  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 phonons, phonons with momentum vectors pointing
to the corners of the hexagonal Brillouin zone, and the resulting exciton
complexes in the monolayer semiconductor WSe2. From Lande 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.
