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...

The energy and momentum selectivity of time-and angle-resolved photoemission spectroscopy is exploited to address the ultrafast dynamics of the antiferromagnetic spin density wave (SDW) transition photoexcited in epitaxial thin films of chromium. We are able to quantitatively extract the evolution of the SDW order parameter Δ through the ultrafast phase transition and show that Δ is governed by the transient temperature of the thermalized electron gas, in a mean field description. The complete destruction of SDW order on a sub-100 fs time scale is observed, much faster than for conventional charge density wave materials. Our results reveal that equilibrium concepts for phase transitions such as the order parameter may be utilized even in the strongly nonadiabatic regime of ultrafast photoexcitation. DOI: 10.1103/PhysRevLett.117.136801 In ultrafast dynamics, the transfer of energy from a short laser pulse can lead to the population of excited states [1][2][3], changes in magnetic ordering [4,5], and even electronic or structural phase transitions [6][7][8][9]. In many cases a transient increase of the electronic temperature occurs that may be tracked, for example, by time-and angle-resolved photoemission spectroscopy (trARPES) [1,10,11]. An open question is still to what extent the electronic temperature alone can be said to govern ultrafast changes, particularly for phase transitions, due to the strongly nonadiabatic nature of pump-probe experiments and the possibility of exciting nonthermal electron distributions on short time scales. Such a description is further complicated in many correlated materials such as high-T c superconductors, charge density waves (CDWs), and ferromagnets, in which lattice degrees of freedom play an important role. In the case of CDWs, the periodic motion of the atomic cores (phonons) can lead to a periodic opening and closing of the spectral gap [6,[12][13][14] independent of the temperature of the electronic system. In ultrafast demagnetization of ferromagnets, a bottle neck for the transition is the transfer of angular momentum, which proceeds through the lattice [15] meaning that a hot electron system may be necessary, but is not sufficient to drive the system from one magnetic phase to another. In contrast, the ordering in spin density waves stems directly from electronic correlations [16] and thus offers an opportunity to study the dynamics of a phase transition in which the role of the lattice is minimized. This allows a more stringent test of the role played by the electronic temperature in driving materials from one phase to another under nonequilibrium conditions.Cr famously undergoes a transition to an antiferromagnetic-SDW phase [17]. Although the SDW in Cr has been widely studied [17][18][19][20][21], very few studies of the timedomain dynamics exist [22][23][24], none of which directly addresses the electronic structure.In this Letter we report the first time-resolved ARPES investigation of the SDW transition in Cr. We directly observe the ultrafast disappearance and...

A 15 W, sub-20 fs OPCPA based on an Yb laser is used to demonstrate 6.3 eV fourth harmonic-based UV photoelectron spectroscopy with sub-60 fs time resolution and XUV high harmonic generation at 500 kHz

The transition-metal dichalcogenide tantalum disulphide (1T-TaS2) hosts a commensurate charge density wave (CCDW) at temperatures below 165 K where it also becomes insulating. The low temperature CCDW phase can be driven into a metastable "mosaic" phase by means of either laser or voltage pulses which shows a large density of CDW domain walls as well as a closing of the electronic band gap. The exact origins of this pulse-induced metallic mosaic are not yet fully understood. Here, we observe the occurrence of such a metallic mosaic phase on the surface of TaS2 without prior pulse excitation using scanning tunnelling microscopy and spectroscopy (STM/STS) over continuous areas larger than 100x100 nm2 and macroscopic areas on the millimetre scale. We attribute the appearance of the mosaic phase to the presence of surface defects which arrange into the characteristic dense domain wall network. Based on our STM measurements, we further argue how the appearance of the metallic behaviour in the mosaic phase can be explained by local stacking differences of the top two layers induced by the large number of domain walls. Thus, we provide a potential avenue to explain the origin of the pulse induced mosaic.

We present a combined angle-resolved photoemission spectroscopy and low-energy electron diffraction (LEED) study of the prominent transition metal dichalcogenide IrTe2 upon potassium (K) deposition on its surface. Pristine IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Supported by density functional theory calculations, we first show that the K atoms dope the topmost IrTe2 layer with electrons, therefore strongly decreasing the work function and shifting only the electronic surface states towards higher binding energy. We then follow the evolution of its electronic structure as a function of temperature across the charge-ordered phase transitions and observe that their critical temperatures are unchanged for K coverages of 0.13 and 0.21 monolayer. Using LEED we also confirm that the periodicity of the related stripe phases is unaffected by the K doping. We surmise that the chargeordered phase transitions of IrTe2 are robust against electron surface doping, because of its metallic nature at all temperatures, and due to the importance of structural effects in stabilizing charge order in IrTe2.