Scaling disparity between superconducting and pseudogap states in very low-$T_c$ Bi-2201 cuprates

Interplay between the normal state pseudogap (PG) and superconductivity in cuprates remains a controversial issue. In this respect it is instructive to compare homologous series of cuprates with a different number of CuO planes. They have similar Fermi energies, resistivities and anisotropies, but exhibit a large variation of $T_c$. Since thermal fluctuations vanish at $T=0$, they are less significant at $T\sim T_c$ in low-$T_c$ cuprates. In this work we compare intrinsic tunneling characteristics of double-layer Bi-2212 ($T_c$=95 K) and single-layer Bi-2201 with a very low $T_c \sim 4$ K. We observe that: (i) The PG characteristics of both cuprates are identical despite a large difference in $T_c$. Thus, the PG phenomenon is universal irrespective of superconducting properties. (ii) In the low-$T_c$ Bi-2201, all superconducting characteristics scale down with $T_c$ in the same proportion as for high-$T_c$ cuprates. This leads to a dramatic disparity between superconducting ($T_c =4$ K, energy gap $< 1$ meV, $H_{c2}\sim 10$ T) and pseudogap (onset $T^* = 90-300$ K, PG energy $\sim 40$ meV, PG suppression field $H^* \sim 250$ T) characteristics in the studied low-Tc cuprate. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.