Title: Electron-phonon coupling and collective electronic phenomena in transition metal dichalcogenides

Speaker: Dr. Sanjoy Kumar Mahatha, University of Aarhus, Denmark

Abstract: Transition metal dichalcogenides (TMDCs) offer a wide range of electronic properties with metallic, semiconducting and insulating behaviours, in addition to collective electronic phenomena such as charge-density wave (CDW) and Mott insulating states. Changing the dimensionality of these materials results in significant modifications of their electronic properties. In single layer (SL) TMDCs with trigonal prismatic structure, the lack of inversion symmetry lifts the spin degeneracy at the K and K’ valleys, providing an opportunity to exploit coupled spin and valley degrees of freedom. This unique situation can result in different electron-phonon coupling strengths for the two spin-split states in the K valleys and has been investigated on epitaxial semiconducting SL WS2 and MoS2 on Au(111), using temperature-dependent angle-resolved photoemission spectroscopy (ARPES) and ab-initio calculations [1, 2]. The electronic properties of these epitaxial SLs are greatly affected by the choice of the substrate. Interestingly, the semiconducting nature of SLWS2 is modified when it is grown on the similar Ag(111) substrate [3] due to hybridization at the interface. This dilemma can be solved by intercalation of Bi atoms which leads to a quasi-free-standing WS2 valence band structure [4]. Bulk 1T-TaS2, unlike MoS2 and WS2, is a metallic TMDC that undergoes a series of temperature-dependent CDW phases [5]. The commensurate CDW phase of 1T-TaS2 has widely been studied as a quasi-2D phenomenon that coexists with a Mott insulating state. However, recent theoretical calculations predicted the coexistence of the CDW phase with a nearly 1D metallic dispersion perpendicular to the crystal planes [6]. Our recent ARPES results confirm the existence of this dispersive band [7] which exists for all the different CDW phases at different temperatures [8].

References:

[1] N. F. Hinsche et al., Phys. Rev. B 96, 121402(R) (2017).

[2] S. K. Mahatha et al., Submitted to 2D Materials.

[3] M. Dendzik et al., Phys. Rev. B 96, 235440 (2017).

[4] S. K. Mahatha et al., In preparation for submission to Nano Letters.

[5] K. Rossnagel, J. Phys.: Condens. Matter 23, 213001 (2011).

[6] P. Darancet, et al., Phys. Rev. B 90, 045134 (2014).

[7] A. S. Ngankeu, S. K. Mahatha, et al., Phys. Rev. B, 96, 195147 (2017).

[8] S. K. Mahatha et al., Manuscript under preparation, A. S. Ngankeu, PhD thesis, Aarhus University, November 2017.