The superconducting transition temperature ($T_{ \mathrm{ c}}$) of trilayer or quadruple-layer cuprates typically surpasses that of single-layer or bilayer systems. This observation is often interpreted within the ``composite picture", where strong proximity effect between inner CuO$_2$ planes (IPs) and outer CuO$_2$ planes (OPs) is crucial. Albeit intriguing, a straightforward scrutinization of this composite picture is still lacking. In this study, using angle-resolved photoemission spectroscopy to investigate (Cu,C)Ba$_2$Ca$_3$Cu$_4$O$_{ 11+\delta}$ (CuC-1234) with a high $T_{ \mathrm{ c}}$ of 110~K, we found that the OPs are not superconducting at the $T_{ \mathrm{ c}}$ of the material. Instead, the large pairing strength and phase coherence concurrently emerge at the underdoped IPs, suggesting that the high $T_{ \mathrm{ c}}$ is primarily driven by these underdoped IPs. Given that the $T_{ \mathrm{ c}}$ of CuC-1234 is comparable to other trilayer or quadruple-layer cuprates, our findings suggest that the conventional ``composite picture" is not universally required for achieving high $T_{ \mathrm{ c}}$. More importantly, we demonstrate that CuO$_2$ planes free of apical oxygen can support superconductivity up to 110~K even at a doping level of 0.07 holes per Cu, a level that lies deep in the underdoped regime of single- and bilayer cuprates. These findings provide new insights into the origin of high $T_{ \mathrm{ c}}$ in multilayer cuprates.
Categories: cond-mat.supr-con