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 principle density functional theory calculations, we obtain a momentum-resolved picture of the energy-transfer from excited electrons to phonons. The measured momentum-dependent phonon population dynamics are compared to first principle 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 non-thermal phonon distribution, which, on longer timescales, relaxes to a thermal distribution via electron-phonon and phonon-phonon collisions. Our results constitute a basis for monitoring and predicting out of equilibrium electrical and thermal transport properties for nanoscale applications of TMDCs.
Due to high binding energy and oscillator strength, excitons in thin flakes of transition metal dichalcogenides constitute a perfect foundation for realizing a strongly coupled light-matter system. In this paper we investigate mono- and few-layer WSe$_2$ flakes encapsulated in hexagonal boron nitride and incorporated into a planar dielectric cavity. We use an open cavity design which provides tunability of the cavity mode energy by as much as 150 meV. We observe a strong coupling regime between the cavity photons and the neutral excitons in direct-bandgap monolayer WSe$_2$, as well as in few-l…
Layered van-der-Waals materials with hexagonal symmetry offer an extra degree of freedom to their electrons, the so called valley index or valley pseudospin. This quantity behaves conceptually like the electron spin and the term valleytronics has been coined. In this context, the group of semiconducting transition-metal dichalcogenides (TMDC) are particularly appealing, due to large spin-orbit interactions and a direct bandgap at the K points of the hexagonal Brillouin zone. In this work, we present investigations of excitonic transitions in mono- and multilayer WSe$_2$ and MoSe$_2$ materials …
We report the direct observation of intervalley exciton between the Q conduction valley and $Γ$ valence valley in bilayer WSe$_2$ by photoluminescence. The Q$Γ$ exciton lies at ~18 meV below the QK exciton and dominates the luminescence of bilayer WSe$_2$. By measuring the exciton spectra at gate-tunable electric field, we reveal different interlayer electric dipole moments and Stark shifts between Q$Γ$ and QK excitons. Notably, we can use the electric field to switch the energy order and dominant luminescence between Q$Γ$ and QK excitons. Both Q$Γ$ and QK excitons exhibit pronounced phonon re…
In nano-device applications using 2D van der Waals materials, a heat dissipation through nano-scale interfaces can be a critical issue for optimizing device performances. By using a time-domain thermoreflectance measurement technique, we examine a cross-plane thermal transport through mono-layered (n=1) and bi-layered (n=2) WSe2 flakes which are sandwiched by top metal layers of Al, Au, and Ti and the bottom Al2O3 substrate. In these nanoscale structures with hetero- and homo-junctions, we observe that the thermal boundary resistance (TBR) is significantly enhanced as the number of WSe2 layers…
We report an experimental study of temperature-dependent spectral lineshapes of phonon side-band emission stemming from dark excitons in monolayer and bilayer WSe$_{2}$. Using photoluminescence spectroscopy in the range from 4 to 100K, we observe a pronounced asymmetry in the phonon-assisted luminescence from momentum-indirect exciton reservoirs. We demonstrate that the corresponding spectral profiles are distinct from those of bright excitons with direct radiative decay pathways. The lineshape asymmetry reflects thermal distribution of exciton states with finite center-of-mass momenta, charac…
Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungsten diselenide (WSe$_2$) shows a multitude of sharp emission peaks below the bright exciton. Some of them have been recently identified as phonon sidebands of momentum-dark states. However, the exciton dynamics behind the emergence of these sidebands has not been revealed yet. In this joint theory-experiment study, we theoretically predict and experimentally observe time-resolved PL providing microscopic insights into thermalization of hot excitons formed after optical excitation. In good agreement between theory and exp…
Quantum entanglement is a fundamental phenomenon which, on the one hand, reveals deep connections between quantum mechanics, gravity and the space-time; on the other hand, has practical applications as a key resource in quantum information processing. While it is routinely achieved in photon-atom ensembles, entanglement involving the solid-state or macroscopic objects remains challenging albeit promising for both fundamental physics and technological applications. Here, we report entanglement between collective, chiral vibrations in two-dimensional (2D) WSe$_2$ host --- chiral phonons (CPs) --…
We present a high-resolution resonance Raman study of hBN encapsulated MoSe$_2$ and WSe$_2$ monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe$_2$ A2s and MoSe$_2$ A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates…
Transconductance as a Probe of Valley Thermodynamics in Multilayer WSe$_2$
Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS)（物质·材料纳米结构研究センター（MANA），国立物質研究所（NIMS））；Department of Nanotechnology for Sustainable Energy, Kwansei Gakuin University（关西学院大学持続可能エネルギーナノテクノロジー学科）
We report first-principles calculations of the structural and vibrational properties of the synthesized two-dimensional van der Waals heterostructures formed by single-layers dichalcogenides MoSe2 and WSe2. We show that, when combining these systems in a periodic two-dimensional heterostructures, the intrinsic phonon characteristics of the free-standing constituents are to a large extent preserved but, furthermore, exhibit shear and breathing phonon modes that are not present in the individual building blocks. These peculiar modes depend strongly on the weak vdW forces and has a great contibut…
The thermoelectric transport coefficients of electrons in two recently emerged transition metal dichalcogenides(TMD), MoS2 and WSe2, are calculated by solving Boltzmann Transport equation and coupled electrical and thermal current equations using Rode iterative technique. Scattering from localized donor impurities, acoustic deformation potential, longitudinal optical (LO) phonons, and substrate induced remote phonon modes are taken into account. Hybridization of TMD plasmon with remote phonon modes is investigated. Dynamic screening under linear polarization response is explored in TMDs sittin…
We study hot carrier transport under magnetic fields up to 15 T in suspended graphitic multilayers through differential conductance (dI/dV) spectroscopy. Distinct high-energy dI/dV anomalies have been observed and shown to be related to intrinsic phonon-emission processes in graphite. The evolution of such dI/dV anomalies under magnetic fields is further understood as a consequence of inter-Landau level cyclotron-phonon resonance scattering. The observed magneto-phonon effects not only shed light on the physical mechanisms responsible for high-current transport in graphitic systems, but also o…
Moiré patterns with angular mismatch in van der Waals heterostructures composed of atomically thin semiconducting materials are a fascinating platform to engineer the optically generated excitonic properties towards novel quantum phenomena. The moiré pattern as a periodic trap potential can give rise to spatially ordered zero-dimensional (0D) exciton ensembles, which offers the possibility for dense coherent quantum emitters and quantum simulation of many-body physics. The intriguing moiré exciton properties are affected by their dynamics and exciton-phonon interaction. However, the moiré exci…
Identifying quantum numbers to label elementary excitations is essential for the correct description of light-matter interaction in solids. In monolayer semiconducting transition metal dichalcogenides (TMDs) such as MoSe$_2$ or WSe$_2$, most optoelectronic phenomena are described well by labelling electron and hole states with the spin projection along the normal to the layer (S$_z$). In contrast, for WSe$_2$/MoSe$_2$ interfaces recent experiments show that taking S$_z$ as quantum number is not a good approximation, and spin mixing needs to be always considered. Here we argue that the correct …
Energy relaxation of photo-excited charge carriers is of significant fundamental interest and crucial for the performance of monolayer (1L) transition metal dichaclogenides (TMDs) in optoelectronics. We measure light scattering and emission in 1L-WSe$_2$ close to the laser excitation energy (down to~$\sim$0.6meV). We detect a series of periodic maxima in the hot photoluminescence intensity, stemming from energy states higher than the A-exciton state, in addition to sharp, non-periodic Raman lines related to the phonon modes. We find a period $\sim$15meV for peaks both below (Stokes) and above …
We report on electron cooling power measurements in few-layer graphene excited by Joule heating by means of a new setup combining electrical and optical probes of the electron and phonon baths temperatures. At low bias, noise thermometry allows us to retrieve the well known acoustic phonon cooling regimes below and above the Bloch Grüneisen temperature, with additional control over the phonon bath temperature. At high electrical bias, we show the relevance of direct optical investigation of the electronic temperature by means of black-body radiation measurements that provide higher accuracy th…
Using non-equilibrium molecular dynamics method(NEMD), we have found that the thermal conductivity of multilayer graphene nanoribbons monotonously decreases with the increase of the number of layers, such behavior can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is found to be proportional to the layer size, which is caused by the increase of phonon resonance. Our results are in agreement with recent experiment on dimensional evolution of thermal conductivity in few layer graphene.
Phonon polaritons (PhPs) in freestanding and supported multilayers (MuLs) of hexagonal boron nitride (hBN) are systematically studied using a macroscopic optical-phonon model. The PhP properties such as confinement, group velocity, propagation quality factor (PQF) and wavelength scaling are studied. Owing to the nonlocal high-frequency screening, there is an upper frequency limit making the two-dimensional (2D) PhPs have a frequency band, and also a maximum PQF occurs near the centre frequency. The substrate's dielectric response should be included to accurately calculate the PhP properties. W…
Using a fully ab-initio methodology, we demonstrate how the lattice vibrations couple with neutral excitons in monolayer WSe2 and contribute to the non-radiative excitonic lifetime. We show that only by treating the electron-electron and electron-phonon interactions at the same time it is possible to obtain an unprecedented agreement of the zero and finite-temperature optical gaps and absorption spectra with the experimental results. The bare energies were calculated by solving the Kohn-Sham equations, whereas G$_{0}$W$_{0}$ many body perturbation theory was used to extract the excited state e…
Calculations combining first-principles electron-phonon ($e$-ph) interactions with the Boltzmann equation enable studies of ultrafast carrier and phonon dynamics. However, in materials with weak Coulomb screening, electrons and holes form bound excitons and their scattering processes become correlated, posing additional challenges for modeling nonequilibrium physics. Here we show calculations of ultrafast exciton dynamics and related time-domain spectroscopies using $ab~initio$ exciton-phonon (ex-ph) interactions together with an excitonic Boltzmann equation. Starting from the nonequilibrium e…
We propose a novel phonon assisted Auger process unique to the electronic band structure of monolayer transition metal dichalcogenides (TMDCs), which dominates the radiative recombination of ground state excitons in Tungsten based TMDCs. Using experimental and DFT computed values for the exciton energies, spin-orbit splittings, optical matrix element, and the Auger matrix elements, we find that the Auger process begins to dominate at carrier densities as low as $10^{9-10}~{\rm cm^{-2}}$, thus providing a plausible explanation for the low quantum efficiencies reported for these materials.
Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe$_2$. We observe the intervalley dark excitons, which can recombine by the assistance of defect scatte…
Bulk layered MX2 transition metal chalcogenides (M = Mo, W and X = S, Se) are known to exhibit an indirect to direct band gap transition as the number of layers is reduced. Previous time-resolved work has principally focused on the investigation of the transient evolution of the band structure after photo-excitation, but additional information on the dynamics of the concomitant lattice rearrangement is needed to fully understand this phenomenon. Here, ultrafast electron diffraction is used to probe the atomic motion and bond dilation in monolayer WSe2 with femtosecond temporal resolution. The …
Structuring materials is one mechanism to influence the thermal conductivity and thus thermoelectric efficiency. In order to investigate the scattering of phonons in multilayer structures we developed a beam matching technique, which is based on the concept of individual phonons and their scattering at interfaces. One of the major goals is to efficiently determine the complex band structure of the bulk materials. The complex band structure is determined using selected k-points on a triangulated grid in the complex plane of wave vectors perpendicular to the interface. Matching the phonon modes …
Magneto Raman scattering study of the E$_{2g}$ optical phonons in multi-layer epitaxial graphene grown on a carbon face of SiC are presented. At 4.2K in magnetic field up to 33 T, we observe a series of well pronounced avoided crossings each time the optically active inter Landau level transition is tuned in resonance with the E$_{2g}$ phonon excitation (at 196 meV). The width of the phonon Raman scattering response also shows pronounced variations and is enhanced in conditions of resonance. The experimental results are well reproduced by a model that gives directly the strength of the electro…
In single-layer WSe$_2$, a paradigmatic semiconducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent $K^{\pm}$ valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a valleytronic device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time-dependent Kerr experiments have provided an accurate way to visualize the v…
Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps, and recent advances in the synthesis, characterization, and device fabrication of the representative MoS$_2$, WS$_2$, WSe$_2$, and MoSe$_2$ TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs …
We analyze resonant magneto-elastic interactions between standing perpendicular spin wave modes (exchange magnons) and longitudinal acoustic phonon modes in free-standing hybrid metal-ferromagnet bilayer and trilayer structures. Whereas the ferromagnetic layer acts as a magnetic cavity, all metal layers control the frequencies and eigenmodes of acoustic vibrations. The here proposed design allows for achieving and tuning the spectral and spatial modes overlap between phonons and magnons that results in their strong resonant interaction. Realistic simulations for gold-nickel multilayers show th…
We investigate the competition between acoustic-phonon-mediated superconductivity and the long-range Coulomb interaction in moiréless graphene multilayers, specifically, Bernal bilayer graphene, rhombohedral trilayer graphene, and ABCA-stacked tetralayer graphene. In these graphene multilayers, the acoustic phonons can realize, through electron-phonon coupling, both spin-singlet and spin-triplet pairings, and the intra-sublattice pairings ($s$-wave spin-singlet and $f$-wave spin-triplet) are the dominant channels. Our theory naturally explains the distinct recent experimental findings in Berna…
Ideal monolayers of common semiconducting transition metal dichalcogenides (TMDCs) such as MoS$_2$, WS$_2$, MoSe$_2$, and WSe$_2$ possess many similar electronic properties. As it is the case for all semiconductors, however, the physical response of these systems is strongly determined by defects in a way specific to each individual compound. Here we investigate the ability of exfoliated monolayers of these TMDCs to support high-quality, well-balanced ambipolar conduction, which has been demonstrated for WS$_2$, MoSe$_2$, and WSe$_2$, but not for MoS$_2$. Using ionic-liquid gated transistors w…
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…
The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible to a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the em…
Lattice dynamics play a crucial role in the physics of Moiré systems. In twisted bilayer graphene (TBG), it was shown that, in addition to the graphene phonons, there is another set of gapless excitations termed Moiré Phonons [Phys. Rev. B, 075416, 2019] reflecting the lattice dynamics at the Moire superlattice level. These modes were later suggested to be phasons due to the incommensurate stacking of the two graphene layers [Phys. Rev. B, 155426, 2019]. In this work, we elucidate the equivalence of these two seemingly distinct perspectives by identifying an underlying symmetry, which we dub m…
We study super-Planckian near-field heat exchanges for multilayer hyperbolic metamaterials using exact S-matrix calculations. We investigate heat exchanges between two multilayer hyperbolic metamaterial structures. We show that the super- Planckian emission of such metamaterials can either come from the presence of surface phonon-polaritons modes or from a continuum of hyperbolic modes depending on the choice of composite materials as well as the structural configuration.
We predict a fast relaxation of photo-excited carriers in monolayer transition metal dichalcogenides (TMDCs), which is mediated by the emission of longitudinal optical (LO) phonons. By evaluating Born effective charges for ${\rm MoS_2, MoSe_2,WS_2}$, and ${\rm WSe_2}$, we find that, due to the polar coupling of electrons with LO phonons, the relaxation times for hot electrons and holes are of the order of a few ps.
The two-fold valley degeneracy in two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) (Mo,W)(S,Se)2 is suitable for "valleytronics", the storage and manupulation of information utilizing the valley degree of freedom. The conservation of luminescent photon helicity in these 2D crystal monolayers has been widely regarded as a benchmark indicator for charge carrier valley polarization. Here we perform helicity-resolved Raman scattering of the TMDC atomic layers. In drastic contrast to luminescence, the dominant first-order zone-center Raman bands, including the low energy…
One of the main tasks in the investigation of 2-dimensional transition metal dichalcogenides is the determination of valley lifetimes. In this work, we combine time-resolved Kerr rotation with electrical transport measurements to explore the gate-dependent valley lifetimes of free conduction band electrons of monolayer WSe$_2$. When tuning the Fermi energy into the conduction band we observe a strong decrease of the respective valley lifetimes which is consistent with both spin-orbit and electron-phonon scattering. We explain the formation of a valley polarization by the scattering of opticall…
In this paper we review thermal properties of graphene and multilayer graphene and discuss the optothermal technique developed for the thermal conductivity measurements. We also outline different theoretical approaches used for the description of phonon transport in graphene and provide comparison with available experimental thermal conductivity data.
Near infrared pump-probe spectroscopy has been used to measure the ultrafast dynamics of photoexcited charge carriers in monolayer and multilayer graphene. We observe two decay processes occurring on 100 fs and 2 ps timescales. The first is attributed to the rapid electron-phonon thermalisation in the system. The second timescale is found to be due to the slow decay of hot phonons. Using a simple theoretical model we calculate the hot phonon decay rate and show that it is significantly faster in monolayer flakes than in multilayer ones. In contrast to recent claims, we show that this enhanced …
We investigate the spin-nonconserving relaxation channel of excitons by their couplings with phonons in two-dimensional transition metal dichalcogenides using $\textit{ab initio}$ approaches. Combining $\text{GW}$-Bethe-Salpeter equation method and density functional perturbation theory, we calculate the electron-phonon and exciton-phonon coupling matrix elements for the spin-flip scattering in monolayer WSe$_{\text{2}}$, and further analyze the microscopic mechanisms influencing these scattering strengths. We find that phonons could produce effective in-plane magnetic fields which flip spin o…
Electron-phonon scatterings in solid-state systems are pivotal processes in determining many key physical quantities such as charge carrier mobilities and thermal conductivities. Here, we report on the direct probing of phonon mode specific electron-phonon scatterings in layered semiconducting transition metal dichalcogenides WSe2, MoSe2, WS2, and MoS2 through inelastic electron tunneling spectroscopy measurements, quantum transport simulations, and density functional calculation. We experimentally and theoretically characterize momentum-conserving single- and two-phonon electron-phonon scatte…
Combining both vertical and in-plane two-dimensional (2D) heterostructures opens up the possibility to create an unprecedented architecture using 2D atomic layer building blocks. The thermal transport properties of such mixed heterostructures, critical to various applications in nanoelectronics, however, have not been thoroughly explored. Herein, we construct two configurations of multilayer in-plane graphene/hexagonal boron nitride (Gr/h-BN) heterostructures (i.e. mixed heterostructures) via weak van der Waals (vdW) interactions and systematically investigate the dependence of their interfaci…
