Can a tiny topological wire act as a switchable superconducting diode?
A topological-insulator nanowire between two superconductors carries more supercurrent one way than the other, and both the size and direction of this diode effect can be flipped with a magnetic field or a gate voltage.
Source
Tunable superconducting diode effect in a topological nano-SQUID
Study at a glance
- Design
- Other — Low-temperature transport on Nb-contacted BiSbTeSe2 nanowire junctions versus parallel field and gate voltage, with tight-binding simulations and a phenomenological SQUID model
- N
- No sample N; main data from device A, rectification from device B, and three further devices in supplementary material
- Population
- Bulk-insulating BiSbTeSe2 topological-insulator nanowires side-contacted by niobium electrodes, measured near 30 mK
- Outcome
- Critical currents in each direction and diode efficiency versus parallel magnetic field and back-gate voltage
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What they did
The authors etched nanowires tens of nanometres across from a bulk-insulating topological insulator, contacted their sides with niobium, and cooled them to about 30 mK. They measured the critical current for positive and negative bias as they varied a magnetic field along the wire and a back-gate voltage, and demonstrated rectification of an alternating current. They compared the data with a 3D tight-binding simulation and a model treating the top and bottom surfaces as two junctions forming a SQUID.
What they found
The critical current oscillated with a period of one flux quantum through the wire, and positive and negative critical currents differed, for example 250 nA versus 390 nA at −2.5 T, giving a diode efficiency near −0.23. The efficiency changed sign near half-flux-quantum points and also with gate voltage, and reached a magnitude of 0.3. Simulations reproduced the effect only when the top and bottom surfaces were made asymmetric, and theory links the field-driven sign change to a topological phase transition that should host Majorana zero modes.
The limits
What it doesn't show
The link to a topological transition and Majorana modes is theoretical and in equilibrium; the authors note current biasing may not preserve the topological phase, and Majoranas were not detected. Results come mainly from a single device, with the ac rectification limited by filters so the intrinsic speed is unknown. The effect requires tesla-scale magnetic fields and millikelvin temperatures, which the authors acknowledge limits practical use. Deviations near a full flux quantum are attributed to other effects not modelled in detail.
Key terms
- Josephson junction
- Two superconductors separated by a weak link through which a supercurrent can flow without voltage up to a critical current.
- SQUID
- A superconducting loop with two junctions whose critical current oscillates with the magnetic flux threading it.
- Superconducting diode effect
- A situation where the critical current differs for opposite current directions, so supercurrent flows more easily one way.
- Flux quantum
- The magnetic flux h/2e; SQUID responses repeat each time the enclosed flux changes by this amount.
- Current-phase relation skewness
- Deviation of supercurrent versus phase from a pure sine wave, adding higher harmonics that are needed for diode behaviour.
- Majorana zero mode
- A predicted zero-energy state at the ends of a topological superconductor, of interest for quantum computing.
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Quiz yourself
What is the period of the critical current oscillations in parallel field?
Common questions
Why does the device behave like a SQUID without a loop?
The top and bottom surfaces of the insulating nanowire each form a junction, and flux threading the wire between them sets a phase difference just as in a two-junction loop.
Which three ingredients are needed for the diode effect here?
Flux breaking time-reversal symmetry, asymmetry between the top and bottom surfaces breaking inversion symmetry, and a skewed current-phase relation.
How is the diode polarity switched?
By changing the parallel magnetic field across a half-flux-quantum point or by gating to swap which surface carries more supercurrent.
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