Momentum-Dark Intervalley Exciton in Monolayer Tungsten Diselenide Brightened via Chiral Phonon

Abstract

Inversion symmetry breaking and three-fold rotation symmetry grant the valley degree of freedom to the robust exciton in monolayer transition metal dichalcogenides (TMDCs), which can be exploited for valleytronics applications. However, the short lifetime of the exciton significantly constrains the possible applications. In contrast, dark exciton could be long-lived but does not necessarily possess the valley degree of freedom. In this work, we report the identification of the momentum-dark, intervalley exciton in monolayer WSe2 through low-temperature magnetophotoluminescence (PL) spectra. Interestingly, the intervalley exciton is brightened through the emission of a chiral phonon at the corners of the Brillouin zone (K point), and the pseudoangular momentum (PAM) of the phonon is transferred to the emitted photon to preserve the valley information. The chiral phonon energy is determined to be ~ 23 meV, based on the experimentally extracted exchange interaction (~ 7 meV), in excellent agreement with the theoretical expectation of 24.6 meV. The long-lived intervalley exciton with valley degree of freedom adds an exciting quasiparticle for valleytronics, and the coupling between the chiral phonon and intervalley exciton furnishes a venue for valley spin manipulation. KEYWORDS: intervalley exciton, chiral phonon, magneto-PL, time-resolved PL, tungsten diselenidethe lifetime of different excitonic complexes by a single exponential function = Ae −t/τ convolved with the response of the laser as a kernel.

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

“…For the D and XK excitons, we observe a much slower population decay rate, consistent with the much smaller oscillator strength compared to X. Specifically, the radiative emission of D and XK requires a spin-flip of electrons 30 and defect/phonon scattering [21][22][23] , respectively. To capture the dynamics of all the excitons populations, especially the one of XK exciton, we perform a universal fitting with the rate equations, Eq.…”

“…We note that the behaviour of Imax should not be confused with the power dependent peak intensity in the continuous-wave (CW) measurement. 22,23 In Supplementary S3, we show the powerdependent PL measurements with a CW laser at 2.33 eV, where the intensity of XK exhibits a linear dependence on the incident power. This discrepancy can be understood as illustrated in the inset of Figure 4b: Although the maximum intensity is growing superlinearly, the lifetime of the exponential decay also decreases (due to more efficient conversion XK → D → X), resulting in a linear dependence of PL intensity.…”

“…As a result, the energy of a photon emitted by the XK exciton is 61 meV below the X exciton emission, even lower than D exciton. [21][22][23][24] In this work, by measuring the time-resolved photoluminescence (Tr-PL) of the high-quality hexagonal boron nitride (hBN) encapsulated 1L-WSe2 at cryogenic temperature, we observe dense populations of D and XK excitons with sub-ns lifetime.…”

Phonon-exciton Interactions in WSe2 under a quantizing magnetic field

“…honons, quasiparticles describing the collective vibrations of lattice, can strongly interact with excitons in semiconductor nanostructures [1][2][3][4][5][6][7][8][9] and significantly influence the light emission and energy relaxation [2][3][4] . The strong Coulomb interaction and many-body effects in monolayer transition metal dichalcogenides (TMDCs) [10][11][12][13] significantly enhance the phonon-exciton interaction, which was shown to reveal the silent Raman modes in boron nitride (BN) encapsulated monolayer WSe 2 9,14 .…”

“…The strong Coulomb interaction and many-body effects in monolayer transition metal dichalcogenides (TMDCs) [10][11][12][13] significantly enhance the phonon-exciton interaction, which was shown to reveal the silent Raman modes in boron nitride (BN) encapsulated monolayer WSe 2 9,14 . The valley degree of freedom of the excitonic complexes also leads to types of phonon-exciton interactions that brighten the spin-forbidden dark exciton and intervalley exciton in WSe 2 through chiral phonon modes at Γ point 5 and K point 1 , respectively, resulting in phonon replica PL with long lifetime and large valley polarization. Due to the valley-spin locking 15,16 and the nontrivial Berry phase [17][18][19] , the phonon-exciton interaction can be even more intriguing with the application of a large out-ofplane magnetic field, which induces valley-polarized Landau levels (LLs) and dictates the unique selection rules of the inter-LL transition [20][21][22] .…”