A single helix for novel quantum phenomena

In most materials the spin, or intrinsic magnetic moment, of the electrons is balanced. For each electron with a spin in one direction there is another electron with exactly opposite spin. Breaking of this equilibrium for the total number of electrons is the basis for the many astonishing phenomena related to magnetism. However, it is also possible to keep this balance, but link the spin to the direction, or momentum, of the electron. In this scenario only electrons with a certain spin direction are allowed to travel in a given direction. As a function of their energy the electrons then form circles in momentum space with their spin direction winding along the circle. For the electrons involved in the conduction of a materials this is referred to as a single helical Fermi surface.

The interest in such a configuration of electrons lies in their response to external perturbations. For example, when a single charge is placed in front of such a material its response will be like that of a magnetic monopole. Even more exotic phenomena happen if this helical Fermi surface is combined with superconductivity. As a result, quasiparticles will form at the edge which behave similar to what would be expected for the elusive Majorana Fermion. These so-called Majorana bound states form the basis for fault tolerant quantum computers, and they have been the object of intense research efforts over the last decade. Initially much was expected of the surface state of topological insulators, but these materials show other problems and it was soon realised that a similar situation can be achieved by combining symmetry breaking, spin-orbit interaction, and magnetism in a single material.

The symmetry breaking is achieved for free at any surface or interface, and also spin-orbit interaction is present in almost all materials and can be enhanced by distortions that occur as a result of the symmetry breaking. The problem is the combination with magnetism and the common brute force approach has been to apply a relatively large external magnetic field. However, for SrTiO₃ the weak magnetism strongly couples to the spin-orbit coupling of the electrons confined at the surface and creates exactly the sought for conditions.

One problem that remained was that the resulting single helix was not at the correct energy and resisted methods to shift the energy level. Surprisingly, researchers from the Laboratory for Topological Matter at the EPFL and PSI have been able to shift the helix to the Fermi level by growing SrTiO₃ thin films on thick wafers of the same material. The difference is due to a different surface structure and a change in the dielectric response.

A further advantage of SrTiO₃ is that it becomes superconducting at ultra low temperatures and thus combines all the ingredients for the formation of Majorana bound states. The formation and observation of these states will require further patterning of the films, but the LTM researchers have made another important step in the realisation of this elusive quantum phenomenon.