Generation and Evolution of Spin-, Valley-, and Layer-Polarized Excited Carriers in Inversion-Symmetric WSe2

Abstract

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...

A First-Principles Time-Dependent Density Functional Theory Framework for Spin and Time-Resolved Angular-Resolved Photoelectron Spectroscopy in Periodic Systems

“…The reason for such agreement is related to the surface sensibility of ARPES experiments. 9 In fact, scattering prevents photoelectrons to be ejected from the lower lying layers of the material and effectively only the topmost layers at the surface contribute to the spectrum. By comparing this spectrum to the experiments reported in Refs.…”

“…Time-resolved ARPES (tARPES) unlocks the time degree of freedom to study non-equilibrium dynamic of solids at the natural time scale of electronic excitations and relaxations [5][6][7][8][9] . Resolving the spin polarization of photoelectrons with spin ARPES (sARPES) provides additional information on the spin character of the sample [10][11][12][13] .…”

Rashba and Dresselhaus Couplings in Halide Perovskites: Accomplishments and Opportunities for Spintronics and Spin–Orbitronics

“…[71][72][73] As a more accurate alternative, one would turn to timeresolved-ARPES (trARPES) experiments, in which circularly-polarized pump pulses are used to access excited states. 74 Chemical engineering of the Rashba-Dresselhaus effects and SOC is possible through metal substitution (Pb, Sn, Ge). The ARIPES technique may provide a nice solution to probe the induced changes in the conduction band.…”

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

“…The linearly polarized pump pulses create electronic excitations in the K and K' valleys of the conduction band. Time-and angle-resolved photoemission spectroscopy studies have shown that carriers scatter from the K valleys to the global minimum of the conduction band (the Σ valleys) within less than 100 fs [17,25]. Electron relaxation in multilayer WSe 2 at times larger than 100 fs is thus dominated by the phononmediated relaxation of electrons to the bottom of the conduction band within the Σ valleys.…”

“…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].…”