Why does a 2D superconductor keep a little resistance near zero kelvin?
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.
Source
Stable vortices in the anomalous metallic state observed on monoatomic-layer superconductors
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.
What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Anomalous metallic state
- A regime in 2D superconductors where resistance levels off at a finite value instead of dropping to zero as temperature approaches zero.
- Superconductor-insulator transition
- A quantum phase transition at zero temperature driven by field, disorder or thickness.
- Vortex
- A whirl of supercurrent threading one flux quantum through a superconductor, with a non-superconducting core.
- Zero-bias conductance map
- An STM image of tunnelling conductance at zero voltage; high values mark vortex cores or suppressed superconductivity.
- Vicinal substrate
- A crystal cut slightly off a low-index plane, giving regularly spaced atomic steps.
Flashcards
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Quiz yourself
What did STM reveal in the field range of the anomalous metallic state?
Common questions
Why does vortex motion cause resistance?
Moving vortices generate an electric field along the current, so a superconductor with mobile vortices dissipates energy even though Cooper pairs remain.
How do atomic steps act as disorder?
They interrupt the lead layer's crystal, weakening phase coherence between terraces and lowering the critical temperature.
What would a quantum vortex liquid predict that was not seen?
Vortices constantly moving due to quantum fluctuations, which would blur them in STM images even without current.
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