Event Details

Quintons and valley phonons in monolayer semiconductors

Abstract:The optical properties of monolayer semiconductors such as transition metal dichalcogenides (TMDs) remarkably change upon thinning from bulk down to a single atomically thin layer. For example, an indirect-to-direct band gap transition is often observed, and accompanied by emphasized excitonic effects - a consequence of the significant loss in electrostatic screening which results in increasing the interaction between charge carriers. The broken inversion symmetry and strong spin-orbit coupling leads to the well-known spin-valley coupling of band edge electrons [1-3]. Therefore they exhibit properties such as optically tunable spin and valley degree of freedom for potential quantum-device applications [4, 5]. The presence of direct optical bandgaps in layered semiconductors as well as different excitonic bound states [6] promises applying such materials in deterministic single-photon emitters [7, 8]. Here we present statistically exact diffusion quantum Monte Carlo (DMC) binding-energy data for a Mott-Wannier model of charged biexcitons or quintons in two-dimensional semiconductors in which charges interact via the Rytova-Keldysh potential [9, 10]. We also provide DMC energies of excitonic charge complexes in the presence of out-of-plane magnetic fields for monolayer semiconductors. Our results are in agreement with experiment [11] and help to identify features in photoluminescence (PL) spectra of layered semiconductors.

Zone edge phonons or valley phonons are another important phenomena in monolayer semiconductors which are emerging from the coupling between spin, charge, and lattice degrees of freedom. Valley phonons are collective lattice oscillations with momentum vectors pointing to the corners of the hexagonal Brillouin zone. These phonons have been predicted to play an important role in spin and valley pseudospin relaxation through phonon-assisted intervalley scattering[12, 13]. This leads to a series of PL peaks as valley phonon replicas of dark excitons and trions. In monolayer WSe2 we have found three valley phonons facilitate spin-conserving intervalley scattering, which results in a series of dark exciton and dark trion phonon-replicas in the low temperature PL spectrum which are in agreement with literature [14].

[1] X. D. Xu et al. Nat. Phys. 10, 343-350 (2014).

[2] D. Xiao et al. Phys. Rev. Lett. 108, 196802 (2012).

[3] G. Wang et al. Rev. Mod. Phys. 90, 021001 (2018).

[4] A. M. Jones et al. Nature Nanotech. 8, 634 (2013).

[5] T. Mueller and E. Malic, Phys. Rev. B 24. 1134 (2018).

[6] E. Mostaani et al. Phys. Rev. B 96, 075431 (2017).

[7] P. Tonndorf et al. Optica 2, 347 (2015).

[8] Y.-M. He, G. Clark et al. Nature Nanotech. 10, 497 (2015).

[9] N. S. Rytova, Dokl. Akad. Nauk. SSSR 163, 1118 (1965).

[10]V. Keldysh, J. Exp. Theor. Phys. 29, 658 (1979).

[11] M. Barbone et al. Nature Comm. 9, 3721 (2018).

[12] Y. Song and H. Dery, Phys. Rev. Lett.111, 026601 (2013).

[13] Y. Song and H. Dery, Phys Rev B 92, 125431 (2015).[14] M. He et al. ArXiv:2001.01769.