Paper

Daily Overview: This post sorts papers by relevance to nickelate superconductors. Summaries are AI-generated and may contain errors. arXiv submission processing window: times are unavailable (UTC). 1. Electronic Nematicity Revealed by Polarized Ultrafast Spectroscopy in Bilayer La$_3$Ni$_2$O$_7$ Relevance Score: 5.7100 Authors: Qi-Yi Wu, De-Yuan Hu, Chen Zhang, Hao Liu, Bo Chen, Ying Zhou, Zhong-Tuo Fu, Chun-Hui Lv, Zi-Jie Xu, Hai-Long Deng, Meng-Wu Huo, H. Y. Liu, Jun Liu, Yu-Xia Duan, Dao-Xin Yao, Meng Wang, Jian-Qiao Meng Link: http://arxiv.org/abs/2601.01702v1 Summary: Using polarized ultrafast pump-probe spectroscopy, the researchers comparatively investigated the normal-state electronic dynamics of bilayer La₃Ni₂O₇ and trilayer La₄Ni₃O₁₀ single crystals under ambient pressure. Both materials exhibit a density-wave transition accompanied by the opening of a quasiparticle relaxation bottleneck, yet their electronic responses display markedly different symmetries: trilayer La₄Ni₃O₁₀ remains optically isotropic across the entire temperature range, whereas bilayer La₃Ni₂O₇ shows clear twofold (C₂) rotational symmetry breaking—i.e., electronic nematicity—at low temperatures. This nematicity manifests in the anisotropy of slow quasiparticle relaxation dynamics and effective gap scale, and below 115 K it competes with a secondary isotropic order, leading to a non-monotonic temperature dependence of the nematic signal. This work reveals the presence of electronic nematic fluctuations in bilayer nickelates, which are absent in the trilayer system, suggesting a close relationship between electronic nematicity and high-pressure superconducting pairing in La₃Ni₂O₇, thereby providing key insights into the microscopic mechanism of this class of nickel-based superconductors. ...

arXiv Daily: nickelate superconductors 2026-01-05

This study utilized X-ray absorption spectroscopy and resonant inelastic X-ray scattering to independently control the strain and oxygen content of (La,Pr)₃Ni₂O₇₋δ thin films, tracking the microscopic evolution from a non-superconducting parent phase to a superconducting phase. The results demonstrate that both tuning methods induce delocalization of the oxygen 2p_z and nickel 3d_z² orbitals, as evidenced by spectral weight transfer from the “upper Hubbard” peak to the hole peak in the O K-edge absorption spectra, accompanied by broadening and weakening of the Ni L-edge absorption spectra and dd excitations. Concurrently, the intensity and correlation length of the long-range spin density wave (SDW) order are significantly suppressed, indicating direct competition with superconductivity; while short-range magnons are damped, their bandwidth remains unchanged. This suggests that the delocalization of oxygen 2p_z and nickel 3d_z² orbitals, along with the robustness of short-range magnons during the melting of the SDW order, are prerequisites for achieving superconductivity, thus providing constraints for theoretical models and pointing toward an orbital-selective pathway for designing nickel-based superconductors.

This study developed a megapascal-level high-pressure oxygen-assisted chemical synthesis route that successfully enabled the effective growth of infinite-layer nickelates extending to heavier rare earth elements, with composition (RE₁₋ᵧRE’ᵧ)₁₋ₓEuₓNiO₂ (RE/RE’: Pr, Nd, Sm, Gd, Dy). Hole doping was realized through Eu²⁺/Eu³⁺ valence variation. At the superconducting dome boundaries of Nd₁₋ₓEuₓNiO₂ and Pr₁₋ₓEuₓNiO₂ systems, robust uniaxial anisotropic magnetic-field reentrant superconductivity was observed, arising from the competition between Eu²⁺ 4f⁷ magnetic moments and magnetic fluctuations, while the optimally doped regions exhibited conventional high-temperature superconductivity with a critical current density reaching ~266 kA/cm² at 2 K, surpassing that of traditional Sr/Ca-doped systems. Further introduction of different RE′ magnetic ions allowed tuning of the exchange field strength and thereby modulation of quantum criticality, both enhancing the reentrant behavior and elevating T_c to 40.1 K. This work reveals the critical role of rare-earth 4f magnetic moments in modulating pairing strength and quantum criticality, establishing a synthetic foundation for utilizing the infinite-layer nickelate platform to investigate 4f-related unconventional superconductivity and quantum phase transitions.

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.