Research Areas
 Quantum Criticality
 Unconventional Superconductivity
Research Description
Prof. Qimiao Si works in the area of theoretical condensed matter physics. His research emphasis is on strongly correlated electron systems, a topic at the forefront of condensed matter physics. These systems present a major challenge in theoretical description and, as such, provide 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 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 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 wellknown 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 "beyondLandau" 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 ironbased superconductors. One important aspect of the work is to address the badmetal behavior in the normal state, which is attributed to a proximity to delocalizationlocalization transition. This line of consideration has opened up studies on orbitalselective Mott phenomena. A corollary of this approach is that magnetism is primarily driven by frustrated spin interactions, a notion that he and his collaborators have pioneered. This approach has led them to theoretically predict a magnetic quantum critical point in isoelectronically tuned iron pnictides, which has since been verified by extensive 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 Fermisurface conditions.
Yet another direction is on topological metals driven by strong correlations. His group has recently advanced a class of Kondodriven Weyl semimetal state. Recent experiments in heavy fermion semimetals have provided thermodynamic and transport evidence for this WeylKondo semimetal.
A variety of other topics on correlated electron systems are also of interest to the group. These range from nonFermi liquid behavior, cuprate superconductivity, quantum entanglement in manybody systems, disordered and interacting electronic systems, metalinsulator transitions, out of equilibrium behavior of electronic systems, spin transport, and the probe of spincharge separation.
Biography
Qimiao Si is the Harry C. and Olga K. Wiess Professor of Physics and Astronomy at Rice University. He 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 UrbanaChampaign. He has been on the faculty of Rice University since 1994 (making the actual move to Rice in 1995, after a year’s leave of absence).
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, and was named a Ulam Distinguished Scholar by the Center for Nonlinear Studies of Los Alamos National Laboratory in 2018.
Selected Publications
H.H. Lai, S. E. Grefe, S. Paschen, and Q. Si, "WeylKondo Semimetal in Heavy Fermion Systems", PNAS 115, 93 (2018).
Yu, J.X. Zhu, and Q. Si, "Orbital selectivity enhanced by nematic order in FeSe", Phys. Rev. Lett. 121, 227003 (2018).
Goswami and Q. Si, "Dynamic zero modes of Dirac fermions and competing singlet phases of antiferromagnetic order", Phys. Rev. B95, 224438 (2017).
Grube, S. Zaum, O. Stockert, Q. Si, H. v. Loehneysen, "Multidimensional entropy landscape of quantum criticality", Nature Phys. 13, 742 (2017).
H.H. Lai, W.J. Hu, R. Yu, and Q. Si, "Antiferroquadrupolar order and rotational symmetry breaking in a generalized bilinearbiquadratic model on a square lattice", Phys. Rev. Lett. 118, 176401 (2017).
M. Nica, R. Yu, and Q. Si, "Orbitalselective pairing and superconductivity in iron selenides", npj Quantum Materials 2, 24 (2017).
Si, R. Yu and E. Abrahams, "High Temperature Superconductivity in Iron Pnictides and Chalcogenides", Nature Rev. Mater. 1, 16017 (2016). http://www.nature.com/articles/natrevmats201617.
Yu and Qimiao Si, “Antiferroquadrupolar and Isingnematic orders of a frustrated bilinearbiquadratic Heisenberg model and implications for the magnetism of FeSe”, Phys. Rev. Lett. 115, 116401 (2015).
H. Pixley, R. Yu, and Qimiao Si, "Quantum phases of the ShastrySutherland Kondo lattice: implications for the global phase diagram of heavy fermion metals", Physical Review Letters 113, 176402 (2014).
Lu, J. T. Park, R. Zhang, H. Luo, A. H. Nevidomskyy, Qimiao Si, and P. Dai, “Nematic spin correlations in the tetragonal state of uniaxial strained BaFe(2x)Ni(x)As(2)”, Science 345, 657660 (2014).
Yu, P. Goswami, Q. Si, P. Nikolic, and JX Zhu, "Superconductivity at the Border of Electron Localization and Itinerancy", Nature Communications 4, 2783 (2013); doi:10.1038/ncomms3783.
Yu and Q. Si, "Orbitalselective Mott phase in multiorbital models for alkaline selenides K(1x)Fe(2y)Se2", Physical Review Letters 110, 146402 (2013).
Goswami and Q. Si, "Effects of Berry Phase and Instantons in One Dimensional KondoHeisenberg Model", Physical Review Letters 107, 126404 (2011).
Si and F. Steglich, "Heavy Fermions and Quantum Phase Transitions'', Science 329, 1161 (2010).
Si, "Quantum Criticality and Global Phase Diagram of Magnetic Heavy Fermions'', Phys. Status Solidi B247, 476 (2010).
Dai, Q. Si, J.X. Zhu, and E. Abrahams, "Iron Pnictides as a New Setting for Quantum Criticality", PNAS 106, 4118 (2009).
Si and E. Abrahams, "Strong Correlations and Magnetic Frustration in the High Tc Iron Pnictides", Physical Review Letters 101, 076401 (2008).
Gegenwart, T. Westerkamp, C. Krellner, Y. Tokiwa, S. Paschen, C. Geibel, F. Steglich, E. Abrahams, and Q. Si, "Multiple energy scales at a quantum critical point", Science 315, 969971 (2007)
J. Yamamoto and Q. Si, "Fermi surface and antiferromagnetism in the Kondo lattice: an asymptotically exact solution in d>1 dimensions'', Physical Review Letters 99, 016401 (2007).
Paschen, T. Luhmann, S. Wirth, P. Gegenwart, O. Trovarelli, C. Geibel, F. Steglich, P. Coleman, and Q. Si, "Halleffect evolution across a heavyfermion quantum critical point", Nature 432, 881885 (2004).
Zhu, M. Garst, A. Rosch and Q. Si, "Universally Diverging Grueneisen Parameter and the Magnetocaloric Effect close to Quantum Critical Points”, Physical Review Letters 91, 066404 (2003).
Si, S. Rabello, K. Ingersent and J. L. Smith, "Locallycritical Quantum Phase Transitions in Strongly Correlated Metals'', Nature 413, 804 (2001).
Recent Publications

06/10/2016
“Straindriven approach to Quantum criticality in AFe2As2 with A= K, Rb, and Cs”, Physical Review Letters 116, 237003 (2016)

03/11/2016
“High Temperature Superconductivity in Iron Pnictides and Chalcogenides”, Nature Reviews Materials 1, 16017 (2016)

01/04/2016
“Emergence of superconductivity in the canonical heavyelectron metal YbRh2Si2”, Science 351, 485488 (2016)

09/08/2015
“Antiferroquadrupolar and Isingnematic orders of a frustrated bilinearbiquadratic Heisenberg model and implications for the magnetism of FeSe”, Physical Review Letters 115, 116401 (2015)

07/23/2015
“Observation of universal strong orbitaldependent correlation effects in iron chalcogenides”, Nature Communications 6, 7777 (2015)

09/30/2014
Rice launches Center for Quantum Materials