Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2

Physical Review Letters 117, 277201 (2016)

Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2

Roman Bertoni, Christopher W. Nicholson, Lutz Waldecker, Hannes Hübener, Claude Monney, Umberto De Giovannini, Michele Puppin, Moritz Hoesch, Emma Springate, Richard T. Chapman, Cephise Cacho, Martin Wolf, Angel Rubio, Ralph Ernstorfer

Manipulation of spin and valley degrees of freedom is a key step towards realizing novel quantum technologies, for which atomically thin transition metal dichalcogenides (TMDCs) have been established as promising candidates. In monolayer TMDCs, the lack of inversion symmetry gives rise to a spin-valley correlation of the band structure allowing for valley-selective electronic excitation with circularly polarized light. Here we show that, even in centrosymmetric samples of 2H-WSe2, circularly polarized light can generate spin-, valley- and layer-polarized excited states in the conduction band. Employing time- and angle-resolved photoemission spectroscopy (trARPES) with spin-selective excitation, the dynamics of valley and layer pseudospins of the excited carriers are investigated. Complementary time-dependent density functional theory (TDDFT) calculations of the excited state populations reveal a strong circular dichroism of the spin-, valley- and layer-polarizations and a pronounced 2D character of the excited states in the K valleys. We observe scattering of carriers towards the global minimum of the conduction band on a sub-100 femtosecond timescale to states with three-dimensional character facilitating inter-layer charge transfer. Our results establish the optical control of coupled spin-, valley- and layer-polarized states in centrosymmetric materials and suggest the suitability of TMDC multilayer materials for valleytronic and spintronic device concepts.

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This work was funded by the Max Planck Society, the European Research Council (ERC-2010-AdG-267374), by the Spanish grant (FIS2013-46159-C3-1-P), and Grupos Consolidados (IT578-13). Access to the Artemis Facility was funded by STFC and LaserlabEurope (EU-FP7 284464). R.B. thanks the Alexander von Humboldt Foundation for financial support. H.H. acknowledges support from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7- PEOPLE-2013-IEF project No. 622934. C.M. acknowledges support by the Swiss National Science Foundation under Grant No. PZ00P2_154867.

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