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Session for “Young” physicists

Date : Thursday, May 27th, 2021 10:30 am - 12:00 pm Place : https://sites.google.com/view/cevis2020/registration (We will send the zoom link to the registered participants) Lecturer : Junyeong Ahn (Harvard Univ), Kazuaki Takasan (UC Berkeley), Jong Yeon Lee (KITP, UCSB) Committee Chair : Masaki Oshikawa (ex.63275)
e-mail: oshikawa@issp.u-tokyo.ac.jp

Speaker: Junyeong Ahn (Harvard Univ)
Title: Quantum Geometry of Light-Matter Interactions

Abstract: The concept of the geometry of quantum states has been useful for characterizing responses of electronic systems to static electromagnetic fields. On the other hand, however, it has been challenging to relate quantum geometry with resonant optical responses. The main obstacle is that optical transitions are properties of a pair of states, while geometrical properties are usually defined for a single state. In this seminar, I will reveal the quantum geometric meaning of resonant optical transitions. This result suggests that light-matter interactions are in general manifestations of the geometry of quantum states. I will take linear, second-order, and third-order responses as examples to demonstrate our theory.

Speaker: Kazuaki Takasan (UC Berkeley)

Title: Current-induced second harmonic generation in inversion-symmetric topological semimetals

Abstract: The rapid developments of the experimental techniques enable us to drive quantum materials into the nonequilibrium states. This opens up a new route to control the states of matter. For example, controls of topological phases by strong laser light have been extensively studied [1]. In this talk, I will present our recent work about a nonlinear response in nonequilibrium states of matter [2]. We studied theoretically the second harmonic generation (SHG) in inversion-symmetric topological semimetals under a DC electric current. While inversion symmetry prohibits even-order responses including SHG, the finite current breaks the symmetry and the SHG is allowed in the nonequilibrium steady state. Based on analytic and numerical calculations, we find that Dirac and Weyl semimetals exhibit strong SHG upon application of finite current. Our experimental estimation for a Dirac semimetal Cd3As2 and a magnetic Weyl semimetal Co3Sn2S2 suggests that the induced susceptibility is 100-10000 times larger than those of typical nonlinear optical materials. We also discuss experimental approaches to observe the phenomena and comment on current-induced SHG in other topological semimetals in connection with recent experiments [3]. If time permits, I will briefly introduce our more recent paper about a nonlinear static response in an interacting many-body system [4].

[1] For example, M. S. Rudner and N. H. Lindner, Nat. Rev. Phys. 2, 229–244 (2020). [2] KT, T. Morimoto, J. Orenstein and J. E. Moore, arXiv:2007.08887. [3] N. Sirica, et al., arXiv:2005.10308. [4] Y. Tanikawa, KT, and H. Katsura, arXiv:2103.05838.

Speaker: Jong Yeon Lee (KITP, UCSB)

Title: A fermionic Quantum Monte Carlo study of Twisted Bilayer Graphene

Abstract: We present a quantum Monte Carlo (QMC) study of twisted bilayer graphene (TBG) near the magic angle at charge neutrality. Due to its strongly interacting nature as well as topological obstructions for a simple lattice model, an unbiased numerical study of a continuum model of TBG has been a challenging problem. In the momentum space, we establish the absence of the sign problem for this model in the determinant QMC method and describe a computationally tractable formulation of this problem. We demonstrate the method by studying the TBG systems up to 9 x 9 Moire unit cells with varying physical parameters, illustrating the hierarchy of symmetry broken phases and the onset of spontaneous symmetry breaking instability as a function of temperature.

Chair: Gil Young Cho

This event is jointly organized by the Korea Institute for Advanced Study and the University of Tokyo. “Correlated Electrons Virtual International Seminars (CEVIS)” https://sites.google.com/view/cevis2020/home


(Published on: Monday May 24th, 2021)