Where superconductors and graphene meet
Mesoscopic physics offers an opportunity to study quantum mechanics in a controlled setting. At the atomic scale, systems cannot be easily engineered, whereas mesoscopic systems — such as graphene devices, single-molecule magnets, nanowire or carbon nanotube quantum dots, and superconducting weak links — can be fabricated to have well-defined quantum states upon cooling to sub-Kelvin temperatures. An EU-funded project, 'Interaction of Cooper pairs and massless Dirac fermions in suspended superconductor-graphene devices' (DIRACOOPER), set out to study the quantum states formed at a graphene-superconductor junction. Along the way, the team developed a powerful spectroscopic technique exploiting superconductivity. Despite a wide range of relevant transition energies in mesoscopic systems, the experimentalist is often restricted to probing them at frequencies below 20 GHz. At higher frequencies, it becomes exceedingly difficult and costly to propagate and detect microwaves in a cryostat. From the far infrared down to the sub-THz range, free space coupling is difficult because of the mismatch between photon wavelength and the size of single nano- or micro-structures. The team proposed and realized an on-chip Josephson-junction (JJ) based spectrometer which overcomes these difficulties and allows investigation of the electronic properties of mesoscopic systems between 2 GHz and 2 THz. Not only does the technique provide access to a frequency range outside the reach of conventional microwave and optical methods, but the spectrometer is expected to have a narrow emission linewidth comparable to that of the best sources, a high sensitivity comparable to that of the best detectors, and the ability to couple on-chip to mesoscopic systems uniformly over the entire bandwidth. The large bandwidth and on-chip coupling allows following transitions tuned by an external parameter, such as the electric field in graphene or the magnetic flux in superconducting circuits. The new technique was first applied to study Cooper pairs in superconductors. Work measuring the excitations of Andreev pairs in superconducting atomic contacts and the development and application of a second spectroscopy technique led to publications in esteemed peer-reviewed journals. Most recently, researchers have worked toward realisation of a second-generation Josephson junction spectrometer and manipulation of Andreev pairs. In a collaborative effort, they have now obtained a high-quality graphene sample for investigation using the Josephson junction spectrometer. The pioneering work has created a powerful new tool for mesoscopic physics research, shed light on Andreev excitations, and helped launch the promising career of the Marie Curie fellow.
