Exploring emergent phenomena at the interface of topological semimetals, magnetism, and superconductivity

Non-technical abstract: A central theme in contemporary condensed matter physics is the design of ‘topological quantum materials’ whose unusual physical properties are best described by an elegant interplay between core concepts from modern physics (quantum mechanics, special relativity) and mathematics (symmetry, topology). In this project, the principal investigator (PI) focuses on designing, synthesizing, and studying a new family of topological quantum materials (‘hybrid topological semimetal heterostructures’) whose topological character is not immutably determined by the crystal composition and structure, but rather can be tuned using an external knob (electrical voltage). This provides a powerful framework for developing a rigorous understanding of emergent quantum phenomena that are predicted to arise in these quantum materials. The hybrid topological semimetals developed in this project also lend themselves to the fashioning of device components that are relevant for future quantum technologies. The PI uses state-of-the-art materials synthesis (‘molecular beam epitaxy’), accompanied by advanced nanopatterning and measurement methodologies, to carry out this research. The project trains students in sophisticated experimental techniques so that they can effectively contribute at the frontiers of quantum science and technology in academia and industry. The PI is committed to developing a research program that combines scientific excellence with an inclusive, supportive, nurturing climate. The PI and graduate students are also developing outreach activities aimed at engaging and educating general audiences about contemporary quantum materials through hands on activities such as a ‘quantum materials escape room’ during Penn State’s Family weekends and during visits to local elementary schools. Technical abstract: Dirac and Weyl semimetals form an important part of the contemporary landscape of topological quantum materials because the presence of Weyl nodes leads to remarkable physical consequences, such as surface Fermi arcs, the chiral anomaly, the anomalous Hall effect, and the axial magnetoelectric effect. Most experimental studies of these topological semimetals focus on materials whose topological band structure is fixed by the crystal lattice structure and composition. This project centers on developing and studying epitaxially engineered hybrid topological semimetal heterostructures wherein the underlying symmetries (time reversal, inversion, gauge) and topology can be electrically tuned in a single material via interfacial proximity effects. Molecular beam epitaxy is used to synthesize heterostructures that interface the canonical Dirac semimetal Cd3As2 with a ferromagnetic III-V semiconductor (such as [In,Mn]As) and with a conventional superconductor (such as Nb), thus creating a hybrid quantum material system wherein electrostatic gating tunes the breaking of time reversal symmetry and gauge symmetry. A comprehensive suite of spin-sensitive probes (anomalous Hall effect, quantum transport, spin-dependent tunneling, polarized neutron reflectometry) and phase coherent measurements (Josephson effect, mesoscopic transport) are marshaled in a search for signatures of topological phase transitions as well as emergent topological states such as skyrmions and monopole superconductivity. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.