Valley phonons and exciton complexes in a monolayer semiconductor

Abstract

The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects because they possess unique spin-valley locking physics for hosting rich excitonic species and the reduced screening for strong Coulomb interactions. Here, we report the observation of multiple valley phononsphonons with momentum vectors pointing to the corners of the hexagonal Brillouin zoneand the resulting exciton complexes in the monolayer semiconductor WSe2. From Landé g-factor and polarization analyses of photoluminescence peaks, we find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncovered an intervalley exciton near charge neutrality, and extract its short-range electron-hole exchange interaction to be about 10 meV. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.

Exciton g-factors in monolayer and bilayer WSe2 from experiment and theory

“…Our results for the bright exciton and the trion doublet in Table 1 agree well with these reports if we discard the magneto-luminescence result for X À 1 that is compromised by both a vanishingly small PL intensity at high magnetic fields and the relatively broad linewidth of 6 meV in our sample. Due to this inhomogeneous broadening, we are unable to track the dispersion of the relatively weak spin-dark exciton peak D 0 , with g-factors ranging between 9.1 and 9.9 in previous reports 20,31,33,34 nor its chiral-phonon replicum with the same g-factor at 65 meV red-shift from X 0 (refs. 33,34,38 ).…”

“…Due to this inhomogeneous broadening, we are unable to track the dispersion of the relatively weak spin-dark exciton peak D 0 , with g-factors ranging between 9.1 and 9.9 in previous reports 20,31,33,34 nor its chiral-phonon replicum with the same g-factor at 65 meV red-shift from X 0 (refs. 33,34,38 ). The signature of the latter is overwhelmed in our spectra by the peak M 0 2 with 60 meV red-shift and a g-factor of −12.9 ± 0.7 in agreement with values reported from samples with spectrally narrow PL 33,34 .…”

“…2 , and D − have been established in previous experiments on a wide range of different samples [12][13][14][15][16][17][18][19][20][31][32][33][34] . Our results for the bright exciton and the trion doublet in Table 1 agree well with these reports if we discard the magneto-luminescence result for X À 1 that is compromised by both a vanishingly small PL intensity at high magnetic fields and the relatively broad linewidth of 6 meV in our sample.…”

“…33 ) and −3.4 (ref. 34 ). This peak of unidentified origin is missing in our spectra from the negative doping regime.…”

Long-lived populations of momentum- and spin-indirect excitons in monolayer WSe2

“…17,18 The intervalley XK exciton (S = 0, marked by the dotted blue line) is composed of electrons and holes with the same energy as the D exciton. However, the exchange interaction raises the binding energy of XK about 10 meV, 19,20 yielding the estimated energy of 30 meV below X exciton. XK exciton has the centre of mass momentum of K. To be radiatively recombined, it requires the coupling to zoneboundary phonons with momentum K (~31 meV) 21 to scatter the exciton into the light cone (the blue zigzag arrow in Figure 1b), further reducing the oscillator strength.…”