Sensitive Microwave Spectroscopy of Van der Waals Materials with Coplanar Waveguides

Strongly correlated states in graphene and transition metal dichalcogenides give rise to electrically tunable insulating, metallic and superconducting phases, making them of great interest for fundamental research and potential applications. Probing the electronic structure of such correlated systems is essential to understand their phase transitions and isospin order. Microwave spectroscopy has proven to be a powerful technique for resolving low energy gaps and studying spin interactions in large area graphene. However, implementing exfoliated van der Waals (vdW) materials into sensitive microwave measurements presents several technological challenges, including optimizing contact resistances, impedance matching, and the small area of exfoliated flakes. Microstructured transmission lines called coplanar waveguides (CPW) allow to efficiently couple microwaves to a two-dimensional material that rests nearby. Graphene Josephson junctions embedded in such a planar microwave circuit are commonly used as microwave bolometer devices, enabling detection of microwaves at the single-photon level due to graphene's broadband absorption and low heat capacity.<br/>Generally, CPWs have to be designed and optimized for a specific purpose to minimize the attenuation (losses) in the desired frequency range. Here, we discuss design choices of CPWs that are optimized for sensitive microwave spectroscopy in two-dimensional vdW materials. We highlight different approaches and their associated technological challenges in coupling microwaves to vdW materials via CPWs. Simulations and fabrication approaches are presented along with the characterization of the finalized CPW. Our designs can be applied in various applications ranging from the investigation of strongly correlated phases and isospin physics in vdW materials using microwave spectroscopy to bolometric applications.