Bulk high-temperature superconductivity in pressurized tetragonal La<sub>2</sub>PrNi<sub>2</sub>O<sub>7</sub>.
basic_science · Level V
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- Also identified by DOI 10.1038/s41586-024-07996-8.
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Abstract
The Ruddlesden-Popper (R-P) bilayer nickelate, La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>, was recently found to show signatures of high-temperature superconductivity (HTSC) at pressures above 14 GPa (ref. <sup>1</sup>). Subsequent investigations achieved zero resistance in single-crystalline and polycrystalline samples under hydrostatic pressure conditions<sup>2-4</sup>. Yet, obvious diamagnetic signals, the other hallmark of superconductors, are still lacking owing to the filamentary nature with low superconducting volume fraction<sup>2,4,5</sup>. The presence of a new 1313 polymorph and competing R-P phases obscured proper identification of the phase for HTSC<sup>6-9</sup>. 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<sub>2</sub>PrNi<sub>2</sub>O<sub>7</sub> polycrystalline samples. We find that substitutions of Pr for La effectively inhibit the intergrowth of different R-P phases, resulting in a nearly pure bilayer structure. For La<sub>2</sub>PrNi<sub>2</sub>O<sub>7</sub>, pressure-induced orthorhombic to tetragonal structural transition takes place at P<sub>c</sub> ≈ 11 GPa, above which HTSC emerges gradually on further compression. The superconducting transition temperatures at 18-20 GPa reach <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow> <msubsup><mrow><mi>T</mi></mrow> <mrow><mi>c</mi></mrow> <mrow><mi>onset</mi></mrow> </msubsup> <mo>=</mo> <mn>82.5</mn> <mspace></mspace> <mi>K</mi></mrow> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow> <msubsup><mrow><mi>T</mi></mrow> <mrow><mi>c</mi></mrow> <mrow><mi>zero</mi></mrow> </msubsup> <mo>=</mo> <mn>60</mn> <mspace></mspace> <mi>K</mi></mrow> </math> , 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. Our results not only resolve the existing controversies but also provide directions for exploring bulk HTSC in the bilayer nickelates.