2D materials and their van der Waals heterostructures have sparked a tremendous interest for their tunable electronic properties due to the electron spin, valley and layer degrees of freedom. New opportunities of control are emerging thanks to electromagnetic cavity resonators such as split-ring resonators with deeply sub-wavelength modal confinement and giant vacuum fields. Such resonators can modify not only the optical response, but also transport phenomena without illumination thanks to ultrastrong light-matter interaction. We will explore: (1) cavity-controlled charge transport in van der Waals heterostructures without illumination; (2) cavity-controlled electronic properties in presence of illumination; (3) novel Moiré superlattices, and their cavity control. This French-Hong Kong project would explore a cutting-edge research field and create synergy by enhancing complementary expertise of two experimental groups and two theoretical teams.

Bose-Einstein condensation in a dilute weakly interacting atomic gas which happens at temperatures only millionth of a degree above absolute zero was first realized in 1995. Because of the deep connections with many important physics phenomena, such as superfluidity and superconductivity, it has since become one of most exciting subjects in physics, awarded by the Nobel Prize in 2001. But many intriguing problems in few-body and many-body physics arise when the particles of the dilute gas interact through strong long-range anisotropic forces, like the so-called polar molecules possessing a permanent electric dipole moment in their own frame. In the proposed joint experimental (CUHK) and theoretical (LAC) project COPOMOL, we will produce a quantum gas of polar 23Na87Rb bosonic molecules to study the ultracold physics with strong and anisotropic interactions, focusing on the precise control of ultracold molecule-molecule collisions with external electromagnetic fields to enter the regime where many-body physics is dominant. Inspired by the amazing achievements of the JILA group with 40K87Rb, we will take advantage of the remarkable properties of the 23Na87Rb ground state molecule, namely a large permanent dipole moment (5 times larger than in KRb) and a chemical stability against mutual collisions, to fully explore dipolar physics with quantum gases. The project will pursue the following objectives planned for the next four years: after the demonstration of the efficient production of Feshbach molecules, we will work out the best strategy for the reliable population transfer to the absolute molecular ground state with stimulated Raman adiabatic passage (STIRAP). Then the control of molecular collisions will be achieved by tuning of the anisotropic long-range dipole-dipole interactions with an external electric field, as a prerequisite to create a molecular quantum degenerate gas. Signatures of many-body physics phenomena in reduced geometries will be searched for by loading the quantum gas in an optical lattice. COPOMOL is timely given the particularly favorable configuration of the consortium. The CUHK group has already successfully prepared binary Na-Rb atomic BECs and investigated their interspecies Feshbach resonances. The LAC team includes several top experts on theoretical molecular structure, spectroscopy, and dynamics. Many high quality calculations related to heteronuclear alkali diatomic molecules, including NaRb, have been carried out while models for elastic, inelastic and reactive collisions between polar molecules have been developed. Another important fact is that the CUHK PI already has a long history of fruitful collaborations and discussions with the LAC PI and co-PI. Mastering these approaches is crucial as many applications proposed with dipolar interactions rely on the collisional properties of the ground-state molecules with and without induced dipole moment. The dangling question, whether trapped polar molecules without chemical reaction channels in the absolute lowest energy level will be really stable, will be answered first. We will measure and model the elastic and inelastic collisions with non-polarized and polarized samples to seek ways of achieving evaporative cooling for the production of a NaRb BEC. We will test the universal model for inelastic collisions with controlled loss channels including nuclear spin flips, rotational quenching, vibrational quenching and chemical reactions. These channels will be created by manipulating the internal state of the molecules with microwaves and lasers. These collisional studies will be further performed with molecules trapped in an optical lattice to explore the anisotropic character of dipolar interactions thus yielding a promising platform for observing several exotic many-body quantum phases.

Correlated Quantum Materials (CQM) encompasses a large variety of materials systems in which electronic correlations and/or topology yield exotic physics, such as unconventional superconductors, multiferroic compounds, with novel magnetic and electronic orders. Engineering such materials through artificial thin-film heterostructures and more generally symmetry breaking is a promising way to tailor their extraordinary properties. Such efforts significantly rely on the guidance from spatially resolved characterisation of various orders (e.g. charge, lattice, orbital, spin) present in these CQM. To this purpose, however, the full potential of achieving sub-nanometre scale or atomic resolution, required for nanomaterials or thin films, is still yet to be unleashed. In this project, we aim to leverage several technical advancements to push the frontier; we will leverage the low temperature high-resolution scanning transmission electron microscopy (STEM) and spectroscopy (e.g. EELS) techniques. The FR team will operate STEM at low temperatures giving access to a broad range of electronic phases that emerge during the cooling of the QCM such as charge ordered phases. Several developments will be implemented to maintain the spatial resolution during STEM-EELS (micro-)spectroscopy. For establishing and testing the state-of-the-art platform, thin films of manganites or layered cobaltates (grown by the HK team, and in some case by the TU-Wien group with a complementary approach based on sputtering) in which electron, spin and orbital orders are relatively known will be firstly investigated as ‘proof-of-concept’ systems. Furthermore, we will search for additional members of the nickelate superconductors, recently discovered by the HK team, with the presence of various possible intertwined phases. Through this project, we expect to gain significant new insights, which are otherwise challenging to obtain by other means, into the origin of high-temperature superconductivity.