Revealing the correlation between superfluid density and transition temperature in an infinite-layer nickelate superconductor

In high-temperature superconductors, electrons form Cooper pairs and flow without resistance below a critical temperature T_c. Identifying the limiting factor of T_c is crucial for deciphering the microscopic mechanism of high-temperature superconductivity. So far, it has been found that many unconventional superconductors, including the cuprates, exhibit a strong correlation between T_c and the zero-temperature superfluid density (ρ_s0). The latter encodes the stiffness of the quantum mechanical phase of Cooper pairs, reflecting the resilience of the superconductor to thermal or quantum phase fluctuations.

“It is natural to pose the same question for the recently discovered infinite-layer nickelate superconductors, which share the same electron configuration and crystal structure as the cuprates but differ from them in the chemical doping phase diagram and pairing symmetry,” said corresponding author, Yihua Wang, a professor in the Department of Physics at Fudan University who leads a team that focuses on the magnetic imaging of quantum materials.

In a recent study published in National Science Review, researchers employed scanning superconducting quantum interference device (sSQUID) microscopy to investigate the spatial distribution of T_c and superfluid density in Nd_1-xSr_xNiO₂ (NSNO) films, from which a strong correlation between T_c and ρ_s0 was unveiled. To their surprise, the correlation is quantitatively similar to that reported in cuprate superconductors, hinting at a close connection between the superconductivity of these two systems.

It is technically challenging to determine the relation between T_c and ρ_s0 in infinite-layer nickelates. “The CaH₂-assisted soft-chemistry topotactic reduction is necessary for inducing superconductivity in infinite-layer nickelates, but it also introduces significant lattice distortion and inhomogeneous chemical doping. Consequently, the infinite-layer nickelates always exhibit inhomogeneous superconductivity, which impedes the accurate determination of T_c and superfluid density using conventional bulk measurement techniques,” explained corresponding author, Liang Qiao, a professor in the Department of Physics at University of Electronic Science and Technology of China who provided the high-quality NSNO films.

To overcome this, Prof. Yihua Wang’s team utilized sSQUID microscopy to probe the magnetic susceptibility of NSNO on the micron scale. They found weakly diamagnetic rings across the sample. Accordingly, the local T_c is markedly inhomogeneous. From the susceptibility data, they extracted spatial distributions of the superfluid density. The statistics for different scan areas further revealed a continuous trend: a linear dependence of local T_c on ρ_s0 for T_c > 8 K, and a parabolic dependence for T_c < 8 K. Remarkably, similar scaling relations between T_c and ρ_s0 were reported in overdoped cuprate superconductors La_2-xSr_xCuO₄.

“The strong correlation between T_c and ρ_s0 is beyond the standard Bardeen-Cooper-Schrieffer scenario in which T_c is determined by the pairing strength. Moreover, the striking similarity in the T_c–ρ_s0 relation of nickelates and cuprates further suggests that the superconductivity in these systems is governed by similar fundamental mechanisms,” said Wang.