Qimiao Si works in theoretical condensed matter physics, with an emphasis on topics in strongly correlated electron systems including quantum criticality, unconventional superconductivity and correlated electronic topology.

Strongly correlated electron systems are at the forefront of condensed matter physics. Their theoretical description is a challenge that provides rich opportunities for creative and original research. The fundamental question is how the electrons are organized and, in particular, whether there are principles that are universal among the various classes of these strongly correlated materials. The overarching goal of the group’s research is to seek such principles of universality. Along the way, it is also fascinating to explore the diversity of the phenomena that result from electron correlations.

One area of Prof. Si’s current interest is quantum criticality. He and his collaborators  have advanced a by now well-known theory of local quantum criticality. Developed in the context of magnetic heavy fermion metals, which is a prototype system for quantum phase transitions, this theory features the “beyond-Landau” physics of critical Kondo destruction. The notion that electronic excitations undergoing a localization-delocalization transition drive quantum criticality has impacted the developments in wider contexts, in high-temperature superconductors and beyond. A related topic of his recent research addresses novel phases that emerge in the vicinity of quantum critical points; for heavy fermion systems, his work here appears in the form of a global phase diagram. He has also been interested in quantum critical physics in a variety of other contexts.

Another focus of Prof. Si’s ongoing research concerns iron-based superconductors. From the very beginning of the field, he recognized that the bad metallicity of these systems implies that strong correlations play an important role. This line of consideration has opened up studies on orbital-selective Mott phenomena. A corollary of this approach is that magnetism is primarily driven by short-range and frustrating (J1-J2) interactions, a notion that he and his collaborators have pioneered. This approach has led them to theoretically predict a magnetic quantum critical point in iso-electronically tuned iron pnictides, which has been verified by extensive subsequent experiments. His recent work has also explored the related magnetic frustration physics in the iron chalcogenides, including its effect on the nematicity in FeSe. Finally, he has been studying the implications of such magnetic interactions for the unconventional superconductivity. He and his collaborators introduced the notion of orbital-selective superconducting pairing. Recent work along this direction has shown how high Tc superconductivity may develop in the iron chalcogenides with seemingly unfavorable Fermi-surface conditions, and how the orbital selectivity gives rise to a new superconducting pairing state.

Yet another direction is on topological metals driven by strong correlations. His group has recently advanced the Kondo-driven Weyl semimetal state. Contemporaneous experiments in heavy fermion semimetals have provided thermodynamic and transfport evidence for this Weyl-Kondo semimetal. Both the theory and the strategy used in establishing the Weyl-Kondo semimetal state promise to open up a new general design principle for strongly correlated topological states.

A variety of other topics on correlated electron systems are also of interest to the group. These range from non-Fermi liquid behavior, cuprate superconductors, quantum entanglement in many-body systems, disordered and interacting electronic systems, metal-insulator transitions, out of equilibrium behavior of electronic systems, spin transport, and the probe of spin-charge separation.