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Publications 2016
Inhalt ausklappen Inhalt einklappen 76. M. Selig, G. Berghaeuser, A. Raja, P. Nagler, C. Schüller, T. F. Heinz, T. Korn, A. Chernikov, E. Malic and A. Knorr, "Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides", Nature Commun. 7, 13279 (2016)76. M. Selig, G. Berghaeuser, A. Raja, P. Nagler, C. Schüller, T. F. Heinz, T. Korn, A. Chernikov, E. Malic and A. Knorr, "Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides", Nature Commun. 7, 13279 (2016)
Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures.
Nature Commun. 7, 13279 (2016)Inhalt ausklappen Inhalt einklappen 75. M. T. Mihnev, F. Kadi, C. J. Divin, T. Winzer, S. Lee, C. Liu, Z. Zhong, C. Berger, W. A. de Heer, E. Malic, A. Knorr and T. B. Norris, "Microscopic Origins of the Terahertz Carrier Relaxation and Cooling Dynamics in Graphene", Nature Commun. 7, 11617 (2016)75. M. T. Mihnev, F. Kadi, C. J. Divin, T. Winzer, S. Lee, C. Liu, Z. Zhong, C. Berger, W. A. de Heer, E. Malic, A. Knorr and T. B. Norris, "Microscopic Origins of the Terahertz Carrier Relaxation and Cooling Dynamics in Graphene", Nature Commun. 7, 11617 (2016)
The ultrafast dynamics of hot carriers in graphene are key to both understanding of fundamental carrier–carrier interactions and carrier–phonon relaxation processes in two-dimensional materials, and understanding of the physics underlying novel high-speed electronic and optoelectronic devices. Many recent experiments on hot carriers using terahertz spectroscopy and related techniques have interpreted the variety of observed signals within phenomenological frameworks, and sometimes invoke extrinsic effects such as disorder. Here, we present an integrated experimental and theoretical programme, using ultrafast time-resolved terahertz spectroscopy combined with microscopic modelling, to systematically investigate the hot-carrier dynamics in a wide array of graphene samples having varying amounts of disorder and with either high or low doping levels. The theory reproduces the observed dynamics quantitatively without the need to invoke any fitting parameters, phenomenological models or extrinsic effects such as disorder. We demonstrate that the dynamics are dominated by the combined effect of efficient carrier–carrier scattering, which maintains a thermalized carrier distribution, and carrier–optical–phonon scattering, which removes energy from the carrier liquid.
Inhalt ausklappen Inhalt einklappen 74. R. Schmidt, G. Berghaeuser, R. Schneider, M. Selig, P. Tonndorf. E. Malic, A. Knorr, S. Michaelis de Vasconcellos and R. Bratschitsch,74. R. Schmidt, G. Berghaeuser, R. Schneider, M. Selig, P. Tonndorf. E. Malic, A. Knorr, S. Michaelis de Vasconcellos and R. Bratschitsch,"Ultrafast Coulomb-induced intervalley coupling in atomically thin WS2", Nano Lett. 16, 2945 (2016)
Monolayers of semiconducting transition metal dichalcogenides hold the promise for a new paradigm in electronics by exploiting the valley degree of freedom in addition to charge and spin. For MoS2, WS2, and WSe2, valley polarization can be conveniently initialized and read out by circularly polarized light. However, the underlying microscopic processes governing valley polarization in these atomically thin equivalents of graphene are still not fully understood. Here, we present a joint experiment–theory study on the ultrafast time-resolved intervalley dynamics in monolayer WS2. Based on a microscopic theory, we reveal the many-particle mechanisms behind the observed spectral features. We show that Coulomb-induced intervalley coupling explains the immediate and prominent pump–probe signal in the unpumped valley and the seemingly low valley polarization degrees typically observed in pump–probe measurements compared to photoluminescence studies. The gained insights are also applicable to other light-emitting monolayer transition metal dichalcogenides, such as MoS2 and WSe2, where the Coulomb-induced intervalley coupling also determines the initial carrier dynamics.
Inhalt ausklappen Inhalt einklappen 73. T. Winzer, R. Jago and E. Malic, "Experimentally accessible signatures of Auger scattering in graphene", Phys. Rev. B 94, 235430 (2016)73. T. Winzer, R. Jago and E. Malic, "Experimentally accessible signatures of Auger scattering in graphene", Phys. Rev. B 94, 235430 (2016)
The gapless and linear electronic band structure of graphene opens up Auger scattering channels bridging the valence and the conduction band and changing the charge carrier density. Here, we reveal experimentally accessible signatures of Auger scattering in optically excited graphene. To be able to focus on signatures of Auger scattering, we apply a low excitation energy, weak pump fluences, and a cryostatic temperature, so that all relevant processes lie energetically below the optical phonon threshold. In this regime, carrier-phonon scattering is strongly suppressed and Coulomb processes govern the carrier dynamics. Depending on the excitation regime, we find an accumulation or depletion of the carrier occupation close to the Dirac point. This reflects well the behavior predicted from Auger-dominated carrier dynamics. Based on this observation, we propose a multicolor pump-probe experiment to uncover the extreme importance of Auger channels for the nonequilibrium dynamics in graphene.
Phys. Rev. B 94, 235430 (2016)Inhalt ausklappen Inhalt einklappen 72. J. C. Koenig-Otto, M. Mittendorff, T. Winzer, E. Malic, A. Knorr, C. Berger, W. A. de Heer, A. Pashkin, H. Schneider, M. Helm and S. Winnerl, "Slow non-collinear Coulomb scattering in the vicinity of the Dirac point in graphene", Phys. Rev. Lett. 117, 087401 (2016)72. J. C. Koenig-Otto, M. Mittendorff, T. Winzer, E. Malic, A. Knorr, C. Berger, W. A. de Heer, A. Pashkin, H. Schneider, M. Helm and S. Winnerl, "Slow non-collinear Coulomb scattering in the vicinity of the Dirac point in graphene", Phys. Rev. Lett. 117, 087401 (2016)
The Coulomb scattering dynamics in graphene in energetic proximity to the Dirac point is investigated by polarization resolved pump-probe spectroscopy and microscopic theory. Collinear Coulomb scattering rapidly thermalizes the carrier distribution in k directions pointing radially away from the Dirac point. Our study reveals, however, that, in almost intrinsic graphene, full thermalization in all directions relying on noncollinear scattering is much slower. For low photon energies, carrier-optical-phonon processes are strongly suppressed and Coulomb mediated noncollinear scattering is remarkably slow, namely on a ps time scale. This effect is very promising for infrared and THz devices based on hot carrier effects.
Phys. Rev. Lett. 117, 087401 (2016)Inhalt ausklappen Inhalt einklappen 71. E. Malic, T. Winzer, F. Wendler and A. Knorr, "Review on carrier multiplication in graphene", Phys. Status Solidi B 253, 2303 (2016)71. E. Malic, T. Winzer, F. Wendler and A. Knorr, "Review on carrier multiplication in graphene", Phys. Status Solidi B 253, 2303 (2016)
The remarkable gapless and linear band structure of graphene opens up new carrier relaxation channels bridging the valence and the conduction band. These Auger scattering processes change the number of charge carriers and can give rise to a significant multiplication of optically excited carriers in graphene. This is an ultrafast many‐particle phenomenon that is of great interest both for fundamental many‐particle physics as well as technological applications. Here, we review the research on carrier multiplication in graphene and Landau‐quantized graphene including theoretical modeling and experimental demonstration.
Phys. Status Solidi B 253, 2303 (2016)Inhalt ausklappen Inhalt einklappen 70. F. Wendler and E. Malic, "Doping-dependent intraband carrier dynamics in Landau-quantized graphene", Phys. Rev. B 93, 035432 (2016)70. F. Wendler and E. Malic, "Doping-dependent intraband carrier dynamics in Landau-quantized graphene", Phys. Rev. B 93, 035432 (2016)
We investigate the intraband carrier dynamics in Landau-quantized graphene after an optical excitation with low-energetic terahertz pulses. Based on a microscopic theory, we calculate time-dependent differential transmission spectra reflecting the Landau-level dynamics. Our calculations reveal a strong dependence on the Fermi energy EF of the graphene sample as well as on the applied magnetic field B. We find that the pump pulse can lead to both absorption bleaching and absorption enhancement depending on B and the position of EF with respect to the resonant Landau-level transition. As a result, positive and negative contributions in differential transmission spectra appear, in good agreement with recent pump-probe measurements.
Phys. Rev. B 93, 035432 (2016)Inhalt ausklappen Inhalt einklappen 69. A. Singh, G. Moody, K. Tran, M. E. Scott, V. Overbeck, G. Berghäuser, J. Schaibley, E. J. Seifert, D. Pleskot, N. M. Gabor, J. Yan, D. G. Mandrus, M. Richter, E. Malic, X. Xu, and X. Li, "Trion formation dynamics in monolayer transition metal dichalcogenides", Phys. Rev. B 93, 041401(R) (2016)69. A. Singh, G. Moody, K. Tran, M. E. Scott, V. Overbeck, G. Berghäuser, J. Schaibley, E. J. Seifert, D. Pleskot, N. M. Gabor, J. Yan, D. G. Mandrus, M. Richter, E. Malic, X. Xu, and X. Li, "Trion formation dynamics in monolayer transition metal dichalcogenides", Phys. Rev. B 93, 041401(R) (2016)
We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond time scale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ∼50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides.
Phys. Rev. B 93, 041401(R) (2016)