Electron-tunneling measurements of low-Tc single-layer Bi-2201 cuprates

The single-CuO$_2$ plane cuprate superconductor Bi$_{2+x}$Sr$_{2-y}$CuO$_{6+\delta}$ (Bi-2201) is characterized by a low critical temperature and a relatively low upper critical field. This allows a complete suppression of superconductivity even at low $T$ and opens a possibility to study the normal-state properties with a relatively low interference of thermal fluctuations. Furthermore, the understanding of $T_c$ suppression in Bi-2201 is of great significance for understanding the mechanism of high $T_c$ in other cuprates. We present intrinsic tunneling and high magnetic field (up to 65 T) transport measurements of Bi-2201 single crystals with a $T_c$ of only $\sim 4$ K. All superconducting characteristics are reduced proportional to $T_c$, but the corresponding $c$-axis pseudogap characteristics remain similar to that in high-$T_c$ Bi-2212 and Bi-2223 compounds with 20-30 times larger $T_c$. This scaling disparity reveals the different origin of superconducting and pseudogap states. We also conclude that the low $T_c$ in our Bi-2201 crystals is not caused by strong thermal fluctuations at low $T$, nor by crystal defects, but is the consequence of a weaker coupling, leading to a small Cooper pair energy.