Fingerprints of spin-fermion pairing in cuprates

TL;DR

This paper proposes that the interaction between fermions and collective spin excitations in cuprates leaves distinct signatures in various spectroscopic measurements, providing evidence for spin-mediated pairing in high-temperature superconductors.

Contribution

It identifies specific spectral features and singularities in experimental data that serve as fingerprints of spin-fluctuation-mediated $d$-wave superconductivity in cuprates.

Findings

Singularities at frequencies Δ + Δ_s in spectral functions and density of states.

Singularities at 2Δ + Δ_s in tunneling and optical conductivities.

Detection of subleading singularities at 4Δ and 2Δ + 2Δ_s in optical measurements.

Abstract

We demonstrate that the feedback effect from bosonic excitations on fermions, which in the past allowed one to verify the phononic mechanism of a conventional, $s-$wave superconductivity, may also allow one to experimentally detect the ``fingerprints'' of the pairing mechanism in cuprates. We argue that for spin-mediated $d-$wave superconductivity, the fermionic spectral function, the density of states, the tunneling conductance through an insulating junction, and the optical conductivity are affected by the interaction with collective spin excitations, which below $T_c$ are propagating, magnon-like quasiparticles with gap $\Delta_s$. We show that the interaction with a propagating spin excitation gives rise to singularities at frequencies $\Delta + \Delta_s$ for the spectral function and the density of states, and at $2\Delta + \Delta_s$ for tunneling and optical conductivities, where $\Delta$ is the maximum value of the $d-$wave gap. We further argue that recent optical measurements also allow one to detect subleading singularities at $4\Delta$ and $2\Delta + 2\Delta_s$. We consider the experimental detection of these singularities as a strong evidence in favor of the magnetic scenario for superconductivity in cuprates.