From The National University of Singapore [新加坡国立大学](SG) : “Predicting high temperature Bose-Einstein condensation of excitons” 

From The National University of Singapore [新加坡国立大学](SG)

December 06, 2021

NUS researchers have predicted that an exotic state of matter, known as a Bose-Einstein condensate, can exist at relatively high temperatures (around 50 K to 100 K) in systems comprising organic molecules on two-dimensional (2D) semiconducting materials.

A Bose-Einstein condensate is a state of matter in which all particles have the same energy and are completely coordinated. From a physical viewpoint, these particles clump together and start to behave as though they are part of a single larger particle. The 2001 Nobel Prize in Physics was awarded for the realisation of Bose-Einstein condensation. This phenomenal breakthrough was first achieved in a collection of rubidium atoms, at an ultra-low temperature of 20 nK. This control of the state of matter is expected to lead to technological breakthroughs, and also enables the realisation of superfluidity.

In this work, Prof QUEK Su Ying from the Department of Physics, National University of Singapore, and her postdoctoral fellow, Dr ULMAN Kanchan, predicted that Bose-Einstein condensation (BEC) can take place at around 50 K to 100 K in organic 2D material systems (see Figure) through their computation. This BEC temperature is orders of magnitude higher than that previously achieved using atoms. The particles that condense in the organic-2D material systems are bound electron-hole pairs (excitons) that are induced in the system through irradiation with light. The electron resides in the 2D semiconductor (molybdenum disulphide, MoS2) and the hole in the organic molecule (zinc phthalocyanine, ZnPc), in what is called a “charge transfer exciton”. The spatial separation between the electron and hole, together with the strongly bound nature of the excitons in these low dimensional materials, results in long exciton lifetimes, which are critical for BEC to take place. Crucially, the predicted BEC temperature is much higher than that in atoms. This is because the BEC temperature is inversely proportional to the particle mass, and the exciton mass is much smaller than typical atomic masses.

Prior to this prediction, BEC of charge transfer excitons was observed at around 100 K in bilayers of 2D materials. However, one practical difficulty in the realisation of BEC in these systems was the need for careful alignment of the two layers of material. Misaligned bilayers host excitons with large momentum, which hinder the formation of the condensate. In the case of organic-2D material systems, the narrow bandwidth of the molecular states imply that the charge transfer excitons have very small momentum, thus favouring BEC formation.

Prof Quek said, “Organic molecules such as transition metal phthalocyanines readily form ordered, self-assembled monolayers on 2D materials. The prediction of high temperature BEC of excitons in organic-2D material systems is expected to lead to more practical realisations of this exotic state of matter, and paves the way for the study of intriguing applications related to Bose-Einstein condensates.”

1
Charge transfer excitons in organic-2D heterostructure: Schematic figure showing charge-transfer excitons in the ZnPc-MoS2 organic-2D heterostructure. The lowest energy charge transfer excitons in ZnPc/MoS2 heterostucture are predicted to undergo Bose-Einstein condensation at around 50 K to 100 K. Credit: National University of Singapore.

Science paper:
Nano Letters

See the full article here.

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The The National University of Singapore (NUS) is the national research university of Singapore. Founded in 1905 as the Straits Settlements and Federated Malay States Government Medical School, NUS is the oldest higher education institution in Singapore. According to a number of surveys, it is consistently ranked within the top 20 universities in the world and is considered to be the best university in the Asia-Pacific by the QS ranking. NUS is a comprehensive research university, offering a wide range of disciplines, including the sciences, medicine and dentistry, design and environment, law, arts and social sciences, engineering, business, computing and music at both the undergraduate and postgraduate levels.

NUS’s main campus is located in the southwestern part of Singapore, adjacent to Kent Ridge, accommodating an area of 150 ha (0.58 sq mi); the Duke-NUS Medical School, a postgraduate medical school jointly established with Duke University, is located at the Outram campus; its Bukit Timah campus houses the Faculty of Law and Lee Kuan Yew School of Public Policy; the Yale-NUS College, a liberal arts college established in collaboration with Yale University (US), is located at University Town (commonly known as UTown). NUS has one Nobel laureate, Konstantin Novoselov, as a professor among its faculty.

Research

Among the major research focuses at NUS are biomedical and life sciences, physical sciences, engineering, nanoscience and nanotechnology, materials science and engineering, infocommunication and infotechnology, humanities and social sciences, and defence-related research.

One of several niche research areas of strategic importance to Singapore being undertaken at NUS is bioengineering. Initiatives in this area include bioimaging, tissue engineering and tissue modulation. Another new field which holds much promise is nanoscience and nanotechnology. Apart from higher-performance but lower-maintenance materials for manufacturing, defence, transportation, space and environmental applications, this field also heralds the development of accelerated biotechnical applications in medicine, health care and agriculture.

Research institutes and centres

Currently, NUS hosts 21 university-level research institutes and centres (RICs) in various fields such as research on Asia, risk management, logistics, engineering sciences, mathematical sciences, biomedical and life sciences, nanotechnology to marine studies. Besides that, NUS also hosts three Research Centres of Excellence, namely, the Cancer Science Institute of Singapore, Centre for Quantum Technologies and Mechanobiology Institute, Singapore – a partner in Singapore’s fifth Research Centre of Excellence. Besides university-level RICs, NUS also has close affiliation with many national research centres and institutes. A special mention is required for The Logistics Institute – Asia Pacific, which is a collaborative effort between NUS and the Georgia Institute of Technology (US) for research and education programmes in logistics. NUS announced its most recent research institute, the Next Age Institute, a partnership with Washington University in St. Louis (US), in February 2015.