Superconductivity: evidence and concepts

Scope

Superconductivity is a state with characteristic electrical, magnetic and collective behavior. Understanding a new claim requires more than finding a resistance drop or a gap-like feature. Start by asking what was measured, on which sample, under which conditions, and which alternative explanations were tested. This overview links the observable evidence to the wiki's theories, materials and control parameters. It does not assign a microscopic pairing mechanism from one probe or claim that every article has completed scientific acceptance. R1 R2 R3

Use zero resistance, the Meissner effect and specific heat to compare transport, magnetic response and thermodynamics. Continue to gap measurements for excitation spectra and to flux quantization for phase-related evidence. These observables constrain different aspects of the state. Agreement on matched samples strengthens a case; one positive result does not automatically supply the missing measurements. R1 R2 R4 R6

Mathematical assumptions

Models turn observations into quantities under explicit assumptions. In a uniform bar carrying homogeneous current, resistivity is inferred from resistance as $\rho=RA/L$, with cross-sectional area $A$ and voltage-contact separation $L$. A heterogeneous film or a filamentary path may violate that conversion. A measurement consistent with zero voltage establishes a resistance bound set by current and voltage sensitivity; it does not mathematically demonstrate an infinitely precise zero or determine superconducting volume fraction. R1 R3

The simplest local London description gives exponential magnetic screening in a uniform half-space, $B(x)=B(0)e^{-x/\lambda}$. Penetration depth is not interchangeable with an uncorrected magnetic shielding fraction: sample shape, demagnetization, vortices and measurement history matter. A homogeneous weak-coupling isotropic BCS model also supplies useful reference scales, but its gap and heat-capacity ratios are not definitions that every superconductor must satisfy. R1 R3 R4

Distinguish pair formation, global phase coherence and a probe's practical transition criterion. In a two-dimensional regime, BKT physics involves phase stiffness and vortex unbinding. A spectroscopic gap or resistance onset can therefore differ from a phase-ordering temperature. A fitted scale must retain its model, dimensionality, current range and uncertainty rather than being relabelled as a universal transition. R7

Experimental criteria

Transport should report contact geometry, applied current, voltage sensitivity, transition width and field dependence. Magnetic evidence should distinguish flux expulsion from shielding, field-cooled from zero-field-cooled protocols, and reversible response from pinning. Thermodynamic evidence should retain background subtraction and sample fraction. Where one probe is impractical, describe the evidence actually obtained and the missing controls rather than either declaring certainty or dismissing a result automatically. R2 R3 R4

Isotope substitution tests how changing nuclear mass affects an observed scale, with chemical and structural controls. It does not prove that phonons are the only pairing interaction. Critical fields involve orbital, spin and vortex effects; a high field alone does not determine spin-triplet order. Critical current additionally depends on pinning, geometry and an operational electric-field criterion, so it is not a universal material ranking independent of field and temperature. R1 R5 R8

Andreev reflection probes electron–hole conversion at an interface, where transparency and the surrounding electronic structure matter. Quasiparticle interference maps scattering-related spatial modulations and constrains electronic states through a model. Neither an isolated conductance peak nor a Fourier feature independently determines pairing phase or establishes a Majorana mode. Follow the relevant article's probe-specific limitations before interpreting a pattern. R9 R10

Linear-temperature resistivity and strange-metal behavior describe normal-state transport, not a superconductivity test by themselves. Residual resistance, temperature window, carrier weight and scattering anisotropy affect the inference. An observed linear curve is not automatically a direct measurement of a universal relaxation time or a unique quantum-critical mechanism. R11

Limits and disputes

Evidence for a superconducting state, the symmetry of its order parameter, the interaction that produces pairing and the topology of its excitations are different claims. Bulk transport or screening does not alone determine gap phase; normal-state band topology does not alone classify the paired state. Disagreement between probes should be traced to samples, conditions, assumptions and uncertainties before being compressed into a single verdict. The core wiki keeps experiments, models, predictions and hypotheses distinguishable. R1 R3 R4 R9 R10

The pinned overview's historical audit label is not a new independent verification. This page reuses the explicitly scoped source checks documented in the linked articles. Complete scientific acceptance remains open, including unresolved final-version comparisons, supplementary controls and raw-data analyses. Numerical illustrations establish only their stated algebra and assumptions; they do not certify experimental reproducibility or exhaustive coverage of the field. R1 R4 R5 R10 R11

Discovery implications

Navigate by the question. Start with BCS pairing and spin fluctuations for interactions, then singlet and triplet pairing for order-parameter constraints. Compare cuprates, nickelates or kagome materials with their sample-specific evidence. Use pressure and chemical substitution to define the state being tested. These are routes into the evidence, not interchangeable explanations. R1 R5 R11

Keep a claim record with material, composition, preparation, pressure, field direction, measurement temperature, transition criterion and source version. Select the next experiment for its ability to distinguish competing interpretations. If a high-pressure, interfacial or transient result lacks a thermodynamic or magnetic cross-check, preserve that limitation in both prose and structured data. A transparent record can be revised as evidence develops without turning an untested hypothesis into a material fact. R3 R4 R8 R9

Related topics

Explore quantum geometry, magnetism and coexistence, charge-density waves, and the SCWiki keyword index.

Material cases

No sample-specific material records are included in this article. Model results are not experimental transition temperatures.