Intrinsic Tunneling Spectra of ${\mathrm{B}\mathrm{i}}_2({\mathrm{S}\mathrm{r}}_{2-x}{\mathrm{L}\mathrm{a}}_x){\mathrm{C}\mathrm{u}\mathrm{O}}_{6+\delta }$

We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped ${\mathrm{B}\mathrm{i}}_2({\mathrm{S}\mathrm{r}}_{2-x}{\mathrm{L}\mathrm{a}}_x){\mathrm{C}\mathrm{u}\mathrm{O}}_{6+\delta }$ ($\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_x$). Despite a difference of a factor of 3 in the optimal superconducting critical temperatures for $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.4}$ and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature $T^{*}\approx 230–300\mathrm{K}$ for both compounds. We find also that, in $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_x$, pseudogap humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.

Figure 1

A low-bias $I\mathrm{\text{−}}V$ characteristic of a $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.4}$ mesa with $N=18$ intrinsic tunnel junctions at $T=4.2\mathrm{K}$. (a) The $I\mathrm{\text{−}}V$ characteristic becomes single valued at high current. The rectangle denotes the region which is zoomed in the main panel. (b) The voltage measured at 0.3 mA for every counted branch ($n=1,\dots ,N$). The perfectly linear dependence indicates the uniformity of the junctions in the mesa.

Figure 2

Upper part: Intrinsic tunneling spectra for (a) $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.4}$ and (b) Bi2212 [6]. The voltage scale is per junction. The curves at $T>4.5\mathrm{K}$ are shifted downwards for clarity. Lower part: The spectra for the overdoped $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.2}$ (c) and underdoped $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.6}$ (d). The voltage scales refer to the whole stacks.

Figure 3

A three-dimensional plot of intrinsic tunneling spectra of $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.4}$ (left: the voltage is per junction) and $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.2}$ (right: the voltage refers to the whole stack). The experimental spectra are shown as thin lines, and the thick lines denote the spectra at $T\approx T_c$ and $T\approx T^{*}$ (the pseudogap temperature). The thick dots mark the conductivity maxima. The temperature is in a logarithmic scale.

Figure 4

The intrinsic tunneling spectra of a $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_{0.4}$ sample at temperatures around the minimum in $R_c(T)$ at $T\approx T^{*}$ (inset).

Figure 5

A pseudogap phase diagram for Bi2212 (dashed line) and $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_x$ (solid lines and dots). The straight solid line represents the best linear fit through the data points for $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_x$. $T^{*}$ and $T_c(p)$ lines for Bi2212 were adopted from Ref. 13, and $T_c(p)$ for $\mathrm{B}\mathrm{i}2201\mathrm{\text{−}}{\mathrm{L}\mathrm{a}}_x$ (from Ref. 9).