Vibrational Andreev bound states in magnetic molecules

TL;DR

This paper predicts vibrational Andreev bound states in magnetic molecules on superconductors, highlighting the effects of anharmonic phonons and electron coupling near quantum phase transitions.

Contribution

It introduces a model with anharmonic vibrational modes affecting Andreev states, revealing new phonon-electron interaction phenomena in magnetic molecules.

Findings

Vibrational Andreev bound states are most visible near the Kondo transition.

Anharmonicity causes competing phonon hardening and softening effects.

Different many-body states experience distinct effective phonon potentials.

Abstract

We predict the existence of vibrational Andreev bound states in deformable magnetic molecules on superconducting surfaces. We discuss the Anderson impurity model with electron-phonon coupling to a realistic anharmonic vibrational mode that modulates the tunneling barrier and show that the vibronic features are spectroscopically most visibile near the transition point between the Kondo-screened singlet and the unscreened doublet ground state. We find competing tendencies between phonon hardening due to anharmonicity and softening due to coupling to electrons, contrary to the Anderson-Holstein model and other models with harmonic local phonon mode where the vibrational mode is always softened. In addition, we find that the singlet and doublet many-body states may experience very different effective phonon potentials.