Zero energy states at a normal-metal/cuprate-superconductor interface probed by shot noise

We report measurements of the current noise generated in the optimally doped, $x=0.15$, Au-${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ junctions. For high transmission junctions on a (110) surface, we observed a split zero-bias conductance peak (ZBCP), accompanied by enhanced shot noise. We observed no enhanced noise neither in low-transmission junctions on a (110) surface nor in any junction on a (100) surface. We attribute the enhanced noise to Cooper pair transport through the junctions.

Figure 1

The measurement circuit. $A_1$ and $A_2$ are the cryogenic and room-temperature amplifiers, respectively. DC current is driven through the load resistor $R_L$, the left-hand side of which is effectively grounded at RF frequencies through the capacitor $C_g$. Noise source was used for calibration at each value of the temperature and the current through a junction.

Figure 2

(a) Differential conductance of a low-transmission ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ junction J1. The $G_{dc}$ is V shaped and the noise in panel (b) is fitted well with charge $e$, Eq. (1).

Figure 3

Noise generated by samples J1, J2, J3 plotted as a function current. The current scale is different for different junctions due to their different resistance, the voltage span is the same: $\pm 4$ mV.

Figure 4

(a) Differential conductance of junction J2 after discharge at 80 V. The ZBCP and minima on its sides are suppressed with heating at temperatures much smaller than the bulk $T_c$. (b) Experimental results for the noise $S$. Dashed lines are the fit $S_{\tilde{\mathrm{\Gamma }}}$ from Eq. (2) with $q=e$ and temperature-independent $\tilde{\mathrm{\Gamma }}=0.4$ for $V<0$ and 0.3 for $V>0$. Solid lines—the maximal possible single-electron generated shot noise $S_0$. Inset: low voltage blowup. The fitting parameter $\tilde{\mathrm{\Gamma }}$ is chosen to match the slope of the noise voltage dependence at large voltages.

Figure 5

(a) Differential conductance and (b) noise for junction J3 after discharge at 70 V; notations are the same as in Fig. 4. In this junction ZBCP is twice wider than in J2 and pronouncedly split. For this junction $\tilde{\mathrm{\Gamma }}$ is temperature dependent: $\tilde{\mathrm{\Gamma }}=0.4,0.4,0.2,0.15$ for $V<0$ and $0.45,0.45,0.3,0.25$ for $V>0$ at temperatures from the lowest to the highest, respectively.