Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe2

Abstract

We investigate the interactions of photoexcited carriers with lattice vibrations in thin films of the layered transition metal dichalcogenide (TMDC) WSe 2 . Employing femtosecond electron diffraction with monocrystalline samples and first-principles density functional theory calculations, we obtain a momentumresolved picture of the energy transfer from excited electrons to phonons. The measured momentumdependent phonon population dynamics are compared to first-principles calculations of the phonon linewidth and can be rationalized in terms of electronic phase-space arguments. The relaxation of excited states in the conduction band is dominated by intervalley scattering between Σ valleys and the emission of zone boundary phonons. Transiently, the momentum-dependent electron-phonon coupling leads to a nonthermal phonon distribution, which, on longer time scales, relaxes to a thermal distribution via electron-phonon and phononphonon collisions. Our results constitute a basis for monitoring and predicting out of equilibrium electrical and thermal transport properties for nanoscale applications of TMDCs. DOI: 10.1103/PhysRevLett.119.036803 Semiconducting transition metal dichalcogenides combine crystal structures of chemically stable twodimensional layers with indirect band gaps in the visible and near infrared optical range. Their intrinsic stability down to monolayer thicknesses [1,2] in combination with the possibility to create artificial stacks [3,4] suggests them for electronic and optoelectronic applications like nanoscale transistors or photodetectors with atomically sharp p-n junctions [5][6][7]. In such devices, the electronic mobilities, electronic coupling, and heat conductivities within the layers and across interfaces are of central interest. Whereas macroscopic heat and electrical transport properties can be measured directly, the observation of the underlying microscopic processes, i.e., the scattering processes of carriers and of phonons, requires methods with time, momentum, and energy resolution to be understood in detail. Such information can be decisive in the correct determination of transport properties [8,9] or energy relaxation [10][11][12]. A momentum-resolved view of scattering processes will in addition be of uttermost importance in conceiving novel quantum technologies harnessing spin and valley degrees of freedom [13][14][15], as they utilize carrier populations at specific positions in momentum space. While time-and angle-resolved photoemission spectroscopy provides this level of detail for electron dynamics, see, for instance, Refs. [16,17], techniques for studying ultrafast structural dynamics have not yet reached the equivalent level of resolution. Recently, the investigation of the time-and momentum-resolved phonon population has been demonstrated with ultrafast x-ray and electron diffraction [18][19][20][21].This work reports a momentum-resolved study of scattering processes and the resulting energy transfer between photoexcited electrons and phonons in thin bulklike films of WS...

Time- and momentum-resolved phonon population dynamics with ultrafast electron diffuse scattering

“…Transient electron diffuse intensity has been used elsewhere[20][21][22][23] as an approximation to the population dynamics of particular modes. However, one can extract the transient wavevector-dependent phonon population dynamics {∆n j,k (t)} by combining the measurements of ∆I(q, t) with the calculations of one-phonon structure factors and associated quantities presented above.For many materials (including graphite), the temperature dependence (and hence time de-q, t 0 )| 2 [a.u.]…”

Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface X-ray scattering

“…Understanding of these emergent phenomena in 2D systems beyond thin films 14 calls for a direct quantitative measurement of the lattice dynamics of monolayer crystals at and across crystalline interfaces on ultrafast time scales.With advances in generating ultrabright and ultrashort electron and x-ray pulses, tracking atomic structural dynamics in 2D materials on femtosecond time scales with high precision becomes possible. For example, ultrafast electron diffraction (UED) and microscopy with large scattering cross section allows measurements of transient structural changes in various multilayer systems [15][16][17][18][19] and at surfaces 8,20,21 . At the monolayer limit, UED in a transmission geometry has been…”

Theory of Thermal Relaxation of Electrons in Semiconductors

“…In semiconductors, the highly heterogeneous electron-phonon interactions (e.g. in polar semiconductors with Fröhlich interactions [9]) and, in some cases, the higher lattice thermal conductivity in comparison to metals weaken the hypothesis of a thermalized phononic subsystem [10,11], hence calling for the reexamination of the 2T physical picture in semiconductors.In this context, the advent of first-principles techniques able to predict the mode-and energy-resolved electronphonon [12][13][14] and phonon-phonon interactions [15,16] provides an important opportunity: In their modern implementations [13,16,17], these methods have been able to predict lattice thermal conductivities [18][19][20][21], the temperature-and pressure-dependence of the electronic bandgap [22][23][24][25][26][27][28], electrical conductivities [29,30], and hot carrier dynamics [31,32]. However, to the best of our knowledge and despite these early successes, these approaches have yet to be applied to the computation of electron-induced, non-equilibrium phonon distributions and their effects on thermal relaxation of electrons.…”

“…In semiconductors, the highly heterogeneous electron-phonon interactions (e.g. in polar semiconductors with Fröhlich interactions [9]) and, in some cases, the higher lattice thermal conductivity in comparison to metals weaken the hypothesis of a thermalized phononic subsystem [10,11], hence calling for the reexamination of the 2T physical picture in semiconductors.…”