When two 2D materials are stacked with a specific twist angle, they form a heterostructure characterized by a moiré superlattice. This superlattice acts as a playground for emergent quantum phenomena, ranging from unconventional superconductivity to the formation of excitonic lattices.

Within the Cluster of Excellence on Chiral Electronics, our goal is to develop heterostructures where the twist angle can be controlled in-situ during optical probing. To achieve this, we are developing an "optical twisting microscope" -- a setup capable of precise mechanical manipulation of 2D materials at cryogenic temperatures.

Our research focuses on:

• Generating chiral polaritons on demand in 2D polaritonic materials.
• Investigating phase transitions within angle-controlled moiré lattices.
• Achieving excitonic confinement via on-demand localized field and strain manipulation.
• Studying the dynamics of twist-angle controlled moiré domains.

 

Two-dimensional materials offer a unique platform for the ultrafast control of spin and magnetization—a prerequisite for next-generation spintronic devices. Their ultrathin nature ensures that external stimuli exert a large influence on material properties compared to bulk systems. Furthermore, the strong intercoupling of different quasiparticles in these systems unlocks novel functionalities.

Within the Research Center on Ultrafast Magnetization Dynamics, we are pioneering methods to investigate magnetization dynamics in 2D materials and heterostructures. We use time-dependent electric fields and mechanical strain as ultrafast "control knobs" to manipulate these systems. Our research focuses on:

• Ultrafast transport of coupled spin and valley degrees of freedom in 2D heterostructures.
• Novel approaches combining ultrafast optical and transport probing techniques.
• Electrical and strain-based control of magnons and their transport in 2D magnetic materials.
• Floquet-type phenomena for magnons driven by periodic electric and strain fields.
• Dynamics and transport of orbital magnetization.

We aim to translate these phenomena into potential applications, including strain-tunable THz emitters based on valley pseudospin and devices such as magnon phase shifters.

Key publications:

[1] Kumar et al., Nano Letters 25, 15164 (2025).

[2] Mittenzwey et al., PRL 134, 026901 (2025).

[3] Stetzuhn et al., arXiv:2506.02185 (2025).

 

Angle-Resolved Photoelectron Spectroscopy (ARPES) is an experimental approach capable of directly resolving a material’s band structure. This powerful technique has been used to unravel correlated states, phase transitions, and excited states in myriads of materials. However, until recently, conducting ARPES measurements on functioning electrical devices was technically prohibitive.

To overcome these challenges, we collaborate with colleagues at BESSY II and RWTH Aachen University to implement a new ARPES setup capable of device-level measurements. Our role is to build device architectures capable of manipulating carrier density, electrical fields, or mechanical strain in 2D material devices compatible with ARPES measurements.

 

Heterostructures of Molecules and 2D Materials

Heterostructures of molecules and 2D materials merge the key features of 2D materials—crystalline order and delocalized excitations—with the chemical tunability, bright light emission, and reactivity inherent to molecules. Within the Research Center CRC1772, we aim to synthesize such heterostructures to identify new types of excitons and discover novel correlated states.

We explore:

• Ultrastrong electric fields generated by charge transfer between organic molecules and 2D materials.
• Widely tunable charge-transfer excitons formed by spatially separated carriers (e.g., an electron on the molecule and a hole on the 2D material).
• The realization of an excitonic insulator state accessible via charge-transfer excitons.
• The interface between nanoconfined water—a rich yet underexplored ubiquitous system—and 2D materials.

Key publications:

[1] Kovalchuk et al., Nature Comm 16, 9893 (2025).

[2] Weintrub et al., Nature Comm 13, 6601 (2022).

 

We aim to establish in-situ symmetry control of two-dimensional (2D) materials and heterostructures by manipulating mechanical strain. We develop nanomechanical platforms providing in-situ control over strain, specifically governing its direction and variation in both space and time.

We investigate "hidden" quantum states in symmetry-controlled materials:

• Rotational Symmetry: Controlled by generating uniaxial strain, this grants access to novel states—for example, valley degrees of freedom behaving as if in ultra-strong magnetic fields, or novel magnets where electron spins propagate without scattering.
• Discrete Translational Symmetry: Controlled by generating inhomogeneous strain, this unlocks excitonic transport, confinement, and superfluidity.
• Moiré Lattice Symmetry: The size and symmetry of moiré lattices in heterostructures are continuously tuned to access hidden phases, including new forms of magnetism.
• Temporal Symmetry: Manipulation reveals topologically protected magnonic states.

These states have far-reaching potential in quantum technologies, ranging from ultralow dissipation spintronics to photon emitters for quantum communication.

Key papers:

[1] Yagodkin et al., Nature Comm. 16, 10232 (2025).

[2] Kumar et al., Nature Comm 15, 7546 (2024).

[3] Harats et al., Nature Photonics 14, 324 (2020).

 

Two-dimensional materials are uniquely suited for the fabrication of Nano-Electromechanical Systems (NEMS). They combine high stiffness and strength with the ability to control mechanical resonance frequencies via externally applied forces.

We create 2D NEMS devices to explore the following:

• Phononic Crystals: We investigate patterned 2D materials as "phononic crystals"—artificial lattices that generate specific band structures for phonons. A key aspect is their electrical tunability. We explore localized defect modes in these crystals for storing classical or quantum information, as well as analogies between solid-state and on-demand phononic systems.
• Excitonic Sensing: We utilize NEMS devices as exquisite sensors capable of detecting fragile excitonic states inside 2D materials and heterostructures that may otherwise remain invisible.

Key papers:

[1] Kirchhof et al., Nano Lett 22, 8037 (2022).

[2] Kirchhof et al., Nano Lett. 21, 2174 (2021).