Can a superconductor induce pairing in a magnetic topological edge?
A narrow superconducting strip on the edge of a quantum anomalous Hall insulator turns incoming electrons into outgoing holes, showing superconducting correlations have leaked into the edge state.
Source
Induced superconducting correlations in a quantum anomalous Hall insulator
Study at a glance
- Design
- Other — Low-temperature three-terminal transport on V-doped (Bi,Sb)2Te3 Hall bars with Nb finger electrodes of different widths, plus tight-binding transport simulations
- N
- Several devices (A to F) with Nb finger widths from 160 to 520 nm; no single sample count
- Population
- Thin-film quantum anomalous Hall insulator devices contacted by superconducting niobium fingers
- Outcome
- Downstream (nonlocal) resistance versus magnetic field, temperature and finger width
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What they did
The team grew thin films of a vanadium-doped topological insulator that shows the quantum anomalous Hall effect without any applied field, and patterned Hall bars with niobium superconducting fingers of different widths touching the chiral edge. They measured the voltage on the edge downstream of the finger while sweeping magnetic field and temperature at dilution-refrigerator temperatures. They also ran tight-binding quantum transport simulations of a proximitized magnetic topological insulator film.
What they found
For the narrowest finger (160 nm) the downstream resistance was negative below about 1 T, the signature of crossed Andreev reflection, and it turned positive once the niobium stopped superconducting. The Andreev contribution shrank exponentially with finger width, with a characteristic length of about 100 nm, much longer than niobium's own coherence length of about 30 nm, suggesting pairing is induced in the topological film under the finger. Simulations showed that both trivial and topological superconducting scenarios can produce this effect.
The limits
What it doesn't show
The data cannot tell whether the induced superconductivity is topological, so it does not demonstrate Majorana modes. The CAR signal is only about 3% of the ideal maximum, varies with magnetic history, and depends on contact quality and film ageing. The simulations use parameters that are qualitative rather than matched to the devices, and they fail to explain why the signal is always negative in experiment.
Key terms
- Quantum anomalous Hall insulator
- A magnetic topological material whose interior is insulating while one-way (chiral) edge channels carry current with quantized Hall resistance, without an external magnetic field.
- Crossed Andreev reflection
- A process in which an electron entering a superconductor pairs with an electron from the far side, so a hole exits into a different edge channel.
- Proximity effect
- Leakage of superconducting pairing from a superconductor into a neighbouring normal material.
- Coherence length
- The characteristic distance over which superconducting pair correlations persist.
- Chiral edge state
- A one-dimensional conducting channel along a sample edge in which electrons move in only one direction.
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Quiz yourself
What was the main observation for the 160 nm niobium finger at low field?
Common questions
Why is a negative resistance evidence for superconductivity in the edge?
When crossed Andreev reflection happens, a hole rather than an electron leaves downstream, carrying the opposite potential, so the measured downstream voltage flips sign.
Why does the width dependence matter?
The signal decays over about 100 nm, much longer than niobium's coherence length, so the pairing that mediates it must live in the proximitized topological film rather than in the niobium.
Did they find Majorana modes?
No. Their data fit both a trivial and a topological superconducting picture; they propose shorter-finger experiments to tell them apart.
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