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Pressure-Induced Superconductivity in Polycrystalline La3Ni2O7−δ

Uwatoko Group

High-Tc superconductors have been at the forefront of scientific exploration due to their immense potential for transformative technological applications. The groundbreaking discovery of high-Tc cuprates, where superconductivity (SC) emerges through doping Mott insulators with strong electron correlations, has motivated numerous endeavors in the past decades to unveil its mechanism and to find more superconducting families with high Tc. Through sharing striking structural and electronic similarities with cuprates, the nickelates with Ni+(3d9) electron configuration offer a tantalizing avenue for uncovering new high-Tc superconductors. However, SC was not experimentally realized in nickelates until 2019, when the infinite-layer Nd1−xSrxNiO2 thin films were found to show SC with Tc ≈ 9-15 K [1]. Since then, considerable dedication has been directed toward finding more nickelate superconductors with higher Tc.

Recently, Sun et al. reported the signature of high-temperature SC in La3Ni2O7 crystals with Tc up to 80 K at pressures above 14 GPa [2]. In contrast to the infinite-layer Nd1-xSrxNiO2, La3Ni2O7 exhibits an exceptionally unique electronic configuration with the nominal oxidation state of Ni2.5+ as a mixed valence state of Ni2+(3d8) and Ni3+(3d7). According to the structural study under high pressure, a structural phase transition from the orthorhombic Amam to Fmmm space group occurs at about 10-15 GPa, where the interlayer Ni-O-Ni bond angle changes from 168° to 180° [2]. Subsequent high-pressure studies on La3Ni2O7 crystals confirmed the presence of a zero-resistance state under better hydrostatic pressure conditions, yet revealed also some issues related with sample-dependent behaviors that remain unclear so far [3,4]. Such a remarkably high Tc has immediately ignited widespread theoretical investigations on the mechanism of high-Tc SC. The significance of interlayer exchange between the dz2 orbitals and intra-layer hybridization of the dz2 and dx2−y2 orbitals on the nearest neighbor sites has received substantial attention. In contrast to the extensive theoretical investigations, experimental progress appears to have lagged behind, presumably due to the challenges associated with obtaining high-quality La3Ni2O7 single crystals with controlled and homogeneous stoichiometry. Depending on the post-annealing process, the oxygen content of La3Ni2O7 can vary from O6.35 to O7.05. In addition, other competitive Ruddlesden-Popper phases are easily formed in the crystals grown using the optical image floating-zone furnace under moderate oxygen pressures. It thus becomes an important issue to perform a comprehensive study on the samples with well-controlled quality. Additionally, an open question remains concerning whether superconductivity can be achieved in La3Ni2O7 polycrystalline samples subjected to high pressure. Therefore, we are motivated to prepare phase-pure polycrystalline La3Ni2O7-δ samples in which oxygen content and chemical homogeneity can be easily controlled, and then to study the pressure effects on its electrical transport properties under high pressure.

In this work [5], we synthesized high-quality La3Ni2O7-δ (δ ≈ 0.07) polycrystalline samples by using the sol-gel method without post-annealing under high oxygen pressure (Fig. 1), and then measured temperature-dependent resistivity under various hydrostatic pressures up to 18 GPa by using the cubic anvil and two-stage multi-anvil apparatus. We find that the density-wave-like anomaly in resistivity is progressively suppressed with increasing pressure and the resistivity drop corresponding to the onset of superconductivity emerges at pressure as low as 6 GPa. Zero resistivity is achieved at 9 GPa below Tczero ≈ 6.6 K, which increases quickly with pressure to 41 K at 18 GPa. However, the diamagnetic response was not detected in the ac magnetic susceptibility measurements up to 15 GPa, indicating a filamentary nature of the observed superconductivity in the studied pressure range. The constructed T-P phase diagram (Fig. 2) reveals an intimate relationship between superconductivity, density-wave-like order, and the strange-metal-like behaviors. The observation of zero resistance state in the polycrystalline La3Ni2O7-δ samples under high pressures not only corroborates the recent report of superconductivity in the pressurized La3Ni2O7-δ crystals but also facilitates further studies on this emerging family of nickelate high-Tc superconductors.


References
  • [1] D. Li et al. and H. Y. Hwang, Nature 572, 624 (2019).
  • [2] H. L. Sun et al., Nature 621, 493 (2023).
  • [3] Y. Zhang et al., Nature Physics (to be published).
  • [4] J. Hou et al., Chin. Phys. Lett. 40, 117302 (2023).
  • [5] G. Wang et al., Phys. Rev. X 14, 011040 (2024).
Authors
  • G. Wanga, N. N. Wanga, X. L. Shena, Y. Uwatoko, and J.-G. Chenga
  • aInstitute of Physics Chinese Academy of Sciences