Summary
This work uses scanning SQUID microscopy to investigate the local superfluid density and magnetic structure of an optimally doped infinite-layer nickelate Nd1−xEuxNiO2 (x = 0.25) thin film grown by molecular beam epitaxy on an LSAT substrate. Simultaneous direct-current magnetometry and alternating-current magnetic susceptibility imaging reveal that superconductivity coexists with a weak magnetic texture, and that the superconducting state is highly spatially inhomogeneous: near the superconducting transition, a robust stripe-like enhancement of superconductivity emerges, accompanied by halo-like defect responses and paramagnetic regions between the stripes. These results indicate that competing magnetic phases are the main factor responsible for the anomalous broadening of the superconducting transition in this optimally doped NENO sample; understanding this superconducting enhancement mechanism may provide a route to raising the superconducting temperature of such materials.
Materials
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Methods
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Keywords
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Highlights
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Conclusions
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Main claims
- Optimally doped NENO exhibits a robust stripe-like enhancement of superconductivity near Tc, indicating a highly non-uniform superfluid density.
- Evidence: AC susceptibility maps show a periodic stripe pattern with period ≈12.3±0.5 μm by direct measurement.,FFT analysis gives a spatial frequency of 0.079 μm^-1, corresponding to a 12.5 μm period.,Stripes appear from ≈8.8 K to 11.1 K and persist under temperature cycling and small out-of-plane fields.
- A weak intrinsic magnetic texture coexists with superconductivity in NENO.
- Evidence: DC magnetometry maps show magnetic texture above Tc and when superconductivity is suppressed by small out-of-plane fields.,Vortex contribution is estimated to be at least an order of magnitude weaker than the observed magnetic signal.,HAADF-STEM and STEM-EELS show no nanoparticle inclusions or secondary phases that could explain the magnetic response.
- Competing magnetic order or electronic phase separation is the primary contributor to the broad superconducting transition.
- Evidence: The global superconducting transition is broad, from about 8 K to 22 K.,Paramagnetic regions appear between superconducting stripes, and halo-like regions indicate local pair-breaking.,Local transition temperature varies by as much as 1.8 K within 100 μm.,Structural and chemical probes show no corresponding periodic inhomogeneity.
- The sample edge hosts an enhanced superconducting response.
- Evidence: A heightened diamagnetic response at the sample edge remains detectable up to ≈13.4 K.,The highest local transition temperature in approach curves was found near the sample edge.
- The ≈12 μm stripe modulation is distinct from known nickelate charge density waves and is not due to SQUID scan artifacts.
- Evidence: Nickelate charge density waves typically have ~three-unit-cell periodicity (≈1 nm), orders of magnitude smaller than the observed ≈12 μm period.,Non-periodic stripes of similar signal strength are also observed independently of the periodic array.,Stripes are detected on a second sample and their orientation relative to the sample edge varies between samples.
Workflow
- sample_preparation — Optimally doped NENO thin films were synthesized on LSAT and reduced to the infinite-layer phase for local magnetic and superconducting characterization.
- Materials: Nd1−xEuxNiO2 (NENO, x=0.25) thin films; commercial LSAT (001) substrates; metallic Al capping layer
- Methods: molecular beam epitaxy (MBE); substrate cleaning with activated oxygen; in situ topotactic reduction using a 2–3 nm Al capping layer
- Observations: NENO films grown on LSAT; infinite-layer structure obtained after topotactic reduction; Al capping oxidizes to Al2O3 and remains on the film surface
- initial_bulk_characterization — Macroscopic measurements show superconductivity with a broad transition and a hysteretic magnetic response, motivating local probes.
- Materials: NENO films on LSAT
- Methods: PPMS transport measurements; MPMS magnetization measurements; zero-field-cooled and field-cooled scans; magnetic field sweeps
- Observations: broad superconducting transition from about 8 K to 22 K; magnetic moment increases as temperature decreases from 100 K; small ZFC/FC hysteresis; no bulk diamagnetic drop in MPMS
- scanning_SQUID_microscopy — Local SSM imaging reveals coexisting magnetic texture and a highly non-uniform, stripe-patterned superconducting state in NENO.
- Materials: NENO film; LSAT substrate; vacuum reference
- Methods: scanning SQUID microscopy (SSM); simultaneous DC magnetometry and AC susceptometry; piezoelectric raster scanning; zero-field cooling; substrate or vacuum subtraction; low-pass filtering of magnetometry
- Observations: weak magnetic texture coexisting with superconductivity; halo-like point-defect responses at low temperature; stripe-like enhancement of diamagnetic response with period ≈12.3–12.9 μm; paramagnetic regions between stripes; edge-enhanced superconductivity; stripes persist under temperature cycling and small magnetic fields
- susceptibility_approach_curves — Approach curves confirm spatially varying effective penetration depth and local Tc, indicating a non-uniform superconducting and magnetic response.
- Materials: NENO film; LSAT substrate
- Methods: height-dependent susceptibility approach curves; fitting to Kirtley-Kogan thin-film diamagnetic and paramagnetic responses
- Observations: effective penetration depth minimum ≈4.05 μm at 6.5 K and approaches infinity past 10 K; transition between diamagnetic and paramagnetic phases between 9.5 K and 10.4 K; model fails in the intermediate regime; local transition temperature varies by as much as 1.8 K within 100 μm
- structural_elemental_characterization — Structural and elemental probes rule out gross structural or chemical inhomogeneity and secondary-phase nanoparticles as the cause of the stripes.
- Materials: NENO lamella prepared by FIB; NENO film surface
- Methods: SEM; HAADF-STEM; STEM-EELS; EDS; FIB lamella preparation; PCA noise reduction for EELS
- Observations: smooth film surface without scratches or cracks; epitaxial growth and atomically flat film-substrate interface; no inclusions or secondary phases; uniform elemental composition at atomic scale; no periodic structural feature matching the ≈12 μm stripes
- data_analysis_and_interpretation — The observed stripe and magnetic textures are intrinsic electronic and magnetic phenomena consistent with mesoscopic phase separation, not instrumental or structural artifacts.
- Materials: SSM images; transport and magnetization data; STEM/EELS data
- Methods: cross-sectional analysis; Fast Fourier Transform (FFT); comparison across samples and positions; vortex-strength estimation; phase-separation interpretation
- Observations: FFT peak at 0.079 μm^-1 corresponding to a 12.5 μm period; stripe orientation varies between samples but is consistent within a sample; magnetic texture spatially correlates with susceptibility stripes; vortex contribution estimated at least an order of magnitude weaker than the observed magnetic signal