Abstract
Resolving the microscopic pairing mechanism and its experimental identification in unconventional superconductors is among the most vexing problems of contemporary condensed matter physics. We show that Raman spectroscopy provides an avenue towards this aim by probing the structure of the pairing interaction at play in an unconventional superconductor. As we study the spectra of the prototypical Fe-based superconductor Ba1−xKxFe2As2 for 0.22 ≤ x ≤ 0.70 in all symmetry channels, Raman spectroscopy allows us to distill the leading s-wave state. In addition, the spectra collected in the B1g symmetry channel reveal the existence of two collective modes which are indicative of the presence of two competing, yet sub-dominant, pairing tendencies of d_{x^2 - y^2} symmetry type. A comprehensive functional Renormalization Group and random-phase approximation study on this compound confirms the presence of the two sub-leading channels, and consistently matches the experimental doping dependence of the related modes. The consistency between the experimental observations and the theoretical modeling suggests that spin fluctuations play a significant role in superconducting pairing.
Highlights
In superconductors such as the cuprates, ferro-pnictides, ruthenates, or heavy-fermion systems, the pairing mechanism is believed to be unconventional and related to direct electronic interactions rather than conventional electron–phonon mediated couplings
The samples with x = 0.62 and x = 0.70 are above the doping level of x = 0.6, where EF reaches the bottom of the inner electron band and the topology of the Fermi surface changes qualitatively
There are essentially four proposals for the explanation of narrow modes close to or below the gap edge 2Δ, where we assume that the gap on a given band is nearly isotropic in BKFA in accordance with experiment:25 (i) Josephsonlike number-phase oscillations of Cooper pairs between the electronic bands are expected for a multi-band system (Leggett mode)
Summary
In superconductors such as the cuprates, ferro-pnictides, ruthenates, or heavy-fermion systems, the pairing mechanism is believed to be unconventional and related to direct electronic interactions rather than conventional electron–phonon mediated couplings. In comparison to other techniques, Raman spectroscopy (which involves inelastic scattering of light) is rather unique as it provides access to both the energy gaps of a superconductor and to bound states inside the gaps that serve as signposts marking the strength of a given pairing interaction. These bound states were predicted a long time ago by Bardasis and Schrieffer (BS) and are collective excitations that correspond to the phase oscillations of the ground state order parameter triggered by the sub-dominant (d-wave) interactions. Theoretical calculations based on spin fluctuations have even argued that d-wave could become the ground state for sufficiently strong holedoping.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
