By Akshay Asija
A new study, published this week in the Proceedings of the National Academy of Sciences (PNAS), describes the existence of a “quantum material” that is yet to be formally discovered. Quantum materials usually have strong electronic correlations along with some form of electronic order, or have their electronic properties linked to non-generic quantum effects. Simply put, the electromagnetic behaviour of quantum materials cannot be explained by the laws of classical physics.
The subject of the study
The study, authored by a group of physicists led by Dr Qimiao Si, a theoretical physicist at Rice University, refers to the material as a “Weyl-Kondo semimetal“. While the semimetal has not been discovered, the predictions made in the study could help experimental physicists synthesise it. This new material is said to exhibit some properties which are shown by distinct materials like topological insulators, heavy fermion metals and high-temperature superconductors. Coincidentally, the existence of the “Weyl-Kondo semimetal” was discovered while the physicists were working on a theory to explain the existence of superconductivity at high-temperature levels.
The subject of Dr Si’s earlier work has been the collective behaviour of electronic materials at points when they undergo a transformation from one quantum state to another, called “quantum critical points“. High-temperature superconductivity is known to occur at these points, owing to the electronic fluctuations between the two different quantum states. Dr Si was working with Dr Hsin-Hua Lai, a postdoctoral fellow, and Sarah Grefe, a graduate student, on some models related to quantum criticality and high-temperature superconductors when they made the breakthrough. They found a model under which the test particles exhibited zero mass, a property that is the defining characteristic of a type of quantum particles called “Weyl Fermions“. Not much is known about these particles, which were first proposed by German physicist Hermann Weyl more than 80 years ago.
Weyl Fermions – A consequence of the Kondo effect
Recent experiments have proven the existence of solid-state conducting materials that are said to contain Weyl Fermions. These materials exhibit some characteristics of topological insulators, but, due to the presence of Weyl Fermions, are capable of conducting large amounts of electricity. According to Si, Lai and Grefe, “the combination of strong electronic correlations and a non-trivial topology” (called the Kondo effect) is the reason behind the presence of the massless Weyl Fermions in these materials, referred to as the “topological conductors”.
What does the future hold?
Prof. Silke Buehler-Paschen, an experimental physicist at the Vienna University of Technology, co-authored the study along with Si, Lai and Grefe. Her team carried out the experimental work of developing the new materials, while the team at Rice University worked out the theoretical aspects. Prof. Buehler-Paschen was one of the researchers who identified the unique experimental signatures of the Weyl-Kondo semimetal. Although the existence of the semimetal does not offer anything that would interest people outside of the quantum researcher community, Prof Buehler-Paschen believes that this discovery has a tremendous potential, with technological applications like quantum computing. Only time will tell how useful this study proves to be.
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