Topic: Electrical Breakdown of Excitonic Insulators
Speaker: Professor Dai Xi, Department of Physics, Hong Kong University of Science and Technology (HKUST)
Date and time: May 15, 16:00
Venue: Room 105, #5 Building of SPST
Host: Liu Jianpeng, Xue Jiamin
Abstract:
In this paper, we propose 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 Dai Xi obtained his bachelor (1993) and master’s degree (1996) from Zhejiang University and PhD (1999) from the Institute of Theoretical Physics, CAS in China. After graduation, he did postdoctoral research in HKUST, Boston college and Rutgers University. From 2004–2007, he worked at the University of Hong Kong as a research assistant professor. In 2007, he joined the Institute of Physics, CAS, as a professor in the theory division. In September 2017, he joined HKUST first as a professor in the Department of Physics, and now is Dr. William M. W. Mong Professor of Nanoscience. 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), and the James C. McGroddy Prize for New Materials (2019). He is a Fellow of American Physical Society (APS), and is an awardee of the Tencent-initiated New Cornerstone Funding.