Recently, a class of materials with novel electronic properties at the surface has been predicted. The so called topological insulators show a unique electronic structure at the surface. The surface states of these materials show a linear dispersion and the spin degeneracy of the bands is lifted. In addition to that the number of fermilevel crossings at the Gamma point is always odd. The structure therefore resembles a Dirac-Cone. This band-structure is famous in graphene, but here we have the advantages of a small parallel momentum of the electrons.
The surface states originate from a band inversion due to the strong spin orbit interaction inside the material. This is different compared to other surface states, which originate from the broken translational symmetry at the surface. The link to the spin orbit interaction makes the surface states of the topological insulators also robust against most pertubations. This means for example that simple adsorbtion at the surface or defects ofthe surface structure itself do not destroy the topological surface states.
contact: Dr. Suguru Ito, Prof. Dr. Jens Güdde, Prof. Dr. Ulrich Höfer
- J. Güdde and U. Höfer
Ultrafast dynamics of photocurrents in surface states of topological insulators
Phys. Status Solidi B, 2000521 (2020). – Feature Article
- K. Sumida, M. Kakoki, J. Reimann, M. Nurmamat, S. Goto, Y. Takeda, Y. Saitoh, K. A. Kokh, O. E. Tereshchenko, J. Güdde, U. Höfer, and A. Kimura
Magnetic-impurity-induced modifications to ultrafast carrier dynamics in the ferromagnetic topological insulators Sb2-xVxTe3
New J. Phys. 21, 093006 (2019).
- C. P. Schmid, D. Peller, F. Langer, S. Schlauderer, C. Lange, T. Cocker, J. Repp, J. Reimann, J. Güdde, U. Höfer, S. W. Koch, M. Kira, and R. Huber
Terahertz lightwave electronics and valleytronics
Proc. of SPIE – Vol. 10916, 1091606 (2019).
- J. Reimann, S. Schlauderer, C. P. Schmid, F. Langer, S. Baierl, K. A. Kokh, O. E. Tereshchenko, A. Kimura, C. Lange, J. Güdde, U. Höfer, and R. Huber
Subcycle observation of lightwave-driven Dirac currents in a topological surface band
Nature 562, 396 (2018).
- K. Kuroda, J. Reimann, J. Güdde, and U. Höfer
Momentum space view of the ultrafast dynamics of surface photocurrents on topological insulators
Proc. SPIE 10102, 101020Q-1 (2017).
- K. Kuroda, J. Reimann, K. A. Kokh, O. E. Tereshchenko, A. Kimura, J. Güdde, and U. Höfer
Ultrafast energy- and momentum-resolved surface Dirac photocurrents in the topological insulator Sb2Te3
Phys. Rev. B 95, 081103(R) (2017).
- K. Kuroda, J. Reimann, J. Güdde, and U. Höfer
Generation of transient photocurrents in the topological surface state of Sb2Te3 by direct optical excitation with mid-infrared pulses
Phys. Rev. Lett. 116, 076801 (2016).
- J. Reimann, J. Güdde, K. Kuroda, E.V. Chulkov, and U. Höfer
Spectroscopy and dynamics of unoccupied electronic states of the topological insulators Sb2Te3 and Sb2Te2S,
Phys. Rev. B 90, 081106(R) (2014).
Charge-carrier dynamics in transition metal dichalcogenides
Heterostructures of transition metal dichalcogenides (TMD) represent a very fascinating model system of well-defined semiconductor interfaces. Recent advances in the isolation and manipulation of atomically thin sheets of two-dimensional crystals have ushered into a new era of basic research and technological innovation. Initiated by the first successful preparation of graphene, this field has been stimulated further by the discovery of the indirect to direct bandgap transition for single-layer TMD. In TMD monolayers, the confinement of electronic motion into two dimensions and the suppression of dielectric screening gives rise to an unusually strong Coulomb interaction resulting in excitons with giant binding energies of several hundred meV and ultrashort radiative lifetimes. As a consequence, many fascinating physical phenomena have been reported in different van-der-Waals heterostructures within the last decade. In particular, TMD monolayers provide an unique platform to investigate and control the valley degree of freedom associated with the energetically degenerate K and K’ valleys, which are formed at the extrema of the valence and conduction bands in the hexagonal Brillouin zone.
Understanding the dynamic interplay and evolution of charge, spin and valley excitations in these van-der-Waals heterostructures is therefore of fundamental scientific interest. But it is also of central importance for many potential applications of TMD materials in optoelectronics, spintronics and valleytronics. We are studying the charge-carrier dynamics in TMD monolayers and heterostructures by means of time-resolved two-photon photoemission and time-resolved second-harmonic generation.
contact: Dr. Gerson Mette, Dr. Robert Wallauer, Prof. Dr. Ulrich Höfer
- J. E. Zimmermann, Y. D. Kim, J. C. Hone, U. Höfer, G. Mette
Directional ultrafast charge transfer in a WSe2/MoSe2 heterostructure selectively probed by time-resolved SHG imaging microscopy
Nanoscale Horizons 5, 1603 (2020) DOI: 10.1039/d0nh00396d
- R. Wallauer, P. Marauhn, J. Reimann, S. Zoerb, F. Kraus, J. Güdde, M. Rohlfing, U. Höfer
Momentum-resolved observation of ultrafast interlayer charge transfer between the topmost layers of MoS2
Phys. Rev. B 102, 125417 (2020) DOI:10.1103/PhysRevB.102.125417 – Editor’s suggestion
- R. Wallauer, N. Armbrust, J. Reimann, J. Güdde, and U. Höfer
Intervalley scattering in MoS2 imaged by two-photon photoemission with a high-harmonic probe