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University of Southern California

1. Song, Kok Wee. Electronic correlation effects in multi-band systems.

Degree: PhD, Physics, 2014, University of Southern California

The recent dominant trends in condensed matter physics research can be roughly summarized into three newly discovered materials: topological insulators, graphene, and iron‐based superconductors. All these materials exhibit many intriguing properties which are fundamentally related to their electronic band structure. Therefore, this lead to many intense investigations on multi‐band electronic system to explore new physics. ❧ The physics of multi‐band electronic structure is fascinating in several aspects. Without many‐body effects, because of the gauge freedom of Bloch states, topological insulators can give rise a robust metallic behavior at its boundaries. In graphene, the touching between conduction and valence band at Fermi level yields a new criticality class which exhibit many unconventional electronic properties, especially its quasi‐relativistic behavior. Turning to the many‐body effects, for instance, the iron‐based superconductors can sustain an superconducting ground state despite of no attractive interactions in the system. Therefore, a deeper understanding for the conventional notions in condensed matter physics has put forward by many of these experimental observations. ❧ In this thesis, the many‐body effects in multi‐band systems are the main focus, especially the study of graphene and iron‐based superconductors which can be compared to experiments. These theoretical studies intend to understand how the underlying electronic bands degree of freedom can give rise to Fermi‐liquid instabilities, and how these effects can be related to intriguing physical properties. ❧ We first study the electrons correlation effects in bilayer graphene by a renormalization group technique. In this study, we build a microscopic model of bilayer graphene from a tight‐binding approach. In our finding, the peculiar Fermi surface configuration leads to critical behavior which is beyond the Fermi‐liquid paradigm. Furthermore, due to the electron‐electron interactions between different bands, excitonic instabilities are found in many different scattering channels. This analysis suggest a collection of competing orders in the system ground states. This result is consistent with the experimental observation that bilayer graphene is an insulator. ❧ Next, we study nematic order in the metallic phase of iron pnictides. In contrast to graphene, the density of states is finite at the Fermi surface. By careful investigating the scattering processes near these Fermi surface, and then identifying the most relevant collective modes from these processes, we find that a Pomeranchuk instability can be driven by magnetic fluctuations. This instability eventually leads to the break down of the isotropic metallic phase which electronic system exhibit broken crystalline rotational symmetry but preserve translation symmetry. As the experiment suggests, this can be a candidate for nematic order in the metallic phase. Advisors/Committee Members: Haas, Stephan W. (Committee Chair), Bickers, Gene (Committee Member), Dappen, Werner (Committee Member), Däppen, Werner (Committee Member), Daeppen, Werner (Committee Member), Haselwandter, Christoph (Committee Member), Jonckheere, Edmond A. (Committee Member).

Subjects/Keywords: excitonic instabilties; Fermi liquid; graphene; iron pnictides; nematic order; superconductivity

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APA (6th Edition):

Song, K. W. (2014). Electronic correlation effects in multi-band systems. (Doctoral Dissertation). University of Southern California. Retrieved from

Chicago Manual of Style (16th Edition):

Song, Kok Wee. “Electronic correlation effects in multi-band systems.” 2014. Doctoral Dissertation, University of Southern California. Accessed April 19, 2019.

MLA Handbook (7th Edition):

Song, Kok Wee. “Electronic correlation effects in multi-band systems.” 2014. Web. 19 Apr 2019.


Song KW. Electronic correlation effects in multi-band systems. [Internet] [Doctoral dissertation]. University of Southern California; 2014. [cited 2019 Apr 19]. Available from:

Council of Science Editors:

Song KW. Electronic correlation effects in multi-band systems. [Doctoral Dissertation]. University of Southern California; 2014. Available from: