Interfacially Controlled Transient Photoinduced Superconductivity

We report on a large transient photoinduced enhancement of the superconducting critical temperature ($\Delta T_c=23\mathrm{K}$) in epitaxial ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.7}/{\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ bilayers upon visible light illumination. The effect relaxes with a characteristic time of 100 s at low temperatures, which is 4 orders of magnitude faster than the persistent photoconductivity or persistent photoinduced superconductivity previously found in single high-$T_c$ superconducting films. This result is discussed in terms of light induced charge transfer through the interface similar to what happens in semiconductor junctions.

Figure 1

(a) Resistance curves vs temperature for a bilayer with $12\mathrm{u}.\mathrm{c}.{\mathrm{YBCO}}_{6.7}/40\mathrm{u}.\mathrm{c}.\mathrm{LCMO}$, in the dark (open symbols) with $T_c=43\mathrm{K}$, and under illumination (solid line). The change in the critical temperature is $\Delta T_c=23\mathrm{K}$. (b) Resistance curves for a single ${\mathrm{YBCO}}_{6.7}$ thin film in the dark (open symbols) and during illumination (line). The enhancement of $T_c$ under illumination is 3.6 K.

Figure 2

Superconducting transition temperature ($T_c$) as a function of YBCO thickness for samples that are fully oxygenated in the dark (open triangles), deoxygenated in the dark (open circles), and deoxygenated during illumination (solid circles). Lines are guides to the eye. $T_c$ is depressed for oxygen deficient bilayers. Notice the $T_c$ enhancement for bilayers with YBCO thickness larger than 4 u.c.

Figure 3

Time dependence of the resistance when the light is switched on ($t=0\mathrm{s}$) and off for (a) a $12\mathrm{u}.\mathrm{c}.{\mathrm{YBCO}}_{6.7}/40\mathrm{u}.\mathrm{c}.\mathrm{LCMO}$ bilayer, and (b) a 60 u.c. LCMO film, both at $T=100\mathrm{K}$. The relaxation times for the bilayer and the single LCMO film are similar and show that the effect is transient even at low (100 K) temperatures.

Figure 4

Time dependence of the resistance for a $12\mathrm{u}.\mathrm{c}.{\mathrm{YBCO}}_{6.7}/40\mathrm{u}.\mathrm{c}.\mathrm{LCMO}$ bilayer at $T=50\mathrm{K}$ signaling the transition from the metallic to the superconducting state under illumination. The effect is transient, since even at low temperatures the initial value of the resistance is recovered on a short time scale when the light is turned off.