Hidden phases revealed at the surface of double-layered $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)}_2{\mathrm{O}}_7$

Double-layered $\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ has received phenomenal consideration because it exhibits a plethora of exotic phases when perturbed. New phases emerge with the application of pressure, magnetic field, or doping. Here, we show that creating a surface is an alternative and effective way to reveal hidden phases that are different from those seen in the bulk by investigating the surface properties of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)}_2{\mathrm{O}}_7$. Driven by the tilt distortion of $\mathrm{Ru}{\mathrm{O}}_6$ octahedra, the surface of $\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ is less metallic than the bulk. In contrast, because of the vanishing of tilt and enhanced rotation with Mn doping, the surface of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{R}{\mathrm{u}}_{0.84}\mathrm{M}{\mathrm{n}}_{0.16}\right)}_2{\mathrm{O}}_7$ is metallic, while the bulk is insulating. Our result demonstrates that the electronic and structural properties at the surface are intimately coupled and consistent with quasitwo-dimensional character.

Figure 1

(a) Tetragonal unit cell of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)}_2{\mathrm{O}}_7$. The cleaving plane is indicated by the dashed line. (b) Planar view of the cleaved surface with octahedral rotation, showing the $\left(\sqrt{2}\times \sqrt{2}\right)\mathrm{R}{45}^{\circ }$ unit cell (black), and the tet-$\left(1\times 1\right)$ unit cell (red). (c) STM topographic image of $x=0.06$ compound (1.2 V bias and 100 K). The different chirality of the Mn sites is indicated by blue and white arrows [34]. (d) Schematic view of octahedral tilt with the tilt angle $\left(\theta \right)$ defined by either the in-plane angle between the O3 plane and $ab$ plane $\left({\theta }_1\right)$, or the $c$-axis angle between the Ru-O2 bond and $c$ axis $\left({\theta }_2\right)$.

Figure 2

(a) Schematic LEED pattern for a surface with octahedral rotation only. The reciprocal lattices from the tet-$\left(1\times 1\right)$ and $\left(\sqrt{2}\times \sqrt{2}\right)\mathrm{R}{45}^{\circ }$ unit cells are indicated by the red dashed square and black solid square, respectively. The two dashed lines are the glide lines where the missing fractional spots (black dashed circles) locate. (b) LEED pattern of $\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ at 86 K and 225 eV. The different colored circles show the spots with different intensities. (c) Comparison of averaged LEED I-V curves from the purple and orange spots in (b). (Inset) ${\left(R_P\right)}_{\mathrm{diff}}$ values from simulated I-Vs with various tilt angles, based on $10.5^{\circ }$ octahedral rotation. The red cross shows the ${\left(R_P\right)}_{\mathrm{diff}}=0.68$ calculated from experimental I-V curves. (d) Normalized line profiles along the dashed lines in Fig. 4. The red and blue shaded areas are the intensities of Gaussian fittings, which are used to estimate the tilt angles, shown by the inset table.

Figure 3

(a) Experimental data (black) and calculated LEED $I\text{−}V$ curves (red) for fractional spots with various energy ranges. The calculated curves are obtained for the optimized structure with global minimum $R_P$ factor. (b) The $R_P$ factor at angles deviating from the optimum solution, blue for rotation and red for tilt, respectively. The dashed lines are used to estimate the error bars.

Figure 4

LEED patterns taken at the same temperature and beam energy (86 K and 225 eV) for four Mn doped samples with doping levels of (a) $x=0.00$, (b) 0.01, (c) 0.06, and (d) 0.16, respectively.

Figure 5

(a) HREELS phonon spectra for the surface of $x=0$ and 0.16 samples at 86 K. Inset shows the atomic displacements of the optical phonon mode associated with ${\omega }_3$. It is an $A_{1g}$ stretching mode of the apical oxygen atoms. (b) The 86 K HREELS data for four different doping levels, with its Fano line-shape fitting after removing the background and other phonons. The line profile becomes more asymmetric when the doping level increases.

Figure 6

(a) Surface phase diagram, comparing the Fano parameter $q$, the tilt angle, and the rotational angle as a function of Mn doping $x$. (b) A portion of the bulk phase diagram. There is a MIT during the cooling process. The insulating phase becomes more dominant as the doping percentage increases and the octahedral rotation decreases at the same time. The color bar on the right indicates the different degree of metallicity. The surface has an opposite trend in the metallicity compared to the bulk.