# Qimiao Si

**Professor & Director of the Rice Center for Quantum Materials**

**Department:**
Physics and Astronomy

**Office Phone:** (713) 348-5204

**Website:** http://faculty.rice.edu/report/FacultyDetail.cfm?DivID=1&DeptID=60&RiceID=705

### Research Areas

### Research Description

Prof. Qimiao Si works in theoretical condensed matter physics, with an emphasis on strongly correlated electron systems. Strongly correlated electron systems are at the forefront of condensed matter physics. Their theoretical description is a challenge that provides rich opportunities for creative 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. A related topic of his recent research addresses novel phases that emerge in the vicinity of quantum critical points. He has also been interested in quantum critical physics in a variety of other contexts.

Another focus of Prof. Si's current research concerns iron-based superconductors. One important aspect of the work is to address the bad-metal behavior in the normal state, which is attributed to a proximity to delocalization-localization transition. 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 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 recent experiments. Finally, the implications of such magnetic interactions for the unconventional superconductivity is being studied; a recent work along this direction has shown how high Tc superconductivity may develop in the iron chalcogenides with seemingly unfavorable Fermi-surface conditions.

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

### biography

Qimiao Si obtained his B.S. degree in Physics from University of Science and Technology of China in 1986, and his Ph.D. degree in Physics from the University of Chicago in 1991. He did his postdoctoral works at Rutgers University and University of Illinois at Urbana-Champaign. In 1994 he joined the faculty of Rice University, where he is the Harry C. and Olga K. Wiess Professor of Physics.

Prof. Si was named a Sloan Research Fellow in 1996, and received a Cottrell Scholar Award from the Research Corporation for Science Advancement in 1998. He was elected a Fellow of the British Institute of Physics in 2004, the American Physical Society in 2005, and the American Association for the Advancement of Science in 2008. He received a Humboldt Prize from the Alexander von Humboldt Foundation in 2012.

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**Selected Publications**:

- J. Wu, Q. Si and E. Abrahams, “Magnetic and Ising quantum phase transitions in a model for isoelectronically tuned iron pnictides'', arXiv:1406.5136.
- P. Goswami and Q. Si, “Topological defects of Neel order and Kondo singlet formation for Kondo-Heisenberg model on a honeycomb lattice'', Phys. Rev. B
**89**, 045124 (2014). - X. Lu, J. T. Park, R. Zhang, H. Luo, A. H. Nevidomskyy, Q. Si, and P. Dai, "Ising-nematic phase in the tetragonal state of uniaxial-strained BaFe
_{2−}*x*Ni*x*As_{2}", <ahref="http://www.sciencemag.org/content/345/6197/657" target="_blank">**Science**345, 657-660 (2014). - R. Yu, P. Goswami, Q. Si, P. Nikolic, and J-X Zhu, "Superconductivity at the Border of Electron Localization and Itinerancy", Nature Communications 4, 2783 (2013); doi:10.1038/ncomms3783.
- X-Y Feng, J. Dai, C-H Chung, and Q. Si, "Competing topological and Kondo insulator phases on a honeycomb lattice", Phys. Rev. Lett. 111, 016402 (2013).
- R. Yu and Q. Si, "Orbital-selective Mott phase in multi-orbital models for alkaline selenides K(1-x)Fe(2-y)Se2", Phys. Rev. Lett. 110, 146402 (2013).
- J. H. Pixley, S. Kirchner, K. Ingersent and Q. Si, "Kondo destruction and valence fluctuations in an Anderson model", Phys. Rev. Lett. 109, 086403 (2012).
- P. Goswami and Q. Si, "Effects of Berry Phase and Instantons in One Dimensional Kondo-Heisenberg Model", Physical Review Letters 107, 126404 (2011).
- Q. Si and F. Steglich, "Heavy Fermions and Quantum Phase Transitions'', Science 329, 1161 (2010).
- P. Goswami, P. Nikolic, and Q. Si, "Superconductivity in Multi-orbital t-J1-J2 Model and its Implications for Iron Pnictides'', EPL 91, 37006 (2010).