Layered nickelates
Nickel oxides (nickelates) with a planar-square NiO2 lattice is a newly (re-)discovered family of materials which hosts superconductivity at ~20 K [Nature 572, 624 (2019)]. The most widely studied nickelate superconductors thus far are the “infinite-layer” compounds, in which the NiO2 sheets are separated by rare-earth (RE) layers. In RENiO2, nickel ion has an uncommon valency of 1+, realized by soft chemistry synthesis known as “topotactic reduction”, which presently limits the preparation of infinite-layer nickelates (ILNs) to thin-film geometry of ≲ 10-nm thickness. The 3d9 configuration of the Ni ion, hosted in a planar-square lattice, makes the ILNs close analogues to the high-Tc cuprates, albeit with a lower Tc and a finite RE contribution to the low-energy electronic structure. Clarifying the commonality and distinctions between the two (nominally) 3d quasi-two-dimensional oxides in their superconducting and normal-state behavior has attracted considerable interest.
+
9
9
9
Structural building block of superconducting cuprates and nickelates, and the Fermi surface (kz = 0) of isostructural CaCuO2 and LaNiO2. Figure reference: Nature 518, 179 & PRX 10, 011024.
Several families of ILNs are known to host a superconducting dome, primarily via chemical substitution of the RE ion with divalent ions (Sr or Ca ) which introduces holes into the Ni site. The Tc values of nominally the same compound reported by various groups show considerable variations, indicating the nickelate superconductivity is highly sensitive to the level of disorder and the challenging nature of its synthesis. As superconductivity in nickelates is currently limited to thin films, most existing experimental investigations are dedicated to transport studies. Nonetheless, systematic studies of the normal-state transport have remained sparse, in large part due to the difficulty in preparing high-quality thin films over a wide range of doping. In collaboration with Prof. Harold Hwang from Stanford University, we have reported an insulator-to-metal crossover near the edge of the superconducting dome in (Nd,Sr)NiO2 [Phys. Rev. Research 3, L042015 (2021)], providing insights into the weakly insulating behavior in underdoped ILNs [Front. Phys. 10, 846639 (2022)], and demonstrated the role of RE magnetism in determining the upper critical field of RENiO2 [Sci. Adv. 9, eadf6655 (2023)].
2+
2+
+
Resistivity versus temperature curves for superconducting Nd1-xSrxNiO2 and La2-xSrxCuO4. The T-linear resistivity slopes dρ/dT of nearly optimally doped samples (x ≈ 0.16) show remarkable similarity between the two systems. Figure reference: Nature 619, 288
More recently, signatures of high-Tc superconductivity has been reported in bulk La3Ni2O7 crystal under high pressure [Nature 621, 493 (2023)], raising the prospect of finding more nickelate superconductors in bulk form with a higher Tc. Recent spectroscopic experiments have also revealed the existence of spin and charge order in ILNs, drawing further parallel to its cuprate counterparts. Whether the nickelates also host pseudogap and/or strange metal phase on its phase diagram remains to be investigated.
Doping evolution of resistivity power-law exponent in Nd1-xSrxNiO2. Region with weakly insulating behavior (dρ/dT < 0) are shown in white. Figure reference: Phys. Rev. Research 3, L042015
Further reading
1. D. Li et al., "Superconductivity in an infinite-layer nickelate", Nature 572, 624 (2019)
2. M. R. Norman, "Entering the nickel age of superconductivity", Physics 13, 85 (2020)
3. B. Y. Wang et al., "Experimental progress in superconducting nickelates", Annual Review of Condensed Matter Physics 15, 305 (2024)