Excitons in van der Waals Antiferromagnet Could Help Improve Quantum Computing

While things can invariably be done faster, nothing can beat light. Calculating with light rather than electricity is considered an advancement to increase the speed of computers.

(Top) The crystal structure of NiPS3. (Down) An exciton state proposed for NiPS3. Image Credit: Institute for Basic Science.

Transistors are essentially the building blocks of data circuits and need to convert electrical signals into light to convey the data through a fiber-optic cable. But such conversion processes require one to invest both time and energy that could be saved by optical computing.

Apart from the high-speed transmission, excellent low-noise characteristics of photons render them perfect for studying the mechanics of quantum. Securing a steady light source, particularly in a quantum state, forms the core of these fascinating applications.

When electrons in a semiconductor crystal are illuminated with light, a conduction electron produces a bound state—the supposed exciton—by integrating with a positively charged hole in the semiconductor.

Excitons can flow just like electrons but produce light when the pair of electron and hole gets back together. They are capable of accelerating the overall data transmission circuits. Moreover, an abundance of unusual physical phases, such as superconductivity, are contemplated as phenomena that emerge from excitons.

In spite of the vast exotic hypothetical predictions and its extended history (initially reported in the 1930s), a major part of physics concerning excitons has been largely about its preliminary theory of “simple” binding of a hole and an electron, which is hardly updated from the discoveries made in the 1930s.

A team of researchers, headed by Professor Park Je-Geun from the Department of Physics and Astronomy in Seoul National University, has now identified a new kind of exciton in NiPS3—a magnetic van der Waals material. Professor Je-Geun was earlier working as an Associate Director of the Center for Correlated Electron Systems within the Institute for Basic Science (IBS, South Korea).

The study was published in the new issue of the Nature journal.

To host such a novel state of an exciton physics, it requires a direct bandgap and most importantly, magnetic order with strong quantum correlation. Notably, this study makes it the latter possible with NiPS3, a magnetic van der Waals material, an intrinsically correlated system.

Park Je-Geun, Study Corresponding Author and Professor, Department of Physics and Astronomy, Seoul National University

Professor Park’s team had used NiPS3 in 2016 and reported the first achievement of exact two-dimensional (2D) magnetic van der Waals materials. Now, with the help of the same material, the researchers have shown that the NiPS3 material hosts a magnetic exciton state that is entirely different from the more traditional excitons known so far.

Such an exciton state is essentially of many-body origin, which is an exact realization of a true quantum state. In this context, the latest study indicates a considerable change in the intense field of research in its eight decades of history.

The entire exotic exciton physics in the NiPS3 material started with peculiarly high peaks detected in early photoluminescence (PL) experiments that were performed by Professor CHEONG Hyeonsik from Sogang University in 2016.

Soon after these experiments, another optical absorption experiment was conducted by Professor KIM Jae Hoon from Yonsei University. The two sets of optical data evidently denoted a couple of points of major significance—one is the highly narrow resonant nature of the exciton and the other is the temperature reliance.

To interpret the unusual discoveries, Professor Park employed a resonant inelastic X-ray scattering method, called RIXS, along with Dr Ke-Jin Zhou from the Diamond Facilities in the United Kingdom.

The latest experiment was crucial for the success of the inclusive project. Firstly, the experiment convincingly proved the existence of the 1.5 eV exciton peak. Secondly, it served as an inspiring guide on how researchers can develop a theoretical model and the resultant computations. This link between the theory and the experiment allowed the researchers to resolve the major mystery in the NiPS3 material.

With the help of the above-described analytical process, Dr KIM Beom Hyun and Professor SON Young-Woo from the Korea Institute for Advanced Study performed extensive theoretical many-body calculations. The duo explored massive quantum states that totaled 1,500,000 in the Hilbert space and ultimately reached a conclusion that the entire experimental outcomes could agree with a specific set of parameters.

When the researchers compared the RIXS data with the theoretical outcomes, it became evident that they were able to fully interpret the highly unusual exciton phase of the NiPS3 material.

Finally, the researchers could hypothetically figure out the magnetic exciton state of many-body nature, that is, a true quantum exciton state.

Many crucial distinctions have to be made regarding the quantum magnetic exciton identified in the NiPS3 material when compared to the more traditional exciton discovered in other types of 2D materials and also all the other insulators that have an exciton state.

Most importantly, the excitons discovered in the NiPS3 material are inherently a quantum state emerging from a shift from a Zhang-Rice triplet to a Zhang-Rice singlet. Secondly, it is virtually a resolution-limited state, indicating that some kind of coherence exists among the states. For comparison purposes, all the other exciton states that were reported in the past come from extended Bloch states.

Perhaps, it is too early to make any conclusive predictions; it might as well usher in the future of the associated field of magnetic van der Waals studies, not to mention the human lives.

But it is evident even at this moment that “The quantum nature of the new exciton state is unique and will attract a lot of attention for its potentials in the field of quantum information and quantum computing, to name only a few. Our work opens an interesting possibility of many magnetic van der Waals materials having similar quantum exciton states,” concluded Professor Park.

Journal Reference:

Kang, S., et al. (2020) Coherent many-body exciton in van der Waals antiferromagnet NiPS3. Nature. doi.org/10.1038/s41586-020-2520-5.

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