Tracing the Electronic Pairing Glue in Unconventional Superconductors via Inelastic Scanning Tunneling Spectroscopy

Abstract

Scanning tunneling microscopy (STM) has been shown to be a powerful experimental probe to detect electronic excitations and further allows to deduce fingerprints of bosonic collective modes in superconductors. Here, we demonstrate that the inclusion of inelastic tunnel events is crucial for the interpretation of tunneling spectra of unconventional superconductors and allows to directly probe electronic and bosonic excitations via STM. We apply the formalism to the iron based superconductor LiFeAs. With the inclusion of inelastic contributions, we find strong evidence for a non-conventional pairing mechanism, likely via magnetic excitations.PACS numbers: 74.55.+v, 74.20.Mn, 74.20.Rp, 74.70.Xa Electron tunneling spectroscopy has turned out to be an outstanding tool for the investigation of superconductors. A classical example is the determination of the electron-phonon pairing interaction in conventional superconductors [1,2]. More recently, quasi-particles interference spectroscopy managed to exploit the local resolution of scanning tunneling microscopy (STM) to obtain momentum space information [3][4][5]. Both examples are based on elastic tunneling theory [6,7] where one interprets the low-temperature conductance to be proportional to the electronic density of states (DOS), including renormalizations of the DOS that occur for example within the strong coupling Eliashberg formalism [8]. An energy dependent coupling to phonons or electronic collective modes and the details of these bosonic spectral features lead to a renormalization of the electronic DOS in form of peak-dip features above the superconducting coherence peaks [1]. Such pronounced peak-dip features have also been observed in cuprate and iron-based superconductors [9][10][11][12][13][15][16][17][18][19][20][21][22][23][24][25]. A frequent interpretation is, based on elastic tunneling theory, in terms of a coupling of electrons to a sharp spin resonance mode with frequency ω res and with momentum at the antiferromagnetic ordering vector of the material [26][27][28].In an interacting system the injection of a real electron may cause both, the creation of a fermionic quasiparticle and the excitation of (bosonic) collective modes as depicted in Fig. 1. The strength of the interaction is usually crucial for the relative weight of the low-energy quasiparticle and the cloud of excitations associated with it. The excitation of the quasiparticle corresponds to the above discussed elastic tunneling, while the creation of real collective modes during the tunneling process corresponds to inelastic tunneling.In this paper we demonstrate that such inelastic tunneling events can lead to important and observable modifications of the STM spectrum in unconventional su- perconductors. In addition to fermionic exitations that are visible via elastic tunneling, inelastic tunneling spectroscopy can be used to identify the bosonic excitations of the system. We show that the fine-structures seen in LiFeAs are predominantly due to such inelastic tunneling processes and th...

Tunneling spectroscopy of c -axis epitaxial cuprate junctions

“…[30] Depending on the relative importance of elastic and inelastic processes involving the mode, one can expect a dip (for dominant elastic corrections to the self-energy) or a peak (for inelastic tunneling contributing an additional channel for conduction) in d 2 I/dV 2 . [31] Inferring the gap from the width of the dI/dV suppression, the relevant bosonic mode energy in this case is around 65 meV. This is close to the experimentally observed out-of-plane oxygen vibrations (∼55 meV) known to couple strongly to the carriers [32,33], and B1g and half-breathing modes in LCO found by neutron scattering in this energy range [34].…”

Constraints on the total coupling strengthto bosons in the iron based superconductors

“…We have provided strong evidence for the presence of low ‐energy electron–boson interactions that affect the superconducting and normal state properties of the FeSC , and these effects can be simulated reasonably well by multi ‐band Eliashberg ‐theory (also including impurity scattering effects . We admit that we cannot rigorously prove that this is the only possible or even the best scenario to describe FeSCs at this time.…”

“…Similar estimates are obtained from several spectroscopies. For example, was obtained from scanning tunneling microscopy data while was obtained from optical data (once a realistic value of the unscreened plasma frequency is adopted).…”

Fermion–boson many-body interplay in a frustrated kagome paramagnet

“…Moreover, the electronic coupling to the bosonic mode can be described within the Eliashberg theory 20,21 with 𝛼 2 𝐹(𝜔) where 𝛼 is the coupling matrix element and 𝐹(𝜔) is the bosonic density of states, and this can be studied in the tunneling experiments. 𝛼 2 𝐹(𝜔) is intimately related to the second differentiation of the tunneling spectra [17][18][19][20][21][22] . Analyzing the measured tunneling spectra (Figs.…”