Paper

This study employs synchrotron X-ray diffraction to systematically investigate the structural evolution of the bilayer nickelate La₂SmNi₂O₇ under low temperature and high pressure. At ambient conditions, single-crystal diffraction reveals a new monoclinic superstructure (space group P2₁/a) with a c-axis doubling primarily driven by antiferrodistortive oxygen displacements; no satellite reflections associated with charge-density-wave order are detected, indicating that any displacive charge ordering, if present, has an amplitude below a few thousandths of an ångström. Under applied pressure, a sequence of structural transitions is observed at room temperature using both powder and single-crystal diffraction: a monoclinic-to-orthorhombic transition at approximately 15 GPa, followed by a further transition to tetragonal symmetry near 21 GPa, with the intermediate orthorhombic phase persisting stably over a finite pressure interval. In the pressure–temperature regime where superconductivity emerges, high-quality single-crystal data enable structural refinement and provide precise lattice parameters and bond angles, establishing a structural basis for understanding the onset of superconductivity. The results demonstrate that the pressure-driven structural transformations in La₂SmNi₂O₇ occur without the participation of a displacive charge-density wave, and the successive symmetry changes impose crucial constraints on theoretical models aimed at exploring the interplay of charge, lattice, and magnetism.

Recently, a signature of high-temperature superconductivity above the liquid nitrogen temperature (77 K) was reported for La3Ni2O7−δ under pressure. This finding immediately stimulated intense interest in the possible mechanism of high-Tc superconductivity in double-layer nickelates. Notably, the pressure-dependent phase diagram inferred from transport measurements indicates that the superconductivity under high pressure emerges from the suppression of density-wave-like order at ambient pressure, which is similar to high-temperature superconductors. Therefore, clarifying the exact nature of the density-wave-like transition is important for determining the superconducting mechanism in double-layer nickelates. Here, nuclear magnetic resonance (NMR) spectroscopy of 139La nuclei was performed to study the density-wave-like transition in a single crystal of La3Ni2O7−δ. At high temperatures, two sets of sharp 139La NMR peaks are clearly distinguishable from a broad background signals, which are ascribed to La(1) sites from two bilayer Ruddlesden-Popper phases with different oxygen vacancy δ. As the temperature decreases, the temperature-dependent 139La NMR spectra and nuclear spin-lattice relaxation rate (1/T1) for both La(1) sites provide evidence of spin-density-wave (SDW) ordering below the transition temperature (TSDW), which is approximately 150 K. The anisotropic splitting in the NMR spectra suggests the formation of a possible double spin stripe with magnetic moments aligned along the c-axis. Furthermore, we studied the pressure-dependent SDW transition up to ∼ 2.7 GPa. Surprisingly, the TSDW inferred from NMR measurements of both La(1) sites increases with increasing pressure, which is opposite to the results from previous transport measurements under pressure and suggests an intriguing phase diagram between superconductivity and SDW. In contrast, the present 139La NMR is insensitive to the possible charge-density-wave (CDW) order in the Ni-O planes. All these results will be helpful for building a connection between superconductivity and magnetic interactions in double-layer nickelates.

In this study, Pr3Ni2O7 thin films were successfully synthesized on LaAlO3 substrates via epitaxial stabilization, overcoming the thermodynamic stability limitations that hinder bulk synthesis of this compound. Under ambient pressure, the Pr3Ni2O7 films exhibited insulating behavior regardless of ozone annealing treatment; however, under a high pressure of 22 GPa, the films displayed T-linear metallic transport and superconductivity, with an onset superconducting transition temperature of 66 K and a zero-resistance temperature of approximately 40 K. Further investigation revealed that while Nd3Ni2O7 films incorporating the smaller rare-earth ion Nd could also be epitaxially stabilized, no superconductivity was observed within the measured pressure range. Comparison of La, Pr, and Nd Ln3Ni2O7 films showed that the critical pressure Pc required for superconductivity increases with decreasing Ln ionic radius, a trend consistent with Ln substitution studies in bulk materials. This work demonstrates that epitaxial stabilization is an effective approach for expanding the bilayer nickelate superconductor family, offering an important pathway for exploring novel superconducting materials.

This study utilized muon spin rotation spectroscopy to investigate the effect of oxygen isotope substitution (16O/18O) on the spin density wave (SDW) transition in the trilayer Ruddlesden-Popper nickelate Pr4Ni3O10. Under ambient pressure, the SDW transition temperatures for the 16O and 18O samples were 158.04 K and 159.81 K, respectively, exhibiting a finite isotope shift. Under hydrostatic pressure, the transition temperatures for both isotopes decreased linearly at nearly identical rates (approximately -4.9 K/GPa), resulting in an essentially pressure-independent isotope shift. This pressure-independent isotope effect indicates that the SDW transition primarily originates from electronic correlations rather than lattice dynamics. Combined with recent inelastic X-ray scattering results that revealed no phonon softening, this study supports a novel mechanism of intertwined charge density wave and spin density wave order stabilized by strong spin interactions in trilayer Ruddlesden-Popper nickelates. This finding contrasts with the doping-enhanced isotope effect observed in cuprates and provides critical constraints for understanding the electronic origin of density wave order and its relationship with superconductivity in nickelates.

This study investigates the electronic structures of La₃Ni₂O₇, Nd₃Ni₂O₇, and LaNiO₃ by comparing Ni 2p and Ni 1s core-level photoelectron spectra. Owing to the severe overlap of La 3d with Ni 2p levels and the presence of La high-energy satellite peaks, conventional Ni 2p spectra fail to reliably extract the intrinsic signal from La-based nickelates. Using hard X-ray photoelectron spectroscopy to probe the deeper Ni 1s core level, which is free of spin–orbit coupling and has negligible multiplet interactions, provides a clean perspective on charge-transfer excitations. The results show that the Ni 1s spectra can clearly distinguish the perovskite LaNiO₃ from the bilayer Ruddlesden–Popper phases and reveal that compared to Nd₃Ni₂O₇, La₃Ni₂O₇ exhibits a broadened main peak with reduced intensity and an enhanced satellite peak. Combined with DFT+DMFT calculations, these spectral changes are attributed to alterations in the charge-transfer energy and hybridization strength, where the tensile strain in La₃Ni₂O₇ weakens the Ni–ligand hybridization. This approach demonstrates the sensitivity of Ni 1s core-level spectroscopy to subtle electronic-structure variations and offers an effective means for systematically characterizing nickelates with different strains, doping levels, or layer numbers.

This review summarizes recent progress in achieving ambient-pressure superconductivity in bilayer nickelate La₃Ni₂O₇ thin films. Through epitaxial strain engineering, compressive strain provided by substrates such as SrLaAlO₄ successfully stabilizes the superconducting phase under ambient conditions, marking a significant breakthrough compared to the high-pressure superconductivity observed in bulk materials. In terms of experimental characterization, angle-resolved photoemission spectroscopy (ARPES) measurements reveal a controversially debated Fermi surface topology, with observations differing among research groups, likely due to variations in thin-film growth conditions. Regarding the enhancement of superconducting transition temperature (Tc), increasing compressive strain and optimizing growth techniques, such as giant oxide atomic layer epitaxy, enable Tc to reach approximately 60 K. Theoretical studies focus on electronic structures and pairing symmetries; weak-coupling approaches, including random phase approximation and functional renormalization group, predict s±-wave or d-wave pairing, while renormalized mean-field theory suggests the possibility of nodal d-wave pairing. However, key issues such as the specific role of electron pockets on the Fermi surface in superconductivity and the relationship between lattice ratio and Tc remain incompletely elucidated. These advances demonstrate that bilayer nickelate thin films serve as a highly tunable and exceptionally promising platform for studying high-temperature superconductivity.

This study systematically investigates the bilayer Kondo lattice model using single-site dynamical mean-field theory (DMFT) to explore the phase diagram of the normal state of bilayer nickelates. In the absence of interlayer tunneling, a non-Fermi-liquid critical point tuned by interlayer spin coupling or hole doping is identified, separating the standard Fermi liquid in the overdoped region from a pseudogap metal in the underdoped region. This pseudogap phase, termed the “second Fermi liquid,” is characterized by small hole pockets and violates the perturbative Luttinger theorem, yet exhibits no symmetry breaking or fractionalization; its behavior resembles that of a heavy Fermi liquid with small quasiparticle residues and large effective masses. Furthermore, an intuitive analytical description of the pseudogap and ground-state wavefunction is provided within the ancilla fermion framework, where the ancilla fermion is interpreted as a spin polaron, and the Kondo resonance peak of this composite fermion is directly shown in DMFT calculations. Extending the analysis to finite interlayer tunneling, the study applies the results to the bilayer nickelate La₃Ni₂O₇, proposing that current experimental samples (x≈0.5) lie in the overdoped Fermi liquid region, while electron doping may drive the system into the pseudogap phase and the non-Fermi-liquid critical regime, offering theoretical predictions for understanding anomalous metallic behavior in such materials.

Infinite-layer nickelate superconductors provide a new platform to study the quantum critical origin of strange metals, yet their thin-film form forbids conventional calorimetry. Using the Seebeck coefficient as a proxy for low-temperature specific heat, we find at the critical doping x* that the high-temperature Seebeck response quantitatively matches the band structure measured by ARPES, indicating well-defined quasiparticles, while below 60 K, S/T develops a logarithmic divergence that persists to the lowest temperatures (after suppressing superconductivity with a magnetic field), marking x* as a quantum critical point that terminates the underdoped phase. Moreover, the Ni-d_{x^2-y^2} carrier density drops abruptly from 1+x above x* to x below x*, reproducing the hallmark signature of the pseudogap phase in cuprates. These results indicate that the underdoped region of infinite-layer nickelates is a pseudogap-like state, at whose end emergent strange metal behavior with T-linear resistivity provides strong evidence for a quantum critical origin of strange metals.

This study systematically analyzes the Raman response of superconducting multi-orbital systems using electronic Raman scattering methods, with nickelates as the application target. For three models—a single-layer and a bilayer two-orbital model involving d_{x^2-y^2} and d_{z^2} orbitals, and a bilayer single-orbital model with only d_{x^2-y^2} orbitals—multiple pairing symmetries including d-wave, s±-wave, and s-wave are considered, and the response characteristics under various Raman symmetries (A1g, B1g, B2g) are calculated. In the two-orbital models, a full multi-orbital approach is employed, incorporating both intra-orbital and inter-orbital scattering, and compared with the additive approximation that simply sums the Raman responses of individual bands. The results reveal distinct fingerprint features in the Raman spectra for different pairing symmetries and model structures, with the full multi-orbital calculations uncovering inter-orbital mixing effects that the additive approximation may overlook. These findings help clarify the minimal model for nickelate superconductivity, determine the magnitude and symmetry of their superconducting gaps, and provide a general theoretical framework for Raman experimental analysis of other multi-orbital superconductors, such as iron-based superconductors.

This review summarizes recent advances in nickelate superconductors, covering infinite-layer, bilayer, and trilayer systems, their superconducting properti