Metallic interfaces in a ${\mathrm{CaTiO}}_3$/${\mathrm{LaTiO}}_3$ superlattice

Apart from a handful of exceptions, all known complex oxide two-dimensional electron gases (2DEGs) are formed in ${\mathrm{SrTiO}}_3$-based heterostructures, and microscopic information about non-${\mathrm{SrTiO}}_3$ 2DEGs systems is scarce. Here, we report on the realization of metallic conductance in a ${\mathrm{CaTiO}}_3$-based system, ${\mathrm{CaTiO}}_3$/${\mathrm{LaTiO}}_3$ superlattices, epitaxially grown in a layer-by-layer way on a ${\mathrm{NdGaO}}_3$(110) substrate by pulsed laser deposition. The high quality of the crystal and electronic structures is characterized by in situ reflection high-energy electron diffraction, x-ray diffraction, scanning transmission electron microscopy, and x-ray photoemission spectroscopy. Electrical transport confirms the formation of metallic interfaces in the ${\mathrm{CaTiO}}_3$/${\mathrm{LaTiO}}_3$ superlattice. In addition, Hall measurements reveal that in the ${\mathrm{CaTiO}}_3$/${\mathrm{LaTiO}}_3$ superlattice the room-temperature carrier mobility is nearly three times higher than that of the ${\mathrm{CaTiO}}_3$/${\mathrm{YTiO}}_3$ superlattice, implying the importance of ${\mathrm{TiO}}_6$ octahedral tilts and rotations on the carrier mobility of a 2DEG. Since doped ${\mathrm{CaTiO}}_3$ is an A-site polar metal, our results provide a materials system for designing synthetic two-dimensional polar metals.

Figure 1

Schematic of (a) CTO/LTO and (b) CTO/YTO interfaces. The yellow lines mark the Ti-O-Ti bond angle $\alpha$, which is close to ${156}^{\circ }$, ${157}^{\circ }$, and ${144}^{\circ }$ for CTO, LTO, and YTO, respectively. (c) Phase diagram of $R{\mathrm{TiO}}_3$ ($R$ is a rare-earth element) adapted from Refs. [21, 22]. As seen, with decreasing Ti-O-Ti bond angle from La to Tm, the magnetic ground state changes from antiferromagnetic (AFM) to ferromagnetic (FM).

Figure 2

(a) Schematic of a CTO/LTO superlattice on NGO(110) substrates. (b)–(d) RHEED patterns of the NGO substrate, CTO layer, and LTO layer during growth, respectively. The white arrows indicate half-order peaks, characteristic of orthorhombic symmetry of the superlattice and substrate.

Figure 3

(a) Wide-range $2\theta -\omega$ scan of the CTO/LTO superlattice. (b) Fine scan near the (002) reflection. The triangle indicates the (002) peak of the CTO/LTO superlattice.

Figure 4

(a) Low-magnification HAADF-STEM image of the CTO/LTO superlattice on a ${\mathrm{NdGaO}}_3$ substrate. (b) Atomic resolution HAADF-STEM image near the interface between the superlattice and substrate.

Figure 5

XPS spectra of CTO/LTO superlattices. The spectrum was obtained along the direction normal to the sample surface. (a) Spectrum from 0 to 600 eV binding energy. (b) Ti $2p$ core level spectrum taken at room temperature. The Shirley background (Bg) function was applied in the fitting (Fit.). The gray dots indicate the experimental (Exp.) data.

Figure 6

Temperature-dependent sheet resistances of (a) 20LTO film and (b) CTO/LTO superlattice. (c) Temperature-dependent carrier density per interface in the CTO/LTO superlattice. (d) Carrier mobility of the CTO/LTO superlattice as a function of temperature.