Re-entrant unconventional superconductivity induced by rare-earth substitution in Nd1-xEuxNiO2 thin films

TL;DR

This study demonstrates that Eu substitution in Nd1-xEuxNiO2 thin films enhances the superconducting gap and induces strong-coupling superconductivity, with magnetic-field effects revealing interactions between magnetic Eu ions and superconducting states.

Contribution

It provides experimental evidence that rare-earth substitution can modify the superconducting properties and induce strong-coupling behavior in nickelate thin films.

Findings

Eu substitution increases the superconducting gap in Nd1-xEuxNiO2.

Magnetic Eu ions interact with superconducting states, enhancing superconductivity under magnetic fields.

The upper critical magnetic field exceeds the Pauli limit, indicating strong-coupling superconductivity.

Abstract

High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, Hc2, and superconducting gap of superconducting NENO thin films with x=0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of interaction between magnetic Eu ions and superconducting states in the Ni dx2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-Tc ratio $2 \Delta k_B T_c \approx 5 - 6$, indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO2. The substitution of Eu in the rare-earth layer provides a method to modify the superconducting gap in Nd-based nickelates, an essential factor in engineering high-Tc superconductivity in infinite-layer nickelates.