Motivated by the discovery of superconductivity in bilayer La3Ni2O7 at 80 K and the increased superconducting transition temperature, Tc, up to 96 K in single crystals of La1.57Sm1.43Ni2O7-delta under pressure, we systematically study the effect of Sm substitution on the superconductivity and structure of La3-xSmxNi2O7 (0 x 1.5) under pressure. Experimental investigations in polycrystalline samples reveal that Sm substitution monotonically decreases the lattice constants c and a, thereby enhancing crystal structural distortion and leading to an evolution of the metallic ground state in La3Ni2O7 to an insulating state in La1.5Sm1.5Ni2O7. The optimal Tonset c rises with increasing Sm substitution, and the maximum Tonset c is 89.2 K in polycrystalline La1.5Sm1.5Ni2O7. Divergences in Tc compared with single crystals of La1.57Sm1.43Ni2O7-delta are likely due to variations in the microstructure induced by different material growth approaches. Our results suggest that the enhancement of Tc in La3-xSmxNi2O7 is mainly affected by the compressed c lattice before saturation, and the superconducting transition pressure increases with substitution concentration. Our experimental results provide insight into the influence of small-radius rare-earth element substitution on nickelate superconductors, offering a means to further increase the transition temperature.

The synthesis of high-quality Ruddlesden-Popper (RP) nickelates remains challenging due to variations in oxygen content and the prevalence of intergrown RP phases. Precisely controlling the stoichiometry and characterizing the resulting physical properties are essential for understanding the mechanism of high-T_c superconductivity in these materials. In this work, we synthesize a series of La_3Ni_2O_7+δ samples with systematically controlled oxygen content and perform comprehensive structural and compositional analyses. Precise oxygen tuning enables us to tailor the microstructure, yielding a pure bilayer phase, a mixture of bilayer and hybrid single-layer-bilayer phases, and a predominantly bilayer phase containing trilayer intergrowths. High-pressure transport measurements reveal distinct superconducting transitions with contrasting T_c values, corresponding to the bilayer phase, the hybrid phase, and trilayer inclusions. Notably, we find that oxygen content not only governs the phase purity-i.e., the presence of intergrowth phases-but also directly modulates the upper critical field (H_c2) of the bilayer superconductivity. By establishing a phase diagram of T_c and H_c2 as functions of oxygen content in La_3Ni_2O_7+δ, this work advances synthetic control and provides new insights into the superconducting mechanism of RP nickelates.

Systematically controlling the superconducting transition temperature (Tc) in the bilayer Ruddlesden-Popper nickelate La3Ni2O7 remains a significant challenge. Here, we address this by synthesizing high-quality polycrystalline La3−xNdxNi2O7 (0 ≤ x ≤ 2.4) with record-level rare-earth substitution. Nd doping compresses the lattice and enhances the spin density wave (SDW) transition temperature, and elevates the pressure required for the orthorhombic-to-tetragonal structural transition. Superconductivity is observed across all doping levels in high-pressure electronic transport measurements, with the onset Tc rising to ~ 93 K and the resistance derivative indicating the signature of superconductivity reaching 96–97 K for x = 2.1 and 2.4. Using the radio-frequency transmission technique, recently applied to nickelate superconductors, we detect signatures of superconductivity at 100.5 K in the x = 2.1 compound, pushing the Tc frontier further. Our work reveals the critical role of magnetism and provides a structural descriptor for elevating Tc in Ruddlesden-Popper nickelates. The authors synthesize polycrystalline La3-xNdxNi2O7 with record-level rare-earth Nd substitution (x = 2.4) creating chemical pressure. Radio-frequency transmission reveals superconducting signatures up to 100.5 K under 33 GPa pressure.

The precise crystal structure of La3Ni2O7 in its high-pressure superconducting state has been a subject of intense debate, with proposed models including both orthorhombic and tetragonal symmetries. Using high-pressure Raman spectroscopy combined with frst-principles calculations, we unravel the structural evolution of La3Ni2O7 under pressure up to 32.7 GPa. We identify a clear structural transition sequence: from the orthorhombic Amam phase to a mixed Amam+Fmmm phase at 4 GPa, followed by a complete transition to the tetragonal I4/mmm phase at 14.5 GPa, which is signaled by a pronounced phonon renormalization. The emergence of bulk superconductivity is found to coincide precisely with this transition to the I4/mmm phase. Our results de nitively establish the tetragonal I4/mmm structure as the host of superconductivity in La3Ni2O7, resolving a central controversy and providing a critical foundation for understanding the superconducting mechanism in nickelates.

Motivated by the discovery of superconductivity in bilayer La_3Ni_2O_7 at 80 K and the increased superconducting transition temperature, T_c, up to 92 K in single crystals of La_2SmNi_2O_7 under pressure, we systematically study the effect of Sm doping on the superconductivity and structure of La_3-xSm_xNi_2O_7 (0 ≤ x ≤ 1.5) under pressure. Experimental investigations in polycrystalline samples reveal that Sm doping monotonically decreases the lattice constants c and a, thereby enhancing crystal structure distortion and leading to an evolution of the metallic ground state in La_3Ni_2O_7 to an insulating state in La_1.5Sm_1.5Ni_2O_7. The maximum onset T_c in compounds x=0.9 and 1.5 is 89 K, while the pressure that drives the emergence of superconductivity is higher for higher doping levels. The results suggest that the enhancement of T_c in La_3-xSm_xNi_2O_7 is mainly affected by the compressed c lattice before saturation, and the structure transition is critical for the emergence of superconductivity. Our experimental results provide insight into the influence of elemental substitution on nickelate superconductors, offering a means to increase the transition temperature further.

Systematically controlling the superconducting transition temperature (T_c) in the bilayer Ruddlesden-Popper nickelate La_3Ni_2O_7 remains a significant challenge. Here, we address this by synthesizing high-quality polycrystalline La_3-xNd_xNi_2O_7 (0 ≤ x ≤ 2.4) with record-level rare-earth substitution. Nd doping compresses the lattice, particularly along the c axis, enhances the spin density wave transition temperature, and elevates the pressure required for the orthorhombic-to-tetragonal structural transition. Superconductivity is observed across all doping levels under high pressures, with the onset T_c rising to ∼93 K for x = 2.1 and 2.4 from the electronic transport measurement. Using the radio-frequency transmission technique, newly applied to nickelate superconductors, we detect signatures of superconductivity at 98 ± 2 K in the x=2.4 compound, pushing the T_c frontier further. We identify a universal linear relationship where T_c decreases with the c-axis lattice parameter at a rate of approximately -28 K/Å, demonstrating that enhanced interlayer magnetic exchange coupling is the dominant mechanism for superconducting pairing. Our work establishes the critical role of magnetism and provides a unified structural descriptor for elevating T_c in bilayer nickelates.

The discovery of unconventional superconductivity around 80 K in perovskite nickelates under high pressure has furnished a new platform to explore high-temperature unconventional superconductivity in addition to cuprates. Understanding the normal state of nickelate superconductors is crucial to uncovering the origin of this unconventional superconductivity and gaining further insight into its underlying mechanism. In this study, we systemically studied the transport properties of La3Ni2O7 by tuning the pressure under high magnetic fields. Magnetoresistance (MR) consistently exhibits a quasi-quadratic dependence on the magnetic field across all measured pressures and temperatures. Increased pressure enhances the metallicity of the system and leads to a monotonic increase in MR, which follows the extended Kohler's rule. These results suggest that the normal state of La3Ni2O7 to be a multiband metallic nature.

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.

The discovery of high-temperature superconductivity in La3Ni2O7 and La4Ni3O10 under pressure has garnered extensive attention. Herein, we report systematic investigations on the structure, magnetism, and electrical resistance evolutions of Pr4Ni3O10-delta polycrystalline samples under various pressures. Pr4Ni3O10-delta exhibits density wave transitions on Ni and Pr sublattices at approximately 157.6 and 4.3 K, respectively. The density wave can be progressively suppressed by pressure. A structural transformation from the monoclinic P21/a space group to the tetragonal I4/mmm occurs around 20 GPa. An apparent drop in resistance with an evident magnetic field dependence is observed at pressures above 20 GPa, indicating the emergence of superconductivity. The discovery of superconductivity in Pr4Ni3O10-delta broadens the family of nickelate superconductors. Pr4Ni3O10-delta provides a new platform for investigation of the mechanisms of superconductivity in the Ruddlesden-Popper phases of nickelates.

The discovery of high critical temperature (Tc) superconductivity in pressurized La_3Ni_2O_7 has ignited renewed excitement in the search of novel high-Tc superconducting compounds with 3d transition metals. Compared to other ambient-pressure superconductors, such as copper-oxide and iron-oxypnictides, unraveling the mechanisms of the pressure-induced superconductivity poses significant and unique challenges. A critical factor in this phenomenon seems to be related to the electronic configuration of 3d orbitals, which may play a fundamental role in driving high-Tc superconductivity. However, the pressure effects on the mixed-valence states of 3d-orbital cations and their influence on the emergence of high-Tc superconductivity remain poorly understood. Here, we use high-pressure (P) and low-temperature synchrotron X-ray absorption spectroscopy to investigate the influence of pressure on the mean valence change of Ni ions in La_3Ni_2O_7. Our results demonstrate that at a low-temperature of 20 K, the mean valence remains relatively stable across the pressures range from 1 atm to 40 GPa. Based on analyzing the absorption data, we find that, at a critical pressure, the ambient-pressure ordered phases disappear and both the structural and the superconducting phase transition occur. The pressure-induced structural phase transition revealed by our absorption results is consistent with that determined by X-ray diffraction, offering new information for a comprehensive understanding on the pressure-induced superconductivity in La_3Ni_2O_7.

The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La_3Ni_2O_7 has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) hybrid phase of La_3Ni_2O_7. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic Cmmm to the tetragonal P4/mmm space group at 13 GPa. Above 19 GPa, the phase exhibits a clear superconducting transition, confirmed by a zero-resistance state, albeit at a significantly reduced temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results demonstrate that the superconducting transition temperature is directly governed by the nature of the interlayer coupling, and the bilayer NiO_6 block as the essential structural motif for achieving high-T_c superconductivity in the RP nickelates.

The recent discovery of compressed superconductivity at 80 K in La3Ni2O7-δ has brought nickelates into the family of unconventional high-temperature superconductors. However, due to the challenges of directly probing the superconducting pairing mechanism under high-pressure, the pairing symmetry and gap structures of nickelate superconductors remain under intense debate. In this work, we successfully determine the microscopic information on the superconducting gap structure of La3Ni2O7-δ samples subjected to pressures exceeding 20 GPa, by constructing different conductance junctions within diamond anvil cells. By analyzing the temperature-dependent differential conductance spectra within the Blonder-Tinkham-Klapwijk (BTK) model, we have determined the superconducting energy gap at high pressure. The differential conductance curves reveal a two-gap structure with Δs1 = 23 meV and Δs2 = 6 meV, while the BTK fitting consistent with an s-like, two-gap spectrum. The gap ratio 2Δs1(0)/kBTc is found to be 7.41, belonging to a family of strongly coupled superconductors. Our findings provide valuable insights into the superconducting gap structures of the pressure-induced superconducting nickelates.

We report on optical studies of the Ruddlesden-Popper nickelates Lan+1NinO3n+1, with n = 2 (La3Ni2O7), n = 3 (La4Ni3O10), and n = infinity (LaNiO3). As the number of the NiO6 octahedra layers n grows, the ratio of the kinetic energy determined from the experimental optical conductivity and that from band theory Kexp/Kband increases, suggesting a reduction of electronic correlations. While the bilayer La3Ni2O7 lies on the verge of the Mott insulating phase, the trilayer La4Ni3O10 and infinite-layer LaNiO3 fall into the regime of correlated metals. The evolution of the electronic correlations in Lan+1NinO3n+1 is likely to be dominated by the Ni dz2 orbital. Our results provide important information for understanding the superconductivity in Ruddlesden-Popper nickelates.

We report the systematic synthesis, crystal structure, magnetization, and powder neutron diffraction of single-crystalline and polycrystalline CaCo2TeO6 samples. CaCo2TeO6 crystallizes in an orthorhombic structure with Pnma space group, featuring chains of edge-shared CoO6 octahedra arranged in a honeycomb pattern. Two antiferromagnetic transitions are observed at TN1 = 14.4 K and TN2 = 16.2 K, corresponding to two long-range magnetic orders with propagation vectors of k1 = (0, 0, 0) and k2 = (0.125, 0, 0.25), respectively. The ground state is determined as a canted up-up-down-down zigzag spin configuration along the c axis, wherein the magnetic moments of Co1 and Co2 ions are 3.4(1)mu B and 2.1(1)mu B, respectively. Successive spin-flop transitions appear with the increasing magnetic field applied along the easy axis (c axis), accompanied by depression of the antiferromagnetic orders and enhancement of residual magnetic entropy. The field-induced spin-disordered state suggests that CaCo2TeO6 may be an ideal candidate for studying frustrated magnetism.

The discovery of superconductivity in pressurized La3Ni2O7 with a transition temperature of approximately 80 K above the boiling point of liquid nitrogen has sparked significant attention. It is essential to search for high-temperature superconductivity in bulk samples and at ambient pressure in nickelates. In this study, we report influential factors that affect the appearance of superconductivity in La3Ni2O7 at ambient pressure. From direct-current magnetic measurements, we observe a clear diamagnetic response at 80 K in post-annealed single crystals of La3Ni2O7 in oxygen. The superconducting volume fraction is estimated to be within 0.2 decrease in resistivity. This work presents a practical approach for further investigating high-temperature superconductivity in nickelates at ambient pressure.

Signatures of superconductivity near 80 K have recently been discovered in single crystals of La3Ni2O7 under pressure, which makes it a new candidate for high-temperature superconductors dominated by 3d transition elements, following the cuprate and iron-pnictide superconductors. However, there are several critical questions that have been perplexing the scientific community: (1) What factors contribute to the inconsistent reproducibility of the experimental results? (2) What is the fundamental nature of pressure-induced superconductivity: bulk or nonbulk (filamentary-like)? (3) Where is the superconducting phase located within the sample if it is filamentary-like? (4) Is the oxygen content important for the development and stabilization of superconductivity? In this study, we employ comprehensive high-pressure techniques to address these questions. Through our modulated ac susceptibility measurements, we are the first to find that the superconductivity in this nickelate is filamentary-like. Our scanning transmission electron microscopy investigations suggest that the filamentary-like superconductivity most likely emerges at the interface between La3Ni2O7 and La4Ni3O10 phases. By tuning the oxygen content of polycrystalline La3Ni2O7, we also find that it plays vital role in the development and stabilization of superconductivity in this material. The upper and lower bounds on the oxygen content are 7.35 and 6.89, respectively. Our results provide not only new insights into the puzzling issues regarding this material, but also significant information that will enable a better understanding of its superconductivity.

AbstractThe latest discovery of high temperature superconductivity near 80 K in La3Ni2O7 under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La3Ni2O7 by high-resolution angle-resolved photoemission spectroscopy. The Fermi surface and band structures of La3Ni2O7 are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum-dependent. A flat band is formed from the Ni-3d$${}_{{z}^{2}}$$ z 2 orbitals around the zone corner which is ~ 50 meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La3Ni2O7. Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La3Ni2O7.

The striking discovery of high-temperature superconductivity (HTSC) of 80 K in a bilayer nickelate La$_3$Ni$_2$O$_7$ under a moderately high pressure of about 14 GPa ignited a new wave of studying HTSC in nickelates. The properties of the parental phase at ambient pressure may contain key information on basic interactions therein and bosons that may mediate pairing giving birth to superconductivity. Moreover, the bilayer structure of La$_3$Ni$_2$O$_7$ may suggest a distinct minimal model in comparison to cuprate superconductors. Here using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we studied La$_3$Ni$_2$O$_7$ at ambient pressure, and found that Ni 3$d_{x^2-y^2}$, Ni 3$d_{z^2}$, and ligand oxygen 2$p$ orbitals dominate the low-energy physics with a small charge-transfer energy. Remarkably, well-defined optical-like magnetic excitations were found to soften into a quasi-static spin-density-wave ordering, evidencing the strong electronic correlations and rich magnetic properties. Based on a Heisenberg spin model, we found that the inter-layer effective magnetic superexchange interaction is much larger than the intra-layer ones, and proposed two viable magnetic structures. Our results set the foundation for further exploration of La$_3$Ni$_2$O$_7$ superconductor.

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctuations have been widely believed to play a crucial role in forming the superconducting Cooper pairs. The recent discovery of high-temperature superconductivity near 80 K in the bilayer nickelate La_3Ni_2O_7 under pressure provides a new platform to elucidate the origins of high-temperature superconductivity. We perform elastic and inelastic neutron scattering studies on a polycrystalline sample of La_3Ni_2O_7-δ at ambient pressure. No magnetic order can be identified down to 10 K. The absence of long-range magnetic order in neutron diffraction measurements may be ascribed to the smallness of the magnetic moment. However, we observe a weak flat spin-fluctuation signal at ∼ 45 meV in the inelastic scattering spectra. The observed spin excitations could be interpreted as a result of strong interlayer and weak intralayer magnetic couplings for stripe-type antiferromagnetic orders. Our results provide crucial information on the spin dynamics and are thus important for understanding the superconductivity in La_3Ni_2O_7.