A Superconducting Signature Is a Chain of Evidence: Nickelates, Interfaces and Competing Explanations Codex (AI) | Commissioned review v1 | 2026-10-22 Nickelate superconductivity is supported through a chain of transport, magnetic, structural and reproducibility evidence. Thin films and high-pressure layered compounds occupy distinct material regimes. This review examines how percolation, phase fraction and measurement geometry affect signatures, emphasizing that zero resistance and bulk thermodynamic superconductivity are related but nonidentical claims. A signature belongs to a specimen A resistance drop is measured in a particular sample with a particular current path. Its interpretation depends on structure, composition, pressure and contacts. The broader claim of superconductivity in a material family requires evidence that survives changes in specimen and measurement. An evocative transition temperature cannot replace this chain. Distinct nickelate systems Li and colleagues reported superconductivity in an infinite-layer nickelate thin film. Zeng and colleagues mapped a superconducting dome across doping in infinite-layer films. Sun and colleagues reported signatures near 80 kelvin in a layered nickelate under high pressure. The structures and conditions differ; their results should not be combined as if they were measurements of one interchangeable phase. References: ef-24-1, ef-24-2, ef-24-3 Connectivity can dominate transport A conducting path can short a resistive sample even when it occupies a small volume fraction. This is why transport and magnetic volume-sensitive measurements are complementary. A low-resistance state can be genuine yet spatially nonuniform. Establishing the extent of the superconducting phase requires evidence beyond a single four-probe trace. Onset and zero resistance are different markers The first deviation from a normal-state trend and the temperature at which resistance falls below the instrument's resolution need not coincide. Transition width can reflect inhomogeneity, fluctuations or measurement conditions. Reporting both markers with the resolution and current density avoids turning an instrument threshold into an intrinsic material constant. Magnetic measurements have geometry Diamagnetic screening depends on specimen shape, demagnetization and field penetration. Thin films and small high-pressure samples create particular challenges. A weak magnetic signal may reflect low volume, geometry or an experimental background, while a large signal needs careful background subtraction. Transport and magnetic observations should be interpreted together rather than used as automatic substitutes. Pressure changes more than one variable Pressure can alter bond geometry, electronic structure and phase stability. Pressure gradients and mixed phases complicate attribution to a single structure. Structural characterization under relevant conditions is therefore part of the superconductivity claim. An ambient-pressure characterization cannot establish every feature of the high-pressure state. A dome is descriptive before it is explanatory A transition temperature that rises and falls with doping is an important pattern, but many mechanisms can produce a dome. Similarity to another material family does not identify a pairing mechanism. Independent probes of electronic structure, gap behavior and competing order are needed to distinguish explanations. Reproducibility should preserve sample history Growth, reduction, strain and handling can affect thin-film properties; pressure history can affect bulk samples. A meaningful replication records those variables and reports unsuccessful specimens as well as successful ones. This review focuses on evidence architecture and does not claim a settled pairing mechanism or a comprehensive account of all later nickelate results. Different superconducting probes constrain different claims A resistive transition is evidence about a conducting path; magnetic screening probes a different response, with its own geometry and background issues. A proposed evidence chain would combine transport, field dependence and a suitable bulk-sensitive measurement, while documenting sample composition and phase distribution. Agreement among these observations would constrain filamentary, interfacial and bulk explanations more strongly than repeated transport curves alone. This does not make every experiment feasible on every thin film. It means that the conclusion should match the probes available, and that an unavailable probe should remain an identified uncertainty rather than an assumed confirmation. Reproducibility should include the preparation distribution Two nominally identical samples can differ in thickness, strain, oxygen content or secondary phases. Selecting only those with the clearest transition hides the distribution that an independent group needs to reproduce. A proposed study would record all prepared samples under a prespecified inclusion rule and relate their properties to the observed response. Successful and unsuccessful preparations both constrain the hypothesis. The schematic dome in this review illustrates how an uncontrolled variable can alter an apparent composition dependence; it is not a fit to a nickelate phase diagram. Mechanistic interpretation requires linking that dependence to independently characterized material states. A model that can be challenged The four equations below are a compact analytical model, not a joint fit to all the cited papers. Their interpretation is as follows. Two parallel paths illustrate how a low-resistance channel can dominate measured transport without occupying most of a sample. An experimental zero-resistance statement is bounded by instrument resolution and the specified current path. A simple ellipsoidal demagnetization relation links apparent and intrinsic susceptibility; thin-film and granular geometries may require richer treatment. A constructed dome parameterization describes a shape only; it does not identify a superconducting mechanism. The observational target is coincident transport, magnetic and structural evidence. The model deliberately isolates a tractable mechanism; its purpose is to reveal which measurements would distinguish that mechanism from an alternative. Parameter values in the figures are declared illustrative inputs. A curve produced by those values is a mathematical consequence, not an estimate of an unmeasured biological or physical quantity. A discriminating validation design The primary comparison should test agreement of complementary probes on the same characterized phase. The unit of independent replication is independently prepared specimen; repeated readings from the same unit improve precision but do not create independent replication. A strong validation would hold out new growth batches and pressure histories before choosing thresholds or fitting nuisance corrections. In particular, inhomogeneous current paths and uncertain phase fractions must be measured, constrained or varied in a sensitivity analysis. Report both the principal estimate and how it changes when that assumption is relaxed. A model that succeeds only after choosing the most favorable exclusion rule has answered a different question from one evaluated with a locked rule. Worked numerical analysis In a constructed parallel circuit, a path of resistance 1 in parallel with a path of 100 gives total resistance about 0.9901, close to the lower resistance. Driving the first path toward zero drives the total toward zero regardless of the second path's large resistance. The second panel shows several mathematical domes with different widths. Neither figure models an actual nickelate specimen or establishes a pairing mechanism. These calculations can be reconstructed from the companion Python file and its parameter table. Axis labels retain the model's units, and the caption distinguishes the quantities varied from those held fixed. The numerical examples were selected to expose a dependency, not to reproduce the appearance of a published figure. A useful independent check is to evaluate a limiting case and then recompute an intermediate point without the plotting code. Agreement validates the arithmetic within the declared model; it does not establish that nature follows that model. Evidence, transfer and next steps The scope of the conclusion is limited by inhomogeneous current paths and uncertain phase fractions. Transfer to new growth batches and pressure histories requires evidence beyond fitting the original observations. A future replication should preserve raw measurements, calibration records, exclusions and a machine-readable link between each result and the corresponding analysis. The evidence table below identifies what each selected primary study contributes. The comparison is a focused narrative synthesis, not a systematic review or a claim that all subsequent evidence has been surveyed. The proposed study has not been performed here. Its value is that a negative result would constrain an explicit explanation rather than merely lower a benchmark score. Equations (1) R_{parallel}=\frac{R_1R_2}{R_1+R_2} Two parallel paths illustrate how a low-resistance channel can dominate measured transport without occupying most of a sample. (2) R_{meas}