3D Topological Insulators

Unconventional Josephson Effect in Hybrid Superconductor-Topological Insulator Devices

Theory predicts the emergence of Majorana fermion excitations in a Josephson junction formed with s-wave superconductor leads separated by a 3D topological insulator. When the phase difference across the junction is π, its quasiparticle spectrum is predicted to become gapless and linear and host Majorana modes [1].


Figure taken from Ref. 1, showing the quasiparticle spectrum of a STIS junction for various phases.

Josephson junctions in hybrid superconducting-topological insulator devices show two striking departures from the common Josephson junction behavior: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent [2]. We mechanically exfoliate thin (50-100 nm) flakes of Bi2Se3 from bulk crystals grown via slowing cooling of a binary melt. We then fabricate on top of these flakes 60 nm-thick aluminum leads of width W ~ 0.5-1.5 μm separated by a gap of length L ~ 30-80 nm.


Figures taken from Ref. 2, showing (left) a schematic and scanning-electron micrograph of a topological insulator Josephson junction and (right) Differential resistance dV/dI as a function of B and I showing an anomalous magnetic diffraction pattern, and a comparison to the expected Fraunhofer pattern for the junction (solid line).

The junctions demonstrate two unconventional behaviors in transport:

  1. The product of critical current and normal state resistance (typically a geometry-independent quantity reflecting the superconducting energy gap of the leads) tends to scale with the reciprocal of the width of the leads.
  2. Applied magnetic fields suppress critical current aperiodically and at a smaller field scale than device geometry should allow, departing from the conventional Fraunhofer pattern.

Normal Probes of 3D Topological Insulator Josephson Junctions

In order to better understand the observations and ascertain whether Majorana physics are involved, new measurements are needed to “peak under the hood” of the proximity effect in Bi2Se3. A recent proposal suggests measuring normal conduction across the junction, perpendicular to supercurrent flow, in a ring configuration allowing the junction to be phase-biased with the application of magnetic flux. Every time the junction phase passes through π the Majorana mode switches between occupied and empty, causing the conductance to change by 2e^2/h [3].

We fabricate aluminum rings with a 70 nm gap on top of Bi2Se3 flakes. To contact the Bi2Se3 next to the junction with a normal metal both inside and outside the ring, we fabricate titanium/gold air bridges. This allows the measurement of normal and normalsuperconductor transport in variety of configurations.


Scanning electron micrograph of a josephson junction in a ring configuration.

We measure prominent oscillations in dV/dI as a function of magnetic field in all configurations, but the oscillation period corresponds to a single superconducting flux quantum, not double. The oscillations appear when DC bias current is smaller than ~1 μA. However, the oscillation amplitude is strongly suppressed around zero bias, but only below 200 mK. This temperature scale appears in all devices.

References

  1. L. Fu and C. L. Kane Phys. Rev. Lett. 100,, 096407 (2008).
  2. J. R. Williams, A. J. Bestwick, P. Gallagher, Seung Sae Hong, Y. Cui, Andrew S. Bleich, J. G. Analytis, I. R. Fisher, and D. Goldhaber-Gordon, "Unconventional Josephson Effect in Hybrid Superconductor-Topological Insulator Devices" Physical Review Letters 109, 056803 (2012) [See accompanying Physics Viewpoint]. Supplementary info
  3. Benjamin J. Wieder, Fan Zhang, and C. L. Kane arXiv:1302.2113
Contact Jimmy Williams (jimi@), Andrew Bestwick (abestwick@), or Eli Fox (ejfox@) for more information.