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Proximity-induced collective modes in an unconventional superconductor heterostructure

Unconventional superconductors have been long sought for their potential applications in quantum technologies and devices. A key challenge impeding this effort is the difficulty associated with probing and characterizing candidate materials and establishing their order parameter. In this Letter, we present a platform that allows us to spectroscopically probe unconventional superconductivity in thin-layer materials via the proximity effect. We show that inducing an s-wave gap in a sample with an intrinsic d-wave instability leads to the formation of bound-states of quasiparticle pairs, which manifest as a collective mode in the d-wave channel. This finding provides a way to study the underlying pairing interactions vicariously through the collective mode spectrum of the system. Upon further cooling of the system we observe that this mode softens considerably and may even condense, signaling the onset of time-reversal symmetry breaking superconductivity. Therefore, our proposal also allows for the creation and study of these elusive unconventional states.