Tunneling measurements of the dependence of the energy gap on critical temperature in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$

We have used a low-temperature, in situ cleaving, scanning tunneling microscope to measure the energy gap in a series of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ samples with different Sr doping concentrations $x$, and therefore different critical temperatures. The current-voltage characteristics vary from measurement to measurement, but consistent results can be obtained by selecting "high-quality" data. We discuss the criteria used for this selection. We find that the measured gaps scale with critical temperature with the ratio $\frac{2\Delta }{k_B}{T}_c\sim 5$.