New Developments for Superconductivity in Cuprates

What attracts a PhD candidate to study at the Université de Sherbrooke? For Sidhartha Dash, it was the world-renowned professionals at the Institut quantique (IQ), as he chose Prof. David Sénéchal as his thesis director. Arriving in the fall of 2017 from India to complete his doctoral studies in condensed matter physics, Sidhartha successfully defended his thesis last November.

From India to Sherbrooke

Pursuing a career in physics was driven by Sidhartha’s passion in understanding natural phenomena in his surroundings. “I was greatly fascinated by the fact that we can have equations to describe most of the things that we see around us, and that’s why I joined physics early on,” says the newly titled PhD. Indeed, before coming to Sherbrooke, Sidhartha completed the Integrated M.Sc. at the National Institute of Science Education and Research (NISER) in India, which is a combined bachelor and master’s degree in physics. “During my studies in physics, I found that the field could be very abstract as well. I chose to study condensed matter physics because it additionally provides a scope for an intuitive understanding of phenomena occurring within a solid,” adds the physicist.

After completing his master’s degree, Sidhartha flew to Sherbrooke to work with Prof. David Sénéchal for his doctoral degree: “Having a good supervisor whose research is in my field of interest was my main criteria to come to the Université de Sherbrooke. He was very helpful throughout the whole process of coming into Canada, and more so with my specific research goals.” Once he joined the lab, Sidhartha grew very passionate towards superconductivity and the mysteries involved, motivating his research interests.

Three Phases of Cuprate Superconductors

“Sidhartha, through his perseverance and scientific rigour, has helped identify the factor that determines the magnitude of superconductivity in high-temperature superconductors, a 35-year-old problem,” says Prof. David Sénéchal. Cuprate superconductors, discovered in 1987, presented researchers with a new dilemma, because the existing theory could not describe the superconductivity in these materials at high temperature. Since then, extensive research has filled the knowledge gap for the most part, but many aspects are yet to be understood. Sidhartha’s work helped fill this gap to some extent. His research concerned three cuprate phases, such as superconductivity, charge density waves, and the pseudo-gap, which is not completely understood.

Most other theorists formulated proposals for the mechanism of superconductivity in cuprates using a model called the one-band Hubbard model. What distinguishes Sidhartha’s research from others is that it used the three-band Hubbard model, a suggestion made by Prof. David Sénéchal. “While our results match existing proposals within the one-band Hubbard model, we see additional characteristics, which were not explainable by this model. Implementing the cluster dynamical mean field theory (CDMFT) on the three-band Hubbard model with parameters suited to cuprates, we found that the critical temperature is correlated to something called the hole content on oxygen orbitals. We also found that there are two types of superconductivity in cuprates,” details the PhD.

His work, collaborated in part with Prof. André-Marie Tremblay, provided new insights that bring direct relevance for experiments to chemists towards building new high-temperature superconductors. Indeed, Sidhartha’s research showed not only that the strength of superconductivity is related to the hole content on oxygen orbitals, but it is also related to the covalent nature of bonding between copper and oxygen. “This finding proposes a new direction to make new superconductors. Instead of using copper and oxygen, you could use materials that form bonds of a more covalent nature, which might lead to a high temperature superconductor,” he adds. This potential improvement could greatly benefit the development of the quantum computer.

A Stepping Stone Towards a Post-Doc

“I greatly appreciated my experience at the Université de Sherbrooke. Of course, there were challenges in studying in a French environment, but the Department was very welcoming, and the group meetings happened in English. During my doctoral defence, the committee members and the jury were supportive, and my newly found friends were happy for me. It was a really good experience,” shares the IQ member.

Now that Sidhartha successfully defended his thesis, he is looking forward to carrying out a postdoctoral project to complete his academic studies and to help enlighten the nature of future work, be in academia or in the industry.

It was a pleasure having you at the IQ, Sidhartha, and good luck in your future endeavours!