Unconventional Superconducting States in Bilayer Semiconductors

Recent experimental and theoretical work shows that it is possible to make doubly charged excitons in bilayer structures of two-dimensional (2D) materials. This complex, which can be called a “quaternion” by analogy with the singly charged “trion,” has been shown to be stable up to 100 K and may be stable in some material systems at room temperature or above.

Since these are charged bosons, a Bose-Einstein condensate of these excitonic complexes would be a non-BCS superconductor, based not on Cooper pairing but instead on a renormalized Coulomb interaction between electrons and holes. This collaborative effort involves joint theoretical and experimental studies to confirm the magnetic and spin properties of these excitonic complexes, to show that they are responsive to electric field, and to bring them to undergo Bose-Einstein condensation (BEC).

The transformative impact of a new type of superconductivity that does not involve Cooper pairing, and which could potentially occur at room temperature, can scarcely be overstated. It could very well lead to new types of inexpensive superconducting devices, such as magnetic field detectors and electromagnetic wave detectors. The principal investigator (PI) and the co-PI of this project, Prof. Igor Bondarev of NCCU and Prof. David Snoke of the University of Pittsburgh, PA, have been at the forefront of the field of excitonic complexes in bilayers for over two decades. Recent experimental work in the labs of David Snoke has shown strong spectroscopic evidence for the existence of doubly charged excitons in 2D transition-metal dichalcogenide (TMD) bilayer structures with hexagonal boron nitride (hBN) used as an insulator layer. Many careful control studies have already been performed, and these confirm the theoretical predictions of Igor Bondarev.

This theoretical-experimental collaborative effort will study experimentally new quantum phenomena for ultrathin TMD bilayers prepared as suggested theoretically. Collaborative work on this project will contribute to the broader knowledge base of these new materials, especially the effect of screening by nearby metals, which are used as contacts in all kinds of applications with electrical transport. This will facilitate the development of these new quantum nanomaterials for novel designs of quantum photonic devices for advanced optoelectronics applications.