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About the Project
Atomically thin layers of various materials can be combined - almost at will - into novel artificial materials, with graphene structures being the most prominent examples. If two (or more) layers are combined at a certain twist angle between the crystal orientations, as illustrated in Fig.1a, an additional periodic modulation of the atomic lattice potential can emerge, often referred to as moiré superlattice. At specific, gate tunable electron fillings, various emergent electronic phases have…
been identified, most notably superconductivity [1,2,3,4], ferromagnetism, or Mott insulators, all driven by the Coulomb interaction between the electrons. Especially the superconducting pairing is not understood, yet, and poses significant theoretical questions, potentially related to high-temperature superconductivity. Similar electronic phases were recently discovered in twisted bilayer structures of atomically thin semiconductors, especially in transitionmetal dichalcogenides (TMDCs), investigated mostly by optical spectroscopy, with the drawback that optical excitations alone are already complicated many-particle states, while low-frequency transport experiments [5,6] are often not directly related to fundamental properties.
In this project, we exploit circuit quantum electrodynamics (cQED) techniques [7] mainly based on magnetic field resilient, high-impedance superconducting resonators in the GHz regime and in the low-photon limit [8] to probe electron-electron interactions in such layered 2D materials. We will optimize resonators we already use for qubit readout [8] to investigate twisted bilayer TMDCs, for which we already established various contacting schemes [9]. The basic concept for our purpose is illustrated in Fig. 1b: the resonator can be understood as a simple LC-circuit, with a resonance frequency 𝑓 1/2π√𝐿𝐶, to which the probed material adds an extra inductance or capacitance, resulting in a shift of the resonance frequency that can be measured with very high precision.
This method gives direct access to certain material properties, especially the quantum capacitance (and thus the density of states at the Fermi energy) of a standard material, or to the kinetic inductance of a superconductor, directly related to the Cooper pair density. In more complex interacting systems, we expect other, more exotic relations that we aim to discover and explore.
In addition, we will work on improving the resonator quality factor and readout with the im to achieve the strong coupling regime, in which the material quantum states hybridize with the photonic quantum states, which we will probe with standard low-frequency transport experiments, and with pump-probe experiments adapted from qubit experiments.
This project allows the prospective PhD student not only to delve into modern nanofabrication and cutting edge material science, but also to actively engage in fundamental physics and quantum technology topics, in the uniquely collaborative effort to go beyond the standard experiments and strategies.
References
[1] Cao et al., Unconventional superconductivity in magic-angle graphene superlattices, Nature 556, 43 (2018)
[2] Banerjee et al., “Superfluid stiffness of twisted trilayer graphene superconductors”, Nature 638, 93 (2025)
[3] Tanaka et al., “Superfluid stiffness of magic-angle twisted bilayer graphene”, Nature 638, 99 (2025)
[4] Han et al., “Signatures of chiral superconductivity in rhombohedral graphene”, Nature 643, 654 (2025)
[5] Huang, et al., Correlated insulating states at fractional fillings of the WS2/WSe2 moiré lattice, Nature Phys. 17, 715 (2021)
[6] Xia, et al., “Bandwidth-tuned Mott transition and superconductivity in moiré WSe2”, Nature 650, 585 (2026)
[7] Blais et al., “Circuit quantum electrodynamics”, Rev. Mod. Phys. 93, 025005 (2021)
[8] Ungerer et al., “Strong coupling between a microwave photon and a singlet-triplet qubit”, Nat Commun 15, 1068 (2024)
[9] Ramezani et al., “Superconducting contacts to a monolayer semiconductor”, Nano Letters 21, 5614 (2021)