Recent progress in nickelate superconductors

Abstract

The discovery of superconductivity in nickelate compounds has opened new avenues in the study of high-temperature superconductors. Here we provide a comprehensive overview of recent progress in the field, covering various nickelate systems, including the reduced-Ruddlesden-Popper-type infinite layer LaNiO[Formula: see text] as well as the Ruddlesden-Popper-type bilayer La[Formula: see text]Ni[Formula: see text]O[Formula: see text] and trilayer La[Formula: see text]Ni[Formula: see text]O[Formula: see text]. We begin by introducing the superconducting properties of the hole-doped LaNiO[Formula: see text] system, which marked the starting point for nickelate superconductivity. We then turn to the bilayer La[Formula: see text]Ni[Formula: see text]O[Formula: see text] system, discussing both its high-pressure and thin-film superconducting phases. This is followed by an examination of the trilayer La[Formula: see text]Ni[Formula: see text]O[Formula: see text] system and other related multilayer nickelates. Throughout the review, we highlight emerging trends, key challenges and open questions. We conclude by addressing current limitations in materials synthesis and characterization, and future directions that may help uncover the mechanisms driving superconductivity in these complex oxide systems.

Keywords: electronic structure; nickelate superconductor; superconductivity.

Figure 1.

Structures, ionic charges and valence states of nickelates. The Ni atom has electron configuration [Ar]3 or [Ar]3. In nickel oxides, the Ni ionic charge typically ranges from to and the most common oxidation state is Ni. The RP nickelate phases LaNiO have ionic charges between Ni and Ni. The bilayer compound LaNiO and the trilayer LaNiO have recently been identified as superconductors. A reduced-RP phase, LaNiO, can be obtained from the RP LaNiO through chemical reduction. These removed oxygen sites are indicated by dashed green circles in LaNiO. Hole-doped LaNiO is the first nickelate superconductor.

Figure 2.

(a) LaNiO is synthesized by reducing LaNiO using CaH, which selectively removes oxygen atoms from the LaO planes [9]. (b) The left panel shows the resistivity of NdNiO (insulating) and NdSrNiO (metallic) thin films. The right panel displays the resistivity of the reduced compounds, NdNiO and NdSrNiO. A superconducting transition with an onset at 14.9 K is observed in NdSrNiO [9]. (c) The global phase diagram of hole-doped LaNiO [30]. (d) The ‘parent’ state of LaNiO is a Mott insulator, in contrast to cuprates, where the parent state is a charge-transfer insulator. We emphasize that the ‘parent’ state here refers to the idealized insulating limit of LaNiO, which may differ from the actual, experimentally realized parent compound.

Figure 3.

(a) The band structure of LaNiO, where the dominate valence electrons are from Ni 3 and La 5d orbitals. (b) The three-dimensional Fermi surface contour of LaSrNiO. (c) The Fermi surfaces at and calculated using tight binding and dynamical mean-field theory [45]. The yellow triangle markers indicate ARPES experimental data points [43].

Figure 4.

(a) Superconducting transition found in different NdNiO samples [38]. (b) In comparison to other 112 superconductors (SCs), SmSrNiO co-doped with Eu and Ca (SECS) achieve higher transition temperatures, reaching up to  K [36].

Figure 5.

(a) Global phase diagram of LaNiO as a function of pressure and temperature, revealing a transition from a low-pressure (LP) phase to a high-pressure (HP) phase. The inset illustrates the distinct crystal structures associated with each phase. The apical oxygen, which connects the two NiO layers, plays a key role in the electronic properties of the 327 compound. (b) Temperature-dependent resistance of LaNiO at 20.5 GPa, showing a superconducting transition onset at 66 K and reaching zero resistance around 40 K [49]. (c) Schematic of the orbital energy levels in bilayer LaNiO. Because of Jahn–Teller (JT) distortion, the orbital lies below the orbital. Strong interlayer coupling further splits the states into bonding () and antibonding () orbitals. (d) The ac magnetic susceptibility of LaPrNiO under various pressures of up to 19 GPa. The dashed line represents the background extrapolated from the high-temperature region [13]. The superconducting shielding volume fraction can reach more than .

Figure 6.

(a) The HP band structure of LaNiO and its projections into orbitals. (b) Fermi surfaces of LaNiO, featuring two prominent sheets: the  FS and the  FS. The presence of a  FS at the BZ corner crossing the Fermi level remains under debate [59,60]. (c, d) The four low-energy bands of LaNiO can be categorized into symmetric (sym) and antisymmetric (anti-sym) sectors. Their irreducible representations have been systematically analyzed [60]. Note that the two bands in the anti-sym sector are strongly entangled. (e) Simulated and experimentally observed STEM structures of LaNiO [61]. Yellow dashed circles indicate inner apical oxygen vacancies. (f) Phase histograms corresponding to different oxygen sites in regions with and , normalized to the average phase of the outer apical oxygen sites [61]. A significant loss of inner apical oxygens is observed in the region.

Figure 7.

(a) RIXS intensity maps along high-symmetry directions in LP LaNiO, with filled red circles marking the peak positions of magnetic excitations. Notably, the magnetic excitation spectrum becomes gapless at [64]. (b) Temperature-dependent NMR linewidths for the La(1)a and La(1)b sites, indicating an SDW transition at 150 K in LP LaNiO [65]. (c) Temperature evolution of the magnetic order parameter measured via SR, confirming magnetic ordering in the LP phase [63]. (d) ARPES measurements of the band structure in LP LaNiO [70]. (e) Laser-based ARPES mapping of the Fermi surface in the same LP phase [70]. (f) Fermi surface calculated using hybrid functional DFT, showing excellent agreement with the experimental ARPES data in (e), as indicated by the red circles and green diamonds [59].

Figure 8.

(a) Schematic illustration of the compressive strain induced by the substrate during thin-film deposition. (b) STEM images of LaNiO thin films grown under compressive and tensile strain conditions [83]. (c) Temperature-dependent resistivity of LaNiO thin films on various substrates [68]. Films grown on compressive SLAO substrates exhibit a broad superconducting transition. A1, A12, A13 and A14 denote different sample identifiers. (d) Resistivity measurements of LaPrNiO thin films, showing a superconducting transition onset above 48.1 K. Sample labels include P75, P78, P81 and P87 [80].

Figure 9.

(a, c) ARPES-measured and schematic Fermi surfaces of LaPrNiO [82], showing that the band does not cross the Fermi level. (b, d) ARPES-measured and schematic Fermi surfaces of LaPrNiO [84], where the band crosses the Fermi level.

Figure 10.

(a) Phase diagram of LaNiO [14], featuring both LP and HP phases. The LP phase exhibits a pronounced DW transition, while superconductivity emerges in the HP phase under pressure. (b) Resistivity of LaNiO measured under varying pressures, showing a superconducting transition [14]. (c) Density wave phases in the 43(10) compound. The trilayer is typically designated as the top (t), middle (m) and bottom (b) layers. Notably, the CDW exists in all three layers, whereas the SDW is present only in the top and bottom layers [88]. (d) Band structure and Fermi surfaces of LaNiO. The presence of the Fermi surface pocket at the Brillouin zone corner remains unresolved. (e) The band structure can also be decomposed into symmetric and antisymmetric sectors, with the symmetric sector resembling that of LaNiO.

Figure 11.

(a) Crystal structure of NdNiO [17], obtained by reducing the Ruddlesden–Popper phase NdNiO. (b) Crystal structure of LaNiO [18], formed by inserting a LaNiO layer into the bilayer LaNiO structure. This compound is also referred to as the 1212 phase. (c) Alternative crystal structure of LaNiO [58], composed of a trilayer LaNiO unit and a LaNiO block. This structure is known as the 1313 phase.