Experimental evidence of Tc enhancement above 50 K and diode and paramagnetic-Meissner effects, in Nickelate films on highly reduced SrTiO₃

Summary

Oxygen-deficient nickelate thin films were fabricated on highly reduced and conductive SrTiO₃ substrates, and through Meissner effect and transport measurements, a superconducting onset temperature of 50–70 K was observed, with zero resistance achieved at 20–25 K, indicating the presence of superconductivity in island-like regions within the film. A giant paramagnetic Meissner effect peak appeared at approximately 48 K, further supporting the occurrence of a superconducting transition near this temperature. Additionally, a non-reciprocal, hysteresis-free superconducting diode effect was observed, with its polarity fully polarizable and reversible. The thin films comprise a mixture of various Ruddlesden–Popper phases, including the infinite-layer phase. These enhanced superconducting properties are attributed to the synergistic effect between the oxygen-deficient films and the highly reduced SrTiO₃ substrates.

Materials

• Nd₀.8Sr₀.2NiO₂ infinite-layer
• mixed Ruddlesden-Popper (Nd₀.8Sr₀.2)n+1Ni_nO₃n+1 phases

Methods

• PLD
• X-ray diffraction (XRD)
• High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)
• Energy-dispersive X-ray spectroscopy (EDS)
• Transport measurements (PPMS)
• SQUID magnetometry (MPMS₃)
• Meissner effect measurements
• Voltage-current (V-I) characteristics

Keywords

• tc enhancement above 50 k
• paramagnetic meissner effect
• superconducting diode effect
• oxygen deficiency
• highly reduced srtio3 substrate

Highlights

• The observed T_c onset of 50-70 K is the highest reported in reduced nickelate films at ambient pressure.
• The fully polarized superconducting diode effect could be developed into a desirable device.
• The highly reduced black STO substrate is essential for these observations and could be used as a substrate for other films needing permanent reduction.

Conclusions

• We observed a T_c onset enhancement to 50-70 K at ambient pressure in oxygen-deficient nickelate thin films on highly reduced black SrTiO₃ substrates.
• We observed a giant paramagnetic Meissner effect peak at about 48 K, supporting a superconducting transition just above it.
• We observed a highly asymmetric, nonreciprocal, and non-hysteretic superconductive diode effect that could be fully polarized and polarity reversed.
• The phase responsible for these effects in our multiphase films could not be singled out; one possibility is an electron-doped 327 phase.

Main claims

• Superconducting transition onset is enhanced to 50–70 K at ambient pressure in oxygen-deficient nickelate films on highly reduced SrTiO₃ substrates.
  • Evidence: Meissner effect measurements showing diamagnetic onset at ≈50 K (Figs. 2b, 5a),Resistance drop starting at ≈70 K and zero resistance at 20–25 K (Figs. 3, 6),Abstract: 'T_c onset of 50-70 K was found in Meissner and transport measurements'
• A giant paramagnetic Meissner effect (PME) peak is observed at ≈48 K under high magnetic fields (2000–4000 Oe).
  • Evidence: SQUID measurements showing a positive magnetization peak just below 50 K (Fig. 5c),PME peak magnitude scales as H^(3/2) (Fig. 5d),Abstract: 'a giant paramagnetic-Meissner effect peak at about 48 K'
• A non-reciprocal, non-hysteretic superconducting diode effect with full polarity reversibility is observed without external magnetic field.
  • Evidence: Asymmetric voltage-current curves: Ohmic in one bias direction, supercurrent in the opposite (Fig. 7),Polarity reversal upon changing contact geometry (Fig. 8a,b),Abstract: 'asymmetric or nonreciprocal and non-hysteretic superconductive diode effect was observed'

Workflow

• Sample Preparation — Reduction under vacuum produces highly reduced black STO substrates and oxygen-deficient nickelate films with multiple RP phases.
  • Materials: Nd₀.8Sr₀.2NiO₃ (NSNO) ceramic target; Ti-terminated (100) SrTiO₃ (STO) wafers; CaH₂ powder for reduction
  • Methods: Pulsed laser deposition (PLD) using fourth-harmonic Nd-YAG laser (266 nm); Reduction annealing in Al packets with CaH₂ at 450-500 °C under vacuum pumping
  • Observations: Virgin films: epitaxial (113) RP perovskite phase; Reduced films: mixed Ruddlesden-Popper phases (n=1,2,3,4,∞), polycrystalline; STO substrates become black and highly conductive (metallic); Ti diffusion from STO into film detected by EDS
• Structural and Compositional Characterization — Films are inhomogeneous with multiple RP phases; Ti diffusion from substrate occurs; dominant phase varies with reduction time.
  • Materials: As-prepared and reduced films
  • Methods: X-ray diffraction (XRD) θ–2θ scans; Atomic force microscopy (AFM); High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM); Energy-dispersive X-ray spectroscopy (EDS)
  • Observations: XRD: (002) perovskite peak disappears after reduction; broad infinite-layer peak appears; polycrystalline rings in longer reductions; STEM: columnar defects, vertical disorder, identification of RP phases (327, 214, 4310, etc.); EDS: Ni aggregation into puddles, Ti substitution for Ni, stoichimetry preserved (Nd+Sr)/(Ni+Ti) ≈ 1; Aging effects: films lose superconductivity and STO loses blackness over months
• Magnetic Characterization — Detection of superconductivity with onset T_c50–70 K and a giant PME peak supporting a superconducting transition near 50 K.
  • Materials: Reduced films on black STO substrates
  • Methods: SQUID magnetometry (MPMS₃); Meissner effect protocol: zero-field cooling (ZFC) then field warming (FW); Paramagnetic Meissner effect (PME) measurements
  • Observations: Meissner diamagnetic signal onset at ≈50 K (films reduced 1 h and 6 h); Low-temperature Meissner transition at ≈7 K in 1 h reduced film; Giant paramagnetic Meissner effect (PME) peak at ≈48 K under fields of 2000–4000Oe; Net PME peak magnitude follows H^(3/2) power law
• Transport Characterization — Observation of a field-free superconducting diode effect with fully polarizable and reversible polarity, and supercurrents consistent with weak-link behavior.
  • Materials: Same films
  • Methods: Four-probe resistance vs temperature (PPMS Dynacool); Voltage-current (V-I) characteristics (Keithley 2450 sourcemeter); Supercurrent extraction from V-I curves
  • Observations: Resistance drop starting at ≈70 K, zero resistance at 20–25K; Highly asymmetric V-I curves: diode effect (non-reciprocal, non-hysteretic); Polarity of diode effect reversible by changing contact geometry; Supercurrents decrease with temperature, vanish at ≈50K
• Data Analysis and Interpretation — The highly reduced black STO substrate is essential for the observed T_c enhancement and novel effects; the mixed RP phases contribute to the inhomogeneous superconductivity.
  • Materials: Raw data from magnetic and transport measurements
  • Methods: Subtraction of STO substrate background (magnetic and resistance); Extraction of T_c onset and zero-resistance temperature; Comparison with literature on nickelates and reduced STO
  • Observations: T_c onset 50–70 K and zero resistance 20–25 K in both 1 h and 6 h reduced films; Diode effect and PME attributed to inhomogeneous film structure and highly reduced STO; Possible role of electron-doped 327 phase or interface effects