Superconductivity
Why does a 2D superconductor keep a little resistance near zero kelvin?
Open access · cc by · source: Europe PMC
In atom-thin lead superconductors, the leftover resistance of the 'anomalous metal' state appears while vortices sit still under the microscope, suggesting the measuring current, not quantum fluctuations, moves them.
Study at a glance
- Design
- Other — Lab experiment: four-probe transport and 0.36 K scanning tunnelling spectroscopy under perpendicular magnetic field on monolayer Pb on flat and vicinal (0.5°, 1.1°) Si(111), with step density as tunable disorder.
- N
- No sample count; flat, 0.5°- and 1.1°-tilted samples measured.
- Population
- Striped incommensurate monolayer Pb on flat and vicinal Si(111) substrates
- Outcome
- Sheet resistance versus temperature and field; zero-bias conductance maps showing vortices; superconducting gap evolution
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
The steps lowered the critical temperature from 1.53 K (flat) to 1.21 K (1.1° tilt). For fields between about 25 and 100 mT, resistance saturated at a finite value near zero temperature, and in this same regime STM showed stable, isolated vortices in a triangular lattice; the saturated resistance grew in proportion to field, consistent with free vortex flow driven by the transport current. Some vortices delocalised from about 120 mT and all liquefied by about 240 mT, while a pseudogap persisted above the upper critical field of 344 mT; disorder turned a metallic regime above that field into an insulating one.
Methodology
The authors grew single atomic layers of lead on silicon, using tilted substrates whose atomic steps add controlled disorder. They measured resistance versus temperature and perpendicular magnetic field to locate the anomalous metallic state, then used a scanning tunnelling microscope at 0.36 K to map the conductance at zero bias, where vortex cores show up as bright spots. They built field–temperature and field–disorder phase diagrams from both techniques.
Limitations
STM sees vortices with no current flowing, so the claim that the transport current moves them is an inference, not a direct observation of vortex motion under current. The transport measurements used no radio-frequency filtering, so stray AC currents could contribute. Only three step densities in one material were examined, and the quantum Griffiths interpretation of the flat-sample metallic phase lacked the diverging critical exponent.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
An 'anomalous metal' can be a superconductor with moving vortices.
Disorder and vortices control low-temperature resistance in atom-thin films: in monolayer Pb on Si, resistance saturated at a finite value between about 25 and 100 mT, the same range in which STM imaged stable vortex lattices, and added step disorder lowered Tc from 1.53 K to 1.21 K.
Evidence for the claim as stated.
Thin-film enhancement looks different in different systems: aluminium's Tc rose on thinning, but monolayer Pb's Tc fell when step disorder was added, so 'thinner is better' depends on disorder and material.
Evidence for the claim as stated.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
Thin-film enhancement looks different in different systems: aluminium's Tc rose on thinning, but monolayer Pb's Tc fell when step disorder was added, so 'thinner is better' depends on disorder and material.
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