A new look at the Superconducting Observables in the High-crystallinity Higher Tc Infinite-Layer Nickelates

The discovery of high-Tc superconductivity in d9 cuprates in 1986 marked a significant breakthrough, which excited theoreticians and experimentalists to reconceive the possible pairing mechanism and maximum energy gap in Cooper pairs. Motivated by this discovery and subsequent theoretical calculations, high-Tc superconductivity in d9 nickelate has been similarly anticipated; yet, experimental demonstration of superconductivity with Tc well above 30 K/40K/77K in d9 nickelate has never been achieved; nevertheless, an exciting discovery was made in 2019, where superconductivity with Tc ~ 15 K was observed in an infinite-layer nickelate. Following these efforts, we hypothesize that increased interlayer interactions that are (hypothetically) pertinent to superconductivity should lead to stronger superconducting pairing in the infinite-layer nickelate system. We preliminarily achieved such a postulated structure by synthesizing the later rare earth nickelates, SmNiO₂, presumably has stronger R-Ni hybridization, while hole-doped with smaller (and similar ionic size) cations Eu2+/3+; additionally, we employ high-entropy synthesis technique: mixed-doped with Ca2+ (and sometimes also with Sr2+), where we have obtained high-crystallinity d9-x infinite-layer nickelates with superconducting Tc in the range of 30 - 40 K at ambient pressure. Following this achievement, recently, we further enhanced their superconducting Tc and correlated them to their structural observables, symmetry breaking states, upper critical fields phase diagram, magnetic exchange interactions, energy gaps and electronic structures, which we will discuss in this presentation.