Emergence of high-temperature superconductivity at the interface of two Mott insulators

Interfacial superconductivity has manifested itself in various types of heterostructures: band insulator–band insulator, band insulator–Mott insulator, and Mott insulator–metal. We report the observation of high-temperature superconductivity (HTS) in a complementary and long-expected type of heterostructures, which consists of two Mott insulators, ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (LCO) and ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (PBCO). By carefully controlling oxidization condition and selectively doping ${\mathrm{CuO}}_2$ planes with Fe atoms, which suppress superconductivity, we found that the superconductivity arises at the LCO side and is confined within no more than two unit cells (∼2.6 nm) near the interface. A phenomenon of “overcome the Fe barrier” will show up if excess oxygen is present during growth. Some possible mechanisms for the interfacial HTS have been discussed, and we attribute HTS to the redistribution of oxygen.

Figure 1

Structural characterizations. (a) X-ray diffraction of a PBCO(6 uc)/LCO(10 uc) heterostructure. For the notation of heterostructures, the right component always denotes the one next to the SLAO substrate. (b), (c) HAADF-STEM images and (d) EDS elemental mapping of a representative PBCO/LCO heterostructure. The dashed line indicates the position of the PBCO/LCO interface. The interface is abrupt and intermixing is not significant.

Figure 2

Temperature-dependent squared resistance, $R_{\mathrm{sq}}$. (a) PBCO and LCO single-phase films grown on SLAO.(b) PBCO/LCO heterostructures (with or without Fe doping. Fe:LCO and Fe:PBCO stand for ${\mathrm{La}}_2{\mathrm{Cu}}_{0.95}{\mathrm{Fe}}_{0.05}{\mathrm{O}}_4$ and ${\mathrm{PrBa}}_2{\mathrm{Cu}}_{2.95}{\mathrm{Fe}}_{0.05}{\mathrm{O}}_7$, respectively.) grown on SLAO. (a), inset: Schematic view of the measurement configuration. (b), inset: Schematic cross view of heterostructures. For comparison, a PBCO(6 uc)/SLAO(1 uc)/LCO heterostructure, in which 1-uc SLAO layer was inserted between LCO and PBCO, and a heterostructure composed of 6.6-nm ${\mathrm{BaTiO}}_3$ (BTO) and 10-uc LCO, are shown in (b).

Figure 3

Replacing one unit cell of LCO with Fe:LCO in PBCO(6 uc)/LCO(10 uc) heterostructures. (a) A sketch of the structure. The positions of LCO unit cells are as labeled. For example, $N=-3$ means the third unit cell from the interface (indicated by the dashed line). Temperature-dependent (b) normalized resistances and (c) the corresponding squared resistances of δ-doped PBCO/LCO heterostructures. The dotted line is the PBCO/LCO heterostructure shown in Fig. 2 (the thick line) for comparison.

Figure 4

Films and heterostructures grown under oxygen-excess condition. (a) PBCO and LCO single-phase films grown on SLAO. The dotted lines are the data shown in Fig. 2 for comparison. (b) PBCO(6 uc)/LCO (10 uc) and PBCO(6 uc)/Fe:LCO(n uc)/LCO[(10-n) uc] heterostructures grown on SLAO. The positions of Fe-doped LCO unit cells are denoted using the same labels as schemed in Fig. 3. The dotted line is the PBCO/LCO heterostructure shown in Fig. 2 for comparison. The gray dashed line illustrates a sample without the top PBCO layer, that is, Fe:LCO(5 uc)/LCO(5 uc), for comparison.