Prof. Jun Sung Kim [POSTECH, Korea]

Title: Flat-band electronic bipolarity and pressure-induced metal-insulator transition of a Janus and Kagome van der Waals semiconductor Nb3TeI7

Language: English presentation

Time: 11:00 - 12:00, Jan 14, 2025

Place:  Onsite：Conference room 410, HPSTAR (Shanghai)

           Online:  Tencent Meeting: https://meeting.tencent.com/dm/poTg60CC4gpx  Meeting ID: 671-775-881

Host: Dr. Duckyoung Kim

Abstract:

Janus materials, a novel class of materials with two faces of different chemical compositions and electronic polarities. A key aspect of Janus materials is face-dependent electronic bipolarity, which is usually limited by the chemical distinction of terminated surfaces and has not been exploited in the semiconducting regime. Here, we show that a Janus and Kagome van der Waals (vdW) material Nb3TeI7 has ferroelectric-like coherent stacking of the Janus layers and hosts strong electronic bipolar states in the semiconducting regime. We observed a large potential difference of ~ 0.7 eV between the I4 and TeI3 terminated surfaces and their face-dependent n-type and p-type field-effect transistor behaviors. These unique properties are attributed to Nb 4d orbital flat bands of the breathing-Kagome lattice, of which significantly large electron mass makes the semiconducting properties immune to impurity doping, and inherent strong electron correlation enhances asymmetric electron distribution, thereby amplifying a built-in electric field [1]. Under high pressures, these flat bands systematically evolve to dispersive bands along the out-of-plane direction, leading to a metal-insulator transition. Due to strong spin orbit coupling and broken inversion symmetry, this metal-insulator transition is associated with topological electronic phase transition to Weyl semimetals [2]. Possible intriguing properties of these pressure-driven Weyl fermions will be discussed. Our findings highlight that naturally-grown Janus and Kagome vdW semiconductors provide a promising material platform for significant potential for novel phenomena and various applications.

[1] J. Yun, M. Sung, M. Choi et al. Adv. Mater. in press (2025)

[2] R. A. Susilo, M. Choi et al. unpublished