Superconducting Pairing Mechanism of Sr$_2$RuO$_4$

Abstract

Recent ultrasound experiments unveiled two-component nature of the superconducting order parameter in Sr2RuO4 and proposed dxz + idyz and the accidentally degenerate d x 2 −y 2 + ig as possible candidates. To clarify the true pairing symmetry, we construct a phenomenological model containing all symmetry-allowed multipole fluctuations as potential pairing glues and make a systematic survey of major pairing states within the Eliashberg framework. This allows us to exclude with certainty dxz + idyz based on the quasi-two-dimensionality of the Fermi surface topology and establish d x 2 −y 2 + ig as the most probable pairing state. The latter is found to arise from the interplay of antiferromagnetic, ferromagnetic, and electric multipole fluctuations and agree well with most current experiments. Our work provides a promising basis for understanding the pairing mechanism in superconducting Sr2RuO4.

Frequency-dependent Inter-pseudospin Solutions to Superconducting Strontium Ruthenate

“…The similar nodal structures of d x 2 −y 2 and g xy(x 2 −y 2 ) could reduce the signature on specific heat, but not remove it entirely. It has been proposed theoretically that this accidental degeneracy is more consistent with ultrasound experiments than the other symmetry-protected d xz + id yz proposal [46]. Moving away from the d x 2 −y 2 state, a d xy ±is * state was proposed [47,48].…”

An emerging global picture of heavy fermion physics

“…To cover this richness, we have developed a general framework based on the Eliashberg theory [99], with realistic band structures and quantum critical pairing glues as the input (see Fig. 5a) [100,101,102,103,104]. Since the latter cannot be easily evaluated in theory, a phenomenological form of quantum critical fluctuations may be used [105],…”

“…This empirical form was first developed for cuprates and derived by expanding the spin interaction around Q [105,106,107,108,109,110,111,112]. We have extended it to other pairing interactions in realistic materials [100,101,102,103,104]. The band structures may be either obtained from first-principles calculations [113,114,115,116] or extracted from ARPES measurement.…”

“…The same framework may also be applied to other unconventional superconductors with quantum critical pairing glues. For example, in 2 RuO 4 [130], our calculations suggest that a candidate d x 2 −y 2 + ig (pseudospin) singlet pairing state [131] could naturally arise from the interplay of AFM, FM, and electric multipole fluctuations in the presence of the spin-orbit coupling [104]. The success of our phenomenological framework indicates that combining quantum critical pairing glues and realistic electronic band structures can already explain the pairing mechanism in many unconventional superconductors without necessarily resorting to other more exotic assumptions.…”