Introduction

The spin-orbit assisted Mott state discovered in layered iridates, e.g. Sr2IrO4, provides a new platform to realize unconventional properties of condensed matter due to the unique coexistence of strong spin-orbit coupling and electron-correlation1. Recent studies have revealed the possibilities of novel electronic and magnetic phases in iridates such as Weyl semimetals2,3 and a potential high-Tc superconducting state with d-wave gap4,5,6,7. However, the fundamental electronic structure of the layered iridate is still under debate; namely, the insulating gap may open due to antiferromagnetic ordering, i.e. Slater scheme8,9, rather than electron-correlation, i.e. Mott picture. Moreover, it is a formidable task to unveil the physics of layered iridates since only Sr2IrO4 and Ba2IrO4 (refs 10, 11, 12, 13) phases are available for experimental characterizations to date.

In this article, we report the systematic changes of the structural, transport and optical properties of layered iridates by investigating meta-stable Ca2IrO4 epitaxial thin-films. Since the Ruddlesden-Popper (R-P) phase of Ca2IrO4 is not thermodynamically stable, its bulk crystals do not exist in nature. However, we have successfully synthesized the R-P phase Ca2IrO4 thin-films (Fig. 1(b)) from a polycrystalline hexagonal (P62m) Ca2IrO4 bulk crystal (Fig. 1(a)) using an epitaxial stabilization technique14. The smaller ionic size of Ca2+ compared to Sr2+ causes increased IrO6 octahedral rotation and/or tilting, hence a reduced electronic band-width (W). Thus, investigating Ca2IrO4 in a comparative study with Sr2IrO4 and Ba2IrO4 provides a unique opportunity to explore the layered iridate system, as it allows for the enhancement of the electronic correlation effect (U/W).

Figure 1
figure 1

Schematic diagram of epitaxial stabilization of tetragonal Ca2IrO4 epitaxial thin-film from (a) the bulk hexagonal phase of Ca2IrO4, i.e. a target used in the pulsed laser deposition, to (b) metastable R-P phase of Ca2IrO4 thin-film grown on a single crystal YAlO3 (110) substrate. (c) Powder x-ray diffraction of our target material, which shows x-ray diffraction peaks from the hexagonal bulk phase of P62m and a couple of small CaO peaks. (d) X-ray 2θ-ω scan of an epitaxial Ca2IrO4 thin-film, where only the (00l)-diffraction peaks of Ca2IrO4 are visible. YAlO3 (110) and (220) peaks are labeled with asterisk (*) symbols.

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Methods

We have grown metastable Ca2IrO4 epitaxial thin-films with the K2NiF4–type crystal structure on YAlO3 (110) substrates by using a custom-built pulsed laser deposition (PLD) system with in-situ spectroscopic monitoring techniques15. The laser ablation is performed on a polycrystalline hexagonal (P62m) Ca2IrO4 target. The powder x-ray diffraction of the target is presented in Fig. 1(c). The samples are grown under the growth conditions of 1.2 J/cm2 laser fluence (KrF excimer, λ = 248 nm) and 700 °C substrate temperature. In order to avoid defects such as oxygen vacancies during the growth, we have used a laser beam imaging technique with reduced laser beam size in PLD to minimize the kinetic energy of the plume16. This technique is essential for the successful growth of Ca2IrO4 thin-films. A relative high oxygen partial pressure of 10 mTorr is also used to minimize oxygen vacancies. The structural properties of the epitaxial Ca2IrO4 thin-films are measured using x-ray diffractometry (Bruker D8 Advance system with Cu-Ka radiation). Transport properties are measured using a Physical Property Measurement System (Quantum Design) with conventional four-probe and Hall geometries. Optical transmission spectra (T(ω)) are taken at normal incidence using a Fourier-transform infrared spectrometer in the photon energy region of 0.2–0.6 eV and a grating-type spectrophotometer in the range of 0.5–7 eV, where the substrates are transparent. The absorption spectra are calculated using where t is the thin film thickness.

Results and Discussion

The metastable R-P phase of the Ca2IrO4 thin films is verified by x-ray diffraction and reciprocal space mapping scans, which indicate that the films are stabilized by the epitaxial strain of the substrates and are of high crystalline quality. Figure 1(d) shows the θ-2θ x-ray diffraction scan with the (00l) peaks of a Ca2IrO4 thin film. The full width at half maximum of the (0012)-reflection rocking curve scan is 0.04°, which clearly shows good crystallinity of the film (Fig. 2(b)). The thickness of the Ca2IrO4 thin films is ca. 6 nm. The crystal quality deteriorates considerably as we increase the thickness further, presumably due to its thermodynamically metastable nature. In x-ray reciprocal space mapping (Fig. 2(a)), the (1118) peak of the film is vertically aligned with the YAlO3 substrate (332)-reflection, indicating Ca2IrO4 films are coherently strained to the substrates, i.e. [110]film//[001]substrate and [001]film//[110]substrate. The lattice parameters obtained from the x-ray diffraction scans show that both in-plane (a) and out-of-plane (c) lattice parameters of Ca2IrO4 films are smaller than those of Sr2IrO4 (ref. 17) and Ba2IrO4 (ref. 10) (Fig. 2(c)). At this moment, the local structural information of Ca2IrO4 films, such as octahedral rotation and tilting, is unknown and requires substantial microscopic characterizations that we plan to perform as a future study. However, by assuming the rigid Ir-O bond-length to be constant, which is a reasonable assumption, we conjecture the reduced lattice constants (from x-ray diffraction) imply that the Ir-O-Ir bond angle is reduced from 158° (Sr2IrO4) to ca. 140° (Ca2IrO4). The reduced bond angle implies a corresponding reduction in bandwidth (W), according to the relation between bandwidth (W) and the Ir-O-Ir bond angle (θ) described by18:

Figure 2
figure 2

(a) Reciprocal space map near the YAlO3 (332)-reflection, which shows the Ca2IrO4 (1118)-reflection. The vertical dashed line indicates that the Ca2IrO4 thin-film is coherently strained to the substrate. (b) The rocking curve scan of Ca2IrO4 (0012)-reflection has a full-width half-maximum of 0.04°. (c) The in-plane (left axis) and out of plane (right axis) lattice parameters of Ca2IrO4, Sr2IrO4 (ref. 17) and Ba2IrO4 (ref. 10) thin films obtained from x-ray diffraction scans, as a function of A-site cation ionic radius. The solid circles and squares present the in-plane and out of plane lattice parameters, respectively. The open symbols indicate the in-plane and out of plane lattice parameters of Sr2IrO4 and Ba2IrO4 single crystals12,23,24.

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where dIr-O is the Ir-O bond length. This will result in an enhanced electron-correlation (U/W) for the Ca2IrO4 compound as compared to that of the Sr2IrO4 and Ba2IrO4 thin films.

Figure 3(a) shows the temperature-dependent resistivity ρ(T) of a Ca2IrO4 thin film, which has an insulating behavior. The room-temperature resistivity of Ca2IrO4 (ca. 170 mΩcm) is about the same as the room temperature resistivity of Sr2IrO4 (ca. 140 mΩcm) and Ba2IrO4 (ca. 130 mΩcm) deposited on SrTiO3 substrates. The energy gap (Δ = 2Ea) of Ca2IrO4 is calculated using an Arrhenius plot (, where kB is the Boltzmann constant) and compared to Sr2IrO4 (ref. 10) and Ba2IrO4 thin films. While the Arrhenius plots of Sr2IrO4 (ref. 10) and Ba2IrO4 show non-linear behaviors, the transport of Ca2IrO4 is quite linear over the entire measured temperature range (300 K to 90 K). An energy gap of 120 meV is extracted from its Arrhenius plot. Due to the increased U/W in Ca2IrO4, we expect its gap energy to be larger than that of Ba2IrO4 and Sr2IrO4. However, the energy gap of Ca2IrO4 obtained from the room temperature transport is smaller than that of Sr2IrO4 and Ba2IrO4. This puzzling observation implies that the transport properties of layered iridates are mostly dominated by impurities or defects and intrinsic bandgap energies should be measured using a spectroscopic technique.

Figure 3
figure 3

(a) Normalized resistivity (ρ) versus temperature data of Ca2IrO4 (red), Sr2IrO4 (blue) and Ba2IrO4 (green) thin-films. The data of Sr2IrO4 is from ref. 10; The inset shows the Arrhenius plot of Ca2IrO4, Sr2IrO4 and Ba2IrO4. Solid black lines present the linear fits at room temperature and low temperature. The estimated gap energies at room temperature are ΔCIO = 120 meV, ΔSIO = 250 meV and ΔBIO = 190 meV. The Arrhenius plots are shifted vertically for clarity. (b) Optical absorption spectra (α (ω)) of Ca2IrO4, Sr2IrO4 and Ba2IrO4 thin-films at room temperature. The plots are shifted vertically by 105 cm−1 for clarity. The α, β and γ represent the optical transition peak energies obtained from the fit with the minimal set of the Lorentz oscillators. The solid black curves are the fit curves using Lorentz oscillators, which match well with the experimental spectra. (c) Fitted absorption spectra of Ca2IrO4, Sr2IrO4 and Ba2IrO4 at low energy using Wood-Tauc’s method21 which clearly confirm the increased energy gap from Ba2IrO4 to Ca2IrO4. The estimated optical gap energies using this method are ΔCIO = 210 meV, ΔSIO = 150 meV and ΔBIO = 110 meV.

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Figure 3(b) presents the optical absorption spectra (α(ω)) of Ca2IrO4 compared with Sr2IrO4 (ref. 17) and Ba2IrO4 (ref. 10) thin films. The absorption spectra are fit using a minimal set of Lorentz oscillators. The common features of strong absorption tails due to the charge-transfer transitions from O 2p to Ir 5d bands are above ca. 2–3 eV. The black solid lines in Fig. 3(b) are the resultant fit curves using Lorentz oscillators, which match well with the experimental spectra. The three absorption peaks indicated by α, β and γ are labeled consistently with previous literature19,20. The α, β and γ absorption bands have been interpreted as the associated Ir 5d transitions, such as Ir-Ir intersite optical transitions1,19,20. Note that as the cation size — and consequently the Ir 5d bandwidth — increases from Ca2IrO4 to Ba2IrO4, the α, β and γ peak-positions are shifted to higher energy. This seemingly counterintuitive peak shift has also been observed in the optical absorption spectra of strain-dependent Sr2IrO4 thin-films17, as the lattice strain changes from compressive to tensile directions. This observation of the peak-energy shift can provide a key to understanding the electronic structures of iridates since the spectral shape is thought to be strong experimental evidence supporting the Mott picture of this system1,19,20. However, we will leave it as a future study since detailed analysis requires theoretical modeling and calculations, which is beyond the scope of this article.

We note the increased optical gap energy of Ca2IrO4 thin-films as compared to that of Sr2IrO4 and Ba2IrO4. To calculate the optical energy gap, each absorption spectrum is fit using the Wood-Tauc’s method21 (Fig. 3(c)). In this method, the strong region of the absorption edge (α > 104 cm−1) can be described by:

where Eg (E) is the optical band gap (incident photon) energy. The estimated optical gap energies using this method are ΔCIO = 210 meV, ΔSIO = 150 meV and ΔBIO = 110 meV. For the exponent γ, we have obtained γ = 1.5 (Ca2IrO4), γ = 3.0 (Sr2IrO4) and γ = 1.5 (Ba2IrO4). While γ = 3 is consistent with the indirect bandgap of Sr2IrO4, γ = 1.5 values in Ca2IrO4 and Ba2IrO4 suggests direct bandgap, of which physical understanding will require further theoretical studies. Nevertheless, as shown in Fig. 3(c), the optical gap energy has clearly increased for Ca2IrO4 compared to that of Sr2IrO4 and Ba2IrO4. Hence, as we decrease the ionic sizes of A-site cations in layered iridates, the Ir-O-Ir bond angle is reduced, which, in turn, increases U/W and manifests itself as the observed increase in the optical bandgap energy.

Our approach of synthesizing meta-stable phase thin-films of strongly correlated systems offers a new route to understanding the physics of complex oxides. For example, the stabilization of metastable phases can provide compounds with larger effective electronic correlations than presently available by producing increased distortion and tilting in lattice. While simple octahedral distortions usually preserve inversion symmetry in the K2NiF4–type structure, the R-P structure of Ca2IrO4 has been proposed as a candidate material featuring a non-centrosymmetric structure due to its low symmetry22. This unique structure, achieved by breaking the inversion symmetry in this system, is expected to induce many interesting phase transitions such as ferroelectricity and multiferroicity. Hence, experimental studies of meta-stable phases allow us to tackle a number of intriguing problems of exotic ground states with novel properties that are theoretically suggested.

Conclusion

We have successfully stabilized Ca2IrO4 thin-films with the K2NiF4–type crystal structure and determined its higher optical gap energy to originate from its enhanced electron-correlation, U/W, with respect to its larger A-site cation isosymmetric compounds. The structural study confirms the good crystallinity and coherent strain state of the epitaxial Ca2IrO4 thin-films on YAlO3 (110) substrates. The transport and optical spectroscopic experiments show that Ca2IrO4 thin-films have an insulating ground state similar to Sr2IrO4 and Ba2IrO4. However, the increased IrO6 octahedral rotation, tilting, or distortion in Ca2IrO4 increases U/W and thus its optical gap energy is larger than the gap energies of Sr2IrO4 and Ba2IrO4. This approach of metastable thin-film phases can greatly expand the number of available materials and can help to unveil the physics of strongly correlated systems.

Additional Information

How to cite this article: Souri, M. et al. Investigations of metastable Ca2IrO4 epitaxial thin-films: systematic comparison with Sr2IrO4 and Ba2IrO4. Sci. Rep. 6, 25967; doi: 10.1038/srep25967 (2016).