Skip to main content

Imaging the Motion of Magneto-Excitons in New Emerging Materials

Periodic Reporting for period 1 - IMME-NEM (Imaging the Motion of Magneto-Excitons in New Emerging Materials)

Reporting period: 2017-10-01 to 2019-09-30

Recently discovered two-dimensional (2D)-honeycomb semiconductor materials have two inequivalent, degenerate valleys in their electronic band structure. This leads to a new “valley” degree of freedom known as pseudospin that, similar to real spin, has been proposed as an extra information carrier for new classes of electronic and optoelectronic devices. Single-layer transition-metal dichalcogenides (TMDs) are an important type of 2D material, in which, due to the strong 2D confinement and a reduced dielectric screening of the Coulomb interactions, electron-hole (e-h) correlations are extremely strong. This results in the creation of e-h pairs (excitons) that are so strongly bound that excitonic effects completely dominate the optical properties of TMDs even up to room temperature. The pronounced excitonic effects in single-layer TMDs, therefore, provide a unique opportunity to investigate strong light-matter interactions associated with valley effects exhibiting exotic behaviour. However, the key fundamental question regarding the exact excitonic band structure, the valley-exciton energy-momentum (dispersion) relationship, and the corresponding excitonic transport properties, remains open. Several theories are proposed concerning the exciton energy-momentum (dispersion) relationship. Two main scenarios can be distinguished: 1) the exciton dispersion is parabolic around K = 0, but split into two branches, each having a different curvature (mass) resulting from the difference in the effective masses of electrons and holes. 2) The exciton dispersion is linear around K = 0, characteristic for massless Dirac particles.
The main purpose of this research project was to provide a fundamental understanding of valley-exciton dispersion and, resulting transport, combining two main research objectives: (R1) to determine the exciton dispersion and the corresponding transport using momentum- and real-space imaging, and (R2) to achieve an external control of the exciton dispersion via applied magnetic fields.
Research objective R1 was addressed in the Work Package (WP) 1, consisting of three tasks designing and testing an imaging setup for the investigation of exciton dispersion in real- and momentum-space in single-layer TMDs. Task 1.1 was to assemble and install a table-top imaging setup at low temperatures (4 K and above) without magnetic field. This task was achieved in two stages: initially, I had a scientific (two weeks) research training in the group of Prof. Daniele Sanvitto at the Advanced Photonic Laboratory (Lecce, Italy). After my return, I designed, realized and tested an imaging setup in the host laboratory. Task 1.2 was to image the exciton dispersion and transport. Numerous experiments were carried out on fresh single-layers (and hetero-bilayer) TMDs samples available through on-going collaborations with Regensburg University. As proposed in the proposal, I measured the exciton diffusion at different temperatures and excitation conditions (laser energy, polarization). The exciton dispersion and the corresponding transport was determined. Furthermore, the effect of (uniaxial) strain on exciton properties was studied. The splitting in the spectral lines of the exciton emission and modification in both the linear and circular polarization properties of the emitted light was observed. Task 1.3 was to analyze the data and write publications. The data has been analyzed using the appropriate models and the relevant reports were generated. These results are published in two international journals.
Research objective R2 was addressed in the WP2, consisting of four tasks investigating the control of valley exciton dispersion using high magnetic fields. Tasks 2.1 and 2.2 was to ‘Design and build of the imaging system for dc magnetic fields’ and ‘Test the imaging system’. These tasks were achieved by re-developing and extending the setup designed in WP1 for the use in dc magnetic fields. Task 2.3 was to manipulate the exciton dispersion and transport via applied magnetic fields. Several experiments were performed on neutral and charged exciton (trion) in single-layers TMDs samples. Two main effects of magnetic field on the photoluminescence (PL) profiles of excitons were observed: (i) the extent of both exciton and trion PL emission decreases with increasing field and, (ii) the halo-like shape of the trion emission PL profiles vanishes in field. Task 2.4 was to analyse the data and write publications. The data has been analysed and the results are summarized in a paper that is now in preparation.

Main Results:
• First development of polarization-resolved optical imaging setup able to take both real- and momentum-space images at low temperatures in high fields of excitons in single-layer TMDs.
• First successful tests of optical imaging setup. The equipment can be used for other material systems as well, such as exciton-polaritons in semiconductor microcavities, black phosphorous, van der Waals heterostructures, and other semiconductor nanostructures.
• First measurements of intra- and inter-layer exciton diffusion in high fields up to 30T.
• Systematic characterization of exciton diffusion at different temperatures and excitation conditions (laser energy, polarization).
• Analysis of neutral and charged exciton photoluminescence profiles evolution as a function of increased perpendicular magnetic field.
• First observation of exciton bright exciton splitting in strained single layer TMDs.
• Observation of anomalous rotation of the linearly polarized emission of bright excitons in strained single layer TMDs under high magnetic fields.
• Observation of magneto-optical alignment of localized excitons in single-layer TMDs quantum dots.
• Observation of chiral exciton-phonon coupling in strained single-layer TMDs.
Conclusions:
This project has helped to determine the exciton dispersion and the corresponding transport in atomically flat single-layers of transition-metal dichalcogenides. In addition, the project has helped to better understand the effect of (uniaxial) strain on exciton properties (the splitting in the spectral lines of the exciton emission, the modification in both the linear and circular polarization properties, caused by a strong mixing between the exciton levels, the variation of g-factors in an applied magnetic field), and on valley coherence.

Impact of Projected Results:
The IMME-NEM project proved to significantly advance our fundamental understanding of valley-exciton dispersion and resulting transport in two-dimensional single-layer transition-metal dichalcogenides, and therefore it advances our understanding of Dirac materials. It provides a wealth of experimental data for the understanding of many-body physics.
It is expected that the project will have a great impact on the research community, as well as on existing and new users of HFML-Nijmegen. The unique combination of optical imaging with high magnetic field opens access to international scientists working on other material systems, such as exciton-polaritons in semiconductor microcavities, black phosphorous, and other semiconductor nanostructures. Furthermore, it offered the possibility to gain fundamental knowledge about the peculiar properties of excitonic particles in 2D TMDs and deepen my understanding of the emerging phenomena crucial for the use of excitons in new photonic and/or valleytronic devices.
Depiction of valley exciton dispersions in single-layer TMDs