Paper

This study finds that zero resistance and a diamagnetic response are observed in stoichiometric PrNiO₂ thin films without cation substitution and in the absence of a capping layer, thereby confirming the intrinsic superconductivity of the parent infinite-layer nickelate. Heterostructure engineering by inserting a non-superconducting buffer layer rules out contributions from substrate interface effects. Isovalent trivalent La substitution likewise preserves superconductivity, demonstrating that the phenomenon is not unique to PrNiO₂. In contrast, trace divalent Sr or Ca substitution (within about 3%) rapidly suppresses superconductivity so that it no longer appears. Combined with angle-resolved photoemission spectroscopy measurements, this reveals that the superconducting phase is confined to an extremely narrow hole-doping range within 3% beyond the parent PrNiO₂. A non-superconducting region exists between this phase and the previously established superconducting dome at approximately 20% divalent doping, giving rise to two separated superconducting regions on the phase diagram. Furthermore, the parent superconducting phase exhibits a significantly stronger upper critical field anisotropy. These results indicate that infinite-layer nickelates are not simply cuprate analogs but possess unique superconducting physics and segregated superconducting domains.

In compressively strained bilayer nickelate thin films, by continuously tuning the oxygen stoichiometry, researchers have discovered a superconducting half-dome. Starting from the optimal superconducting state, increasing the oxygen content progressively suppresses superconductivity, driving a transition toward a metallic phase; conversely, decreasing the oxygen content induces a granular superconductor-insulator transition while the onset superconducting temperature remains unchanged. This half-dome structure originates from the distinct roles of interstitial oxygen and oxygen vacancies: the former primarily regulates carrier concentration through doping effects, whereas the latter introduces strong scattering that leads to electronic inhomogeneity. Experiments show that this half-dome consistently appears across different rare-earth combinations and with or without alkaline-earth doping, revealing a universal feature of the bilayer nickelate phase diagram. This finding offers new perspectives for understanding the emergence and suppression of superconductivity in correlated electron systems.

This study proposes a unified theory based on a two-component model to explain a series of key experimental phenomena in pressurized bulk and thin-film bilayer nickelate superconductors. Centered on the interlayer superexchange coupling and hybridization between strongly correlated localized electrons and itinerant electrons at the nickel orbitals, the theory predicts two distinct behaviors of the superconducting transition temperature with doping: when the interlayer superexchange coupling is strong, electron or hole doping respectively produce two superconducting domes, with a non-superconducting interlayer valence-bond state appearing near half-filling; when the coupling is weak or moderate, the two domes merge into a single dome that spans half-filling but has a lower maximum temperature. Increasing doping drives the normal state from a Fermi liquid to a non-Fermi liquid or weakly insulating state, with a quasi-linear resistivity scattering rate emerging near optimal doping. Oxygen vacancies or chemical substitutions can disrupt the interlayer valence bond, simultaneously suppressing superconductivity and inducing local Kondo scattering of itinerant electrons, which explains the logarithmic temperature dependence of resistivity and the negative magnetoresistance observed in non-superconducting samples. This framework uniformly accounts for the differences in superconducting transition and normal state between bulk and thin films, the effects of hole doping and oxygen stoichiometry on the dome shape, and the competitive relationship between superconductivity and the Kondo effect. Based on the theory, the authors propose that bulk superconductivity at ambient pressure can be achieved through doping or by reducing the interlayer magnetic coupling, and predict that electron doping will yield higher transition temperatures.

Based on the previously proposed effective d_{x^2-y^2} orbital bilayer t-J∥-J⊥ model with model parameters input from first-principles calculations, this paper provides a unified explanation for a series of experiments on the regulation of the superconducting transition temperature (Tc) in La₃Ni₂O₇ via oxygen stoichiometry, elemental substitution, pressure, or strain, using slave-boson mean-field and density matrix renormalization group methods. The model reveals that, due to the near quarter-filling of the d_{x^2-y^2} orbital, its Tc tuning behavior resembles that of hole-doped overdoped cuprates. In terms of doping dependence, the system exhibits particle-hole asymmetry: hole doping suppresses Tc by making the system more overdoped, while electron doping has the opposite effect, explaining the Tc suppression caused by excess oxygen or Ca/Sr substitution for La, as well as the “half-dome” behavior in oxygen stoichiometry tuning. Regarding interaction dependence, Tc varies with the interlayer antiferromagnetic superexchange interaction J⊥, accounting for the enhancement of bulk Tc by Sm/Nd substitution for La, the “right-triangle” shape of pressure-dependent bulk Tc, and the enhancement of Tc under compressive strain in thin films. Compared with weak-coupling theory (where Tc depends mainly on the density of states) and the d_{z^2} orbital-dominated pairing mechanism (where Tc is proportional to the d_{z^2} hole density), this model provides a more natural and unified explanation. The paper further proposes that Tc can be increased through electron doping that does not introduce disorder, such as substituting La with higher-valent elements.

Absence of Ni₂/Ni₃ charge disproportionation and possible roles of O₂ p holes in La₃Ni₂O₇−δ revealed by hard x-ray photoemission spectroscopy

Recently, the Ruddlesden-Popper bilayer nickelate $La_3Ni_2O_7$ has emerged as a superconductor with a transition temperature ($T_c$) of ~ 80 K above 14 GPa$^{[1-4]}$. Efforts to search for nickelate superconductors with higher $T_c$$^{[5,6]}$, to grow reproducible high-quality single crystals$^{[2,7-10]}$, and to eliminate reliance on demanding high gas pressure synthesis conditions$^{[11]}$, remain significant challenges. Here we report superconductivity up to 92 K under high pressure in single crystals of bilayer nickelates synthesized at ambient pressure using flux methods. High quality $La_2SmNi_2O_{7-δ}$ single crystals with dimensions up to 220 μm on edge were successfully grown. At ~ 15 GPa, these crystals exhibit superconductivity with an onset transition temperature ($T_c^{onset}$) of 68 K and zero-resistance temperature ($T_c^{zero}$) of 47 K. Increasing pressure further enhances both transition temperatures, reaching record values for nickelates: $T_{c,max}^{onset}$ = 92 K and $T_{c,max}^{zero}$ = 73 K @ 21 GPa. Notably, higher $T_c$ correlates with larger in-plane lattice distortion at ambient conditions for bilayer nickelates. Furthermore, we observed a structural transition from monoclinic $P2_1/a$ to tetragonal $I4/mmm$ at ~ 18 GPa, indicating that tetragonal structure is not a prerequisite for superconductivity to appear in this bilayer nickelate. This study provides an easy-to-access method for growing reproducible high-quality bilayer nickelate single crystals and offers new insights into achieving higher Tc superconductivity.

Using polarization-resolved electronic Raman scattering measurements on high-quality La₃Ni₂O₇ single crystals, we observe a pronounced, symmetry-dependent spectral weight redistribution across the density-wave transition below 150 K: the B₁g channel exhibits an asymmetric peak, while the B₂g channel shows a symmetric broad peak, corresponding to electronic excitations near the X/Y points of the Brillouin zone and along the diagonal directions, respectively. Quantitative analysis extracts two sets of SDW gap values, with the B₁g channel gap approximately 37.5–40.4 meV (2Δ/kBTc ≈ 5.5–5.9) and the B₂g channel gap about 23.0 meV (2Δ/kBTc ≈ 3.4), indicating intermediate-to-strong coupling for the former and weak coupling for the latter. This momentum-selective anisotropic coupling strength cannot be explained by simple weak-coupling nesting theory, revealing that the unconventional SDW originates from anisotropic electronic correlations. The temperature dependence of the gap is significantly weaker than mean-field expectations, and the isotropy of the B₂g channel along with the weak anisotropy of the B₁g channel further support the coexistence of two distinct coupling mechanisms. This work establishes the electronic characteristics of the SDW in La₃Ni₂O₇, providing a microscopic foundation for understanding the emergence of high-temperature superconductivity in nickelates under pressure.

This study employs first-principles density functional theory (GGA and GGA+U) to investigate the electronic and magnetic properties of La3Ni2O5F, which features a bilayer NiO2 infinite-layer structure where La(O/F)La blocking layers achieve strict isolation of the NiO2 bilayers, forming a purely two-dimensional electronic and magnetic system. Calculations reveal an E* single band composed of electron density in the interstitial region, which is not associated with any atomic orbital, dips below the Fermi level along the M-A direction, and provides a self-doping of 0.09 holes per Ni, resulting in an actual Ni valence of +1.09; the Fermi surface of this E* band is cylindrical, occupying 9% of the Brillouin zone area. The dpσ band is nearly half-filled but is shifted near a Van Hove singularity due to the self-doping, and the magnetic response exhibits anomalous characteristics distinct from previous nickelates, with the magnetic susceptibility tending to vanish under a large magnetic field. The absence of a magnetic phase transition can be attributed to strong two-dimensional spin fluctuations and the self-doping effect away from half-filling, revealing the unique behavior of Ni¹⁺ ions in this system and the critical influence of the interfacial blocking layer on the interstitial band morphology.

This study employs atomic-resolution scanning tunneling microscopy and spectroscopy to characterize 1.5 unit-cell-thick (La,Pr)₃Ni₂O₇ ultrathin films grown on SrLaAlO₄. Through low-temperature ultrahigh vacuum sample transfer, an ordered √2×√2 surface reconstruction is preserved, and a U-shaped spectrum with two gap scales (approximately 14 and 20 meV) and a flat zero-bias conductance is observed in the tunneling spectra, indicating a nodeless superconducting gap. In contrast, if the sample is exposed to ultrahigh vacuum for a longer time during transfer without cooling, although the surface reconstruction and a transport superconducting onset temperature above 40 K are maintained, the tunneling spectrum becomes V-shaped, and the wide-energy spectrum shows that oxygen deficiency mixes spectral weight related to density waves. By comparing samples with different transfer times, it is determined that controlling the oxygen content is necessary to obtain an intrinsic superconducting gap, providing atomic-scale observational evidence for the intrinsic nodeless superconducting gap in bilayer nickelate ultrathin films.

This study realized a superconductor-insulator transition by fabricating infinite-layer samarium nickel oxide superconducting thin films into a spatially periodic network structure, thereby modulating the phase coherence of Cooper pairs. The observation of magnetoresistance oscillations with a period of h/2e in the experiments directly confirmed the existence of 2e Cooper pairs in nickel oxides. The transition was primarily driven by enhanced superconducting fluctuations, with Cooper pairs participating in charge transport throughout the entire transition process. Two anomalous metallic states were also identified: one emerging under finite magnetic fields and the other appearing even at zero magnetic field; both states could be characterized by bosonic excitations, suggesting the dynamic role of vortices in the ground state. This work establishes nickel oxides as a key platform for studying the rich bosonic phases arising from the modulation of Cooper pair phase coherence.