Giant Proximity Effect in Cuprate Superconductors

Using an advanced molecular beam epitaxy system, we have reproducibly synthesized atomically smooth films of high-temperature superconductors and uniform trilayer junctions with virtually perfect interfaces. We found that supercurrent runs through very thick barriers. We can rule out pinholes and microshorts; this “giant proximity effect” (GPE) is intrinsic. It defies the conventional explanation; it might originate in resonant tunneling through pair states in an almost-superconducting barrier. GPE may also be significant for superconducting electronics, since thick barriers are easier to fabricate.

Figure 1

Trilayer $S{N}^{\prime }S$ devices studied in this work and their transport characteristics. Inset: The device geometry. The top and bottom HTS electrodes were made of LSCO. The barriers were made of LCO, and their thickness was varied from 13 to 200 Å. The circular mesa diameter was varied from 10 to 80 mm. Gold contacts allowed for 4-point contact transport measurements. Main panel: The current density as a function of voltage dependence (the $j\mathrm{\text{−}}V$ characteristics), at $T=6.4\mathrm{K}$, for a set of ten sandwich junctions on the same chip. The plot illustrates very good device uniformity; in the best such set, the $1\mathrm{\text{−}}\sigma$ spread in $j_c$ was merely 2.5%. In the particular set shown here, the LCO barrier was 100 Å thick.

Figure 2

The temperature dependence of critical current density of $S{N}^{\prime }S$ trilayer junctions shown in Fig. 1. (The diameter of circular mesa was $20\mu \mathrm{m}$ in this particular junction, but $j_s$ in other junctions in this set did not differ by more than a few percent.) As the temperature was raised, the $I\mathrm{\text{−}}V$ characteristics gradually changed from flux-flow–like (large junction, inhomogeneous current flow) at low temperature, to resistively shunted junctionlike (small junction, homogeneous current flow) at high temperature close to $T_c$ of the LSCO electrodes.

Figure 3

Shapiro steps induced by microwave ($v=20\mathrm{G}\mathrm{H}\mathrm{z}$) radiation in the device shown in Fig. 2, at $T=30\mathrm{K}$. (Note that the voltage scale is a hundred times smaller than in Fig. 1.) The steps occur exactly at the voltages given by $V=nhv/2e$, for $n=1,2,3,4,\dots$, as expected from a single Josephson junction. We have also observed the expected scaling of the step height with the power of microwave irradiation, and, in few cases, nearly complete modulation of $j_c$ by the applied external magnetic field, with minima that corresponded well to the nominal junction area.