Bridging ambient- and high-pressure superconductivity in La₂LnNi₂O₇ films

Summary

This study systematically investigates the superconductivity of compressively strained La₂LnNi₂O₇ thin films (Ln = lanthanide) under ambient and high pressure to clarify the relationship between ambient-pressure and high-pressure superconducting mechanisms. B_y suppressing the superconducting state with a 59 T magnetic field, the normal-state resistivity at ambient pressure tends toward T² behavior; after applying high pressure in a cubic anvil chamber, T_c increases from 41–42 K at ambient pressure to 67–73 K at 16 GPa. However, lattice compression induced by Ln substitution, although it may mimic pressure effects, lowers T_c. In both cases, T_c correlates with the evolution of normal-state transport between T² and T-linear behavior, indicating that lattice structural changes in bilayer nickelates have an important influence on superconductivity and providing clues for understanding the intrinsic connection between ambient-pressure strained films and high-pressure bulk superconducting states.

Materials

• La₂SmNi₂O₇
• La₂NdNi₂O₇
• La₂PrNi₂O₇
• La₂EuNi₂O₇
• La₂D_yNi₂O₇
• La₂YNi₂O₇
• La₃Ni₂O₇

Methods

• PLD
• ozone annealing
• X-ray diffraction
• STEM
• energy-dispersive X-ray spectroscopy
• electrical transport measurements
• Hall effect measurements
• high-magnetic-field transport up to 59T
• cubic-anvil-cell high-pressure transport
• DFT
• Wannier function construction

Keywords

• ambient pressure superconductivity
• high pressure superconductivity
• compressively strained films
• t2 behaviour
• t linear behaviour
• lattice compression
• lanthanide substitution
• two step superconducting transition

Highlights

• This study reports a systematic investigation of superconductivity in compressively strained bilayer nickelate films at both ambient and high pressures, bridging thin-film and bulk regimes.
• High-pressure measurements reveal T_c as high as 67-73 K at 16 GPa in La₂SmNi₂O₇ and La₂NdNi₂O₇ films, approaching bulk high-T_c values.
• An anomalous two-step superconducting transition emerges in films above about 8-12 GPa, which may have extrinsic origins but could also reflect multiband pairing.
• Hall coefficient sign reversal with c-axis lattice constant suggests carrier redistribution among d3z2-r2 and dx2-y2 orbitals.

Conclusions

• In compressively strained La₂LnNi₂O₇ films, high pressure enhances T_c from 41-42 K at ambient pressure to 67-73 K at 16 GPa, while lattice compression induced by lanthanide substitution lowers T_c.
• At ambient pressure, suppressing superconductivity with 59 T fields reveals normal-state resistivity tending toward T₂ behaviour.
• In both pressure- and substitution-induced lattice changes, T_c correlates with the evolution of normal-state transport between T₂ and T-linear behaviour, with smaller exponent alpha accompanying higher T_c.
• The pressure-induced two-step superconducting transition may be extrinsic in origin, possibly from spatial inhomogeneities, although intrinsic electronic-structure contributions remain possible.
• The results clarify the relationship between ambient-pressure strained-film superconductivity and high-pressure bulk superconductivity.

Main claims

• Compressively strained La₂LnNi₂O₇ films show ambient-pressure superconductivity with onset up to 47 K.
  • Evidence: La₂SmNi₂O₇ film onset T_c = 41 K and zero resistance at 6 K,Ln = Pr, Nd, Sm, Eu, D_y show onset T_c = 47 K, 42 K, 41 K, 10 K, 20K
• Hydrostatic pressure enhances T_c in the films from 41–42 K at ambient pressure to 67–73 K at 16 GPa.
  • Evidence: La₂SmNi₂O₇ T_c[high] reaches 67 K at 16 GPa,La₂NdNi₂O₇ T_c reaches 73 K at 16 GPa
• Higher T_c is associated with a smaller normal-state resistivity power-law exponent alpha, i.e. more nearly T-linear transport.
  • Evidence: ambient normal state at 59 T shows alpha ≈1.86,under pressure alpha approaches unity as T_c increases,higher-T_c Ln-substituted samples tend to have exponents approaching unity
• The superconductivity in La₂SmNi₂O₇ films is quasi-two-dimensional and anisotropic.
  • Evidence: Hc₂||ab(0 K) = 86 T and Hc₂||c(0 K) = 53 T,coherence lengths xi_ab = 25 Å and xi_c = 15 Å, shorter than c-axis constant 20.5 Å,anisotropy parameter gamma_H = 1.62
• The pressure-induced two-step superconducting transition is likely extrinsic, though an intrinsic multiband origin cannot be excluded.
  • Evidence: Discussion proposes oxygen deficiency, La/Sm or La/Nd composition fluctuations, and non-uniform strain as plausible extrinsic origins,Direct microscopic evidence for intrinsic multiband pairing is currently unavailable

Workflow

• sample_preparation — Compressively strained, ozone-annealed La₂LnNi₂O₇ thin films were fabricated on SrLaAlO₄ substrates.
  • Materials: La₂LnNi₂O₇ films (Ln = La, Pr, Nd, Sm, Eu, D_y, Y); SrLaAlO₄ (001) substrates; KrF excimer laser; ozone treatment apparatus
  • Methods: pulsed-laser deposition; ex-situ ozone annealing; deposition of SrTiO₃ capping layer
  • Observations: La₂SmNi₂O₇ films were the most successfully stabilized; STEM shows coherent bilayer 2222-structure with Sm preferentially on Ln(2) rock-salt sites; without ozone annealing films are insulating
• measurement — Superconducting and normal-state transport were measured as functions of lanthanide substitution, magnetic field, and hydrostatic pressure.
  • Materials: La₂LnNi₂O₇ films; Au electrodes; PPMS; non-destructive pulse magnet; cubic-anvil cell with Daphne oil 7575
  • Methods: temperature-dependent resistivity; Hall effect measurement; high-magnetic-field electrical resistance up to 59T; high-pressure electrical resistivity up to 20 GPa; XRD; STEM-EDS
  • Observations: ambient-pressure T_c onset: Sm₄₁ K, Pr₄₇ K, Nd₄₂ K, Eu₁₀ K, Dy₂₀K; La₂SmNi₂O₇ reaches zero resistance at 6K; T_c enhanced to 67 K (Sm) and 73 K (Nd) at 16 GPa; two-step resistive transitions above 8–12 GPa
• analysis — Analysis links lattice parameters, normal-state transport, and electronic structure to the superconducting transition temperature.
  • Materials: resistivity data; Hall coefficient data; DFT calculations using Quantum ESPRESSO and RESPACK
  • Methods: power-law fitting rho = rho0 + A T^alpha; Abrikosov-Gorkov fitting of upper critical fields; normal-state exponent analysis under pressure; density functional theory electronic structure calculations
  • Observations: ambient normal-state exponent alpha ≈1.86 at 59T; under pressure alpha approaches unity as T_c increases; higher-T_c Ln samples show alpha approaching unity; Hc₂||ab(0 K) = 86 T and Hc₂||c(0 K) = 53T; coherence lengths: xi_ab = 25 Å and xi_c = 15 Å; DFT shows Fermi surface changes from alpha/beta sheets at ambient pressure to an additional gamma sheet under high pressure
• interpretation — Ambient-pressure strained films and high-pressure bulk-like superconductivity are linked through lattice-dependent transport, but c-axis-only compression by Ln substitution is not equivalent to hydrostatic pressure.
  • Materials: ambient-pressure Ln-substitution series; high-pressure transport data; DFT-derived orbital-energy differences
  • Methods: comparison of Ln-substitution chemical pressure and hydrostatic pressure effects; evaluation of possible extrinsic and intrinsic origins of two-step transitions
  • Observations: Ln substitution compresses the c-axis and lowers T_c; hydrostatic pressure compresses both a and c axes and raises T_c; in both cases smaller power-law exponent alpha corresponds to higher T_c; two-step transition may arise from oxygen/La-Sm inhomogeneity or non-uniform strain, but intrinsic multiband contributions remain possible