Virtual and Real Bosons in Unconventional Superconductors

• Applicants: Professor Dr. Jörg Schmalian; Professor Dr. Wulf Wulfhekel
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: from 2016 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 298697451

Superconductivity is caused by an attractive interaction between electrons that is mediated by virtual Boson exchange. In conventional superconductors, the relevant Bosons are phonons, while high temperature or unconventional superconductors often show too high transition temperatures and have sufficiently non-trivial pairing states to be caused by phonon exchange. In many of the high temperature superconductors a competition between magnetically ordered and superconducting phases exist hinting for magnetic excitations as the bosonic glue. Besides the exchange of virtual phonons, hot electrons can also inelastically scatter and create real bosons. When investigating superconductors with tunneling spectroscopy, signatures of both real and virtual bosons are contained in the tunneling spectra. For phonon mediated superconductors, typically the emission of real phonons in the superconducting state has been neglected in planar junctions and the electron-phonon coupling could be determined from the renormalization of the BCS quasi-particle density of states (McMilan method). In the normal state, renormalization of the electronic density of states can be neglected but emission of real phonons can be detected with inelastic tunneling spectroscopy. In a recent work, we showed that when tunneling spectroscopy is carried out using STM, both real (inelastic) and virtual (elastic) phonon emission have to be taken into account in the superconducting state, while only real phonons can be detected in the normal state. While this complicates the determination of the electron-phonon coupling in conventional superconductors, this finding can be very useful to determine the coupling in unconventional superconductors. Here, magnetic excitations can be detected in the normal state. In the superconducting state, both the electronic density of states is drastically modified by the gap formation and the magnetic excitation spectrum is altered as it represents low energy many body excitation of the electron gas near the Fermi surface. The key idea of the project is to disentangle the elastic and inelastic tunneling contributions in STM-spectra of unconventional superconductors and to utilize this new information to identify a natural pairing interaction that is consistent with these experiments and other existing observations.

• DFG Programme: Research Grants