A research team at Chalmers University of Technology is now among the first in the world to submit results indicating that they have actually succeeded in manufacturing a topological superconductor. In solid state materials Majorana fermions only appear to occur in what are known as topological superconductors – a new type of superconductor that is so new and special that it is hardly ever found in practice.
Quantum computers with topological superconductors should have very long coherence times.
“Our experimental results are consistent with topological superconductivity,” says Floriana Lombardi, Professor at the Quantum Device Physics Laboratory at Chalmers.
To create their unconventional superconductor they started with what is called a topological insulator made of bismuth telluride, Be2Te3. A topological insulator is mainly just an insulator – in other words, it does not conduct current – but it conducts current in a very special way on the surface. The researchers have placed a layer of a conventional superconductor on top, in this case aluminium, which conducts current entirely without resistance at really low temperatures.
Proximity-induced OP on the surface of Bi2Te3. We consider only the chiral p x + ip y part of the total p + s OP. Close to the interface I the chiral term is modified: the p x (dashed line) and p y (straight line) components exhibit different magnitude. In case of high transparency interface (a), the two components are both suppressed in equal manner and the resulting interface OP still preserve the p x + ip y wave character. For a low transparency interface (b), the p x component is highly suppressed while the p y slightly enhanced. The resulting OP has a predominantly p y character. c shows the amplitude of the resulting interface OP by considering a mixture p y + iαp x , with α = 0.1. This corresponds to the reduction of p x on distances of the order of the coherence length of S′28. The corresponding phase is shown in d
“The superconducting pair of electrons then leak into the topological insulator which also becomes superconducting,” explains Thilo Bauch, Associate Professor in Quantum Device Physics.
However, the initial measurements all indicated that they only had standard superconductivity induced in the Bi2Te3 topological insulator. But when they cooled the component down again later, to routinely repeat some measurements, the situation suddenly changed – the characteristics of the superconducting pairs of electrons varied in different directions.
“And that isn’t compatible at all with conventional superconductivity. Suddenly unexpected and exciting things occurred,” says Lombardi.
Unlike other research teams, Lombardi’s team used platinum to assemble the topological insulator with the aluminium. Repeated cooling cycles gave rise to stresses in the material (see image below), which caused the superconductivity to change its properties.
After an intensive period of analyses the research team was able to establish that they had probably succeeded in creating a topological superconductor.
“For practical applications the material is mainly of interest to those attempting to build a topological quantum computer. We ourselves want to explore the new physics that lies hidden in topological superconductors – this is a new chapter in physics,” Lombardi says.
The results were recently published in the scientific journal Nature Communications: Induced unconventional superconductivity on the surface states of Bi2Te3 topological insulator
More about quantum computers and the Majorana particle
A large Quantum computer project in the Wallenberg Quantum Technology Centre is underway at Chalmers University of Technology. It is, however, based on technology other than topological superconductors.
The Majorana particle was predicted by the Italian physicist Ettore Majorana in 1937. It is a highly original fundamental particle which – like electrons, neutrons and protons – belongs to the group of fermions. Unlike all other fermions the Majorana fermion is its own antiparticle.
Topological superconductivity is central to a variety of novel phenomena involving the interplay between topologically ordered phases and broken-symmetry states. The key ingredient is an unconventional order parameter, with an orbital component containing a chiral p x + ip y wave term. Here we present phase-sensitive measurements, based on the quantum interference in nanoscale Josephson junctions, realized by using Bi2Te3 topological insulator. We demonstrate that the induced superconductivity is unconventional and consistent with a sign-changing order parameter, such as a chiral p x + ip y component. The magnetic field pattern of the junctions shows a dip at zero externally applied magnetic field, which is an incontrovertible signature of the simultaneous existence of 0 and π coupling within the junction, inherent to a non trivial order parameter phase. The nano-textured morphology of the Bi2Te3 flakes, and the dramatic role played by thermal strain are the surprising key factors for the display of an unconventional induced order parameter.