Phases of quantum materials
We use quantum materials as a playground to study emergent phases and quantum phase transitions. This includes complex material systems, such as layered 2D materials, Van der Waals heterostructures and proximity induced states.
Common among these platforms is their small physical size, which limits the sensitivity of conventional probes at this scale. To fully access their delicate quantum phases, we use techniques developed from cQED, where small materials are embedded in the strong electromagnetic fields of microwave resonators, which can be engineered to realize different types of coupling. The material is coupled through either the magnetic field or electric field, which allows us to probe both superconducting and insulating regimes of materials as intrinsic material parameters – the compressibility or superfluid density - perturb the resonator field leading to frequency shifts of our microwave probe.
An example of a quantum phase transition is the famous Berezinskii-Kosterlitz-Thouless (BKT) transition in 2D superconductors where vortex anti-vortex pairs unbind at the critical temperature. A well-known signature of this transition is a jump in superfluid density at the critical temperature. Thus, a direct measure of superfluid density allows one to observe novel transitions such as the BKT transition even in small-scale samples and proximity structures that are challenging to access with conventional probes.
Many of these materials are even thought to undergo a transition to a superconducting phase with an unconventional pairing symmetry. Such phases are unique in that the superconducting gap has nodes (eg. p-wave, d-wave) as opposed to conventional spherically symmetric s-wave pairing (a good example of such is aluminum). The presence of gap nodes introduces - even at the lowest temperatures - a finite density of so-called ‘nodal quasiparticles’ whose thermal distribution follows a simple power law set by the nodal character. Accessing the residual unpaired electrons or alternatively the scaling of superfluid density as the system cools to near zero temperature then allows one to map out the exact gap symmetry. This is a powerful tool for us to access information about material’s pairing symmetry and speed up a search for unconventional superconductors such as spin-triplet or topologoical superconductors.