Parity of the Pairing Bosons in a High-Temperature $\mathrm{Pb}\mathrm{\text{−}}{\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ Bilayer Superconductor by Angle-Resolved Photoemission Spectroscopy

We report the observation of a novel effect in the bilayer $\mathrm{Pb}\mathrm{\text{−}}{\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ (Pb-Bi2212) high-$T_c$ superconductor by means of angle-resolved photoemission with circularly polarized excitation. Different scattering rates, determined as a function of energy separately for the bonding and antibonding copper-oxygen bands, strongly imply that the dominating scattering channel is odd with respect to layer exchange within a bilayer. This is inconsistent with a phonon-mediated scattering and favors the participation of the odd collective spin excitations in the scattering mechanism in near-nodal regions of the $\mathbf{k}$ space, suggesting a magnetic nature of the pairing mediator.

Figure 1

ARPES spectra taken using the circularly polarized light of (a) positive and (b) negative helicity. (c) Their sum (color plot) and $E_F$—momentum distribution curve. (d) Their difference (color plot) and $E_F$—momentum distribution (black line). The inset in (c) shows the location in the $\mathbf{k}$ space.

Figure 2

Spectra from Fig. 1 after mutual subtraction with the corresponding weighting factor (shown in the insets) which correspond to the (a) bonding and (b) antibonding bands. Red dashed lines emphasize the difference in the energy dependence of the MDC’s maxima.

Figure 3

(a) MDC heights as functions of energy. (b) MDC widths as functions of energy. (c) Two pairs of MDC’s from Figs. 1a, 1b taken at $-31\mathrm{meV}$ (upper pair) and 8 meV (lower pair) showing changing difference in the scattering rates. (d) $(\pi ,0)$ spectra taken at two photon energies at which one of the bands is suppressed. Background is subtracted and spectra are normalized to the integrated intensity.