Contrasted Sn Substitution effects on Dirac line node semimetals SrIrO$_{3}$ and CaIrO$_{3}$
Masamichi Negishi Naoka Hiraoka Daisuke Nishio-Hamane Hidenori Takagi







 
















Department of Physics, University of Tokyo, 
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan



Department of Physics, University of Tokyo, 
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan



Institute for Solid State Physics, University of Tokyo, 
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan


[E-mail: ]h.takagi@fkf.mpg.de

Department of Physics, University of Tokyo, 
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Max Planck Institute for Solid State Research, 
Heisenbergstraße 1, 70569 Stuttgart, Germany

Institute for Functional Matter and Quantum Technologies, University of Stuttgart, 
Pfaffenwaldring 57, 70550 Stuttgart, Germany










Perovskite-type iridates SrIrO_3 and CaIrO_3 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 SrIr_1-xSn_xO_3 and CaIr_1-xSn_xO_3 thin films epitaxially on SrTiO_3(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 SrIr_1-xSn_xO_3 above a critical doping level (x_c∼ 0.1) shows a well-defined transition from the semimetal to a weakly ferromagnetic insulator at T = T_c. In contrast, the ρ (T) of CaIr_1-xSn_xO_3 with increasing x shows a rapid increase of magnitude but does not show 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.






Contrasted Sn Substitution effects 

on Dirac line node semimetals SrIrO_3 and CaIrO_3

    Hidenori Takagi
    December 30, 2023
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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 <cit.>. 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 t_2g <cit.>, splits the t_2g bands with five d electrons into upper half-filled J_eff = 1/2 band and lower completely filled J_eff = 3/2 bands. In the two-dimensional layered perovskite Sr_2IrO_4, a modest on-site Coulomb U of ∼ 2 eV <cit.> brings the system to a spin-orbital Mott state with the J_eff = 1/2 moments <cit.>. Such a spin-orbital Mott state has been identified in many two-dimensional layered iridium oxides, where exotic magnetism of J_eff = 1/2, particularly the Kitaev spin liquid state, has been explored <cit.>. In the three-dimensional perovskites SrIrO_3 and CaIrO_3, in contrast, the J_eff = 1/2 band remains metallic marginally due to the increased band width <cit.> and forms a “correlated” topological semimetal.



SrIrO_3 and CaIrO_3 have a GdFeO_3-type distorted perovskite structure where the rotation and the buckling of IrO_6 octahedra give rise to a unit cell with the size of √(2) a_c×√(2) a_c× 2 a_c (a_c is the lattice constant of the original cubic lattice) as shown in Fig. <ref>a <cit.>. The results of band calculations indicate that SrIrO_3 and CaIrO_3 have Dirac electron bands with a line of nodes and heavy hole bands at the Fermi level <cit.>. The line nodes of Dirac bands are protected by the time-reversal symmetry and the gliding symmetry of the GdFeO_3-type perovskite structure <cit.>. The existence of Dirac electrons has been supported by experiments, for example, by ARPES <cit.> and transport measurements <cit.>. 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 SrIrO_3 layers in (SrIrO_3)_n, (SrTiO_3)_1 superlattice structures <cit.>. 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 Ca^2+ (1.34 Å) is smaller than that of Sr^2+ (1.44 Å) <cit.>, the lattice is more distorted in CaIrO_3 than in SrIrO_3, which reduces the band width of CaIrO_3 appreciably as compared with that of SrIrO_3. The effect of electron correlation should be larger in CaIrO_3 than SrIrO_3. 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 CaIrO_3 bring its Fermi level closer to the Dirac node and therefore further reduces its density of electrons and holes when compared to SrIrO_3 <cit.>.

A transition from the Dirac semimetal to a magnetic insulator was also discovered in bulk polycrystalline SrIrO_3 by substituting Ir with Sn <cit.>. As Sn ions are tetravalent like Ir ions in SrIrO_3, 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 J_eff = 1/2 band, as in the case of the superlattice structure. We note here that Sn doping should modify not only the effective band width, 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 SrIr_1-xSn_xO_3 and CaIr_1-xSn_xO_3 and measured their resistivity to probe the effects of Sn doping. A transition from a semimetal to a (magnetic) insulator is observed in both SrIr_1-xSn_xO_3 and CaIr_1-xSn_xO_3 thin films, as in the bulk SrIr_1-xSn_xO_3. We discovered a sharp contrast in the transition behavior between the Sr and Ca iridium perovkites: the appearance of well-defined transition from the Dirac semimetal to a magnetic insulator as function of temperature above a critical Sn concentration x_c∼ 0.1 for SrIrO_3, and the continuous increase of resistivity without a clear signature of the semimetal-insulator transition for CaIrO_3, which we ascribe to the interplay of the electron correlation and the disorder effect.

SrIr_1-xSn_xO_3 and CaIr_1-xSn_xO_3 thin films were epitaxially grown in the range of 0 ≤ x ≤ 0.2 on SrTiO_3(001) substrates by pulsed laser deposition technique using polycrystalline targets with 5 % excess B (Ir/Sn) cations. The film deposition of SrIr_1-xSn_xO_3 (CaIr_1-xSn_xO_3) was conducted at 650 ^∘C (750 ^∘C) in a 100 mTorr O_2 atmosphere. Typical thickness of our samples estimated from 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 SrIr_1-xSn_xO_3 and CaIr_1-xSn_xO_3 films crystalize in the perovskite structure without any trace of impurity phase within the given resolution (Fig. <ref>b). The full width at half maximum of the rocking curve is typically as narrow as 0.1^∘ at the pseudo-cubic (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 SrTiO_3 substrate (Figs. <ref>c, d). Those observations clearly demonstrate the epitaxial growth and high crystallinity of the grown films. The out-of-plane lattice constant of the CaIr_1-xSn_xO_3 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 Sn^4+ (0.690 Å) than that of Ir^4+ (0.625 Å) <cit.> (Fig. <ref>e)





The film orientation was identified by RSM measurements of XRD and transition electron microscopy (Figs. <ref>a-c). We will describe the lattice orientation of films by the pseudo-cubic unit cell in this paper using lattice parameters a_pc = √(a^2 + b^2) / 2 and c_pc = c/2, where a, b and c denote the orthorhombic unit cell parameters of the distorted GdFeO_3 structure <cit.>. In case of SrIr_1-xSn_xO_3, the c_pc axis (|| orthorhombic c) lies within the substrate plane independent of Sn doping (Fig. <ref>d). This is natural because c_pc is closer to that of SrTiO_3 substrate than a_pc (See Table <ref>.). In contrast, a_pc is closer to that of SrTiO_3 in CaIrO_3. The c_pc axis (|| orthorhombic c) therefore aligns perpendicular to the substrate plane in the CaIrO_3 case (Fig. <ref>f) <cit.>. The inclusion of minority domains with the c_pc axis lying within the substrate plane as in SrIr_1-xSn_xO_3 are observed for the high Sn content films (x ≥ 0.1) (Fig. <ref>g), which very likely reflects the expansion of c_pc due to Sn doping and the resultant proximity to the lattice constant of SrTiO_3.



As a_pc is larger than c_pc, the orientation of c_pc within the substrate plane can be controlled by introducing additional epitaxial strain using step edges of vicinal substrate <cit.>. We used a vicinal SrTiO_3(δ01) substrate with the substrate plane 0.4^∘ rotated from (001) towards the [100] direction for the growth of SrIr_1-xSn_xO_3. Because of the epitaxial strain from the side (100) plane at the step edges, the c_pc (|| orthorhombic c) axis prefers to align along the edge, namely [010]_STO direction (Fig. <ref>e). The RSM measurements clearly indicate that more than 95 % of domains have c_pc axis parallel to the substrate [010] 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 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 SrIr_1-xSn_xO_3 films indicate the presence of a metal-insulator transition accompanied by a weak ferromagnetism as in the bulk <cit.>. The SrIrO_3 (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. <ref>a). This agrees well with previous reports on SrIrO_3 thin films <cit.> and can be understood as a typical behavior of semimetals with extra conductivity at high temperature from thermally excited electrons and holes. With Sn doping (Fig. <ref>b), we do not observe an appreciable change of the ρ (T) up to the critical concentration x_c = 0.1. Above x_c = 0.1, however, the resistivity shows a transition from the semimetal to a weak insulator at a transition temperature T_c where we observe a kink in ρ (T) and a well-defined peak of the second derivative d^2ρ (T) / d T^2 (Fig. <ref>c). T_c increases rapidly with increasing x. The metal-insulator transition below T_c is accompanied by a weak ferromagnetism with the easy axis parallel to the pseudo-cubic c_pc axis (|| orthorhombic c), as seen in Fig. <ref>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 contour map of the magnitude of ρ (T) (Fig. <ref>e). The stabilization of the magnetic insulator phase at low temperatures above x_c = 0.1 highly likely originates from the increase of the effective electron correlation. We argue that this is because of the reduced hopping of J_eff = 1/2 electrons by the introduction of Sn^4+ without conduction electrons.

We find contrasting behavior in the Sn doping effect on ρ (T) in CaIr_1-xSn_xO_3 compared to SrIr_1-xSn_xO_3. For CaIrO_3 (x = 0) films (Fig. <ref>a), the overall behavior of ρ (T) is similar to that observed in SrIrO_3. The magnitude of resistivity is, however, appreciably larger than that of SrIrO_3. A weak increase of ρ (T), reminiscent of a weak localization and not observed in SrIrO_3, is seen below 20 K, which is suggestive of the presence of appreciable disorder effect. It was discussed in the recent transport study on single crystal CaIrO_3 that the Fermi level is much closer to the Dirac nodes and hence the electron and the hole densities are lower in CaIrO_3 than SrIrO_3 due to the enhanced correlation effect originating from the narrow band in CaIrO_3 <cit.>. The effect of disorder should be enhanced in CaIrO_3 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. <ref>a, b). We do not see a well-defined semimetal-insulator transition as a function of T and x in contrast to the case for SrIrO_3. 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 SrIrO_3. It is natural that the effect of disorder associated with Sn-doping is much more profound in CaIrO_3 than in SrIrO_3 because of the lower carrier density and Fermi energy of CaIrO_3. For CaIr_0.8Sn_0.2O_3, a very weak ferromagnetic moment appears to emerge below T_mag∼ 100  K with B perpendicular to the film plane, smaller in magnitude and lower in temperature than SrIr_0.8Sn_0.2O_3 as shown in Fig. <ref>c. No clear anomaly can be identified in ρ (T) at T_mag and ρ (T) shows an insulating behavior above T_mag, implying that the magnetic ordering is not a trigger of the semimetal-insulator transition in CaIr_1-xSn_xO_3 in contrast to that in SrIr_1-xSn_xO_3. The emergence of magnetism in the disordered insulator may suggest a Mott insulator character and therefore a Mott-Anderson type metal-insulator transition in CaIr_1-xSn_xO_3. 

In summary, we have successfully grown thin films of Dirac semimetals SrIr_1-xSn_xO_3 and CaIr_1-xSn_xO_3 epitaxially on SrTiO_3(001), with their orthorhombic c axis parallel and perpendicular to the substrate plane respectively. In the case of SrIr_1-xSn_xO_3, 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 x_c∼ 0.1 in SrIr_1-xSn_xO_3, we observe in CaIr_1-xSn_xO_3 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 SrIr_1-xSn_xO_3 and CaIr_1-xSn_xO_3 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 SrIr_1-xSn_xO_3, the disorder effect and the carrier localization dominate in the CaIr_1-xSn_xO_3 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 Number JP24224010, JP17H01140, JP15H06092, JP17K14335 and JP18J21922.

























































































