Terahertz Fingerprint of Monolayer Wigner Crystals – Publication by B9 (Malic) in Nano Letters

The Ultrafast Quantum Dynamics group of Ermin Malic (Project B9) together with Rudolf Bratschitsch from the University of Münster revealed unexpected transport behavior of excitons in ultrathin semiconductors

Sketch of the 2D Wigner crystal with a honeycomb lattice and alternating spin polarization. The colored curves underneath the particles illustrate their wave functions. Reprinted with permission from Brem et al. Copyright 2022 American Chemical Society.

Wigner crystals are solid, crystalline phases of electrons, formed at low temperatures in order to minimize their repulsive energy. This formation is one of the most intriguing quantum phase transitions and their experimental realization remains challenging since their theoretical prediction. However, the strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields.

In this work, the group of Ermin Malic predicts that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. The density matrix formalism was used to derive the internal quantum structure and the optical conductivity of the Wigner crystal and to microscopically analyze the multipeak shape of the obtained terahertz spectrum. Moreover, a characteristic shift of the peak position as a function of charge density for different atomically thin materials was predicted and showed how the results can be generalized to an arbitrary two-dimensional system.

The results will guide future experiments toward the detection of Wigner crystallization and help to study the interaction dynamics in pure and generalized Wigner crystals in twisted bilayers.

Publication

S. Brem, E. Malic
Terahertz Fingerprint of Monolayer Wigner Crystals
Nano Lett. (2022) DOI:10.1021/acs.nanolett.1c04620

Contact

Prof. Dr. Ermin Malic
Philipps-Universität Marburg
SFB 1083 project B9
Tel.: 06421 28-22640
EMAIL

Dark exciton anti-funneling in atomically thin semiconductors – Publication by B9 (Malic) in Nature Communication

The Ultrafast Quantum Dynamics group of Ermin Malic (Project B9) together with Rudolf Bratschitsch from the University of Münster revealed unexpected transport behavior of excitons in ultrathin semiconductors

Adapted from Rosati et al. (full citation see below) licensed by CC BY 4.0.

Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by applying external electric fields. However, the optoelectronic properties of the emerging material class of atomically thin semiconductors are governed by tightly bound excitons. These are neutral Coulomb-bound electron-hole pairs and as such their propagation cannot be controlled by electrical fields. Recently, strain engineering has been introduced to manipulate the propagation of excitons in these technologically promising materials. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. Excitons have been observed to propagate towards spatial regions with the strongest strain gradient, where the energy is minimal. However, the transport of dark excitons, which govern the optoelectronic response of these materials, has remained literally in the dark up till now.

In this joint theory-experiment work, the research groups of Ermin Malic and Rudolf Bratschitsch combined spatiotemporal photoluminescence measurements with microscopic many-particle theory to track the way of excitons in time, space and energy. They found that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be traced back to the dominating role of propagating dark excitons, which possess an opposite strain-induced energy variation compared to bright excitons. The findings open new possibilities to control the transport in materials dominated by excitons.

See also the press release by Philipps-University Marburg (in German).

Publication

R. Rosati, R. Schmidt, S. Brem, R. Perea-Causín, I. Niehues, J. Kern, J.A. Preuß, R. Schneider, S.M. de Vasconcellos, R. Bratschitsch, E. Malic
Dark exciton anti-funneling in atomically thin semiconductors
Nat. Commun. 12 (2021) 7221 DOI:10.1038/s41467-021-27425-y

Contact

Prof. Dr. Ermin Malic
Philipps-Universität Marburg
SFB 1083 project B9
Tel.: 06421 28-22640
EMAIL

Publication of new SFB 1083 Image Brochure

SFB 1083 published a new image brochure introducing the projects and the principle investigators in the third funding period.

Cover of the image brochure of the third funding period. Design by Bosse&Meinhard.

In October 2021, the SFB 1083 updated its image brochure to feature the goals and the focus of the research center in the third funding period. The image brochure gives a general introduction to the research on internal interfaces and portraits the participating researchers mainly for interested students and for the general public.  The numbers on the SFB for the past two as well as the current funding period can also be found in the booklet.

The image brochure (German) can be downloaded here.

A printed version of the image brochure is available upon request.

Contact

Sonderforschungsbereich 1083
Philipps-Universität Marburg
Hans-Meerwein-Str. 6
35043 Marburg
Tel.: 06421 28-24223
EMAIL