In 2023, signatures of high-temperature superconductivity (HTSC) with $T_c\approx$80K were reported at pressures above 14 GPa in La₃Ni₂O₇ [1]. This discovery has immediately attracted tremendous research interest for a new family of HTSC. La₃Ni₂O₇ is a bilayer member ($n=$2) of the Ruddlesden-Popper (R-P) perovskite nickelates denoted as La_$n$+1Ni_$n$O_3$n$+1. In contrast to well-known cuprate HTSCs, and to infinite-layer Nd_1−$x$Sr_$x$NiO₂ thin films superconductros with $T_c\approx$9-15 K (2019), La₃Ni₂O₇ exhibits an exceptionally unique electronic valence state with the nominal oxidation state of Ni^2.5+ (3$d$^7.5) instead of 3$d$⁹. The 3$d$^7.5 electronic configuration sitting on bilayers with moderate interlayer exchange has been theoretically proposed as an alternative HTSC playground to that of cuprate. The prediction was reported prior to the discovery in real materials. However, HTSC in La₃Ni₂O₇ has not been experimentally established yet, because Meissner diamagnetic shielding effect, inherent to a genuine superconductor, was not confirmed in any of bilayer nickelate HTSC candidates.

Subsequent high-pressure studies on La₃Ni₂O₇ crystals confirmed the presence of a zero-resistance state under better hydrostatic pressure conditions, but issues related to sample-dependent behaviors remain unclear so far. ISSP Activity Report 2023 12 successfully reproduced zero-resistivity using high-quality La₃Ni₂O_7−$\delta$ ($\delta \approx$0.07) polycrystalline samples by using the sol-gel method and under hydrostatic pressure condition up to 18 GPa [2]. Although diamagnetic response was not detected during ac-susceptibility measurement, the report constructed pressure-temperature ($P$-$T$) phase diagram and intimate relationship between superconductivity, density-wave-like order, and the strange-metal-like behaviors. However, again, still HTSC as bulk was not obtained even with the high-quality sample, and identification of the exact superconductivity phase/structure is the most prominent task at present.

La₃Ni₂O₇ is known to include considerable amount of intergrowths of other R-P phases, La₂NiO₄ (monolayer) and La₄Ni₃O₁₀ (trilayer), resulting in stacking faults against the bilayer-bilayer stacking sequence. To overcome this problem, we synthesized the polycrystalline samples of bilayer nickelates La_3-$x$Pr_$x$Ni₂O_7-$\delta$ via sol-gel method [3]. In this work, we found that the above-mentioned sample-quality issues of La₃Ni₂O₇ can be effectively resolved via substitution of smaller Pr ions for La, leading to the successful synthesis of high-purity La₂PrNi₂O₇ with nearly an ideal bilayer R-P structure. Combined resistivity and ac magnetic susceptibility measurements under hydrostatic pressures provided the key evidence of bulk HTSC, including the zero resistance with high $T_{\mathrm{c}}^{\text{onset}} =$82.5 K and $T_{\mathrm{c}}^{\text{zero}} =$60 K, along with a clear diamagnetic response correspondinZg to a superconducting shielding volume fraction of 97% (Fig. 1,2). The key parts of the measurements were carried out by a two-stage 6/8 multianvil apparatus of the high-pressure measurement section in ISSP. These results provide critical experimental evidence for bulk HTSC in the pressurized La₂PrNi₂O₇, confirming the bilayer R-P phase as the source of HTSC for the first time.

References

• [1] H.L. Sun et al., Nature 621, 493 (2023).
• [2] G. Wang et al., Phys. Rev. X 14, 011040 (2024).
• [3] N. Wang et al., Nature 634, 579 (2024).

Authors

• N. Wang^a, G. Wang^a, X. Shen^b, J. Hou^a, J. Luo^a, X. Ma^a, H. Yang^a, L. Shi^a, J. Dou^a, J. Feng^a, J. Yang^a, Y. Shi^a, Z. Ren^a, H. Ma^a, P. Yang^a, Z. Liu^a, Y. Liu^a, H. Zhang^a, X. Dong^a, Y. Wang^a, K. Jiang^a, J. Hu^a,c, S. Nagasaki, K. Kitagawa, S. Calder^d, J. Yan^d, J. Sun^a, B. Wang^a, R. Zhou^a, Y. Uwatoko &  J. Cheng^a
• ^aChinese Academy of Sciences
• ^bShanghai Jiao Tong University
• ^cNew Cornerstone Science Laboratory
• ^dOak Ridge National Laboratory