DCMPMS Seminars

Interlayer excitons: (spatially) indirect evidence of two-dimensionality in few-layer and bulk van der Waals semiconductors

by Dr. Ashish Arora (Institute of Physics and Center for Nanotechnology University of Muenster, Germany)

Thursday, February 21, 2019 from to (Asia/Kolkata)
at AG80
Description
Coulomb-bound electrons and holes with spatially displaced wave functions called ‘interlayer excitons’ are important for Bose-Einstein condensation, superfluidity, dissipationless current flow, and the light-induced exciton spin Hall effect. Here, we demonstrate that interlayer excitons exist in bulk transition metal dichalcogenides (TMDCs) by using valley-resolved reflectance spectroscopy under high magnetic fields of up to 29 T, combined with GW-BSE ab initio calculations [1,2]. Interlayer excitons are formed due to the specific spin-valley coupling of the charge carriers within the individual layers of the crystal. The constituent electrons and holes are individually localized in the neighboring layers of the van der Waals crystal (Fig. (1)). We find that due to weak van Waals interactions, bulk crystals provide a cleaner and simpler system to study interlayer phenomena compared to artificially stacked TMDC heterostructures, which are difficult to fabricate with high quality. 

Fig.1 Schematic drawing of an interlayer exciton and an intralayer exciton in a transition metal dichalcogenide crystal. 

Our discovery also solves the long-standing puzzle of positive g-factors in van der Waals semiconductors, and paves the way for studying collective phenomena in these materials at elevated temperatures due to the large interlayer excitonic binding energies of tens of meV. Furthermore, interlayer excitons in bulk TMDCs provide an evidence of the two-dimensional nature of excitons in TMDCs, irrespective of the layer thickness.

[1] A. Arora et al., Nat. Commun. 8 (2017), 639
[2] A. Arora et al., Nanoscale 10 (2018), 15571