Paper

Researchers have developed a technique to integrate free-standing infinite-layer Nd₀.₈₅Sr₀.₁₅NiO₂ thin films into diamond anvil cells, thereby overcoming the difficulties of measuring such films under high-pressure conditions. By applying pressures up to approximately 90 GPa to the films, they observed that the superconducting transition temperature (T_c) increased monotonically and linearly from about 17 K at ambient pressure to roughly 74.2 K, with an enhancement rate of approximately 0.65 K/GPa and no signs of saturation. This linear, non-saturating pressure dependence of T_c markedly differs from the pressure-induced overdoping that leads to T_c suppression in most copper oxide superconductors and bilayer nickelates, suggesting that the pairing strength in infinite-layer nickelates can be elevated to unexpectedly high levels. Furthermore, measurements of the upper critical field and coherence length confirm the pressure-induced enhancement of the superconducting state. This study provides a new pathway for continuously enhancing superconductivity through lattice compression, and the developed free-film high-pressure technique holds promise for broad application to other two-dimensional materials.

Recent experimental observations have showed some signatures of superconductivity close to 80 K in La3Ni2O7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La3Ni2O7−δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T-linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system.

This paper combines density functional theory with the Hubbard U correction (DFT+U) and the finite displacement method to achieve a full Hubbard-corrected calculation of phonon spectra and electron-phonon matrices in strongly correlated materials. The authors apply this method to two representative systems: infinite-layer nickelate LaNiO₂ and ruthenium dioxide RuO₂. The results show that in 20% hole-doped LaNiO₂, the Hubbard U correction weakly enhances the electron-phonon interaction, but the total coupling strength remains small and insufficient to explain the experimentally observed superconducting transition temperature of approximately 10–30 K; this contradicts recent predictions from the GW correction, with the discrepancy arising from differences in the Fermi surface topology obtained by DFT+U and GW methods. In RuO₂, the Hubbard U correction eliminates imaginary phonon modes under TiO₂ substrate strain and significantly reduces the electron-phonon coupling, alleviating the contradiction between the theoretically overestimated electron-phonon coupling and the experimentally observed low superconducting transition temperature. This work provides a computational scheme that fully incorporates the Hubbard U correction for electron-phonon properties and highlights the critical influence of Fermi surface shape and correlation effects on phonon spectra and electron-phonon matrices.

Identification of superconductivity in the Ruddlesden-Popper phases of nickelates under high pressure remains challenging. Here, we report a comprehensive study of the crystal structure, electrical resistance, and Meissner effect in single crystals of bilayer nickelate La3Ni2O7 under hydrostatic pressures up to 104 GPa. Using high-pressure X-ray diffraction, we observe a structural transition from an orthorhombic to a tetragonal phase above 40 GPa. Superconductivity emerges with a maximum onset transition temperature Tconset of 83 K at 18.0 GPa, accompanied by zero resistance. The superconducting phase is gradually suppressed and vanishes above 80 GPa, forming a right-triangle-like superconducting region. Direct-current magnetic susceptibility measurements demonstrate the Meissner effect and reveal a superconducting volume fraction of ∼41% at 22.0 GPa and 20 K, confirming the bulk nature of superconductivity in La3Ni2O7. Our results highlight the intricate relationship between superconductivity, oxygen content, and structural transitions in this material.

Using high-pressure variable-temperature Raman spectroscopy and polarization analysis, this work systematically tracks the structural evolution of a La₃Ni₂O₇ single crystal down to 3 K and up to 32.7 GPa. Based on rigorous symmetry selection rules, the disappearance and renormalization of multiple phonon modes in the spectra indicate a first-order structural phase transition from the orthorhombic Amam phase to the orthorhombic Fmmm phase at approximately 14.5 GPa, precisely coinciding with the emergence of bulk superconductivity. Polarized Raman measurements further reveal that above 1.92 GPa the sample recovers its intrinsic D₂h symmetry through detwinning, and in the superconducting state (3 K, 19.45 GPa) phonon modes are still observed in polarization channels, directly ruling out the tetragonal I4/mmm phase. These results confirm that the intrinsic crystal structure of the pressurized superconducting state below 19.45 GPa is orthorhombic Fmmm, rather than the previously disputed tetragonal phase, and disclose that the 180° Ni–O–Ni bond angle along the c-axis is a key structural prerequisite for achieving a high superconducting transition temperature, thereby establishing a vital structural foundation for understanding the superconducting mechanism of bilayer nickelates.

This study employed spin-polarized scanning tunneling microscopy to perform real-space imaging of the stripe order in the trilayer nickelate La₄Ni₃O₁₀, revealing its local magnetic and charge distributions. The experiments showed that the stripe order exhibits a four-unit-cell periodicity, highly reminiscent of the stripe order in cuprate high-temperature superconductors, and opens a nearly complete energy gap of approximately 66 meV near the Fermi level. More importantly, when the tunneling electron energy exceeds a threshold of about 20 meV, discrete phase slips can be triggered, enabling atomic-scale imaging of stripe dynamics. These results underscore the crucial role of correlated physics in driving stripe-like order in lanthanum nickelates and reveal striking similarities to cuprate superconductors, providing important clues for understanding the pairing mechanism in nickel-based superconductors.

This study employs a multi-orbital tight-binding model to analyze non-Fermi liquid transport phenomena in the bilayer nickelate La₃Ni₂O₇, focusing on the influence of multiband effects on the Hall coefficient. Using the Green’s function method, a rigorous formula for the Hall coefficient incorporating the quasi-quantum metric (qQM) term is derived, revealing that the temperature dependence of this qQM term is crucial in strongly correlated multiband systems. Calculations show that spin fluctuations in the Ni d₂² orbital lead to stronger quasiparticle damping, while the Ni dₓ²₋ᵧ² orbital forms cold spots. The pronounced temperature dependence of the Hall coefficient in La₃Ni₂O₇ originates from the competition between the positive contribution of the hole band and the negative contribution of the electron band, with the qQM term enhancing the positive Hall coefficient at low temperatures and explaining the experimentally observed T-linear resistivity and the increase of the Hall coefficient upon cooling. Furthermore, the qQM term also plays a key role in describing the Nernst coefficient and other transport phenomena involving second derivatives of velocity. This study reveals the core mechanism of spin-fluctuation-induced orbital-selective renormalization in non-Fermi liquid transport, providing a theoretical framework for understanding the anomalous transport properties of this system.

The bilayer nickelate La3Ni2O7 under pressure has recently emerged as a promising system for high-Tc superconductivity. In this work, we investigate the fate of the superconducting properties in La3Ni2O7 under pressure, focusing on the effects of structural deformation and apical oxygen vacancies. Employing a low-energy effective t-J∥-J⊥ model for the $$3{d}_{{x}^{2}-{y}^{2}}$$orbitals within the slave-boson mean-field approach, we demonstrate that the pairing strength is significantly enhanced in the high-pressure tetragonal I4/mmm phase compared to the ambient pressure orthorhombic Amam phase. Furthermore, by simulating random configurations of apical oxygen vacancies, we show that oxygen vacancies suppress both pairing strength and superfluid density. These results underscore the critical role of pressure and oxygen stoichiometry in tuning the SC of La3Ni2O7, providing key insights into optimizing its high-Tc behavior.

The recent discovery of superconductivity in the bilayer Ruddlesden-Popper nickelate La 3 Ni 2 O 7 under high pressure has generated much interest in the superconducting pairing mechanism of nickelates. Despite extensive work, the superconducting pairing symmetry in La 3 Ni 2 O 7 remains unresolved, with conflicting results even for identical methods. We argue that different superconducting states in La 3 Ni 2 O 7 are in close competition and highly sensitive to the choice of interaction parameters as well as pressure-induced changes in the electronic structure. Our study uses a multiorbital Hubbard model, incorporating all Ni 3 d and O 2 p states. We analyze the superconducting pairing mechanism of La 3 Ni 2 O 7 within the random phase approximation and find a transition between d -wave and sign-changing s -wave pairing states as a function of pressure and interaction parameters, which is driven by spin fluctuations with different wave vectors. These spin fluctuations with incommensurate wave vectors cooperatively stabilize a superconducting order parameter with d x 2 − y 2 symmetry for realistic model parameters. Simultaneously, their competition may be responsible for the absence of magnetic order in La 3 Ni 2 O 7 , demonstrating that magnetic frustration and superconducting pairing can arise from the same set of incommensurate spin fluctuations.

The effects of Co doping in a series of La3Ni2−xCoxO7−δ samples before and after high oxygen pressure annealing have been investigated. The structural refinemen