Observation of both pair and quasiparticle tunneling in intrinsic junction stacks fabricated on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ single crystals

We have investigated the tunneling properties of intrinsic Josephson junction stacks with various numbers $N$ of Cu${\mathrm{O}}_2$ bilayers fabricated on underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ single crystals. The effect of gap suppression based primarily on nonequilibrium superconductivity can be significantly reduced with a reduction in $N$. For an $N$ as few as 40, both pair and quasiparticle tunneling characteristics are clearly observed. It is shown that a clear gap $2\Delta$ of larger than 34 meV, the substantial conductance below the gap, and the $I_c{R}_N$ product of less than 5 mV are the intrinsic features of tunneling along the $c$ axis. Numerical calculation indicates that the quasiparticle current-voltage characteristics are consistent with the $d$-wave order parameter which is still suppressed by the nonequilibrium effect in a higher current region.