Paper

Bosonic Phases across the Superconductor-Insulator Transitions in Infinite-Layer Samarium Nickelate

The Ruddlesden–Popper (R–P) bilayer nickelate, La3Ni2O7, was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa (ref. 1). Subsequent investigations achieved zero resistance in single-crystalline and polycrystalline samples under hydrostatic pressure conditions2–4. Yet, obvious diamagnetic signals, the other hallmark of superconductors, are still lacking owing to the filamentary nature with low superconducting volume fraction2,4,5. The presence of a new 1313 polymorph and competing R–P phases obscured proper identification of the phase for HTSC6–9. Thus, achieving bulk HTSC and identifying the phase at play are the most prominent tasks. Here we address these issues in the praseodymium (Pr)-doped La2PrNi2O7 polycrystalline samples. We find that substitutions of Pr for La effectively inhibit the intergrowth of different R–P phases, resulting in a nearly pure bilayer structure. For La2PrNi2O7, pressure-induced orthorhombic to tetragonal structural transition takes place at Pc ≈ 11 GPa, above which HTSC emerges gradually on further compression. The superconducting transition temperatures at 18–20 GPa reach $${T}_{{\rm{c}}}^{{\rm{onset}}}=82.5\,{\rm{K}}$$and $${T}_{{\rm{c}}}^{{\rm{zero}}}=60\,{\rm{K}}$$, which are the highest values, to our knowledge, among known nickelate superconductors. Importantly, bulk HTSC was testified by detecting clear diamagnetic signals below about 75 K with appreciable superconducting shielding volume fractions at a pressure of above 15 GPa. Our results not only resolve the existing controversies but also provide directions for exploring bulk HTSC in the bilayer nickelates.

Recently, the Ruddlesden–Popper bilayer nickelate La3Ni2O7 has emerged as a superconductor with a transition temperature (Tc) of approximately 80 K above 14 GPa (refs. 1–3). Achieving a higher Tc in nickelate superconductors, along with the synthesis of reproducible high-quality single crystals without relying on high-oxygen-pressure growth conditions, remains a significant challenge4–7. Here we report superconductivity up to 96 K under high pressure in bilayer nickelate single crystals synthesized at ambient pressure. Energy-dispersive spectroscopy, single-crystal X-ray diffraction, nuclear quadrupole resonance and scanning transmission electron microscopy evidenced high crystal quality of the flux-grown La2SmNi2O7−δ single crystals. La2SmNi2O7 exhibits clear bulk superconductivity, including zero resistivity ( $${T}_{{\rm{c}},\max }^{{\rm{onset}}}$$ = 92 K and $${T}_{{\rm{c}},\max }^{{\rm{zero}}}$$ = 73 K at 21.6 GPa) and the Meissner effect (Tc = 60 K at 20.6 GPa). A low-temperature high-pressure structural study indicates that both monoclinic and tetragonal structures can support superconductivity in this bilayer nickelate. Furthermore, we established a correlation between higher Tc under high pressures and larger in-plane lattice distortion under ambient conditions, corroborated by observing even higher $${T}_{{\rm{c}}}^{{\rm{onset}}}$$of 96 K in La1.57Sm1.43Ni2O7−δ. This study overcomes key limitations in growing nickelate superconductor crystals, resolves the crystal structure in the superconducting state and demonstrates an effective pathway towards achieving higher Tc.

Based on the effective three-orbital model, this study systematically analyzes the interplay between multiorbital and nonlocal self-energy effects in the normal state of the high-pressure superconducting bilayer nickelate La₃Ni₂O₇ using the D-TRILEX many-body framework beyond dynamical mean-field theory. The results reveal that the low-energy physics is highly dependent on the interorbital interaction strength: when the interaction is weak, the renowned γ quasiparticle flat band lies below the Fermi level; as the interaction strengthens, this flat band crosses the Fermi level, causing electrons to scatter with ferromagnetic paramagnon excitations, thereby forming spin-polaron bound states. These bound states manifest as incoherent spectral weight shadow bands below the Fermi level. The findings unveil the existence of additional competing electronic states in bilayer nickelates, providing a theoretical basis for resolving recent controversies in angle-resolved photoemission spectroscopy experiments regarding spectral structures near the Fermi surface.

Resonant inelastic X-ray scattering (RIXS) at the Ni L edge on single crystals of the trilayer nickelate La4Ni3O10 reveals collective spin excitations with a bandwidth of approximately 60 meV, comparable to that of the bilayer nickelate La3Ni2O7, but with significantly reduced spectral weight, indicating weaker electronic correlations in the trilayer system. Localized spin excitations at around 100 and 200 meV are also observed, originating from local dipole and quadrupole excitations. The dispersive magnetic excitations exhibit three-dimensional characteristics, and fitting with linear spin-wave theory yields comparable in-plane and out-of-plane exchange coupling parameters, with the interlayer coupling being the strongest. The results indicate that La4Ni3O10 possesses stronger three-dimensional magnetism, with its spin dynamics consistent with spin-density-wave order, while the reduced electronic correlations and three-dimensional multi-orbital character are key factors leading to differences in its magnetic excitation spectrum compared to the bilayer nickelate, providing important insights into the evolution of magnetism and its connection to superconductivity in the Ruddlesden-Popper nickelate family.

Using polarization-resolved electronic Raman scattering, this study systematically maps the momentum-selective spin-density-wave (SDW) gap in the trilayer nickelate La4Ni3O10. The experiments reveal that SDW-induced spectral weight redistribution occurs simultaneously on the α pocket at the Brillouin zone center and on part of the β pocket near the zone boundary, with a corresponding gap energy of approximately 55 meV, whereas no comparable spectral weight suppression is observed in the diagonal region of the β pocket, indicating that this region remains nearly gapless. This momentum-space gap topology contrasts sharply with that of the bilayer nickelate La3Ni2O7, where only the β pocket exhibits an anisotropic SDW gap. These results establish distinct momentum-space gap topologies between bilayer and trilayer nickelates, providing new constraints on the ordering wave vector of the density-wave instability and the mechanism related to superconductivity.

Using Ni L3-edge resonant inelastic X-ray scattering (RIXS), this study compares low-energy spin excitations in the octahedral Ruddlesden-Popper (RP) phase Nd9Ni8O25 (non-superconducting) and its reduced planar phase Nd9Ni8O18 (exhibiting superconducting correlations at approximately 5 K). The results show that the octahedral phase exhibits a spin-density wave (SDW) ground state with an ordering wave vector of (1/4,1/4), where the low-energy spectrum is dominated by weakly dispersive paramagnons along the (0,π) and (π,π) directions; in contrast, the planar phase displays an elastic peak at (1/3,0) with dispersionless magnetic excitations at an energy of about 65 meV. Polarization-resolved RIXS further confirms the distinct nature of magnetic excitations in the two phases. These findings systematically reveal fundamental differences in the ground states and spin excitations between the two structural families, providing critical insights into the mechanism of nickelate superconductivity.

This study employs high-pressure synthesis and hydrostatic high-pressure transport techniques to systematically modulate the bandwidth and band filling in the bilayer nickelate La₃Ni₂O₇ family, aiming to investigate their effects on superconductivity and non-superconducting state properties. By partially substituting La with smaller Nd (which increases NiO₆ octahedral tilting and reduces bandwidth), the pressure required for the superconducting phase is significantly elevated; conversely, co-introducing Sr for hole doping reverses this trend, lowering the onset pressure of superconductivity. In the non-superconducting state, up to three characteristic resistance anomalies are observed, evolving with pressure, likely corresponding to charge density wave and spin density wave orders that compete with superconductivity. A comprehensive comparison of phase diagrams across samples with different compositions indicates that independent control of bandwidth and filling is key to unraveling the mechanism of unconventional superconductivity and its competing orders in this system.

The recent observation of high-𝑇𝑐 superconductivity in the bilayer nickelate La3⁢Ni2⁢O7 under pressure has garnered significant interest. While researches have predominantly focused on the role of electron-electron interactions in the superconducting mechanism, the impact of electron-phonon coupling (EPC) has remained elusive and unexplored. In this Letter, we perform first-principles calculations to study the phonon spectrum and electron-phonon coupling within La3⁢Ni2⁢O7 under pressure and explore the interplay between EPC and electronic interactions on the superconductivity by employing functional renormalization group (FRG) approach. Our calculations reveal that EPC alone is insufficient to trigger superconductivity in La3⁢Ni2⁢O7 under pressure. We identify unique out-of-plane and in-plane breathing phonon modes which selectively couple with the Ni 𝑑𝑧2 and 𝑑𝑥2−𝑦2 orbitals, showcasing an orbital-selective EPC. Within the bilayer two-orbital model, it is revealed that solely electronic interactions foster 𝑠±-wave pairing characterized by notable frustration in the band space, leading to a relatively low transition temperature. Remarkably, we find that the out-of-plane EPC can act in concert with electronic interactions to promote the interlayer pairing in the 𝑑𝑧2 orbital, partially releasing the pairing frustration and thus elevating 𝑇𝑐. In contrast, the inclusion of in-plane EPC only marginally affects the superconductivity, distinct from the cuprates. Potential experimental implications in La3⁢Ni2⁢O7 are also discussed.

The discovery of pressurized superconductivity at 80 K in La3Ni2O7 officially brings nickelates into the family of high-temperature superconductors, which gives rise to not only new insights but also mysteries in the strongly correlated superconductivity. More recently, the sibling compound La4Ni3O10 was also shown to be superconducting below about 25 K under pressure, further boosting the popularity of nickelates in the Ruddlesden-Popper phase. In this study, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structures of La4Ni3O10 at ambient pressure. We reveal a high resemblance of La4Ni3O10 with La3Ni2O7 in the orbital-dependent fermiology and electronic structure, suggesting a similar electronic correlation between the two compounds. The temperature-dependent measurements imply an orbital-dependent energy gap related to the density-wave transition in La4Ni3O10. By comparing the theoretical pressure-dependent electronic structure, clues about the superconducting high-pressure phase can be deduced from the ambient measurements, providing crucial information for deciphering the unconventional superconductivity in nickelates.