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Semiconducting nanowires have opened up the field of quantum transport, and this is largely due to their confined geometry and electrostatic tunability. For many reasons, they are now at the forefront to realize topological superconductivity and Majorana modes, with the main challenge lying in reducing the disorder within hybrid nanowires
An international team of Scientists have shown ballistic superconductivity in InSb semiconductor nanowires, with the intention of using it to improve the superconductivity of semiconducting nanowires and pave the way for disorder-free Majorana devices.
Semiconducting nanowires have been a hotbed of research for the superconductivity field, which has now led to the realization of hybrid nanowire systems. These materials combine the quantum properties seen in a superconductor, with the ability to control charges as small as a single electron. Their potential to realize disorder-free Majorana devices is also a very important reason for their continued development.
Majorana nodes are zero-energy quasiparticles that emerge at the interface of a topological superconductor. Current technology involves the coupling of a semiconducting nanowire to a superconductor, to show characteristic Majorana signals.
To strengthen the signatures and realize the predicted topological properties of Majoranas, the disorder within the hybrid system needs to be reduced. Disorder within these systems can cause a mimicking of the zero-energy signatures found in Majoranas. This confusion results in a state where the superconducting energy gap renders the topological properties inaccessible through experimental means. As such, the removal of these defects is paramount to further understanding these hybrid systems.
The Researchers grew InSb nanowires using an Au-catalysed vapor–liquid–solid mechanism, in a metal organic vapor phase epitaxy reactor. The nanowires were then deposited, one-by-one, onto a micromanipulator, patterned and deposited using oxygen plasma cleaning. Superconducting regions (composed of NbTiN) were mounted onto the chip using a sputtering system (AJA International ATC 1800 sputtering system).
The Researchers performed structural and chemical analyzes on the hybrid chips using focused ion beam milling (FIB, FEI Nova Nanolab 600i Dualbeam), high-resolution transition electron microscopy (HRTEM) and scanning transmission electron microscopy (TEM, JEM ARM200F) and energy dispersive X-ray spectroscopy (EDX).
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Compared to other semiconducting wires, InSb is cleaner and possesses higher electron mobility, hence, it was chosen. InSb also has a 5-fold larger g-factor, lower in the external magnetic field required to induce the topological phase transition. The Researchers chose NbTiN as the superconducting material, as it possesses a high critical magnetic field exceeding 10 T.
The Researchers were able to achieve and characterize ballistic superconductivity in the InSb hybrid nanowires, i.e. when the electrons, which act as fermions, in the chip have a negligible electrical resistivity (normally caused by scattering) due to the mean free path of the electron being much longer than the dimension of the chip where the electrons travel through.
The Researchers' analyzes found there to be a high-quality interface between the InSb semiconducting nanowire and the NbTiN superconducting material. The interface allowed for the ballistic transport to occur, and arose as a function of the quantized conductance for the normal carriers. This was found to strongly enhance the conductance in the Andreev reflection carriers at energies below the superconducting gap. Andreev reflection carriers occur when an incident electron (hole) from one material is injected with a subgap energy, onto a superconducting layer, forming an Andreev hole (electron).
The Researchers also performed a numerical analysis which showed a mean free path of several micrometers and implied that ballistic transport had occurred in the proximity of Andreev pairs in the nanowire. The Researchers also found the material to possess an induced hard gap with a significantly reduced density of states.
This research has made great strides towards creating disorder-free Majorana devices and future work looks to envelop the development of a quantitative model for magnetic field-induced deviations arising from Andreev transport. This transport is thought to play a crucial role in realizing a topological quantum bit based on semiconductor nanowires, so will be the main focus in new research.
Sources and Further Reading
- “Ballistic superconductivity in semiconductor nanowires”- Zhang H., Nature Communications, 2017, DOI: 10.1038/ncomms16025