Search for Majorana fermion

The goal of this work is to design a system which supports Majorana excitations and study properties of this new unconventional state of matter. Specifically, we will fabricate one dimensional semiconductor wires where superconductivity is induced by proximity effects from an ordinary superconductor. In the presence of strong magnetic field in a properly designed system topological superconductivity can be realized, where Majorana modes are formed at the ends of the wire.

People involved

Zhong Wan

Current progress

In 1928 Dirac reconciled quantum mechanics and special relativity in a set of coupled equations which became the cornerstone of quantum mechanics. Its main prediction that every elementary particle has a complex conjugate counterpart -- an antiparticle -- has been confirmed by numerous experiments. A decade later Majorana showed that Dirac's equation for spin-1/2 particles can be modified to permit real wavefunctions. The complex conjugate of a real number is the number itself, which means that such particles are their own antiparticles. The most intriguing feature of Majorana particles is that in low dimensions they obey non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence. While the search for Majorana fermions among elementary particles is still ongoing, excitations sharing their properties may emerge in electronic systems. It has been predicted that Majorana excitations may be formed in some unconventional states of matter.

What are the experimental signatures of Majorana particles? Majorana particles come in pairs, and zero energy Andreev end-modes localized at the ends of a wire can be probed in tunneling experiments. Indeed, there are reports of zero bias anomaly observed in topological insulator/superconductor and semiconductor/superconductor structures. However, conductivity enhancement near zero bias can also be a signature of diverse phenomena in mesoscopic physics, such as the Kondo effect in quantum dots or the ``0.7 anomaly'' in nanowires. Fusion of two Majorana modes produces an ordinary fermion and, uniquely to Majorana particles, modifies periodicity of the Josephson relation from 2π (Cooper pairs) to 4π (Majorana particles).

We fabricated InSb/Nb hybrid nanowires using self-alignment fabrication techniques, Figure 1. When the junction is irradiated with rf frequency at zero external magnetic field, quantized voltage steps (Shapiro steps) are observed, Figure 2, as is expected for conventional superconductor junctions where the supercurrent is carried by charge-2e Cooper pairs. At high fields the height of the first Shapiro step is doubled, suggesting that the supercurrent is carried by charge-e quasiparticles. This is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago.

Figure 1 Device fabrication. a self-aligning fabrication of a InSb/Nb nanowire.

Figure 2 ac Josephson effect. Shapiro steps when sample is irradiated with 3 GHz rf. At high magnetic fields the 6 μ step disappears.