Perovskite-type iridates SrIrO3 and CaIrO3 are a Dirac line node semimetal protected by crystalline symmetry, providing an interesting playground to investigate electron correlation effects on topological semimetals. The effect of Sn doping was examined by growing SrIr1−xSnxO3 and CaIr1−xSnxO3 thin films epitaxially on the SrTiO3(001) substrate using pulsed laser deposition. Upon Sn doping, the semimetallic ground state switches into an insulator. As temperature is lowered, the resistivity, ρ(T), of SrIr1−xSnxO3 above a critical doping level (xc ∼ 0.1) shows a well-defined transition from the semimetal to a weakly ferromagnetic insulator at T = Tc. In contrast, ρ(T) of CaIr1−xSnxO3 with increasing x shows a rapid increase in magnitude but does not show a clear signature of metal-insulator transition in the temperature dependence. We argue that the contrasted behavior of the two closely related iridates reflects the interplay between the effects of electron correlation and disorder enhanced by Sn doping.
Recently, 5d iridium oxides with perovskite-related structures have been explored extensively as a mine for exotic quantum phases, partly because of an interplay of strong spin-orbit interaction and electron correlation of the 5d electrons.1 The strong spin-orbit coupling of ∼0.4 eV for 5d electrons, which is larger than the typical crystal field splitting of ≲0.1 eV within t2g,2 splits the t2g bands with five d electrons into the upper half-filled Jeff = 1/2 band and lower completely filled Jeff = 3/2 bands. In the two-dimensional layered perovskite Sr2IrO4, a modest on-site Coulomb U of ∼2 eV3 brings the system to a spin-orbital Mott state with the Jeff = 1/2 moments.4,5 Such a spin-orbital Mott state has been identified in many two-dimensional layered iridium oxides, where an exotic magnetism of Jeff = 1/2, particularly the Kitaev spin liquid state, has been explored.6 In the three-dimensional perovskites SrIrO3 and CaIrO3, in contrast, the Jeff = 1/2 band remains metallic marginally due to the increased bandwidth3 and forms a “correlated” topological semimetal.
SrIrO3 and CaIrO3 have a GdFeO3-type distorted perovskite structure where the rotation and the buckling of IrO6 octahedra give rise to a unit cell with the size of (ac is the lattice constant of the original cubic lattice), as shown in Fig. 1(a).7,8 The results of band calculations indicate that SrIrO3 and CaIrO3 have Dirac electron bands with a line of nodes and heavy hole bands at the Fermi level.9 The line nodes of Dirac bands are protected by the time-reversal symmetry and the gliding symmetry of the GdFeO3-type perovskite structure.10 The existence of Dirac electrons has been supported by experiments, for example, by ARPES11,12 and transport measurements.13,14 Their proximity to the spin-orbital Mott state and moderately strong electron correlations are evident, for example, from the observation of a transition from a Dirac semimetal to a magnetic insulator by decreasing the number of SrIrO3 layers in superlattice structures.15 The presence of an apparent correlation effect is a prominent feature of the two Ir perovskites when compared with many other topological semimetals which are only weakly correlated and makes them an interesting arena to explore the effect of electron correlation in topological semimetals. As the ionic radius of Ca2+(1.34 Å) is smaller than that of Sr2+(1.44 Å),16 the lattice is more distorted in CaIrO3 than in SrIrO3, which reduces the bandwidth of CaIrO3 appreciably as compared with that of SrIrO3. The effect of electron correlation should be larger in CaIrO3 than SrIrO3. Recently, it was shown that the strong electron correlation modifies the semimetallic band structures appreciably in the two three-dimensional iridium perovskites. The stronger electron correlation effects of CaIrO3 bring its Fermi level closer to the Dirac node and therefore further reduces its density of electrons and holes when compared to SrIrO3.14
A transition from the Dirac semimetal to a magnetic insulator was also discovered in bulk polycrystalline SrIrO3 by substituting Ir with Sn.17 As Sn ions are tetravalent like Ir ions in SrIrO3, Sn doping does not change the valence of Ir ions and therefore reduces the hopping path of the Ir 5d electrons in real space and, hence, the effective width of the Jeff = 1/2 band, as in the case of the superlattice structure. We note here that Sn doping should modify not only the effective bandwidth but also the degree of disorder. With this unique opportunity of controlling the electron correlations and disorder in mind, we synthesized epitaxial thin films of SrIr1−xSnxO3 and CaIr1−xSnxO3 and measured their resistivity to probe the effects of Sn doping. A transition from a semimetal to a (magnetic) insulator is observed in both SrIr1−xSnxO3 and CaIr1−xSnxO3 thin films, as in the bulk SrIr1−xSnxO3. We discovered a sharp contrast in the transition behavior between the Sr and Ca iridium perovskites: the appearance of well-defined transition from the Dirac semimetal to a magnetic insulator as a function of temperature above a critical Sn concentration of xc ∼0.1 for SrIrO3 and the continuous increase in resistivity without a clear signature of the semimetal-insulator transition for CaIrO3, which we ascribe to the interplay of the electron correlation and the disorder effect.
SrIr1−xSnxO3 and CaIr1−xSnxO3 thin films were epitaxially grown in the range of 0 ≤ x ≤ 0.2 on SrTiO3(001) substrates by the pulsed laser deposition technique using polycrystalline targets with 5% excess B (Ir/Sn) cations. The film deposition of SrIr1−xSnxO3 (CaIr1−xSnxO3) was conducted at 650 °C (750 °C) in a 100 mTorr O2 atmosphere. The typical thickness of our samples estimated from the X-ray reflectivity measurement is ∼15 nm (10 nm). Magnetization and transport measurements were conducted using a commercial SQUID magnetometer (MPMS, Quantum Design) and in a physical property measurement system (PPMS, Quantum Design). The X-ray diffraction (XRD) measurements were performed using SmartLab, Rigaku.
The results of 2θ-θ scans of XRD indicate that all the grown SrIr1−xSnxO3 and CaIr1−xSnxO3 films crystallize in the perovskite structure without any trace of impurity phase within the given resolution [Fig. 1(b)]. The full width at half maximum of the rocking curve is typically as narrow as 0.1° at the pseudocubic (001) peak, and the Laue oscillations around the Bragg peaks are clearly observed. Reciprocal space mapping (RSM) measurements reveal that the in-plane lattice of the films is locked to that of the SrTiO3 substrate [Figs. 1(c) and 1(d)]. Those observations clearly demonstrate the epitaxial growth and high crystallinity of the grown films. The out-of-plane lattice constant of the CaIr1−xSnxO3 films as a function of Sn content x, as an average, increases almost 0.7% from x = 0 to x = 0.2 which should reflect the larger ionic radius of Sn4+ (0.690 Å) than that of Ir4+ (0.625 Å)16 [Fig. 1(e)].
The film orientation was identified by RSM measurements of XRD and transition electron microscopy [Figs. 2(a)–2(c)]. We will describe the lattice orientation of films by the pseudocubic unit cell in this paper using lattice parameters and cpc = c/2, where a, b, and c denote the orthorhombic unit cell parameters of the distorted GdFeO3 structure.19 In the case of SrIr1−xSnxO3, the cpc axis (∥ orthorhombic c) lies within the substrate plane independent of Sn doping [Fig. 2(d)]. This is natural because cpc is closer to that of the SrTiO3 substrate than apc (see Table I). In contrast, apc is closer to that of SrTiO3 in CaIrO3. The cpc axis (∥ orthorhombic c) therefore aligns perpendicular to the substrate plane in the CaIrO3 case [Fig. 2(f)].20 The inclusion of minority domains with the cpc axis lying within the substrate plane as in SrIr1−xSnxO3 is observed for the high Sn content films (x ≥ 0.1) [Fig. 2(g)], which very likely reflects the expansion of cpc due to Sn doping and the resultant proximity to the lattice constant of SrTiO3.
As apc is larger than cpc, the orientation of cpc within the substrate plane can be controlled by introducing additional epitaxial strain using step edges of the vicinal substrate.21,22 We used a vicinal SrTiO3(δ01) substrate with the substrate plane 0.4° rotated from (001) toward the  direction for the growth of SrIr1−xSnxO3. Because of the epitaxial strain from the side (100) plane at the step edges, the cpc (∥ orthorhombic c) axis prefers to align along the edge, namely, STO direction [Fig. 2(e)]. The RSM measurements clearly indicate that more than 95% of domains have the cpc axis parallel to the substrate  direction for the films on the vicinal substrates. Consistent with the in-plane preferred orientation, a clear anisotropy of magnetization within the substrate plane was observed for the x = 0.2 sample on the vicinal substrate as we describe below. Pronounced anisotropy in the magnetization was not observed in the resistivity ρ(T).
The resistivity ρ(T) measurements on the SrIr1−xSnxO3 films indicate the presence of a metal-insulator transition accompanied by a weak ferromagnetism as in the bulk.17 The SrIrO3 (x = 0) film shows only weakly temperature-dependent behavior of ρ(T), where a gradual increase followed by the temperature-independent behavior is observed with decreasing temperature [Fig. 3(a)]. This agrees well with previous reports on SrIrO3 thin films15,23 and can be understood as a typical behavior of semimetals with extra conductivity at a high temperature from thermally excited electrons and holes. With Sn doping [Fig. 3(b)], we do not observe an appreciable change in ρ(T) up to the critical concentration xc = 0.1. Above xc = 0.1, however, the resistivity shows a transition from the semimetal to a weak insulator at a transition temperature Tc where we observe a kink in ρ(T) and a well-defined peak of the second derivative d2ρ(T)/dT2 [Fig. 3(c)]. Tc increases rapidly with increasing x. The metal-insulator transition below Tc is accompanied by a weak ferromagnetism with the easy axis parallel to the pseudocubic cpc axis (∥ orthorhombic c), as seen in Fig. 3(d). This behavior, the emergence of the magnetic insulator out of the Dirac node semimetal with Sn doping, can be summarized as a Sn content x-T phase diagram on top of the contour map of the magnitude of ρ(T) [Fig. 3(e)]. The stabilization of the magnetic insulator phase at low temperatures above xc = 0.1 highly likely originates from the increase in the effective electron correlation. We argue that this is because of the reduced hopping of Jeff = 1/2 electrons by the introduction of Sn4+ without conduction electrons.
We find contrasting behavior in the Sn doping effect on ρ(T) in CaIr1−xSnxO3 compared to SrIr1−xSnxO3. For CaIrO3 (x = 0) films [Fig. 3(a)], the overall behavior of ρ(T) is similar to that observed in SrIrO3. The magnitude of resistivity is, however, appreciably larger than that of SrIrO3. A weak increase in ρ(T), reminiscent of a weak localization and not observed in SrIrO3, is seen below 20 K, which is suggestive of the presence of an appreciable disorder effect. It was discussed in the recent transport study on single crystal CaIrO3 that the Fermi level is much closer to the Dirac nodes, and hence, the electron and the hole densities are lower in CaIrO3 than in SrIrO3 due to the enhanced correlation effect originating from the narrow band in CaIrO3.14 The effect of disorder should be enhanced in CaIrO3 because of the reduced carrier density, which may account for the larger resistivity and the weakly localized behavior.
With Sn substitution, ρ(T) gradually increases and shows a poorly insulating behavior with a power-law divergence [Figs. 4(a) and 4(b)]. We do not see a well-defined semimetal-insulator transition as a function of T and x in contrast to the case for SrIrO3. We argue that the gradual transition from the Dirac node semimetal to a weak insulator is driven by the disorder and perhaps the inhomogeneity and that the nature of semimetal-insulator transition is distinct from that of SrIrO3. It is natural that the effect of disorder associated with Sn-doping is much more profound in CaIrO3 than in SrIrO3 because of the lower carrier density and Fermi energy of CaIrO3. For CaIr0.8Sn0.2O3, a very weak ferromagnetic moment appears to emerge below Tmag ∼ 100 K with B perpendicular to the film plane, smaller in magnitude and lower in temperature than SrIr0.8Sn0.2O3, as shown in Fig. 4(c). No clear anomaly can be identified in ρ(T) at Tmag, and ρ(T) shows an insulating behavior above Tmag, implying that the magnetic ordering is not a trigger of the semimetal-insulator transition in CaIr1−xSnxO3 in contrast to that in SrIr1−xSnxO3. The emergence of magnetism in the disordered insulator may suggest a Mott insulator character and therefore a Mott-Anderson type metal-insulator transition in CaIr1−xSnxO3.
In summary, we have successfully grown thin films of Dirac semimetals SrIr1−xSnxO3 and CaIr1−xSnxO3 epitaxially on SrTiO3(001), with their orthorhombic c axis parallel and perpendicular to the substrate plane, respectively. In the case of SrIr1−xSnxO3, the c axis can be aligned within the substrate plane using a vicinal substrate. While a well-defined Dirac node semimetal to a magnetic insulator transition with Sn doping is observed at a critical Sn concentration of xc ∼ 0.1 in SrIr1−xSnxO3, we observe in CaIr1−xSnxO3 that the Dirac node semimetal changes only gradually to a poor insulator without any well-defined semimetal-insulator transition. We argue that the contrast between SrIr1−xSnxO3 and CaIr1−xSnxO3 is a consequence of the interplay between the enhanced effective electron correlation and the disorder effect by Sn doping. While the correlation effect dominates in the case of SrIr1−xSnxO3, the disorder effect and the carrier localization dominate in CaIr1−xSnxO3 due to the proximity of the Fermi level to the Dirac nodes and the resultant low carrier concentration. These results clearly indicate that Sn-doped iridium perovskite oxides are an interesting playground to study the effect of electron correlations and disorders in a topological semimetal.
This work was supported by JSPS KAKENHI, Grant Nos. JP24224010, JP17H01140, JP15H06092, JP17K14335, and JP18J21922.
Strictly speaking, in both SrIrO3 and CaIrO3, cells of film layers on SrTiO3(001) slightly deform from orthorhombic to monoclinic due to epitaxial strain.