Structural origin of enhanced critical temperature in ultrafine multilayers of cuprate superconducting films

The interface layers of ${\mathrm{La}}_{2-x}$${\mathrm{Sr}}_x$${\mathrm{CuO}}_4$ (LSCO) thin films epitaxially grown on ${\mathrm{LaSrAlO}}_4$ substrates by molecular beam epitaxy were investigated using transmission electron microscopy (TEM). In single-phase LSCO film, we observed an irregular layering sequence near the interface between the film and the substrate, as well as an abundance of oxygen vacancies in ${\mathrm{CuO}}_2$ layers. A multilayer LSCO film with a high critical temperature ($T_c$ = 44.5 K) showed perfect interfaces between the sublayers. Furthermore, by combining scanning TEM, electron energy-loss spectroscopy, and electron holography, we show that there is little or no interdiffusion between the sublayers. The interfacial defects and oxygen vacancies reduce $T_c$, while the compressive strain in high-quality multilayers enhances $T_c$.

Figure 1

Temperature dependence of susceptibility, measured by the mutual inductance technique, of a multilayer LSCO film, showing a superconducting transition at 44.5 K.

Figure 2

(a), (b) Low-magnification cross-section TEM images of a single-layer LSCO sample A and a multilayer LSCO sample B, respectively. In both samples, sharp and flat top surfaces and interfaces between the films and the substrates are observed. Insert: electron diffraction shows a split peak only in the out-of-plane direction, indicating that both films are epitaxialy strained.

Figure 3

(a) A lattice image of sample A showing formation of an unusual “interface compound”; i.e., a sequence of reconstructed atomic layers that compensate for the polarization discontinuity at the interface. A possible interface model and a simulated image (red dotted square) are shown in the inset. (b), (c) The line profiles from regions in Fig. 3 enclosed in red-dotted and blue-dotted rectangles. (d) A model of ${\mathrm{CuO}}_2$ layer (top) based on the simulated image (middle) and the scanning profile with different oxygen vacancies (numbers 2–9 correspond to the oxygen occupancy from 20–90% in the atomic columns).

Figure 4

(a) A cross-section STEM image of sample B. The red-dotted square is the enlarged STEM image of the corresponding part. (b) A projected intensity profile from the boxed are in (a). (c) EEL spectra in the energy range from 780 to 890 eV obtained from the eight positions in Fig. 4.

Figure 5

(a) The electrostatic potential distribution of an ideal multilayer thin film and its derivative dI/dx. (b) The reconstructed phase image. (c) The phase shift profile of the multilayer thin film (sample B) in Fig. 5. (d) The derivative dI/dx from the boxed region in Fig. 5.