【T. D. Lee Colloquium No. 14】Electrical Breakdown of Excitonic Insulators

by Prof. Xi Dai (The Hongkong University of Science and Technology)

Thursday 16 May 2024, 14:00 → 15:00 Asia/Shanghai

S500 lecture hall （previous 541)

Abstract

In this talk, I will introduce a new electrical breakdown mechanism for exciton insulators in the BCS limit, which differs fundamentally from the Zener breakdown mechanism observed in traditional band insulators. Our new mechanism results from the instability of the many-body ground state for exciton condensation, caused by the strong competition between the polarization and condensation energies in the presence of an electric field. We refer to this mechanism as “many-body breakdown”. To investigate this new mechanism, we propose a BCS-type trial wave function under finite electric fields and use it to study the many-body breakdown numerically. Our results reveal two different types of electric breakdown behavior. If the system size is larger than a critical value, the Zener tunnelling process is first turned on when an electrical field is applied, but the excitonic gap remains until the field strength reaches the critical value of the many-body breakdown, after which the excitonic gap disappears and the system becomes a highly conductive metallic state. However, if the system size is much smaller than the critical value, the intermediate tunnelling phase disappears since the many-body breakdown happens before the onset of Zener tunnelling. The sudden disappearance of the local gap leads to an “off-on” feature in the current-voltage (I − V) curve, providing a straightforward way to distinguish excitonic insulators from normal insulators.

Biography

Professor Xi Dai obtained his Bachelor and Master’s degree (1993 &1996) from Zhejiang University and PhD (1999) from Institute of Theoretical Physics, CAS in China. After graduation, he did postdoctoral research in HKUST, Boston college and Rutegrs University. From 2004-2007, he worked in University of Hong Kong as a research assistant professor. In 2007, he joined institute of physics, CAS as a professor in the theory division. In September 2017, he joined HKUST as a professor in the Department of Physics. Professor Dai received several important awards during the past ten years including the OCPA “Achievements in Asia Award” (2011), the CAS prize for outstanding scientific achievements and the KC Chou foundation “Fundamental Physics Prize” (2014). He is a Fellow of American Physical Society (APS).

Most of professor Dai’s previous research activities are concentrated in two fields. One is to develop computational methods for the electronic structure of the strongly correlated materials. The other one is to search for the topological non-trivial quantum state in realistic materials. In the first research field, with the collaboration with professor Z. Fang, they developed the Gutzwiller density functional theory (LDA+Gutzwiller) and applied it to study many of correlated materials, i.e. ferromagnetic metals, iron based superconductors and topological Kondo insulators. In the second field, together with the collaborating team formed by scientists from both IOP and Stanford University, they have proposed several very important topological insulators, i.e. Bi2Se3, Bi2Te3 and Sb2Te3. They also proposed that magnetic doped topological insulator thin film can realise quantum anomalous Hall effect, which has been confirmed by experiments in 2013. In 2015, professor Dai’s research team predicted the first Wely semimetal material TaAs family, which has soon been confirmed by both photo emission and transport experiments.

Chair

Prof. Hong Ding （丁洪）

Division

Institution

Other information

Tencent Meeting Link: https://meeting.tencent.com/dm/8UID9uwl5qRe

Meeting ID: 514 155 347 (no password)

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