Superconductivity at the metal-insulator phase boundary in a bulk nickelate at ambient pressure

Summary

At ambient pressure, signatures of superconductivity were observed in bulk single crystals of the nickelates (La_1−xPr_x)₄Ni₃O₈ and (La1−xYx)4Ni3O8, whose structures consist of alternating square-planar nickel-oxide sandwich layers and fluorite-like spacer layers. The parent La₄Ni₃O₈ is an insulator in which electrons form intertwined charge/spin stripe order; substitution with smaller lanthanide ions disrupts this order and ultimately leads to a metallic ground state. Magnetotransport and magnetic susceptibility measurements indicate that superconductivity appears within a narrow window near the insulator-to-metal transition, where a metallic phase coexists with the charge-stripe phase. The observed superconducting volume fraction is low and non-percolative, suggesting that superconductivity may nucleate in filamentary form at phase boundaries; TEM also reveals intergrowth defects of a nearly infinite-layer nickelate, so it cannot be ruled out that superconductivity originates from these defects rather than the sandwich matrix. The electronic phase diagrams of the Y and Pr series coincide when plotted as a function of the fluorite-like spacer volume, indicating that this steric parameter strongly tunes the electronic structure of the nickel-oxide sandwich layers. A deeper understanding of the coexistence region between the metallic and charge-ordered phases is considered key to expanding superconductivity in bulk nickelates at ambient pressure, and spacer-layer engineering is established as a design approach for exploring this competing-phase region.

Materials

• (La_1−xPr_x)₄Ni₃O₈
• (La1-xYx)4Ni3O8
• La₄Ni₃O₈
• Pr₄Ni₃O₈
• (La_1−xPr_x)₄Ni₃O₁₀
• (La1-xYx)4Ni3O10
• infinite-layer-like nickelate intergrowth defects

Methods

• floating zone crystal growth
• hydrogen reduction
• single-crystal X-ray diffraction (SXRD) refinements
• AC susceptibility measurements (SQUID magnetometer)
• magnetotransport measurements (PPMS)
• temperature-dependent resistivity measurements
• isothermal magnetoresistance measurements
• current-voltage (I-V) and differential resistance (dV/dI) measurements
• STEM
• focused-ion beam (FIB)/scanning electron microscopy (SEM)
• Ginzburg-Landau analysis of upper critical fields

Keywords

• ambient pressure superconductivity
• metal insulator phase boundary
• charge/spin stripes
• charge density wave
• spin density wave
• filamentary superconductivity
• non percolative superconductivity
• phase coexistence
• phase competition
• insulator metal transition
• percolative transition
• spacer layer engineering
• steric tuning
• two dimensional superconductivity
• superconducting volume fraction
• intergrowth defects
• oxygen sensitivity
• phase segregation
• electronic inhomogeneity
• critical current
• upper critical field anisotropy

Highlights

• The paper reports signatures of superconductivity at ambient pressure in bulk nickelate single crystals, whereas superconductivity in nickelates had previously been realized only in thin films or under extreme pressure in the bulk.
• Superconducting signatures appear only in an extremely narrow composition region at the metal-insulator phase boundary, where charge/spin stripe order is suppressed toward T = 0 by chemical pressure.
• The low superconducting volume fraction (a few percent for LPNO and ≈0.03% for LYNO) is non-percolative, yet well-defined critical currents are observed.
• TEM reveals a high density of intergrowth defects with n = 7, 31, and 44; the n = 44 and 31 intergrowths are reduced square-planar slabs approximating infinite-layer 1-1-2 nickelates and may be a source of superconductivity.
• The estimated superconducting layer thickness of ≈10 nm from the in-plane upper critical field falls in the range of observed intergrowth thickness.
• Marginal oxidation of fully reduced LPNO-438 suppresses the diamagnetic response, showing that the superconductivity is highly sensitive to oxygen content.
• The Y and Pr series share a common phase diagram only when the transition temperatures are plotted against the fluorite-like spacer layer volume, not against total unit-cell volume, lattice constants, Ni-O bond lengths, or bond angles.
• The study rules out surface superconductivity and oxygen inhomogeneity as likely origins, but leaves interface superconductivity and competing stripe/superconductivity scenarios as plausible.

Conclusions

• Signatures of superconductivity at ambient pressure are observed in bulk single crystals of (La_1−xPr_x)₄Ni₃O₈ and (La1-xYx)4Ni3O8.
• The parent compound La₄Ni₃O₈ is an insulator with intertwined charge/spin stripe order, and substitution of smaller lanthanide ions disrupts this order and leads to a metallic ground state.
• Superconductivity emerges in a narrow window proximate to the insulator-metal phase boundary where metallic and charge-stripe phases coexist.
• The observed superconductivity has a low volume fraction and is non-percolative, suggesting filamentary superconductivity nucleated at boundaries between the coexisting phases.
• TEM observations reveal intergrowth defects approximating infinite-layer nickelates, leaving open the possibility that superconductivity resides in these defects rather than in the trilayer matrix.
• The electronic phase diagrams of the Y and Pr series coincide when parameterized by the fluorite-like spacer layer volume, showing that this steric parameter strongly modifies the nickel oxide trilayer electronic structure.
• The superconductivity is two-dimensional in nature, as indicated by anisotropic magnetotransport and the absence of a detectable diamagnetic response for H // ab.
• Spacer-layer engineering is established as a design tool for exploring the regime of phase competition and for expanding bulk ambient-pressure nickelate superconductivity.
• The data do not allow unambiguous distinction between intrinsic phase-boundary superconductivity and superconductivity from intergrowth defects.

Main claims

• Ambient-pressure superconductivity signatures occur in bulk single crystals of the reduced trilayer nickelates LPNO and LYNO near the insulator-metal phase boundary.
  • Evidence: Abrupt resistivity drops at T_c about 7 K for LPNO x=0.55 and about 15 K for LYNO x=0.11.,Applied magnetic fields suppress the resistive downturn.,AC susceptibility shows a weak diamagnetic response at the same T_c.,I-V curves show nonlinearity and critical-current peaks that disappear near T_c.
• The superconductivity is non-percolative, low-volume-fraction, and two-dimensional or anisotropic.
  • Evidence: Finite residual resistance and small diamagnetic shielding fractions: about 2% for LPNO and about 0.03% for LYNO.,H // c suppresses superconductivity much more effectively than H // ab for LPNO.,No detectable diamagnetic response for H // ab in AC susceptibility.,Estimated superconducting layer thickness d about 10 nm from the in-plane H_c2 curve.
• The electronic phase diagrams of the Pr and Y series coincide when parameterized by fluorite-like spacer-layer volume, identifying steric spacer-layer engineering as a tuning parameter.
  • Evidence: T_DW and T_c evolve faster with x in LYNO than LPNO, tracking the smaller Y3+ ionic radius.,Attempts to parameterize by lattice constants, Ni-O bond lengths, bond angles, or total unit-cell volume did not make the series coincide.,Both series overlap when plotted against the fluorite-like spacer-layer volume.
• The origin of the superconductivity is not uniquely determined: filamentary superconductivity at phase boundaries and intergrowth defects approximating infinite-layer nickelates are both plausible.
  • Evidence: Superconductivity appears only in a narrow window where metallic and charge-stripe phases coexist.,STEM reveals a high density of intergrowth defects with n=7, 31, and 44; some reduced intergrowths approximate infinite-layer nickelate slabs.,The estimated about 10 nm superconducting layer thickness overlaps with observed intergrowth thicknesses.,The authors state that the data do not unambiguously distinguish between these possibilities.

Workflow

• sample_preparation — Reduced trilayer nickelate single crystals of LPNO and LYNO were synthesized for ambient-pressure study.
  • Materials: (La_1−xPr_x)₄Ni₃O₁₀ and (La1-xYx)4Ni3O10 single crystals; La₄Ni₃O₈ parent compound; LPNO and LYNO reduced crystals
  • Methods: high-oxygen-pressure floating zone growth; hydrogen reduction to convert 4-3-10 to 4-3-8; single-crystal X-ray diffraction for composition and structure refinement
  • Observations: XRD refinements showed Y3+ substitutes into La3+ sites within fluorite-like spacer layers; La₄Ni₃O₈ is an insulating parent with charge/spin stripe order
• measurement — Magneto-transport, susceptibility, I-V, and microscopy measurements probe superconducting signatures and microstructure in LPNO and LYNO.
  • Materials: LPNO x=0.55 single crystals; LYNO x=0.11 single crystals
  • Methods: temperature-dependent and isothermal magnetotransport in PPMS; AC susceptibility in MPMS3 SQUID magnetometer; current-voltage and differential resistance measurements; STEM/HAADF/LAADF imaging
  • Observations: abrupt resistivity drops at T_c about 7 K for LPNO and about 15 K for LYNO; magnetic field suppresses the resistive downturn; weak diamagnetic response for H // c and no detectable diamagnetic response for H // ab; nonlinear I-V and critical-current peaks that disappear near T_c; STEM shows intergrowth defects and phase-separated regions
• analysis — The superconducting response is low-volume-fraction, non-percolative, anisotropic, and linked to the insulator-metal phase boundary.
  • Materials: resistivity, magnetoresistance, AC susceptibility, I-V, XRD, and STEM data
  • Methods: determination of T_c from resistive drops; construction of H-T phase diagrams and H_c2(T); Ginzburg-Landau fit for anisotropic or two-dimensional superconductivity; estimation of superconducting volume fraction and layer thickness; comparison of T_DW phase diagrams parameterized by unit-cell and fluorite-layer volume
  • Observations: superconducting volume fraction is low: about 2% for LPNO and about 0.03% for LYNO; LPNO shows strong anisotropy with H_c2//c about 9 T and estimated layer thickness d about 10 nm; T_DW for LPNO and LYNO coincide when plotted versus fluorite-like spacer-layer volume
• interpretation — The most plausible intrinsic scenario is filamentary superconductivity nucleated at boundaries between metallic and charge-stripe phases, but an intergrowth-defect origin cannot be ruled out.
  • Materials: LPNO and LYNO phase diagrams; STEM intergrowth observations
  • Methods: comparison of possible origins: phase-boundary filamentary superconductivity, intergrowth defects, interfaces, oxygen inhomogeneity, surface superconductivity; spacer-layer engineering framework
  • Observations: surface superconductivity and oxygen inhomogeneity were excluded as likely origins; intergrowth defects approximating infinite-layer nickelates remain an alternative source; superconductivity appears only in a narrow composition window near the phase boundary