Understanding Topological Insulators to Build the Next Generation of Electronics
06 Aug 2021
- Shikha Gianchandani



Using MuSR, Professor Patnaik from Jawaharlal Nehru University, cracked how to study fundamental properties of topological superconductors.


​ZF-MuSR spectra collected at 0.1 K and 3.7 K (top) and temperature variation of the muon spin relaxation rate above and below TC


Our phones are extraordinary – what was once used just to make calls, can now be used to do everything from telling you the weather in a foreign country, helping you set reminders for tasks and, not to mention, entertaining you for hours. Although that's just the beginning: researchers are constantly on the lookout for new ways to improve device performance.

The computation going on in our phone requires massive amounts of data processing (with great speed), which is hidden from us since it's happening on a server somewhere externally. A majority of our phone chips are made of silicon. With the growing demand to improve device performances and include new features, scientists are finding that they might be reaching a limit with how much use they can get out of these chips. This raises the question: if our gadgets are getting faster and smaller, what kind of change do scientists need to implement to keep up?

One option is to work with new quantum materials that will help build the next generation of devices. In the electronics world, there are two common types of materials (you might even be familiar with these from school-level physics) – conductors and insulators. These act in juxtaposition because the former can conduct electricity, whilst the latter can't. In the last couple of years, scientists have been working on a new class of quantum materials called topological insulators that are able to have both properties at once – the interior is insulated, but they act as conductors on the exterior.

Professor Satyabrata Patnaik from Jawaharlal Nehru University, New Delhi, collaborated with ISIS scientist Pabitra Biswas to study a new class of quantum materials known as topological superconductors. It is predicted that these materials could provide the material basis for realisation of Quantum Computers. In a normal computer, information is stored and processed in 'bits', which only have two possible states: 'On' or 'Off' (corresponding to 1 and 0 of Boolean Algebra). But, because of superposition principle, 'qubits' can have infinite combinatorial possibilities. This is the source of next revolution in computational technology.

Topological superconductors require specific characteristics with regard to the pairing of electrons if they are to be used as a chip in quantum computer. In physics, all phenomena can be associated with symmetry principles. For example, simple phenomena like conversion from ice (solid) to water (liquid) can be understood as a consequence of translational symmetry breaking. In particular, the electron-pairing symmetry of topological materials has been of great interest for some time. In a normal superconductor, electrons pair up below certain temperature, inducing the associated phenomena of zero electrical resistance and perfect diamagnetism.

The question is: how do the electrons pair up in a topological superconductor? Several theoretical models have been proposed but, prior to this study, no unequivocal confirmation has been achieved experimentally regarding the “px + ipy" type order parameter that is the pre-requisite for the use of topological superconductors as the building block of a futuristic Quantum Computer.

Using the MuSR beamline, Professor Patnaik and collaborators have established that the pairing symmetry of the topological superconductor SrxBi2Se3 is indeed the “px + ipy" type that is a basic requirement for the realisation of quantum computers. The MuSR technique is based on highly resolved measurement of magnetic moments; it was previously impossible to decipher the pairing mechanism of a topological superconductor with such great accuracy.

The researchers measured high-quality single crystals of SrxBi2Se3 at true zero field and transverse fields in a temperature range from 0.05 K to 3 K. Carrying out these measurements allowed them to calculate the temperature dependence of the muon penetration depth, which informed them of the true symmetry pairing in crystals.

“It was unbelievable that we could measure magnetic moments associated with Cooper pairs which are constituted by quasi-particles and therefore quasi-existence." Says Professor Patnaik; “This is only possible at ISISNot just because of the high-end facility but more so because of the extremely talented and knowledgeable group of scientists associated with it."

By establishing the pairing symmetry in SrxBi2Se3, the researchers have developed our understanding of these quantum materials, which can be used to build and improve the next generation of computational devices.

Currently, Professor Patnaik is working on magneto-resistance in Weyl and Dirac semi-metals. These quantum materials also show signatures of topological superconductivity in the presence of external pressure. Most interestingly, they provide a fertile landscape for the detection of Majorana fermions; particles that are their own anti-particle!

Contact: de Laune, Rosie (STFC,RAL,ISIS)