Ambient pressure growth of bilayer nickelate single crystals with superconductivity over 90 K under high pressure

Summary

Recently, the Ruddlesden-Popper bilayer nickelate La₃Ni₂O₇ has emerged as a superconductor with a transition temperature (T_c) of ~ 80 K above 14 GPa[1-4]. Efforts to search for nickelate superconductors with higher T_c[5,6], to grow reproducible high-quality single crystals[2,7-10], and to eliminate reliance on demanding high gas pressure synthesis conditions[11], remain significant challenges. Here we report superconductivity up to 92 K under high pressure in single crystals of bilayer nickelates synthesized at ambient pressure using flux methods. High qualityLa₂SmNi₂O₇-δ single crystals with dimensions up to 220 μm on edge were successfully grown. At ~ 15 GPa, these crystals exhibit superconductivity with an onset transition temperature (T_conset) of 68 K and zero-resistance temperature (T_czero) of 47 K. Increasing pressure further enhances both transition temperatures, reaching record values for nickelates: T_c,maxonset = 92 K and T_c,maxzero = 73 K @ 21 GPa. Notably, higher T_c correlates with larger in-plane lattice distortion at ambient conditions for bilayer nickelates. Furthermore, we observed a structural transition from monoclinic P₂₁/a to tetragonal I₄/mmm at ~ 18 GPa, indicating that tetragonal structure is not a prerequisite for superconductivity to appear in this bilayer nickelate. This study provides an easy-to-access method for growing reproducible high-quality bilayer nickelate single crystals and offers new insights into achieving higher T_c superconductivity.

Materials

• La₂SmNi₂O₇-δ

Methods

• Flux method
• High-pressure transport measurements
• X-ray diffraction

Keywords

• record tc for nickelates
• in plane lattice distortion
• structural transition from monoclinic p21/a to tetragonal i4/mmm

Highlights

• Observation that tetragonal structure is not a prerequisite for superconductivity.
• Provides easy-to-access method for growing high-quality single crystals.

Conclusions

• Superconductivity up to 92 K onset and 73 K zero resistance at 21 GPa is achieved in La₂SmNi₂O₇-δ single crystals.
• Larger in-plane lattice distortion correlates with higher T_c.

Main claims

• La₂SmNi₂O₇ single crystals exhibit superconductivity with Tc_onset=92 K and Tc_zero=73 K at 21.6 GPa, the highest among known nickelates.
  • Evidence: Resistivity measurements show a sharp superconducting transition and zero resistance (Fig. 3a-c)
• There is a structural transition from monoclinic P₂₁/m to tetragonal I₄/mmm at approximately 18 GPa.
  • Evidence: Synchrotron XRD patterns show merging of (201) and (020) peaks at ≈18 GPa (Fig. 4b)
• Superconductivity appears before the structural transition, indicating tetragonal symmetry is not required.
  • Evidence: Superconductivity observed at 14.3 GPa in the monoclinic phase (Fig. 3a)
• Higher T_c under high pressure correlates with larger in-plane lattice distortion at ambient conditions.
  • Evidence: Comparison of Tc_max with Δ for La₃Ni₂O₇, La₂PrNi₂O₇, and La₂SmNi₂O₇ shows increasing trend (Fig. 5a)

Workflow

• single_crystal_growth — Successful growth of bilayer nickelate single crystals at ambient pressure.
  • Materials: La₂O₃; Rare-earth oxides (Pr-Er); NiO; K₂CO₃ flux
  • Methods: Ambient pressure flux method (evaporation of flux)
  • Observations: Single crystals of La_3-xR_xNi₂O₇-δ (R=Pr-Er) obtained; La₂SmNi₂O₇-δ crystals up to 220 µm on edge
• crystal_characterization — La₂SmNi₂O₇-δ single crystals are of high quality with pure bilayer phase.
  • Materials: As-grown single crystals
  • Methods: SEM; EDS; SXRD; PXRD; NQR; STEM-HAADF
  • Observations: High homogeneity and phase purity for Sm-substituted samples; No intergrowth of other R-P phases; Preferential Sm occupation at interlayer La sites
• post_annealing_optimization — Post-annealing reduces oxygen defects and enhances superconductivity.
  • Materials: La₂SmNi₂O₇-δ single crystals
  • Methods: Post-annealing in oxygen at controlled pO₂
  • Observations: Optimal condition for superconductivity; Zero resistivity achieved
• high_pressure_resistivity — Record high T_c in nickelates achieved in La₂SmNi₂O₇ under high pressure.
  • Materials: Annealed La₂SmNi₂O₇-δ single crystals
  • Methods: DAC with helium as PTM; Four-probe/van der Pauw method
  • Observations: Superconductivity emerges at 14.3 GPa with Tc_onset=68 K, Tc_zero=4K; Maximum Tc_onset=92 K, Tc_zero=73 K at 21.6 GPa
• high_pressure_synchrotron_xrd — Tetragonal structure is not a prerequisite for superconductivity; both phases support it.
  • Materials: Pulverized La₂SmNi₂O₇ single crystals
  • Methods: Synchrotron XRD with helium as PTM; Rietveld refinement
  • Observations: Structural transition from monoclinic P₂₁/m to tetragonal I₄/mmm at ≈18 GPa; Monoclinic phase persists in superconducting state at 17.9 GPa down to 64K