Structural and electronic properties of SrCuO_2+δ thin films

The crystal structure of superconducting cuprates¹ is characterized by a layered arrangement of CuO₂ atomic planes separated by planes of cations; a charge-transfer mechanism between these planes provides the charge carriers that lead to electrical conduction.² In its simplest form, where the ordering of planes is a regular alternation of CuO₂ planes and atomic A planes (A usually being an alkaline metal), the structure is called infinite layer and has received considerable attention for its key role in the mechanism of superconductivity.³ Additionally, the recent discovery of superconductivity in nickel compounds with the same infinite-layer structure⁴ has reignited interest in the ACuO₂ phases, whose synthesis has always been challenging. The synthesis of superconducting nickelates requires a reduction of the oxygen content from the perovskite RNiO₃ phase (R being La, Pr, or Nd) to the infinite-layer RNiO₂ via a soft chemical route.^5–10 In contrast, the cuprates can be synthesized directly in the infinite-layer structure. In the bulk form, however, they require extreme conditions of temperature and pressure,^11–13 while epitaxy enables their growth as crystalline layers in thin film form.^14–16 

Achieving superconductivity in the infinite-layer cuprates is a difficult task: when the separating cation is an alkaline metal (CaCuO₂, SrCuO₂, or BaCuO₂), Cu is in a 2+ valence state and the compounds are antiferromagnetic insulators; they can be made superconducting through electron doping, by replacing the divalent alkaline metal with a trivalent lanthanide.^11,17,18 In hole doped systems, cation vacancies or interstitial oxygen atoms, apical to the Cu, have been reported as effective mechanisms to induce superconducting transitions.¹⁹ Superlattices of BaCuO₂/SrCuO₂^20,21 and BaCuO₂/CaCuO₂^22,23 have superconducting critical temperatures as high as 70 K, attributed to the doping effect of apical oxygen inserted into the Ba planes. More recently, the proximity of a perovskite phase to the infinite-layer structure in SrTiO₃/CaCuO₂ bilayers and superlattices has been shown to be another heterostructuring approach to observe superconductivity via the introduction of apical oxygen.^24,25 These studies have demonstrated that precise manipulation of the oxygen content and the arrangement of oxygen atoms in infinite-layer cuprates is crucial to engineer their electronic properties.

In this work, we show that control of the oxygen configuration in infinite-layer cuprate thin films can be achieved by tuning the oxidizing power during film growth using ozone-assisted pulsed laser deposition. The oxidizing power can be varied by fixing the O₃/O₂ percentage and varying the gas mixture flow during the growth of SrCuO_2+δ films. As the oxidizing power is increased, we find that a different phase with an elongated c axis appears (SrCuO_2+δ): the macroscopic phase fraction of SrCuO_2+δ and SrCuO₂ can be controlled precisely with the O₃/O₂ flow. With reference to previous experimental and theoretical work, we discuss the possible structures and oxygen content values of the high c-axis SrCuO_2+δ phase.

Planar-view aberration-corrected scanning transmission electron microscopy (STEM) measurements show that the SrCuO_2+δ thin films are characterized by a modulation of the in-plane spacing between consecutive Sr atoms. Regions with expanded in-plane lattice parameter appear as dark stripes along either the [100] or the [010] direction in Z-contrast images and arise due to the ordering of oxygen vacancies (with respect to the stoichiometric SrCuO_2.5 structure) along one of the two crystallographic axes when δ is smaller than 0.5. Using x-ray absorption spectroscopy (XAS), we show that this high c-axis SrCuO_2+δ phase is associated with a more isotropic orbital coordination for Cu compared to the infinite-layer structure, as well as with hole doping. Transport measurements as a function of temperature reveal that this hole doping leads to a dramatic decrease in the resistivity, with a magnitude that depends on the precise oxygen content of the structure.

SrCuO_2+δ thin films were deposited on (001)-oriented SrTiO₃ substrates (Crystec GmbH) using pulsed laser deposition with a KrF excimer laser (248 nm, 25 ns) by ablating from a stoichiometric ceramic target (Praxair Surface Technologies). The films were grown at a constant pressure of 0.25–0.5 mbar, in a pure oxygen (O₂) atmosphere, or in a mixture of oxygen and ozone (O₂ + 5% O₃). The gas mixture was delivered into the chamber through a stainless steel tube pointing toward the substrate and placed 3–4 cm away from it. The oxidizing power during the growth was varied by controlling the flow through the tube using a flow meter, with O₂ + O₃ flows ranging from 10 to 20 standard cubic centimeters per minute (sccm). During growth, the substrate was kept at a constant temperature of 600 °C, heated using an infrared laser. Following deposition, the samples were quenched to room temperature in the same atmosphere used during the growth.

Structural characterization of the SrCuO_2+δ thin films was conducted using a PANalytical X’Pert Pro diffractometer with a Cu Kα source. STEM measurements were used to get information on the local structure of the films using a double-corrected Thermofisher TITAN Themis electron microscope, equipped with a double-corrector DCOR (CEOS) and a high brightness field emission gun with monochromator, operated at 300 kV in STEM mode using a semi-convergence angle of 20 mrad. The monochromator allowed us to acquire the data using low beam currents (around 30–40 pA), ensuring no beam damage. To improve the image quality and reduce scanning distortions, we acquire stacks of images that are then cross-correlated using the SmartAlign plug-in for DigitalMicrograph.²⁶ The TEM specimen was prepared following mechanical polishing and an ion milling process (carried out at a temperature of −100 °C to minimize sample damage) to thin the specimen down to electron transparency. The coordinates associated with each Sr column have been estimated using a 2D Gaussian fitting procedure using the Atomap script.²⁷ 

XAS was used to determine the orbital configuration of the thin films. XAS measurements were performed in total electron yield mode across the oxygen K and copper L_2,3 edges in the BL29-BOREAS beamline²⁸ of the ALBA Synchrotron (Barcelona, Spain) at room temperature, under ultra-high vacuum conditions (2 × 10⁻¹⁰ mbar). The photon flux on the samples was ∼10¹² photons/s with an energy resolution of 50 meV at the O K edge and about 120 meV at the Cu L₃ edge. Low-temperature resistivity measurements were performed using a probe slowly dipped in liquid helium as well as an Oxford Instruments Teslatron low-temperature system. For both types of measurements, a current of 1–100 μA was supplied using a Keithley 224 or 622X Current Source, and a voltage was measured in four-point van der Pauw geometry using a Keithley 2000 Multimeter or a Keithley 2182 A Nanovoltmeter, with the four corners of the sample contacted using Al wire-bonding.

Figure 1(a) shows large-range θ-2θ scans around the 001 and 002 peaks of SrTiO₃ for SrCuO_2+δ thin films with a thickness of ∼20 nm (50–59 unit cells). The film deposited in oxygen (bottom curve, blue) shows finite-size oscillations around the 00L reflections, which correspond to the tetragonal infinite-layer phase with an out-of-plane lattice parameter of c = 3.45 Å. The oxidizing power during the growth has been progressively increased by introducing ozone into the chamber (using a mixture of O₂ + 5% of O₃) and raising the flow rate from 10 to 20 sccm. At 15 sccm, an additional Bragg peak appears, associated with an out-of-plane lattice parameter of 3.64 Å. Further increasing the flow rate of the oxygen/ozone mixture (20 sccm), we observe that only the high c-axis peak remains while its position moves slightly to lower 2θ values. Multiple samples of the high c-axis phase were deposited at this high O₂/O₃ mixture flow, for which we found a variation in their out-of-plane lattice parameter that ranges from 3.63 to 3.71 Å. The variation in lattice parameters indicates a range of oxygen contents in these films, as will be discussed below. We note that the jump in the c axis when ozone is introduced into the chamber indicates that the additional oxygen is not gradually incorporated into the structure, but rather that a new phase occurs. Despite the large change in the lattice parameter from the infinite-layer to this high c-axis phase (∼6% increase), both structures remain strained to the SrTiO₃ substrate as seen in the reciprocal space maps around the $1̄03$ peak of SrTiO₃ [Figs. 1(b) and 1(c)]. Additionally, the reciprocal space map around the $01̄3$ peak of SrTiO₃ for the high c-axis phase (not shown here) is identical to Fig. 1(c), indicating that no in-plane asymmetry occurs.

To determine the structure that can give rise to this larger out-of-plane lattice parameter, we have to consider all possible phases of SrCuO₂, SrCuO_2+δ and Sr_n+1Cu_nO_2n+1. First, in addition to the tetragonal structure, SrCuO₂ can crystallize in an orthorhombic phase²⁹ when deposited at very high temperatures.^30,31 This structure would give rise to a very large change in the unit cell size, with c = 16.311 Å, leading to additional diffraction peaks. We observe this phase only when we deposit SrCuO₂ in pure oxygen at temperatures higher than 650 °C, as shown in the supplementary material Fig. S1, showing diffraction signatures that are distinct from the observed elongated c-axis phase in Fig. 1(a).

Second, a chain-type SrCuO₂ structure with a c axis of 3.9 Å was previously reported when the thickness of SrCuO₂ is reduced to below approximately five unit cells.^32–34 This structure results from a rotation of the CuO₂ planes by 90°. Our films are much thicker than this, and our observed c-axis value is smaller.

Additionally, phases in the Sr_n+1Cu_nO_2n+1 system can be easily excluded because they have larger lattice parameters than those we observe: the n = 1 phase (Sr₂CuO₃) would already show additional diffraction peaks corresponding to an out-of-plane lattice parameter of 12.548 Å.^35–37 These phases would also change the Sr/Cu stoichiometry very drastically, not something we expect simply by introducing ozone into the system.

Finally, an elongated out-of-plane lattice parameter has previously been observed in SrCuO_2+δ when deposited or post-annealed under highly oxidizing growth conditions,^38–41 or when electron doped using La or Nd substitution.^42–44 This phase was attributed to the incorporation of additional oxygen atoms in the Sr planes, leading to a SrCuO_2+δ structure. In thin films with two distinct Bragg peaks (arising from the infinite-layer and high c-axis structures), a macroscopic phase separation was also previously observed.⁴⁴ In Ref. 39, the authors used transmission electron microscopy to characterize the high c-axis phase in Na-doped Ca_1−xSr_xCuO_2+δ thin films and have found evidence for a $22ap×22ap$ superstructure (where a_p is the in-plane lattice parameter of the SrTiO₃ substrate), leading them to determine a value of δ = 0.25. Nevertheless, due to the extreme sensitivity of the samples to the measurement conditions and their fast degradation, the origin of this superstructure and the precise oxygen content of the high c-axis phase could not be confirmed.

Seko and Ishiwata,⁴⁵ using density functional theory and Bayesian optimization, determined the most stable SrCuO_2+δ structures for various values of δ. Out of the calculated lowest energy structures, only two have been observed experimentally: these are the structures with the two extreme values of δ equal to 0 and 0.5. For δ = 0 (SrCuO₂), the most stable structure is the infinite-layer with tetragonal unit cell lattice parameters a_t = b_t = 3.927 Å, c_t = 3.435 Å [shown in Figs. 2(a)–2(c)].⁴⁶ For δ = 0.5, the most stable structure is one consisting of corner-sharing CuO₅ pyramids [shown in Figs. 2(d) and 2(e)] that has been observed when SrCuO_2.5 is prepared at 950 °C and a pressure of 100 kbar.⁴⁷ This phase has a similar structure to CaMnO_2.5⁴⁸ and LaCuO_2.5,⁴⁹ and has an orthorhombic unit cell, with a_o = 5.424 Å, b_o = 10.837 Å, c_o = 3.731 Å. The SrCuO_2.5 structure can be represented using a pseudocubic unit cell as shown in red in Fig. 2(d), with a_p = a_o/$2$ = 3.835 Å, b_p = b_o/$(22)$ = 3.831 Å, c_p = c_o = 3.731 Å.⁴⁷ We note that, in contrast to the infinite-layer SrCuO₂ structure, the SrCuO_2.5 phase does not exhibit continuous CuO₂ planes in the in-plane a_p and b_p directions.

Two other structures have been predicted depending on the precise oxygen content in SrCuO_2+δ. For δ = 0.375, a structure with local CuO₆ octahedra, CuO₅ pyramids, and CuO₄ squares that are parallel or perpendicular to the c axis, is predicted, shown in the supplementary material Fig. S2. This structure has a tetragonal unit cell with a = b = 10.6056 and c = 3.7362 Å.⁴⁵ For δ = 0.25, a similar tetragonal structure is predicted, with a = b = 10.6212 and c = 3.6919 Å,⁴⁵ shown in the supplementary material Fig. S2. For both of these phases, a pseudocubic unit cell can be defined with a_p = b_p = a_t/2$2$ = 3.755 and 3.750 Å, respectively. Both the SrCuO_2.375 and SrCuO_2.25 phases are consistent with the $22ap×22ap$ superstructure observed by Wen et al. using transmission electron microscopy.³⁹ However, the two phases have not been observed in bulk samples so far.

The common feature in all the possible structures of SrCuO_2+δ is the incorporation of oxygen in the Sr planes that are parallel to the interface with the substrate. The incorporation of additional oxygen in our high c-axis structure is confirmed by in situ x-ray diffraction measurements performed while annealing a SrCuO_2+δ sample in a N₂ atmosphere (see Fig. S3 in the supplementary material).

We would expect to see additional peaks in x-ray diffraction either for the orthorhombic SrCuO_2.5 or for the larger unit cell of the tetragonal SrCuO_2.375 or SrCuO_2.25 structures that originate from the displacements of the Sr and Cu ions from their positions in the infinite layer phase. Using our laboratory diffractometer, however, we have only observed peaks related to the infinite layer structure. We attribute this to the low intensity of the bulk orthorhombic peaks compared to the infinite layer reflections,⁴⁷ as well as the possible formation of domains with local oxygen ordering that suppress the intensity of the reflections that originate from a long-range order of the additional oxygen.

To investigate the local structure of the films, we use STEM. Figure 3(a) is a planar-view (viewing axis along the growth direction) high-angle annular dark field (HAADF) image of a SrCuO_2+δ thin film deposited on SrTiO₃ with c = 3.626 Å and a thickness equal to 85 unit cells. Due to the Z-contrast nature of the HAADF imaging mode, the brightest spots in the image are Sr atoms, while the second brightest spots are Cu atoms. We find that a modulation of the HAADF contrast appears, with domains of dark stripes aligned along either the $100$ or $010$ crystallographic axes of the substrate, as indicated by the green and red arrows, respectively. These stripes are generally regularly spaced and lead to the appearance of a superstructure with an in-plane lattice parameter that is on average three times that of SrTiO₃, as observed by the additional spots—marked by the corresponding arrows—in the Fourier transform of the HAADF image [Fig. 3(b)].

Dark stripes similar to the ones in Fig. 3(a) have been observed in STEM studies of cobaltite,^50–52 manganite,⁵³ and ferrite⁵⁴ perovskite thin films, and have been attributed to expanded unit cells that are associated with ordered oxygen vacancies. For example, in the brownmillerite La_0.6Sr_0.4MnO_2.5 system, the distance between A-site cations varies from 3.783 to 4.527 Å depending on the local coordination of Mn—tetrahedral or octahedral;⁵⁵ LaCoO_3−x and La_0.5Sr_0.5CoO_3−x show an alternation of expanded (up to 20%) oxygen-depleted layers with CoO₄ triangular pyramids and of perovkite layers with CoO₆ octahedra.⁵⁵ 

To detect the in-plane modulation of the SrCuO_2+δ structure, we have mapped the atomic positions of Sr in a smaller region of the sample where the dark stripes run along the $100$ direction [Fig. 3(c)]. Figure 3(d) is a plot of the distance between consecutive Sr ions as a function of layer number for both the $100$ and $010$ directions: along the $100$ direction, this spacing remains constant, while along the $010$ direction a large variation in the Sr–Sr distance is recorded. In the dark stripe regions, the spacing between consecutive Sr atoms has an average value of 3.98 ± 0.01 Å; in the other regions, it decreases to an average value of 3.87 ± 0.01 Å. We note that since no substrate area is available to be used as a reference for image calibration, we have calibrated the pixel size so that the averaged in-plane lattice parameters match those of the SrTiO₃ substrate, in agreement with the reciprocal space maps [Fig. 1(c)].

In the SrCuO_2+δ system, the only experimentally observed configurations for copper are CuO₅ square pyramids (in orthorhombic SrCuO_2.5⁴⁷) and CuO₄ squares (in tetragonal SrCuO₂⁴⁶). Our current STEM measurements indicate that SrCuO_2+δ structures with intermediate values of δ between 0 and 0.5 do not adopt the SrCuO_2.375 or SrCuO_2.25 structures. However, they most likely manifest as SrCuO_2.5 structures periodically interrupted by infinite-layer SrCuO₂ planes [shown in the sketch of Fig. 3(c)] that appear to accommodate the oxygen off-stoichiometry and to minimize the tensile strain imposed on SrCuO_2.5 by the SrTiO₃ substrate.

The average oxygen content of the SrCuO_2+δ thin film with c = 3.626 Å can be estimated based on the Fourier transform of Fig. 3(b). Considering the tripling of the in-plane unit cell, we can assume the superstructure consists of two unit cells of SrCuO_2.5 and one unit cell of SrCuO₂, and therefore, δ is evaluated to be equal to 0.33. We expect the spacing between the infinite-layer SrCuO₂ planes and their phase fraction compared to SrCuO_2.5 to depend not only on the value of the epitaxial strain but also on the precise oxygen content of the SrCuO_2+δ structure. Therefore, the average out-of-plane lattice parameter of the high c-axis SrCuO_2+δ phase should give an indication of the precise oxygen content, with a higher c-axis value indicating a larger value of δ, due to a smaller fraction of infinite-layer SrCuO₂ planes in the structure. Finally, we note that the periodic insertion of an infinite layer unit cell breaks the long range order of the oxygen in the SrCuO_2.5 regions, which explains the absence of orthorhombic peaks in x-ray diffraction measurements.

To understand the electronic properties of the SrCuO_2+δ system and its differences from the infinite-layer SrCuO₂ structure, we use XAS. Figure 4(a) displays a schematic of the measurement geometry: linear vertical (LV) and linear horizontal (LH) polarisations result in the electric field of the incident X rays having a purely in-plane (I_ab) or majority out-of-plane (I_c) component, respectively. Figure 4(b) is a plot of the Cu L_2,3 XAS spectra measured for two orthogonal light polarization directions for an infinite-layer SrCuO₂ thin film deposited on SrTiO₃ (c = 3.460 Å, thickness = 39 unit cells). The peaks at the L₂ and L₃ edges correspond to transitions from Cu 2p_1/2 to Cu 3d and from Cu 2p_3/2 to Cu 3d orbitals, and occur at ∼952 and 932 eV, respectively. For an undoped infinite-layer structure, the predominant transitions are labeled 2p⁶3d⁹ → 2p⁵3d¹⁰.^4,56–62

The absorption intensity in XAS is proportional to the number of unoccupied electronic orbitals coupling to the polarization of the incoming light. Thus, the dramatically lower Cu L_2,3 peak intensities seen in Fig. 4(b) for LH polarization indicate that the occupation of the out-of-plane electronic orbitals [3$d3z2−r2$ and 3d_xz(3d_yz)] is much higher than the in-plane ones (3$dx2−y2$ and 3d_xy). This agrees with the orbital configuration expected for Cu in planar tetrahedral coordination where the in-plane orbitals are pushed to higher energy by crystal field effects and are thus unoccupied for Cu 2+ [see Fig. 4(e)], as previously observed in infinite-layer structures.^24,33,63,64

Figure 4(c) is a plot of the Cu L_2,3 XAS spectra for the SrCuO_2+δ structure (c = 3.679 Å, thickness = 38 unit cells). In contrast to the spectra for SrCuO₂, the difference in intensity between LV and LH is much smaller, indicating that the Cu coordination is more isotropic. The large differences between the SrCuO₂ and SrCuO_2+δ orbital configuration can be better appreciated by calculating the x-ray linear dichroism at the Cu L_2,3 edges (XLD = I_ab − I_c), plotted in Fig. 4(d). Considering the crystal field for Cu 3d orbitals in a CuO₄ square and in a CuO₅ pyramid (with apex in the in-plane direction), we expect the two orbital configurations sketched in Fig. 4(e). Note that since the apex of the CuO₅ pyramid in Fig. 4(e) is in the x direction, the orbital pointing from Cu to the apical oxygen in the pyramid is 3$d3x2−r2$⁠. Similarly, for a CuO₅ pyramid with an apex in the y direction, the corresponding orbital is 3$d3y2−r2$⁠.

For the CuO₅ pyramid sketched in Fig. 4(e), the energy splitting depends on the exact atomic position of Cu and its distance from the basal plane, δx,⁶⁵ with the in-plane orbitals (3$d3x2−r2$ and 3d_xy) going up in energy for increasing δx. In the experimentally observed structure of SrCuO_2.5, the average O–Cu–O bond angle is 172.7°,⁴⁷ making δx and δy finite, in agreement with our observation of a lower occupation of in-plane orbitals for SrCuO_2+δ.

Coincident with the change in the Cu 3d orbital configuration from the infinite-layer SrCuO₂ to SrCuO_2+δ, we expect a significant degree of hole doping and a change in the formal valence of copper with the incorporation of additional oxygen. To investigate this doping further, we calculate the XAS spectra [XAS = (I_ab + I_c)/2] for the two structures and plot them in Fig. 5.

First, we focus on the Cu L_2,3 edges, shown in Fig. 5(a). As mentioned above, in the infinite-layer SrCuO₂ structure, two distinct peaks are observed at ∼932 and 952 eV, attributed predominantly to 2p⁶3d⁹ → 2p⁵3d¹⁰ transitions. In SrCuO_2+δ, additional intensity appears at higher energies for both Cu L₂ and L₃. This contribution is most likely associated with $2p63d9L̲→2p53d10L̲$ transitions, where $L̲$ denotes a hole in the oxygen ligand p-bands and has been observed in various hole-doped cuprates as a function of hole doping.^{4,56,59–63,66–68} Increasing hole doping causes an increase in the amplitude of the higher energy peak, making it possible to roughly estimate the concentration of holes per Cu atom, using the formula $p=I|3d9L̲〉/I|3d9〉+I|3d9L̲〉$⁠, where I is the integrated area of each Cu L₃ peak component.^59,61,69 By fitting the Cu L₃ peak with two pseudo Voigt functions to extract $I|3d9〉$ and $I|3d9L̲〉$ (see the supplementary material Fig. S5), we can get an estimate of the hole doping p = 0.6–0.7 (δ = 0.30–0.35), with an uncertainty in the value arising from the choice of background subtraction. However, additional uncertainty in the determination of the hole doping can arise due to the large structural differences between SrCuO₂ and SrCuO_2+δ (this uncertainty is discussed further in the supplementary material).

The spectra at the O K edge for the two structures are shown in Fig. 5(b). The pre-edge region (E $<$ 531 eV) of SrCuO₂ is characterized by a single peak at ∼528.3 eV, due to a transition to the upper Hubbard band with a Cu 3d character hybridized with O 2p (referred to as peak U⁴). In SrCuO_2+δ, an additional peak appears at lower energies, and peak U shifts to a lower energy. This additional peak can be attributed to two effects. First, this change in the O K-pre-edge region may reflect the modification of the crystal structure when additional oxygen atoms are inserted. In other oxide systems, the number of peaks in the O K-edge region that are due to the hybridization of O 2p with transition metal 3d orbitals appears to be correlated with the number of nonequivalent oxygen sites in the structure.^70–76 These sites have different distances to the transition metal, and their orbitals hybridize differently with the transition metal 3d orbitals, giving rise to multiple peaks at different energies. In the case of bulk SrCuO_2.5, the average copper-oxygen distance is ∼1.866 Å in the c (out-of-plane) direction, while it is ∼1.917 Å in the a/b (in-plane) directions, suggesting that Cu 3d − O 2p hybridization will be different for different oxygen sites.

Second, the appearance of a low-energy feature in the O K edge is consistent with x-ray absorption studies of other hole-doped cuprates.^{4,58,60,62,63,68} The intensity of this low-energy peak was observed to increase as a function of hole doping. Considering that in our case, both structural and electronic changes occur between SrCuO₂ and SrCuO_2+δ, a one-to-one comparison of the O K-edge region with other hole-doped cuprates like La_2-xSr_xCuO₄ is more difficult. However, we can confirm that the SrCuO_2+δ structure is hole doped using field-effect experiments using the SrTiO₃ substrate as the gate dielectric, as shown in the supplementary material Fig. S6.

Doping in the infinite-layer cuprate systems can lead to drastic changes in their resistivity and even to superconductivity. Figure 6(a) shows the behavior of the resistivity of different studied films as a function of temperature, revealing the sensitivity of this compound to the oxidizing growth conditions. In the plot, the infinite-layer SrCuO₂ film deposited in pure oxygen has a resistivity of the order of 1 × 10⁴ μΩ cm at room temperature and exhibits insulating behavior as a function of temperature, reaching values of 1 × 10⁷ μΩ cm at low temperatures.

As additional oxygen is introduced into the structure and the transformation to the high c-axis SrCuO_2+δ phase occurs, the resistivity decreases very drastically and reaches values lower than 1 mΩ cm at room temperature. This decrease in resistivity does not come from a drastic change in sample topography, as the surface roughness of the films of Fig. 6(a) does not change significantly, as shown in the supplementary material Fig. S7. A similar decrease in resistivity has been observed in high c-axis SrCuO_2+δ thin films that have been plasma-annealed,³⁸ cooled in atomic oxygen⁴⁰ or ozone.⁴¹ Nevertheless, superconductivity is not observed in the SrCuO_2+δ system, probably due to the very high level of doping (that goes beyond the superconducting dome of the cuprates²), as well as the absence of continuous CuO₂ planes.

For SrCuO_2+δ films in the high c-axis phase, we find a variation in the temperature dependence of the resistivity that is correlated with the value of the out-of-plane lattice parameter of each film. Figures 6(b) and 6(c) show the resistivity behavior of two SrCuO_2+δ thin films with c = 3.63 Å and c = 3.67 Å, respectively: the film with the lower lattice parameter exhibits metallic behavior down to ∼100 K, with an upturn at lower temperatures; in contrast, the film with the larger lattice parameter exhibits semiconducting/insulating behavior at all temperatures below 300 K. We note that, while it is possible that differing degrees of disorder could impact the behavior of the resistivity, x-ray diffraction measurements reveal no significant difference in the quality of the two films.

This is the first indication that the resistivity is highly dependent on the precise oxygen content of the structure, as in conventional hole-doped cuprates.¹⁹ Among the high c-axis structure thin films, lower out-of-plane lattice parameters signify a smaller number of oxygen atoms in the SrO_x planes and, therefore, a nominal copper valence smaller than 3+.² In contrast, films with larger out-of-plane lattice parameters have a higher oxygen content and are closer to SrCuO_2.5, a structure with a nominal Cu 3+ valence.

In summary, we have used ozone-assisted pulsed laser deposition to fabricate high-quality SrCuO_2+δ thin films on SrTiO₃ substrates. At low oxidizing power (deposition using pure oxygen), the infinite-layer SrCuO₂ structure is stabilized. As the oxidizing power is increased, a different phase with an elongated c axis appears (SrCuO_2+δ), with a macroscopic SrCuO₂ vs SrCuO_2+δ phase fraction that can be varied by controlling the O₃/O₂ flow. Planar-view scanning transmission electron microscopy measurements have shown that the SrCuO_2+δ thin films are characterized by regions of an expanded in-plane lattice parameter, appearing due to the ordering of oxygen vacancies along either the $100$ or the $010$ direction when δ is smaller than 0.5. X-ray absorption spectroscopy measurements around the Cu L_2,3 and O K edges indicate that the high c-axis phase has a more isotropic Cu orbital configuration and is hole doped compared to the infinite-layer SrCuO₂ phase. The resistivity of the SrCuO_2+δ thin films was measured as a function of temperature, revealing a dramatic decrease with hole doping, with a magnitude that depends on the precise oxygen content of the structure.

Our work provides useful insight concerning the role of oxygen and the way it can be accommodated in epitaxially strained infinite-layer cuprate structures, uncovering a new way in which these SrCuO_2+δ thin films accommodate oxygen vacancies compared to stoichiometric SrCuO_2.5 structures. We believe that these results will contribute to further understanding of the behavior of infinite-layer cuprate thin films and will also inspire further studies on the use of ozone to control the precise oxygen content and Cu coordination in these systems.

See the supplementary material for more information on the growth conditions of SrCuO₂ thin films, the other possible structures of SrCuO_2+δ, annealing experiments of the SrCuO_2+δ phase, x-ray linear dichroism at the O K edge, fitting of the Cu L₃ edge, gating experiments on SrCuO_2+δ, and atomic force microscopy characterization of the thin films.

We thank Marco Lopes for technical support, Jérémy Bettex for the preparation of the STEM specimen, Giuseppe Balestrino and Daniele Di Castro for their advice concerning the use of ozone during sample growth, and Atsuto Seko for providing the calculated structure files of Ref. 45. We also acknowledge Philippe Ghosez, Yajun Zhang, Alexandre Gloter, and Marc Gabay for fruitful discussions. This work was supported by the Swiss National Science Foundation—Division II (Grant No. 200020_179155) and by the European Research Council under the European Union Seventh Framework Programme, Grant No. FP7/2007–2013 and ERC Grant Agreement No. 319286 (Q-MAC). We acknowledge the ALBA synchrotron facility and the BL29 beamline staff for support during the x-ray absorption spectroscopy experiment. M.G. and G.D.L. acknowledge support by the Swiss National Science Foundation under Grant No. PP00P2_170564.

The authors have no conflicts to disclose.

Marios Hadjimichael and Adrien Waelchi contributed equally to this work.

Marios Hadjimichael: Conceptualization (lead); Formal analysis (lead); Investigation (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Adrien Waelchli: Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (supporting); Writing – review & editing (equal). Bernat Mundet: Formal analysis (supporting); Investigation (supporting); Methodology (supporting). Siobhan McKeown Walker: Formal analysis (supporting); Investigation (supporting). Gabriele De Luca: Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – original draft (supporting); Writing – review & editing (supporting). Javier Herrero-Martín: Investigation (supporting); Methodology (supporting); Writing – original draft (supporting); Writing – review & editing (supporting). Marta Gibert: Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – original draft (supporting); Writing – review & editing (supporting). Stefano Gariglio: Conceptualization (supporting); Formal analysis (equal); Funding acquisition (supporting); Investigation (supporting); Methodology (supporting); Project administration (supporting); Resources (supporting); Writing – original draft (equal); Writing – review & editing (equal). Jean-Marc Triscone: Conceptualization (supporting); Funding acquisition (lead); Project administration (lead); Resources (lead); Supervision (lead).

The data that support the findings of this study are openly available in the Yareta repository at https://doi.org/10/gqx6bd.