Elucidating the driving force of superconductivity increase in compressed optimally doped Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+x}}$

An optimally doped cuprate Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+x\thinspace }}$is as a perfect model system to explore the mechanism of superconductivity by applying pressure as one can avoid complicated competing orders in the underdoped regime and explore pure intrinsic effects rather than secondary effects related to change in the carrier concentration. Here, by carefully examining the collected high-pressure Raman spectra at low temperatures, we have observed an enhanced two-magnon mode and connected this to the observed 10 K increase in $T_{\mathrm{c}}$ (reaching more than 100 K for the first time) in the optimally doped Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+x\thinspace }}$upon compression clearly delineating the effect of pressure-induced charge transfer that must suppress $T_{\mathrm{c}}$ for this optimally doped sample. Our finely designed experiments offer the direct and convincing evidence for identifying the magnetic fluctuations as the pairing interaction in cuprate superconductors.