At the forefront of quantum materials research is the goal to understand how new quantum phenomena can emerge from the topology of electronic wavefunctions or correlations arising from electron-electron interactions. The Yazdani Lab has contributed significantly to this research paradigm by harnessing the power of high-resolution scanning tunneling microscopy (STM) techniques to directly visualize electronic wavefunctions in topological and correlated quantum materials. These studies have provided information that is impossible to obtain using conventional macroscopic averaging techniques typically used in condensed matter physics. Our studies have provided unique atomic-resolution data that has enabled validation or constraint of theoretical models for a variety of topological and correlated phenomena in materials.
In the study of topological phases, a major breakthrough by our group was to demonstrate how topological superconductivity and Majorana zero modes (MZMs) emerge in chains of magnetic atoms on a superconductor. This platform was specifically developed by our group and collaborators to enable application of high-resolution spectroscopic mapping with the STM to directly visualize MZMs as the end mode excitation of a one-dimensional topological superconductor. Furthermore, we were able to exploit novel spectroscopic techniques with superconducting and spin-polarized STM tips to unequivocally distinguish MZMs from trivial low-energy excitations that may occur in superconductors.[3,4] Most recently, we have extended studies of topological superconductivity to realize MZMs on a topological hinge mode that has been put in contact with a superconductor, where we visualized and uniquely identified MZM signatures in STM experiments.[5,6] In other studies of topological phases of matter, our group was first to directly visualize Landau orbits in quantum Hall phases and to use this capability to directly visualize nematic quantum Hall liquid phases and the presence of novel topological boundary modes at the interfaces between such phases.[7,8,9] The application of STM to topological semimetals was also used to demonstrate the unusual bulk-boundary connectivity between Fermi arc states at the surface and Weyl electrons in the bulk of such materials.
In the study of correlated materials and superconductors, our work on high-Tc cuprates and heavy fermions established an extensive range of novel behavior in these materials. We demonstrated properties ranging from ubiquitous interplay between charge order and superconductivity in cuprates, to how heavy fermions initially form and then transition into unconventional superconducting states.[11,12,13] During this past year, we redirected our study of electronic correlations to examine the newly discovered correlated, insulating and superconducting states in magic-angle twisted bilayer graphene. We performed spectroscopy with the STM as function of carrier concentration (adjusted in situ with gate voltage), the results of which were the first to establish a spectroscopic signature of a strong electronic correlation at electron densities where superconductivity emerges in this system. This study firmly establishes the connection between this bilayer system and high-Tc cuprates, beyond the phenomenological resemblance of their transport phase diagrams. We have also developed new experimental techniques, such as Josephson STM spectroscopy, which can be used to map variation of the superconducting order parameter on the atomic scale. Finally, in recent years a major project in our lab has been the design and construction of a versatile millikelvin high-field STM system. The primary objective of this system is to achieve modularity by partitioning the STM system into a set of easily separable, interchangeable components. This system allows for interchanging the scanning probe module without disassembly or warm up of the system to room temperature, thereby allowing for development and testing of new scanning probe technologies using the same vacuum and low temperature system. This and other unique instrumentation that we have developed in our lab makes our experimental research program as described in these pages possible.