High-$T_c$ Superconductivity at the Interface between the ${\mathrm{CaCuO}}_2$ and ${\mathrm{SrTiO}}_3$ Insulating Oxides

At interfaces between complex oxides it is possible to generate electronic systems with unusual electronic properties, which are not present in the isolated oxides. One important example is the appearance of superconductivity at the interface between insulating oxides, although, until now, with very low $T_c$. We report the occurrence of high $T_c$ superconductivity in the bilayer ${\mathrm{CaCuO}}_2/{\mathrm{SrTiO}}_3$, where both the constituent oxides are insulating. In order to obtain a superconducting state, the ${\mathrm{CaCuO}}_2/{\mathrm{SrTiO}}_3$ interface must be realized between the Ca plane of ${\mathrm{CaCuO}}_2$ and the ${\mathrm{TiO}}_2$ plane of ${\mathrm{SrTiO}}_3$. Only in this case can oxygen ions be incorporated in the interface Ca plane, acting as apical oxygen for Cu and providing holes to the ${\mathrm{CuO}}_2$ planes. A detailed hole doping spatial profile can be obtained by scanning transmission electron microscopy and electron-energy-loss spectroscopy at the O $K$ edge, clearly showing that the (super)conductivity is confined to about 1–2 ${\mathrm{CaCuO}}_2$ unit cells close to the interface with ${\mathrm{SrTiO}}_3$. The results obtained for the ${\mathrm{CaCuO}}_2/{\mathrm{SrTiO}}_3$ interface can be extended to multilayered high $T_c$ cuprates, contributing to explaining the dependence of $T_c$ on the number of ${\mathrm{CuO}}_2$ planes in these systems.

Figure 1

(a) $R(T)$ for NGO/CCO/STO, NGO/CCO/CTO, NGO/STO/CCO, NGO/STO/CCO/STO, and NGO/STO/CCO/amSTO with the top amorphous STO (amSTO) layer. (b)–(d) Schematic interface structures for the bilayers NGO/CCO/STO (b), NGO/STO/CCO (c), and NGO/CCO/CTO (d). The extra oxygen ions introduced in the Ca plane at the interface in NGO/CCO/STO and NGO/CCO/CTO are shown in black together with the corresponding apical coordination with the Cu ions (dashed black line).

Figure 2

Structure of interface $\alpha$ in NGO/CCO/STO bilayer: (a) Inverted ABF images showing O columns. (b) HAADF image showing cations with Z contrast. (c) Overlay of (a) in green and (b) in red, showing all the cations in orange and the O columns in green. (d), (e), and (f) Line profiles of the inverted ABF image at the magenta (interface Ca plane), green and blue arrow, respectively. The black arrow in (d) indicates the intensity from oxygen columns in the interfacial ${\mathrm{CaO}}_x$ plane. (h) EELS elemental profiles. (g) Intensity profile of the HAADF image [taken from the dashed blue line in (b)]. (i) EELS elemental maps for Ti and Ca. Dashed cyan lines indicate the interface.

Figure 3

O $K$-edge measurements in the trilayer NGO/STO/CCO/STO: (a) O $K$ edge from the upper STO layer in (d). (b) O $K$ edge from the CCO close to interface $\alpha$. (c) CCO O $K$ edge close to interface $\beta$. (d) HAADF image of the trilayer from which the spectrum image in (e) was taken. (e) Spectrum image (raw data) showing decreasing peak $A$ amplitude in going from interface $\alpha$ to interface $\beta$ [big white arrows indicate the range of data shown in (f), small white arrows indicate the position of peak $A$ and $B$ as indicated in panel (b)]. (f) Amplitude of $A$ as a function of the distance $d$ from the $\alpha$ interface. The line is a fit with $1/d$ dependence.