Fröhlich exciton-phonon coupling. Picture: U. Wurstbauer (Uni Münster)

Essential in the development of new ultrathin information carriers is the stabilization of the excited state, which carries the information. The NIM physicists Prof Ursula Wurstbauer, Prof Alexander Holleitner and Prof Alexander Högele now found a tunable method, even at room temperature. This enables applications in so called “valleytronics”.

Important feature of semiconductor materials are their energetic properties, especially with regard to an application in optoelectronics and information technologies.

Light is used to optically generate the information within the semiconductor crystal. Irradiation with visible light excites a so called exciton, a mobile excited state that propagates like a wave. So far, in practice such excited, information carrying states of the exciton, also called valley polarization, were stable at low temperatures (100 K, liquid nitrogen) only.

The NIM physicists Professor Ursula Wurstbauer, Professor Alexander Holleitner and Professor Alexander Högele now clarified the underlying mechanism resulting in the depolarization at increasing temperatures. In parallel, they developed a tunable method to suppress that process and stabilize the valley polarization at elevated temperatures (220 K) and even at room temperature.
In Nature Communications, they present impressive results with an increased degree of valley polarization of 20 % and more.

Efficient single layers

Molybdenum disulfide (MoS₂) is a member of the emerging material class of two-dimensional transition metal dichalcogenides. Its properties include stability in the presence of oxygen, water and diluted acids as well as a high melting temperature. All making the semiconductor an ideal candidate in the development of new information technology devices.

Additionally, even single-layered MoS₂ is very sensitive to light-dependent processes. So called valley degrees of freedom, specific energy levels within the lattice structure, allow for valley-specific selective optical excitation and local increase of the charge density. Stabilization of the valley polarization is prerequisite for lossless devices and their commercial applicability.

Stabilization of excited states

Theoretically, the valley polarization should be stable after the excitation with light at room temperature, in the practice this was not the case. The physicists now could elucidate the nearly-breakdown of the polarization of excitons in semiconductor crystals is due to the coupling of excitons to specific lattice oscillations, so called phonons.

“One could picture the depolarization process, especially at elevated temperatures, as the coupling of around themselves rotating excitons and phonons,” Ursula Wurstbauer explains, “and this movement is propagating like a wave front on a water.” The coupling of the excitons to longitudinal optical phonons is called Fröhlich exciton-phonon interaction and enhances the depolarization.

“Doping of the semiconductor with electrons can longlastingly suppress that coupling,” Wurstbauer describes the basis of the new strategy to stabilize the valley polarization.

“Mechanistically, the added electrons shield the electrical field induced by lattice oscillations. Hence, the coupling of excitons is suppressed,” the physicist further explains, “Comparing the system to a water again, we fill the lake with electrons and thereby smooth the oscillations of the electric field. The scattering of a valley-polarized exciton on phonons afterexcitation with light is suppressed.”

The gained basic understanding of the depolarization mechanism enabled the scientists to develop a strategy to stabilize the valley polarization. Doping of the semiconductor with electrons makes the depolarization process tunable. Another extra of their method is the reduction of scattering due to lattice imperfections and disorder. (IA)

Publication:

Tuning the Fröhlich exciton-phonon scattering in monolayer MoS₂. Miller B, Lindlau J, Bommert M, Neumann A, Yamaguchi H, Holleitner A, Högele A, Wurstbauer U. Nature Communications 10, 807 (2019), DOI: 10.1038/s41467-019-08764-3

Contacts:

Prof Dr Ursula Wurstbauer
Dynamics at Interfaces
Westfälische Wilhelms-Universität Münster
Wilhelm-Klemm-Straße 10
48149 Münster
Germany

Phone: +49 (0)251-83 33609

E-Mail: wurstbauer(at)uni-muenster.de

Web: www.uni-muenster.de/Physik.PI/Wurstbauer/

Web: www.wsi.tum.de/views/sub_group.php

Prof Dr Alexander Holleitner
Physics Department
Walter Schottky Institute and Center for Nanotechnology and Nanomaterials
Technische Universität München
Am Coulombwall 4
85748 Garching
Germany

Phone: +49-(0)89-289 11575

E-Mail: holleitner(at)wsi.tum.de

Web: www.wsi.tum.de/views/sub_group.php

Prof Dr Alexander Högele
Nanophotonics
Fakulty of Physics
Ludwig-Maximilians-Universität München
Geschwister-Scholl-Platz 1
80539 Munich
Germany

Phone: +49-(0)89-2180 1457

E-Mail: alexander.hoegele(at)lmu.de

Web: www.nano.physik.uni-muenchen.de/nanophotonics/