Quantum excitation found in van der Waals magnetic material

Things can always be done faster, but can anything beat the light? Computing with light instead of electricity is considered an advance to increase the speed of the computer. Transistors, the basic components of data circuits, must change electrical signals into light to transmit information through a fiber optic cable. Optical computing could potentially save the time and energy used for such a conversion. In addition to high-speed transmission, the excellent low-noise properties of photons make them ideal for exploring quantum mechanics. The core of these attractive applications is to ensure a stable light source, especially in a quantum state.

When light is illuminated on the electrons in a semiconductor crystal, a conduction electron can combine with a positively charged hole in the semiconductor to create a bound state, the so-called exciton. Flowing like electrons but emitting light when the electron-hole pair rejoins, excitons could speed up general data transmission circuits. Furthermore, many exotic physical phases like superconductivity are speculated as phenomena arising from excitons. Despite the richness of exotic theoretical predictions and its long history (first reported in the 1930s), much of exciton physics has focused primarily on its initial concept of ‘simple’ binding of an electron and a hole, rarely updated from the findings in the 1930s.

In the last issue of the magazine. Nature, a research team led by Professor Park Je-Geun of the Department of Physics and Astronomy at Seoul National University, formerly Associate Director of the Center for Correlative Electronic Systems within the Institute of Basic Sciences (IBS, South Korea), found a new kind of excitation in magnetic material from van der Waals, NiPS₃. “To host such a novel state of exciton physics, a direct band gap and, most importantly, a magnetic order with strong quantum correlation is required. It should be noted that this study makes this possible with NiPS₃, a van der Waals magnetic material, an intrinsically correlated system, “says Professor Park Je-Geun, corresponding author of the study. Professor Park’s group reported the first realization of exact 2-D magnetic van der Waals materials using NiPS₃ in 2016. Using the same material, they have shown that NiPS₃ It houses a completely different state of magnetic excitation than the most conventional excitons known to date. This state of excitement is intrinsically of origin of many bodies, which is a real realization of a genuine quantum state. As such, this new work marks a significant change in the vibrant field of research in its 80-year history.

These unusual physical excitons in NiPS₃ It started with strangely high peaks seen in the first PL (photoluminescence) experiments conducted in 2016 by Professor Cheong Hyeonsik from Sogang University. It was soon followed by another optical absorption experiment conducted by Professor Kim Jae Hoon of Yonsei University. Both sets of optical data clearly indicated two points of significant importance: one is temperature dependence and the other is the extremely narrow resonant nature of the exciton.

To understand the unusual findings, Professor Park used a resonant inelastic X-ray scattering technique, known as RIXS, in conjunction with Dr. Ke-Jin Zhou at the Diamond, UK facility. This new experiment was critical to the overall success of the project. First, it confirmed the existence of the 1.5 eV excitation peak beyond question. Second, it provided inspiring guidance on how we might arrive at a theoretical model and subsequent calculations. This connection between experiment and theory played a fundamental role in solving the puzzle in NiPS₃.

Using the analytical process shown above, Dr. KIM Beom Hyun and Prof. SON Young-Woo of the Korea Institute for Advanced Studies carried out massive theoretical calculations of many bodies. By exploring massive quantum states totaling 1,500,000 in Hilbert space, they concluded that all experimental results could be consistent with a particular set of parameters. When they compared the theoretical results with the RIXS data, it became clear that they came to a complete understanding of the very unusual excitation phase of NiPS.₃. Finally, the team could theoretically understand the magnetic arousal state of many-body nature – that is, a genuine quantum arousal state.

There are several vital distinctions to be made about the quantum magnetic exciton discovered in NiPS.₃ compared to the more conventional exciton found in other 2-D materials and all other insulators that have an exciton state. First of all, the excitons found in NiPS₃ it is intrinsically a quantum state that arises from a transition from a Zhang-Rice triplet to a Zhang-Rice singlet. Second, it is almost a state of limited resolution, indicative of some sort of coherence present between the states. For comparison, all other excitation states reported above are from extended Bloch states.

It is probably too early to make definitive predictions; It could also bring the future of the related field of van der Waals magnetic research, not to mention our lives. However, it is clear even at this time that “The quantum nature of the new excitonic state is unique and will attract much attention for its potentials in the field of quantum information and quantum computing, to name but a few. Our work opens up an interesting possibility that many van der Waals magnetic materials have similar quantum excitation states, “explains Professor Park.