Coupling Lattice Instabilities Across the Interface in Ultrathin Oxide Heterostructures.

Oxide heterointerfaces constitute a rich platform for realizing novel functionalities in condensed matter. A key aspect is the strong link between structural and electronic properties, which can be modified by interfacing materials with distinct lattice symmetries. Here, we determine the effect of the cubic-tetragonal distortion of SrTiO3 on the electronic properties of thin films of SrIrO3, a topological crystalline metal hosting a delicate interplay between spin-orbit coupling and electronic correlations. We demonstrate that below the transition temperature at 105 K, SrIrO3 orthorhombic domains couple directly to tetragonal domains in SrTiO3. This forces the in-phase rotational axis to lie in-plane and creates a binary domain structure in the SrIrO3 film. The close proximity to the metal-insulator transition in ultrathin SrIrO3 causes the individual domains to have strongly anisotropic transport properties, driven by a reduction of bandwidth along the in-phase axis. The strong structure-property relationships in perovskites make these compounds particularly suitable for static and dynamic coupling at interfaces, providing a promising route towards realizing novel functionalities in oxide heterostructures.

Engineering matter with tailored properties is one of the main objectives in materials science. Perovskite oxides have been at the center of attention due to the combination of a flexible lattice structure and strong structure-property relationships. At heterointerfaces, structural phases and domain patterns that are not present in bulk can manifest.[1−3] Such artificial phases can have a marked effect on electronic and magnetic properties and have been shown to modify features, such as magnetic anisotropy,[4,5] interfacial ferromagnetism,[6−8] and ferroelectricity.[9] Recent years have seen an increasing amount of attention focused on the exploration of nanoscale domains, which have emerged as an abundant source of novel physical properties.[10−14] Control of such domain patterns, however, remains an open challenge. A possible way forward is to incorporate materials that undergo structural phase transitions. A canonical example is SrTiO3, a widely used material that undergoes a transition from a cubic to a tetragonal phase when lowering the temperature below 105 K. At this temperature, SrTiO3 breaks up into ferroelastic domains in which TiO6 octahedra rotate about one of three possible directions.[15] When SrTiO3 is used as a substrate for heteroepitaxial growth, the rotational distortion and resulting domain pattern can interact with the thin film due to octahedral connectivity across the interface.[16] In this context, semimetal SrIrO3 is of particular interest, since dimensionality and octahedral rotations have been shown to be pivotal in the delicate interplay between spin-orbit coupling (SOC) and electronic correlations.[17−19] Efforts to study SrIrO3 have primarily been fueled by theoretical predictions of a Dirac nodal ring, which is at the boundary between multiple topological classes, depending on the type of lattice symmetry-breaking.[20−22] In this respect, the interplay between the correlation strength and electronic bandwidth is crucial as it determines the position of the Dirac point with respect to the Fermi level.[17,21,23] The bandwidth is, among other things, governed by the Ir–O–Ir bond angle, which may be controlled through cation substitution, pressure tuning,[24] or heteroepitaxy.

Here, we demonstrate manipulation of the structural domain pattern of SrIrO3 thin films, through interaction with the tetragonal distortion in SrTiO3. We find that tetragonal domains in the substrate couple directly to orthorhombic domains in the film, forcing a binary domain structure in SrIrO3. In ultrathin films, the SrTiO3 tetragonal distortion induces a strong anisotropy in the longitudinal resistivity of SrIrO3, manifesting as a metal-to-insulator transition. Ab initio calculations on ultrathin films corroborate the anisotropic character of the domains, revealing a depletion of states at the Fermi level along one lattice axis, while along the other the system remains metallic.

The resistivity (ρ) versus temperature (T) characteristics of three SrIrO3/SrTiO3 heterostructures measured in a Hall bar (HB) geometry are shown in Fig. a. The film thicknesses were chosen to be just above the critical point for the metal–insulator transition,[18] such that the properties of the films are most sensitive to interface effects while maintaining a semimetallic ground state. At T = 105 K, ρ displays a sudden change of slope. Note that the change in resistivity of the SrIrO3 film occurs simultaneously with the structural phase-transition in the SrTiO3 substrate, indicating a strong octahedral connectivity across the interface that couples the lattice degrees of freedom of the SrTiO3 substrate to the electronic properties of the SrIrO3 film. The bulk phase diagrams and lattice structures of SrIrO3 and SrTiO3 are shown in Figure b and 1c.[25] Perovskite SrIrO3 has an orthorhombic structure (space group Pbnm) from 300 K down to low temperature, with rotation angles of typically 10° or larger about the pseudocubic lattice axes.[26,27] SrTiO3 is cubic (Pm3̅m) but transforms into a tetragonal phase (I4mcm) below 105 K, where it forms three possible domains. Its transition temperature, as well as the magnitude of the distortion can be controlled by, for example, Ca- or Ba-doping.[28−30]

Figure 1

Simultaneous structural and electronic transition. (a) ρ (T) curves of SrIrO3 films of different thicknesses, measured in a HB geometry oriented along the (100) lattice axis. (b) Bulk phase diagram of SrIrO3 and SrTiO3. Perovskite SrIrO3 is orthorhombic at all temperatures, while SrTiO3 undergoes a transition from a cubic to a tetragonal phase below 105 K. (c) Octahedral rotations and cation displacements of orthorhombic SrIrO3 viewed along the pseudocubic [001] (top) and [100] (bottom) directions.

Octahedral rotations double the perovskite unit cell, a phenomenon that gives rise to half-order Bragg peaks in X-ray diffraction measurements. The presence of specific half-order peaks is governed by symmetry,[31] and the measurement of a set of half-order peaks can be used to fully determine the rotational pattern of the film.[32] SrTiO3 is characterized by a0a0c–, that is, an out-of-phase rotation about the c-axis, which is slightly elongated.[33] Bulk SrIrO3 is denoted by a–a–c+ , having out-of-phase rotations of the same amplitude about two axes and in-phase rotations of different amplitude about the third axis.[34] To study the octahedral rotations in the SrIrO3/SrTiO3 heterostructures, we performed low temperature (4 K) synchrotron X-ray diffraction measurements. The films have thicknesses of 40, 25, and 15 u.c. and are capped by an amorphous SrTiO3 layer, preventing an additional diffraction signal from the capping layer while shielding the film from exposure to ambient conditions. Measurements of the (002) diffraction peak of these films are shown in Figure a, which demonstrate that the films are compressively strained. We first consider (h, k, l) Bragg conditions where one of the three reciprocal lattice positions is an integer and the other two are unequal half-order positions (1/2, 1, 3/2). This peak is present if the integer reciprocal lattice vector is parallel to the real-space direction of the in-phase axis.[35] As shown in Figure b, a peak is present when the integer reciprocal lattice vector is along h and k, but not along l. From this, we infer that the in-phase rotation (+) axis lies in the plane of the film, and it exhibits a mixed population of a+a–c– and a–a+c– domains, consistent with previous reports.[17,36] In the ABO3Pbnm structure, the B–B distance, along the in-phase axis is slightly shorter compared to the out-of-phase axis. Therefore, to minimize the lattice mismatch with the compressive substrate, the in-phase axis should lie in-plane. The a– axis, which experiences the largest strain, should then be oriented along the c– axis of SrTiO3 tetragonal domains, such that a–a+c– (a+a–c–) domains in the film couple to c–a0a0 (a0c–a0) domains in the substrate. This is supported by ab initio calculations (Figure c), which show (1) that forming a–a–c+ domains is energetically unfavorable due to a larger in-plane lattice parameter when the in-phase axis is oriented out-of-plane (apc = 3.9430 Å) as compared to in-plane (apc = 3.9411 Å)[26] and (2) that the energy is minimized for the aforementioned domain configuration (see section VI of the Supporting Information for further details). Different rotational domains arise depending on whether the octahedron closest to the origin rotates clockwise or counterclockwise about each axis. This is probed by the {1/2, 1/2, 3/2} series of half-order peaks, which provide the a (or b) direction along which the displacement of Sr ions occurs. Peaks are present for all reflection conditions (Figure d), indicating that the SrIrO3 film consists of two orthorhombic domains with a aligned along [100] and [010].

Figure 2

Binary domain structure. (a) XRD L-scans of SrIrO3 films of different thicknesses, measured in the vicinity of the (002) reflection of the SrTiO3 substrate. (b) Half-order peaks arising from in-phase octahedral rotations. (c) DFT calculated energy difference per formula unit for the in-phase axis (red) parallel and (blue) perpendicular to the c-axis (growth axis) as a function of lattice constant for supercells consisting of four formula units of SrTiO3 and SrIrO3. (d) Half-order peaks from different rotational domains.

Having established a coupling between the binary domain structure in the SrIrO3 film and the tetragonal domains in the SrTiO3 substrate, we turn to the question of how this interfacial domain coupling affects the electronic properties and the connection with the observed anomaly in the ρ–T curve. While in the Pbnm structure, the B–B distance along the in-phase axis is shorter compared to the out-of-phase axis, the B–O–B bond angles are slightly more tilted.[37] Accordingly, one would expect a reduction of bandwidth along the in-phase axis due to a reduced orbital overlap,[38] with anisotropic transport properties as a consequence. Figure a and 3b show the DFT-calculated electronic structure, assuming a correlation strength U = 1.47 eV, similar to previous work.[18] The out-of-phase (−) axis is oriented along Γ–X and the in-phase (+) axis along Γ–Y, with Γ the center of the primitive orthorhombic Brillouin zone. Electron wavepackets along Γ–X have a group velocity oriented purely along the out-of-phase axis and along X–S include a component along the in-phase axis, which is smaller than or equal to the component along the out-of-phase axis. Accordingly, Γ–X–S (gray region) comprises carrier transport oriented either fully or predominantly along the out-of-phase axis (and analogously for S−Γ–Y and the in-phase axis). Two electron-like pockets are present along X–S and S–Y. However, only the former intersects the Fermi level and the latter remains unoccupied. As a consequence, electronic bands along the in-phase axis are depleted at the Fermi level and the system is anticipated to favor insulating behavior along the in-phase axis but remain metallic along the out-of-phase axis. This is a remarkable scenario, where the electronic structure is finely tuned between a metallic and insulating phase by a reduction of bandwidth along the in-phase axis. In AIrO3 iridates, time-reversal symmetry protects the nodal line and thus safeguards metallic behavior. A metal–insulator transition, therefore, necessarily coincides with the onset of G-type antiferromagnetic order.[18,22,39] Our DFT calculations confirm that AFM is required to realize any type of insulating behavior, even if it is anisotropic in nature. Experimentally, we indeed observe strongly anisotropic electronic properties. Figure c shows ρ (T) measured in a HB geometry and in two patterned van der Pauw (VdP) squares with sizes of 375 and 750 μm for two electrical configurations. We directly observe that the anomaly in ρ is much more pronounced in the VdP geometry than in the Hall bar and that a strong anisotropy develops below 105 K. As shown in Figure d, the transition can be remarkably sharp and manifest as a metal-insulator transition. The derivative dρ/dT is shown in the bottom panel, which shows opposite behavior in the two electrical configurations, that is, a positive (metallic) or negative (insulating) slope depending on the orientation. Microscopically, this can be viewed as current traversing an unequal domain population in the probing region of the VdP device (see Figure e and 3f). Domains in SrTiO3 can be sized up to 100 μm (see also section IV of the Supporting Information), which suggests, in accordance with our observations, that the anisotropic character should be most pronounced in small devices and reduced in larger devices due to statistical averaging over complex domain patterns.[11,12,40] The ρ (T) anomaly at 105 K can then be ascribed to a sudden reconfiguration of the current paths as the SrIrO3 domains adapt to the onset of the tetragonal multi-domain state of the SrTiO3 substrate. We remark that at the boundaries between adjacent structural domains, the crystal unit cells are typically distorted.[12] Considering the strong structure–property relationship in iridates, it is likely that the domain walls have different electronic properties compared to the undistorted areas. However, because of the ∼45° angle with respect to the crystal lattice axes, any enhanced or suppressed conductivity would be projected equally onto the (100) and (010) directions. Hence, the devices shown in Figure are only sensitive to the domains and not to the domain walls. Probing transport in nanoscale devices oriented at 45° could elucidate their electronic properties.

Figure 3

Anisotropic electronic transport. (a) DFT-calculated band structure with the out-of-phase (−) axis along Γ–X and the in-phase (+) axis along Γ–Y. (b) Enlarged view around the Fermi energy. The inset shows the Brillouin zone of the primitive orthorhombic unit cell. (c) ρ (T) curves of a 5 u.c. film comparing (light blue) a large (750 μm) and (red) small (375 μm) VdP geometry, measured in two mutually orthogonal configurations of current and voltage probes. The dark blue curve represents the ρ (T) curve recorded in a 150 μm wide Hall bar (aspect ratio 3:1). (d) Enlarged view of ρ (T) around the cubic-to-tetragonal transition of SrTiO3 at 105 K (top) and the corresponding dρ/dT curves (bottom). (e) Optical microscope images of (left) the 375 μm VdP device and (right) c–a0a0 and a0c–a0 tetragonal domains in SrTiO3 in a 375 μm square area. (f) Illustration of current traversing a binary domain population in the probing region of the device.

To further explore the effect of the SrTiO3 tetragonal distortion on the octahedral rotations in SrIrO3, we performed temperature-dependent diffraction measurements across the transition temperature (see Figure ). By fitting the half-order Bragg peaks with a Gaussian function and comparing the areas under the curves, we quantify the octahedral rotation angles and cation displacements as a function of temperature.[41] The oxygen positions are obtained by comparing the intensities of the peaks with the calculated structure factor of the oxygen octahedra. Standard nonlinear regression is used to determine the optimal values of α and γ, defined in Figure . The determined in- and out-of-plane rotation angles α and γ, respectively, are plotted versus temperature for SrIrO3 films of different thicknesses in Figure a. The angles are found to be nearly constant over the entire temperature range and weakly dependent on the film thickness (Figure b). Figure c visualizes the low temperature lattice structure. The rotational angles are substantially reduced with respect to bulk SrIrO3. Considering that SrTiO3 has been reported to strongly suppress octahedral rotations in other oxide heterostructures,[4] we attribute this to the interaction with the SrTiO3 substrate.[42] We also find an enhancement of orthorhombicity for the thinner films, possibly pointing to larger rotational distortions in the unit cells closest to the SrTiO3/SrIrO3 interface (see also section VI of the Supporting Information). Interestingly, we do not observe a clear deviation of the SrIrO3 rotation angles across 105 K, further pointing to the reconfiguration of the multi-domain state as the underlying cause of the observed resistivity anomaly at 105 K.

Figure 4

Temperature dependence of octahedral rotations. (a) Rotation angles of the 15, 25, and 40 u.c. films as a function of temperature. (b) Temperature-averaged rotation angles as a function of film thickness. (c) Visualization of the octahedral rotation pattern as seen (from left to right) along the c–, a–, and a+ axes, respectively.

In summary, we established an interfacial coupling in ultrathin SrTiO3/SrIrO3 heterostructures and demonstrated the emergence of a binary orthorhombic domain pattern in SrIrO3 that couples directly to the tetragonal domains in the SrTiO3 substrate. For each domain, the electronic bandwidth along the in-phase rotational axis is suppressed, resulting in strongly anisotropic transport properties that manifest as a metal–insulator transition. This coupling mechanism is not limited to iridates, but can be extended to control physical properties, such as magnetism, multiferroicity, and superconductivity in a wide variety of orthorhombic materials, for example, ferrites, manganites, and nickelates.[43−49]

Experimental Methods

SrIrO3 thin films were synthesized by pulsed-laser deposition on (001) TiO2-terminated SrTiO3 substrates. The growth conditions are described in detail in previous work, including the requirement of a protective capping layer to prevent degradation of the films resulting from exposure to ambient conditions.[50] Samples measured in transport were capped by a 10 u.c. crystalline layer of SrTiO3, whereas samples measured in XRD were capped by amorphous SrTiO3, to prevent an additional contribution in diffraction. Hall bar (HB) and van der Pauw (VdP) geometries were patterned by e-beam lithography. The SrIrO3 layer was contacted by Ar etching and in situ deposition of Pd and Au, resulting in low-resistance Ohmic contacts (see also section V.A of the Supporting Information). Low temperature transport measurements were performed in an Oxford flow cryostat, by sourcing a low frequency (∼17 Hz) 10 μA current and measuring the resulting voltage with a lock-in amplifier. Details regarding the synchrotron X-ray diffraction measurements, half-order peak analysis, polarized-light microscopy measurements and ab initio calculations are described in the Supporting Information.