We report the spin-selective optical excitation of carriers in inversion-symmetric bulk samples of the transition metal dichalcogenide (TMDC) WSe 2 . Employing time-and angle-resolved photoelectron spectroscopy (trARPES) and complementary time-dependent density functional theory (TDDFT), we observe spin-, valley-, and layer-polarized excited state populations upon excitation with circularly polarized pump pulses, followed by ultrafast (< 100 fs) scattering of carriers towards the global minimum of the conduction band. TDDFT reveals the character of the conduction band, into which electrons are initially excited, to be two-dimensional and localized within individual layers, whereas at the minimum of the conduction band, states have a three-dimensional character, facilitating interlayer charge transfer. These results establish the optical control of coupled spin-, valley-, and layer-polarized states in centrosymmetric materials with locally broken symmetries and suggest the suitability of TMDC multilayer and heterostructure materials for valleytronic and spintronic device concepts. DOI: 10.1103/PhysRevLett.117.277201 Manipulation of spin and valley degrees of freedom is a key step towards realizing novel quantum technologies [1][2][3][4], for which semiconducting two-dimensional (2D) TMDCs have been established as promising candidates. In monolayer TMDCs, the lack of inversion symmetry in 2H polytypes gives rise to a spin-valley correlation of the band structure which, in combination with strong spin-orbit coupling in those containing heavy transition metals [5], lifts the energy degeneracy of electronic bands of opposite spin polarizations, allowing for valley-selective electronic excitation with circularly polarized light [1,2,[5][6][7][8]. While such an effect should be forbidden in inversion symmetric materials, recent theoretical work suggests that the absence of inversion symmetry within moieties of the unit cell locally lifts the spin degeneracy [9,10]. The lack of inversion symmetry and the presence of in-plane dipole moments within individual TMDC layers can be seen as atomic site Dresselhaus and Rashba effects and can cause a hidden spin texture in a globally inversion symmetric material [9]. This is supported by the observation of spin-polarized valence bands in 2H-WSe 2 by photoelectron spectroscopy [11] and spin-resolved ARPES [12]. Polarization-resolved photoluminescence experiments on inversion-symmetric bilayer samples [1,2,[13][14][15] have shown varying degrees of circular dichroism. This has primarily been explained by symmetry breaking induced by applied or intrinsic electric and magnetic fields.In this Letter, we demonstrate that in centrosymmetric samples of 2H-WSe 2 , it is possible to generate spin-, valley-and layer-polarized excited states in the conduction band. By employing time-and angle-resolved photoemission spectroscopy (trARPES) with circularly polarized pump pulses, we observe spin-polarized excited state populations in the K valleys, which are in addition localized to a single...
Ultrafast non-equilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photo-induced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a non-equilibrium population of excited electronic states. Utilizing femtosecond photoemission we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photo-holes in shaping the potential energy landscape, and enables a combined momentum and real space description of PIPTs, including the ultrafast formation of chemical bonds.Artists view of the excitation and formation of chemical bonds along Indium nanowires (red balls) on a Silicon(111) surface during the ultrafast photoinduced phase transition between the 8x2 and 4x1 structures. This real space view of atoms and bonds is complemented by detailed measurememets of the electronic structure of electrons in their "momentum space" exhibiting the evolution of the band stuctrue providing a complete picture of the phase transition. 3 In/Si(111) undergoes a transition from an insulating (8x2) to a metallic (4x1) structure above 130 K (27,28), r-space schematics of which are shown in Fig. 1, C and D, respectively. The bonding motif in the insulating phase (Fig. 1C) consists of distorted hexagons, while in the conducting phase the In atoms rearrange into zig-zagging chains (Fig. 1 D).The k-space band structures of the two phases calculated within the GW approximation are given below the relevant structures in Fig. 1, E and F. In contrast to the (4x1) phase which has three metallic bands (m1 -m3) that cross EF (17), the (8x2) phase is gapped at the Γ 8x2 and X 8x2 points. Upon increasing the temperature across the (8x2) to (4x1) phase transition, the states initially lying far above EF at Γ 8x2 shift down in energy and eventually cross EF, forming the metallic m1 band of the (4x1) phase. Concurrently the energy gap in the m2 and m3 bands at the X 8x2 point closes, while at the same time the bands shift apart in momentum along the kx direction (23). We note that the three metallic bands predicted from the calculation in the (4x1) phase are clearly observed in Fig. 1B. The Fermi surface of the (4x1) phase in Fig. 1G shows the momentum cut along which our data are obtained.
Time-and angle-resolved photoemission spectroscopy (trARPES) employing a 500 kHz extreme-ultraviolet (XUV) light source operating at 21.7 eV probe photon energy is reported. Based on a high-power ytterbium laser, optical parametric chirped pulse amplification (OPCPA), and ultraviolet-driven high-harmonic generation, the light source produces an isolated high-harmonic with 110 meV bandwidth and a flux of more than 10 11 photons/second on the sample. Combined with a stateof-the-art ARPES chamber, this table-top experiment allows high-repetition rate pump-probe experiments of electron dynamics in occupied and normally unoccupied (excited) states in the entire Brillouin zone and with a temporal system response function below 40 fs. function A(k,ω) and a matrix element between the initial and final state |M k if | 2 ; here k and ω denote the electron's wavevector and angular frequency, respectively. Many-body effects are encoded in the spectral function A(k,ω) and manifest themselves in renormalization of the bare electronic bands and in the observed lineshape 1 . In a trARPES experiment, the distribution I(k,ω) is collected for a series of delays (τ ) between pump and probe pulses: after perturbation, the population distribution f(k,ω,τ ) evolves towards a quasi-thermal distribution and energetically relaxes on femto-to picosecond timescales 2 . During relaxation, the concomitant many-body interactions affect the transient spectral function A(k,ω,τ ) and even the photoemission matrix elements might change, if the final state's orbital symmetry is altered 3 . trARPES accesses at once the population dynamics, the evolution of the spectral function and the evolution of matrix elements. trARPES has found increasingly successful applications in the past few decades 4-6 : among many examples, trARPES was used to study photo-induced phase transitions 7-11 and to observe electronic states above the Fermi level, unoccupied under equilibrium conditions [12][13][14][15][16] . Energy conservation in the photoemission processes imposes that a femtosecond light source for trARPES must possess a photon energy ω ph exceeding the work function Φ, which in most materials lies in the range between 4 to 6 eV. Ultraviolet femtosecond light sources are thus required for these experiments.The conservation of the electrons' in-plane momentum ( k ) in the photoemission process allows reciprocal space resolution. The advantage of a probe with high photon energy is Journal of Electron Spectroscopy and Related Phenomena 200, 15 (2015). 79 SPECS Surface Nano Analysis GmbH, product spectrometer PHOIBOS TM 150 (2013), see
Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr 3 by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26 AE 0.02 m e , 50% heavier than the bare mass m 0 ¼ 0.17 m e predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fröhlich coupling parameter α ¼ 1.81. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.