- Quantum Criticality
- Unconventional Superconductivity
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 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 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 Fermi-surface conditions.
Yet another direction is on topological metals driven by strong correlations. His group has recently advanced a class of Kondo-driven Weyl semimetal state. Recent experiments in heavy fermion semimetals have provided thermodynamic and transport evidence for this Weyl-Kondo semimetal.
A variety of other topics on correlated electron systems are also of interest to the group. These range from non-Fermi liquid behavior, cuprate superconductivity, 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.
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 Urbana-Champaign. 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.
H.-H. Lai, S. E. Grefe, S. Paschen, and Q. Si, "Weyl-Kondo 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 bilinear-biquadratic model on a square lattice", Phys. Rev. Lett. 118, 176401 (2017).
M. Nica, R. Yu, and Q. Si, "Orbital-selective 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 Ising-nematic orders of a frustrated bilinear-biquadratic 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 Shastry-Sutherland 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(2-x)Ni(x)As(2)”, Science 345, 657-660 (2014).
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.
Yu and Q. Si, "Orbital-selective Mott phase in multi-orbital models for alkaline selenides K(1-x)Fe(2-y)Se2", Physical Review Letters 110, 146402 (2013).
Goswami and Q. Si, "Effects of Berry Phase and Instantons in One Dimensional Kondo-Heisenberg 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, 969-971 (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, "Hall-effect evolution across a heavy-fermion quantum critical point", Nature 432, 881-885 (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, "Locally-critical Quantum Phase Transitions in Strongly Correlated Metals'', Nature 413, 804 (2001).