Quantum materials in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. In Shen group, we are able to precisely engineer a variety of materials at atomically thin limit using molecular beam epitaxy (MBE), and investigate their extreme properties by performing in-situ ARPES measurements.
One representative example in our current research is a single unit cell (UC) thick film of iron selenide (FeSe) grown on strontium titanate (STO). This system has demonstrated superconducting energy gaps opening at temperatures near the boiling point of liquid nitrogen. Surprisingly this phenomenon exists only on the first unit cell, and further layers do not show superconductivity at all. One of the goals of our MBE/ARPES experiment is to understand how the transition temperature can be enhanced from the bulk superconducting transition temperature (Tc) of 8 K at this 1UC FeSe/STO interface.
We have recently demonstrated a large electron-phonon coupling present in the 1 UC FeSe/STO system. This coupling manifests as extremely sharp replica bands in the photoemission spectra, separated by an energy on the order of the phonon. The phonon in question comes from STO itself. The sharpness of the replica bands imply that emission or absorption of a phonon by the electron occurs with very little momentum transfer (q=0 scattering). This kind of forward scattering can help boost the Tc even in superconductors whose order parameter has a sign change, as is the case in cuprates, and possibly the iron-based superconductors. Our calculations show that this coupling, which is constrained to the first unit cell, is what enhances the Tc of the FeSe film.
The 1UC FeSe/STO system is a dramatic demonstration of a more general phenomenon: modification of bulk electronic properties via substrate interactions. The addition of the STO substrate allows for a wide number of parameters to be tuned, including the phonon coupling, doping via substrate, lattice strain, and possible ferroelectricity. Our current efforts involve both systematic studies of the effects of the substrates and discovering new systems where one can apply this anomalous coupling.
Another system we are interested in is the transition metal dichalcogenides MX2 (M=Mo, W; X= S, Se, Te). Recently we successfully achieved layer by layer growth of MoSe2 thin films using MBE. In ARPES measurements, we discovered a distinct transition from an indirect bandgap to a direct bandgap when the layer thickness changes from 8 monolayers to one monolayer. Moreover, we found a significant spin-splitting at the valence band maximum of the monolayer film, suggesting its possible application to spintronic devices.
 J. J. Lee et al. Significant Tc enhancement in FeSe films on SrTiO3 due to interfacial mode coupling. arXiv: 1312.2633; accepted to Nature.
 Y. Zhang et al. Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2. Nature Nano 9, 111 (2014)