Bulk high-temperature superconductivity in pressurized tetragonal La 2 PrNi 2 O 7

Abstract

The Ruddlesden–Popper (R–P) bilayer nickelate, La 3 Ni 2 O 7 , was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa . Subsequent investigations achieved zero resistance in single-crystalline and polycrystalline samples under hydrostatic pressure conditions. Yet, obviousdiamagnetic signals, the other hallmark of superconductors, are still lacking owing to the flamentary nature with low superconducting volume fraction. The presence of a new 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. Here we address these issues in the praseodymium (Pr)-doped La 2 PrNi 2 O 7 polycrystalline samples. We find that substitutions of Pr for La efectively inhibit the intergrowth of diferent R–P phases, resulting in a nearly pure bilayer structure. For La 2 PrNi 2 O 7 , pressure-induced orthorhombic to tetragonalstructural transition takes place at P c ≈ 11 GPa, above which HTSC emerges gradually on further compression. The superconducting transition temperatures at 18–20 GPa reach $T$ $^{onset}_{c}$ = $82.5$ $K$ and $T$ $^{zero}_{c}$ = $60$ $K$, which are the highest values, to our knowledge, among known nickelate superconductors. Importantly, bulk HTSC was testified by detecting clear diamagnetic signals below about 75 K with appreciable superconducting shielding volume fractions at a pressure of above 15 GPa. Further, our results not only resolve the existing controversies but also provide directions for exploring bulk HTSC in the bilayer nickelates.