Microscopic Exploration of Electronic States in Nickelate Superconductors

In 2023, the discovery of the superconductivity of La₃Ni₂O₇ with a transition temperature (T_c) of ~80 K under pressure led to nickelates joining the family of high-T_c superconductors. La₃Ni₂O₇ has a two-layered perovskite structure resembling cuprates, which are well-known high-T_c superconductors. However, it actually possesses different characteristics from those of cuprates, anticipating a new superconducting mechanism.

One of the major mysteries of La₃Ni₂O₇ is what kind of electronic orders and/or fluctuations exist and how they lead to the superconducting phase from the non-superconducting phase at ambient pressure. In addition, La₃Ni₂O₇ belongs to the Ruddlesden–Popper (RP) phase La_n+1Ni_nO_3n+1, which includes similar compounds with different numbers of layers (n). Systematic studies of the RP series are expected to provide clues regarding the origin of superconductivity and the exploration of new superconductors.

We investigated the electronic states of (La₃Ni₂O₇) (n=2) and La₄Ni₃O₁₀ (n=3) at ambient pressure using nuclear magnetic resonance (NMR) experiments, which can be used to microscopically observe the orders and/or fluctuations of charge and spin. The NMR spectra at high temperatures exhibited multiple distinct peaks, which allowed us to identify two crystallographically inequivalent sites. At temperatures below 150 K for La₃Ni₂O₇ and 130 K for La₄Ni₃O₁₀, significant broadening of their spectra, particularly in the satellite peaks, was observed. This suggests the existence of a density-wave transition accompanied by a spatial charge distribution. We also measured the nuclear spin relaxation rate (1/T₁), showing that, in addition to the sharp drop in 1/T₁ associated with the density wave transition, the relaxation remains finite at lower temperatures. This result indicates that the system has a metallic ground state with a partial gap at the Fermi energy owing to the density-wave transition.

The fact that these characteristics are commonly observed in La₃Ni₂O₇ and La₄Ni₃O₁₀ provides important insights. These observations are likely associated with the different features of the multiple bands composed of the d_x²-y² and d_3z²-r² orbitals, which are common to these compounds. Recently, superconductivity with a of ~25 K under pressure was also discovered in ​La₄Ni₃O₁₀. This finding suggests a connection between the multiband nature, density wave phase, and emergence of the superconducting phase under high pressure. The possibility of spin order coexisting with charge order remains debatable, and further investigations are required to understand how the charge and spin degrees of freedom evolve toward the superconducting phase under high pressure.

Written by M. Kakoi and H. Mukuda on behalf of all authors.