Doping evolution and polar surface reconstruction of the infinite-layer cuprate ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$

We use angle-resolved photoemission spectroscopy to study the doping evolution of infinite-layer ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of the superconducting cuprate parent compounds. As carriers are added to the system, a continuous evolution from charge-transfer insulator to superconductor is observed, with the initial lower Hubbard band pinned well below the Fermi level and the development of a coherent low-energy band with electron doping. This two-component spectral function emphasizes the important role that strong local correlations play even at relatively high doping levels. Electron diffraction probes reveal a $p(2\times 2)$ surface reconstruction of the material at low doping levels. Using a number of simple assumptions, we develop a model of this reconstruction based on the polar nature of the infinite-layer structure. Finally, we provide evidence for a thickness-controlled transition in ultrathin films of ${\mathrm{SrCuO}}_2$ grown on nonpolar ${\mathrm{SrTiO}}_3$, highlighting the diverse structural changes that can occur in polar complex oxide thin films.

Figure 1

The low-energy electronic structure of ${\mathrm{Sr}}_{0.99}{\mathrm{La}}_{0.01}{\mathrm{CuO}}_2$. (a) Energy distribution curves (EDCs), offset for clarity, along a diagonal cut from $(0,0)$ to $(\pi ,\pi )$ through the LHB, which is visible as a bump at the foot of the valence band. The bold red line shows the background EDC subtracted from the data in the remaining panels in order to enhance the LHB feature. (b) Momentum-space map of spectral weight at a binding energy of 0.5 eV showing the LHB at $(\pi /2,\pi /2)$ and equivalent points. (c) EDC at $(\pi /2,\pi /2)$ after background subtraction. The peak has a Franck-Condon line shape (demonstrated schematically in the lower left inset) and can be fit to a Gaussian (red curve). The “foot” in the low-binding-energy region deviates slightly from the Gaussian and most likely reflects low-energy levels occupied by the small amount of dopants added to the sample. (d) Dispersion of the LHB as a function of momentum along cut I from $(0,0)$ to $(\pi ,\pi )$ (red points) and along perpendicular cut II from $(\pi ,0)$ to $(2\pi ,-\pi )$ (blue points), as determined by Gaussian fitting. The smooth curves show the dispersion predicted by the $t\text{−}{t}^{\prime }\text{−}{t}^{\prime \prime }\text{−}J$ model with $J=150$ meV. (e) Experimental spectrum along cut I after background subtraction. The white dots reproduce the dispersion shown in (d).

Figure 2

Momentum-space evolution of spectral intensity with doping. Constant energy spectral maps for (a) $x=0.01$, (b) $x=0.05$, and (c) $x=0.10$, integrated within $\pm 50$ meV of the specified binding energy. At the Fermi level, the insulating $x=0.01$ sample shows no spectral weight, while the $x=0.05$ sample shows an accumulation of weight at $(\pi ,0)$. By $x=0.10$, the electron pocket at $(\pi ,0)$ is well established and additional spectral weight is apparent at $(\pi /2,\pi /2)$. This weight is due to the finite integration region of the map rather than a true band crossing at the Fermi level [5]. At higher binding energies, there is clear evidence for a coexisting LHB with intensity near $(\pi /2,\pi /2)$ for all three doping levels. The circled regions at the top of (b) show shadow band reflections from spectral weight at $(\pi ,0)$ due to a $p(2\times 2)$ surface reconstruction in this sample, as discussed in Sec. 5.

Figure 3

Evolution of electronic structure with doping. Spectra along $(0,0)$ to $(\pi ,\pi )$ after background subtraction (a) for $x=0.01$, (b) for $x=0.05$, and (c) for $x=0.10$. As the doping level increases, the spectral weight of the LHB shifts to lower binding energy and gradually fills in the charge-transfer gap. At $x=0.10$, a coherent band near the Fermi level is visible. (d) Momentum-distribution curve (MDC)-derived dispersion for $x=$ 0.01, 0.05, and 0.10 (also shown as white lines in the preceding panels). The LHB maximum appears to shift away from $(\pi /2,\pi /2)$ with increased doping. The “boomerang” phenomenon is clearly visible for $x=0.10$. (e) Schematic diagram showing the qualitative form of the spectral function for $x=0.10$. Spectral weight fills in the charge-transfer gap, forming a coherent band on top of the remnant LHB. The boomerang phenomenon in MDC-derived dispersions, shown by the black dashed line, is an artifact arising from the presence of two bands.

Figure 4

Energy distribution curves. (a) Doping dependence of EDCs at $(\pi /2,\pi /2)$ for $x=$ 0.01, 0.05, and 0.10. As the doping level increases, states are filled in near the Fermi level. (b) An illustration of the two-component spectral function for $x=0.10$. The EDC is made up of a coherent low-energy band and an incoherent high-energy remnant LHB.

Figure 5

Structural change induced by the oxygen reduction step. (a) RHEED image along the [$100{\mathrm{]}}_p$ azimuth before the vacuum annealing step for an $x=0.10$ film. White arrows highlight extra diffraction streaks present in all as-grown films. (b) RHEED image after oxygen reduction for the same film. The extra RHEED streaks vanish during the annealing step. (c) LEED image of an unannealed $x=0.10$ film taken with 100 eV electrons. Circles show where Bragg peaks are located for films with the proper infinite-layer structure; some Bragg peaks are missing, while a number of incommensurate peaks are visible.

Figure 6

Evidence of surface reconstruction by electron diffraction. RHEED images along the [$100{\mathrm{]}}_p$ azimuth after the vacuum annealing step for ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ films with (a) $x=0.10$ and (b) $x=0$. The latter image shows extra diffraction streaks consistent with a doubled lattice constant. LEED images taken with 100 eV electrons for films with (c) $x=0.10$ and (d) $x=0.05$. The latter image again shows very clear evidence of a $p(2\times 2)$ surface reconstruction causing a doubling of the unit cell in both the $a$ and $b$ directions.

Figure 7

Proposed model of surface reconstruction. (a) One orientation of the surface structure composed of oxygen vacancies and consistent with a $p(2\times 2)$ reconstruction. Three other orientations related by in-plane $90{}^{\circ }$ rotations are also permitted. Upper-left and lower-right shaded squares shows the original unit cell and the doubled unit cell, respectively. (b) Layer-by-layer view showing the proposed oxygen vacancy reconstruction of the terminal ${\mathrm{CuO}}_2$ plane (highlighted in yellow). The vacancies result in a net $-1$ charge per unit cell on the topmost plane and a transfer of $-1$ charge per unit cell to the bottom of the film, avoiding the electric potential divergence associated with a polar catastrophe.

Figure 8

Fermi-surface map for a ${\mathrm{Sr}}_{0.95}{\mathrm{La}}_{0.05}{\mathrm{CuO}}_2$ film taken at 20 K showing spectral weight within $E_F\pm 50$ meV and normalized to a featureless background at high binding energy. The Fermi surface is composed of small electron pockets centered at $(\pi ,0)$ and equivalent points. Weak spectral weight visible at $(0,0)$ and $(\pi ,\pi )$ is a result of shadow band reflections (shown by yellow arrows) due to a $p(2\times 2)$ surface reconstruction in this film.

Figure 9

Oscillations of the intensity of the [10] diffraction rod (RHEED oscillations) at the start of film growth. (a) Thickness-controlled transition for a ${\mathrm{SrCuO}}_2$ film grown on nonpolar (001) ${\mathrm{SrTiO}}_3$. Deposition of the first four unit cells results in a dramatically varying RHEED intensity (highlighted in gray). Stable oscillations are observed only after depositing the fourth unit cell. (b) Stable oscillations for a film grown on (110) ${\mathrm{GdScO}}_3$. Diagrams at right illustrate individual atomic layers for a two-unit-cell-thick film, with arrows representing the growth direction.