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...

The atomic structure and electronic and vibrational properties of glassy Ga 11 Ge 11 Te 78 have been studied by combining density functional (DF) simulations with x-ray (XRD) and neutron diffraction (ND), extended x-ray absorption fine structure (EXAFS), and Raman spectroscopies. The final DF structure (540 atoms) was refined using reverse Monte Carlo methods to reproduce the XRD and ND data as well as Ge and Ga K-edge EXAFS spectra, while maintaining a semiconducting band gap and a total energy close to the DF minimum. The local coordination of Ga is tetrahedral, while Ge has twice as many tetrahedral as defective octahedral configurations. The average coordination numbers are Ga, 4.1, Ge, 3.8, and Te, 2.6. The chemical bonding around Ga involves Ga 4s, Ga 4p, Te 5s, and Te 5p orbitals, and the bond strengths show bonding close to covalent, as in Ge. There are fewer Te chains and cavities than in amorphous Te, and a prepeak in the structure factor at 1.0Å −1 indicates medium-range order of the Ga/Ge network. Density functional calculations show that contributions of Te-Te, Ga-Te, and Ge-Te bonds dominate the experimental Raman spectra in the 110-150 cm −1 range.

Phononic crystals (PnCs) control the transport of sound and heat similar to the control of electric currents by semiconductors and metals or light by photonic crystals. Basic and applied research on PnCs spans the entire phononic spectrum, from seismic waves and audible sound to gigahertz phononics for telecommunications and thermal transport in the terahertz range. Here, we review the progress and applications of PnCs across their spectrum, and we offer some perspectives in view of the growing demand for vibrational isolation, fast signal processing, and miniaturization of devices. Current research on macroscopic low-frequency PnCs offers complete solutions from design and optimization to construction and characterization, e.g., sound insulators, seismic shields, and ultrasonic imaging devices. Hypersonic PnCs made of novel low-dimensional nanomaterials can be used to develop smaller microelectromechanical systems and faster wireless networks. The operational frequency, compactness, and efficiency of wireless communications can also increase using principles of optomechanics. In the terahertz range, PnCs can be used for efficient heat removal from electronic devices and for novel thermoelectrics. Finally, the introduction of topology in condensed matter physics has provided revolutionary designs of macroscopic sub-gigahertz PnCs, which can now be transferred to the gigahertz range with advanced nanofabrication techniques and momentum-resolved spectroscopy of acoustic phonons.

Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density-functional theory, and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs and shed light on why SEF occurs on ultrafast time scales.

We combine ultrafast electron diffuse scattering experiments and first-principles calculations of the coupled electron–phonon dynamics to provide a detailed momentum-resolved picture of lattice thermalization in black phosphorus. The measurements reveal the emergence of highly anisotropic nonthermal phonon populations persisting for several picoseconds after exciting the electrons with a light pulse. Ultrafast dynamics simulations based on the time-dependent Boltzmann formalism are supplemented by calculations of the structure factor, defining an approach to reproduce the experimental signatures of nonequilibrium structural dynamics. The combination of experiments and theory enables us to identify highly anisotropic electron–phonon scattering processes as the primary driving force of the nonequilibrium lattice dynamics in black phosphorus. Our approach paves the way toward unravelling and controlling microscopic energy flows in two-dimensional materials and van der Waals heterostructures, and may be extended to other nonequilibrium phenomena involving coupled electron–phonon dynamics such as superconductivity, phase transitions, or polaron physics.

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Black phosphorus has recently attracted significant attention for its highly anisotropic properties. A variety of ultrafast optical spectroscopies has been applied to probe the carrier response to photoexcitation, but the complementary lattice response has remained unaddressed. Here we employ femtosecond electron diffraction to explore how the structural anisotropy impacts the lattice dynamics after photoexcitation. We observe two time scales in the lattice response, which we attribute to electron-phonon and phonon-phonon thermalization. Pronounced differences between armchair and zigzag directions are observed, indicating a nonthermal state of the lattice lasting up to ∼ 60 ps. This nonthermal state is characterized by a modified anisotropy of the atomic vibrations compared to equilibrium. Our findings provide insights in both electron-phonon as well as phonon-phonon coupling and bear direct relevance for any application of black phosphorus in nonequilibrium conditions. Layered van der Waals (vdW) materials have attracted significant research interest in recent years due to their potential device applications [1][2][3][4]. The most prominent 2D material, graphene, exhibits high carrier mobility, but lacks a band gap, which is required in many applications. In contrast, transition metal dichalcogenides possess a band gap in the visible range, but a lower carrier mobility. With a thickness-dependent band gap extending from the infrared to the visible [5-7] and a high carrier mobility [8][9][10], black phosphorus provides an important complementary building block for vdW heterostructure devices. A central aspect of black phosphorus is its inplane anisotropic structure, shown in Figure 1a. The layers have two inequivalent high-symmetry directions, the so-called zigzag and armchair directions. This structural anisotropy is also reflected in many macroscopic material properties, such as optical absorption [9,[11][12][13][14] and in-plane anisotropic thermal [15][16][17][18] and electrical [5,9,19,20] conductivities. These anisotropic properties offer additional tunability in device design.Since any device operates in nonequilibrium conditions, a microscopic understanding of nonequilibrium states in vdW materials is of particular interest. For optoelectronic devices, knowledge of the evolution of the system after optical excitation is desired. Carrier dynamics in black phosphorus have been studied using a variety of time-resolved optical spectroscopies [20-26] as well as time-and angle-resolved photoemission [27,28] (trARPES). An important relaxation pathway for excited carriers is via coupling to the lattice. However, to date, no study has directly reported on the ultrafast lattice response of black phosphorus upon photoexcitation, which reflects the strength of electron-phonon as well as phonon-phonon interactions. In this work, we employ femtosecond electron diffraction [29] (FED) to directly probe the structural dynamics of photoexcited black phosphorus.The measurement principle is sketched in Figure 1b. The sample...

Compression of electron pulses with terahertz radiation offers short pulse durations and intrinsic subcycle stability in time. We report the generation of 12-fs (rms), 28-fs (FWHM) electron pulses at a kinetic energy of 75 keV by using single-cycle terahertz radiation and a simple planar mirror. The mirror interface provides transverse velocity matching and spatially uniform compression in time with purely longitudinal field-electron interaction. The measured short-term and long-term temporal drifts are substantially below the pulse duration without any active synchronization. A simple phase-space model explains the measured temporal focusing dynamics for different compressor strengths and shows a path toward generating isolated attosecond electron pulses from beams of almost arbitrary transverse emittance.