Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride
English
Galambosi, S.[University of Helsinki > Department of Physics]
Wirtz, Ludger[Institute of Electronics > > > ; Microelectronics and Nanotechnology > Department ISEN, CNRS Villeneuve d'Ascq]
Soininen, J. A.[University of Helsinki > Department of Physics]
Serrano, J.[Universitat Politecnica de Catalunya > ICREA - Departamento de Fisica Aplicada]
Marini, A.[IKERBASQUE, Basque Foundation for Science, Spain / European Theoretical Spectroscopy Facility (ETSF) and Dipartimento di Fisica, Universita di Roma “Tor Vergata", via della Ricerca Scientifica 1, I-00133 Roma, Italy / Nano-Bio Spectroscopy Group, ETSF Scientific Development Centre, Universidad del Pais Vasco, CFM-CSIC-UPV/EHU-MPC and DIPC, Avenida Tolosa 72, E-20018 San Sebastian, Spain]
Watanabe, K.[National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan]
Taniguchi, T.[National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan]
Huotari, S.[University of Helsinki, Depatment of Physics / European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France]
Rubio, A.[Universidad del Pais Vasco > Nano-Bio Spectroscopy Group, ETSF Scientific Development Centre]
Hamalainen, K.[University of Helsinki > Department of Physics]
[en] The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with momentum transfer parallel to the BN layers is dominated by strongly bound, quasi-two-dimensional excitons. In contrast, excitations with momentum transfer perpendicular to the layers are influenced by weakly bound three-dimensional excitons. This striking phenomenon is revealed by a combined study using high-precision nonresonant inelastic x-ray scattering measurements supported by ab initio calculations. The results are relevant in general to layered insulating systems.