenon where the Fermi surface changes topology, leading to a crossover to a self-doping regime characterized by partial occupation of the La $5d$ bands.
In terms of magnetic correlations, the study suggests the possible formation of spin and charge (or bond) density wave stripes, implying strong spin and charge correlations in LNO. Interestingly, the researchers found that moderate electron doping of the Ni$^{2.5+}$ ions, such as through oxygen deficiency, significantly enhances the strength of in-plane spin and charge fluctuations.
For the energy sector, these findings could have implications for the development of high-temperature superconductors, which are highly sought after for energy transmission and storage applications. The study’s insights into the electronic structure and magnetic correlations of LNO could guide the design of new materials with enhanced superconducting properties. Moreover, the understanding of doping effects on electronic correlations could inform strategies for optimizing the performance of existing superconducting materials in energy technologies.
This article is based on research available at arXiv.