Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS₂

Abstract

In monolayer Transition Metal Dichalcogenides (TMDs) the valence and conduction bands are spin split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark exciton, consisting of an electron and a hole with opposite spin orientation, has lower energy than the A exciton. A possible mechanism leading to the transition from bright to dark excitons involves the scattering of the electrons from the upper to the lower conduction band state in K. Here we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics of electrons in the conduction band of single-layer WS2. This process occurs on a sub-ps time scale and it is significantly dependent on the temperature, indicative of a phonon-assisted relaxation. These experimental results are supported by time-dependent ab-initio calculations which show that the intra-valley spin-flip scattering occurs on significantly longer time scales only exactly at the K point. In a realistic situation the occupation of states away from the minimum of the conduction band leads to a dramatic reduction of the scattering time.Transition metal dichalcogenides (TMDs) are promising for opto-electronics[1-4], valleytronics[5-9] and quantum information processing [10]. Monolayers (1L) of TMDs are direct bandgap semiconductors[11] whose optical properties are dominated by excitons with binding energies of up to several hundred meVs [12][13][14][15][16][17][18]. The valence/conduction (VB/CB) band extrema lie at the non-equivalent K and K' points on the edge of the Brillouin zone [19,20]. Their spin-degeneracy is lifted by strong spin-orbit (SO) interaction [21]. For the VB the energy splitting ∆ v ranges between 150 and 400meV [22], while for the CB ∆ c it is one/two orders of magnitude lower (∼1-30meV) [23,24]. Together with a broken inversion symmetry, this results in spin-polarized bands and valley-dependent dipole-allowed interband optical transitions, as first detected by helicity resolved photoluminescence (PL) measurements [25][26][27].The large VB splitting gives rise to two distinct interband transitions (called A and B) strongly renormalized by excitonic effects [16], dominating the optical response in the visible range [22]. Recent theoretical calculations predicted that, while the spin orientation of the upper and lower CB states is antiparallel in K/K', in W-based 1L-TMDs, the upper CB state has the same spin orientation as the upper VB. A direct consequence of this spin-ordering is the formation of an intravalley (i.e. zero momentum) dark exciton with a lower energy than the bright exciton [28][29][30][31][32][33]. The bright to dark exciton transition reduces the PL quantum yield in W-based 1L-TMDs for decreasing temperatures, as recently shown [33]. The formation of dark excitons requires scattering processes such as intravalley relaxation. We stress that other excitonic comple...

Ultrafast dynamics in monolayer transition metal dichalcogenides: Interplay of dark excitons, phonons, and intervalley exchange

“…figure 2 (a-d), which is required for the formation of the B exciton σ + response. This result is in full agreement with recent experiments [13,18,20,21]. We present detailed numerical calculations for the exemplary material MoSe 2 , but find that our results are also applicable to other TMDC materials.…”

“…figure 2 (d). Such a dynamics is observed as a blocking of the probe pulse and contribute similar to the proposed exciton-upconversion or spin-flips [21,23]. Figure 3 (b) shows the temporal evolution of the bleaching of the B transition in the unpumped valley.…”

“…There are several mechanisms which might explain these results: (a) A spin flip within the pumped valley [25,26] would lead to a population of the opposite-spin conduction band, visible as a B transition bleaching in the same valley [21]. (b) Dexter-like intervalley exciton coupling due to electronic wavefunction overlap in k-space where an A exciton in the FIG.…”

Layer-engineered interlayer excitons

“…5F). We should note that this valley mixing time scale is much slower than that in monolayer TMDs (42). Our result represents the first direct observation of valley mixing in the time domain from PL emission, thanks to the prolonged exciton lifetimes in multilayer heterostructure.…”

Exciton–phonon coupling strength in single-layer MoSe2 at room temperature

“…Exciton-phonon coupling (EXPC) plays a key role in determining the T-dependent optoelectronic and transport properties of 1L-TMDs [9][10][11] . It is responsible for, e.g., nonradiative exciton decay 9,10,12 , limiting the fluorescence quantum yield 13 , the formation of darkexciton phonon replicas 14 , and it mediates spin-flip processes, thus decreasing the lifetime of spin/valley-polarized charge carriers 15 . For T<100 K, the interaction between excitons and acoustic phonons induces linewidth broadening and dominates the excitonic resonance of 1L-TMDs 9,16,17 .…”