Bulk high-temperature superconductivity in the high-pressure tetragonal phase of bilayer La2PrNi2O7

TL;DR

This study demonstrates bulk high-temperature superconductivity in Pr-doped La2PrNi2O7 under high pressure, achieving record transition temperatures and confirming the phase responsible for superconductivity.

Contribution

The paper reports the discovery of bulk HTSC in a nearly pure bilayer nickelate phase, with high transition temperatures and clear diamagnetic evidence, resolving previous controversies.

Findings

Superconductivity emerges above 11 GPa after structural transition.

Transition temperatures reach up to 82.5 K (onset) and 60 K (zero resistance).

Bulk superconductivity confirmed with ~57% volume fraction at 20 GPa.

Abstract

The Ruddlesden-Popper (R-P) bilayer nickelate, La3Ni2O7, was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa. Subsequent investigations achieved zero resistance in single- and poly-crystalline samples under hydrostatic pressure conditions. Yet, obvious diamagnetic signals, the other hallmark of superconductors, are still lacking owing to the filamentary nature with low superconducting volume fraction. The presence of a novel "1313" polymorph and competing R-P phases obscured proper identification of the phase for HTSC. Thus, achieving bulk HTSC and identifying the phase at play are the most prominent tasks at present. Here, we address these issues in the praseodymium (Pr)-doped La2PrNi2O7 polycrystalline samples. We find that the substitutions of Pr for La effectively inhibits the intergrowth of different R-P phases, resulting in nearly pure bilayer structure. For La2PrNi2O7, pressure-induced orthorhombic-to-tetragonal structural transition takes place at Pc ~ 11 GPa, above which HTSC emerges gradually upon further compression. The superconducting transition temperatures at 18-20 GPa reach Tconset = 82.5 K and Tczero = 60 K, which are the highest values among known nickelate superconductors. More importantly, bulk HTSC was testified by detecting clear diamagnetic signals below ~75 K corresponding to an estimated superconducting volume fraction ~ 57(5)% at 20 GPa. Our results not only resolve the existing controversies but also illuminate directions for exploring bulk HTSC in the bilayer nickelates.