What is the ``glue'' for high temperature superconductivity?

It has been 20 years since the high-temperature superconductivity (HTSC) was discovered in La$_{2-x} $Ba$_x$CuO$_4$, by Bednorz and M\"{u}ller. Since then, many different HTSC compounds, all containing copper-oxide planes, were synthesized and HTSC became one of the most studied problems in science. However, the mechanism of HTSC still remains unknown. Recent advancements in Angle-Resolved Photoemission Spectroscopy (ARPES) have enabled a direct probing of effects of interactions between electrons and different bosonic excitations in a system, raising the expectations that a ``pairing boson'' responsible for HTSC could finally be identified. Here, we will present the study of single-particle excitations in two different cuprates: Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, a high-temperature superconductor with $T_C=91$ K and La$_{1.875}$Ba$_{0.125}$CuO$_4$, a system with suppressed superconductivity ($T_C\le 2.5$ K) due to the spin/charge ordering. The extracted self-energies and single-particle gaps will be compared and possible coupling mechanisms will be discussed. This work was supported by the DOE under contract number DE- AC02-98CH10886.