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

• Final Report Year: 2021

Final Report Abstract

Our analysis clearly established that scanning tunneling spectroscopy data in superconductors are affected by inelastic tunneling contributions and that the widely held view that STM measures the singleparticle spectral function is, at least in this generality, not correct. In addition, we proposed several scenarios that allow to extract the additional inelastic contribution to the STM spectrum. In most cases this is based on the comparison of spectra for the normal and superconducting state and requires some knowledge about the normal state (is it particle hole symmetric or not) and whether collective bosonic excitations change upon entering the superconducting state. This is why the project heavily relied on a close experiment-theory collaboration. We developed the theoretical framework for inelastic tunneling in superconductors based on electron-phonon and electron-electron scattering. We were then able to demonstrate that inelastic contributions are crucial for the iron-based superconductors and determined a new collective mode in the pairing glue of FeSe. In addition, wee extended the approach to cuprates and, again, managed to extract the pairing-boson of this state.

Publications

• Coupling to real and virtual phonons in tunneling spectroscopy of superconductors, Phys. Rev. B 93, 060505R (2016)
  J. Jandke, P. Hlobil, M. Schackert, W. Wulfhekel, J. Schmalian
  
• Scanning tunneling spectroscopy on SrFe2 (As1−x Px )2 , Phys. Rev. B 93, 104528 (2016)
  J. Jandke, P. Wild, M. Schackert, S. Suga, T. Kobayashi, S. Miyasaka, S. Tajima, W. Wulfhekel
  
• Tracing the Electronic Pairing Glue in Unconventional Superconductors via Inelastic Scanning Tunneling Spectroscopy, Phys. Rev. Lett. 118, 167001 (2017)
  P. Hlobil, J. Jandke, W. Wulfhekel, J. Schmalian
  
• Orbital loop currents in iron-based superconductors, Phys. Rev. B 97, 155130 (2018)
  M Klug, J. Kang, R. M. Fernandes, and J. Schmalian
  
• Unconventional pairing in single FeSe layers, Phys. Rev. B 100, 020503R (2019)
  J. Jandke, F. Yang, P. Hlobil, T. Engelhardt, D. Rau, K. Zakeri, C. Gao, J. Schmalian, W. Wulfhekel
  
• Microscopic charging and in-gap states in superconducting granular aluminum, Phys. Rev. B 102, 104502 (2020)
  F. Yang, T. Gozlinski, T. Storbeck, L. Grünhaupt, I. M. Pop, W. Wulfhekel
  
• Pairing glue in cuprate superconductors from the self-energy revealed via machine learning, Phys. Rev. B 101, 180510(R) (2020)
  A. V. Chubukov and J. Schmalian