Main Content

2022

D. Schmitt, J. P. Bange, W. Bennecke, A. AlMutairi, G. Meneghini, K. Watanabe, T. Taniguchi, D. Steil, D. R. Luke, R. T. Weitz, S. Steil, G. S. M. Jansen, S. Brem, E. Malic, S. Hofmann, M. Reutzel, and S. Mathias, Formation of moiré interlayer excitons in space and time, Nature 608, 499 (2022).

Moiré superlattices in atomically thin van der Waals heterostructures hold great promise for extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moiré lattices and the formation of exotic quantum phases. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. To exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. First, we elucidate that interlayer excitons are dominantly formed through femtosecond exciton–phonon scattering and subsequent charge transfer at the interlayer-hybridized Σ valleys. Second, we show that interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moiré modification. Third, we reconstruct the wavefunction distribution of the electronic part of the exciton and compare the size with the real-space moiré superlattice. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.
DOI: 10.1038/s41586-022-04977-7
M. Düvel, M. Merboldt, J. B. Bange, H. Strauch, M. Stellbrink, K. Pierz, H. W. Schumacher, D. Momeni, D. Steil, G. S. M. Jansen, S. Steil, D. Novko, S. Mathias, and M. Reutzel, Far-from-equilibrium electron-phonon interactions in optically-excited graphene, Nano Letters 22, 4897-4904 (2022).

Comprehending far-from-equilibrium many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equilibrium functions. In this work, we use time- and angle-resolved photoelectron spectroscopy to study such far-from-equilibrium many-body interactions for the prototypical material graphene. In accordance with theoretical simulations, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photoinduced nonequilibrium conditions. These observations can be understood by ultrafast scatterings between nonequilibrium electrons and strongly coupled optical phonons, which signify the crucial role of ultrafast nonequilibrium dynamics on many-body interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create non-equilibrium states of matter.
DOI: 10.1021/acs.nanolett.2c01325
A. Li, M. Reutzel, Z. Wang, D. Schmitt, M. Keunecke, W. Bennecke, G. S. M. Jansen, D. Steil, S. Steil, D. Novko, B. Gumhalter, S. Mathias, and H. Petek, Multidimensional multiphoton momentum microscopy of the anisotropic Ag(110) surface, Physical Review B 105, 075105 (2022).

We investigate the strongly anisotropic near-surface electronic band structure of Ag(110) by multidimensional momentum microscopy within the energy-momentum space of the Brillouin zone accessible to multiphoton photoemission (mPP). The momentum imaging of the near-surface band structure reveals an unexpected plethora of energy-momentum mPP features arising from surface-surface, surface-bulk, and bulk-bulk optical transitions. The nonlinear excitation enables the imaging of the surface and bulk bands over a wider energy-momentum range than is available to linear photoemission spectroscopy. The mPP spectra record the unusual in-surface-plane responses involving resonances with the known Shockley surface states of Ag(110) at the point, as well as the strongly anisotropic surface state in a minigap at the point, which mediates the plasmonic photoemission of Ag(110). In addition, image potential surface states and resonances of Ag(110) appear with different contrast that is defined by their parallel momentum-dependent projections onto the projected bands and gaps, the resonance conditions from the lower surface and bulk bands, as well as the order of the nonlinear photoemission process.
DOI: 10.1103/PhysRevB.105.075105