Surface reconstructions and electronic structure of metallic delafossite thin films

The growing interest in the growth and study of thin films of low-dimensional metallic delafossites, with the general formula AB O 2 , is driven by their potential to exhibit electronic and magnetic characteristics that are not accessible in bulk systems. The layered structure of these compounds introduces unique surface states as well as electronic and structural reconstructions, making the investigation of their surface behavior pivotal to understanding their intrinsic electronic structure. In this work, we study the surface phenomena of epitaxially grown PtCoO 2 , PdCoO 2 , and PdCrO 2 films, utilizing a combination of molecular-beam epitaxy and angle-resolved photoemission spectroscopy. Through precise control of surface termination and treatment, we discover a pronounced √ 3 × √ 3 surface reconstruction in PtCoO 2 films and PdCoO 2 films, alongside a 2 × 2 surface reconstruction observed in PdCrO 2 films. These reconstructions have not been reported in prior studies of delafossites. Furthermore, our computational investigations demonstrate the B O 2 surface’s relative stability compared to the A -terminated surface and the significant reduction in surface energy facilitated by the reconstruction of the A -terminated surface. These experimental and theoretical insights illuminate the complex surface dynamics in metallic delafossites, paving the way for future explorations of their distinctive properties in low-dimensional studies.


ARTICLE pubs.aip.org/aip/apm
The CoO 2 layers adjacent to these conductive platinum (or palladium) layers serve as insulating spacers, leading to the out-of-plane resistivity being more than 1000 times higher than the in-plane resistivity in PdCoO 2 single crystals at low temperature. 1,7Notably, PtCoO 2 films maintain relatively high conductivity even with reduced dimensionality, exhibiting diminished sensitivity to film thickness in comparison to copper. 8Among delafossite compounds, PdCrO 2 is particularly remarkable for its unique combination of The film shows high crystalline quality with abrupt interfaces.(g) A higher magnification view of the region outlined by the orange box in (f).The inset of (g) displays an overlay with the structural model where platinum gives the strongest contrast, followed by cobalt due to its lower atomic number, whereas oxygen is invisible in the STEM-HAADF image.
antiferromagnetic order (AFM) at around 37 K while maintaining its metallic conductivity. 9,10][11][12][13] With their unique layered structures, metallic delafossites offer opportunities to create novel low-dimensional materials with distinctive properties and functionalities.The polar layers in delafossites, denoted as A 1+ (A = Pt, Pd) and BO 1− 2 (B = Co, Cr) in the bulk, exhibit distinct surface states when cleaved in vacuum, effectively addressing the challenges posed by polar surfaces. 14The A polar surfaces facilitate pronounced electron doping and stabilize the surface states of platinum and palladium layers, whereas hole-doped surface states emerge at the BO 2 terminations.The CoO 2 -terminated surface displays a substantial spin splitting of the surface states, arising due to a strong breaking of the inversion-symmetry at the surface. 15he ferromagnetism observed in the palladium surface state opens up possibilities for creating two-dimensional ferromagnets. 14,16The superlattice structures of PdCoO 2 and PdCrO 2 , characterized by structural symmetry breaking, provide a potential avenue for studying interlayer electron interactions.0][11][12][13] Despite the significant promise held by low-dimensional delafossites, interpretation of surface reconstruction in thin films is complicated by the simultaneous presence of surface states and electronic reconstructions driven by AFM order.This complexity underscores the need for further detailed investigations to decipher the intricate phenomena underlying these observations.
Through the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we have grown films of PtCoO 2 , PdCoO 2 , and PdCrO 2 with controlled terminations, enabling a detailed investigation into their electronic structures.These delafossites exhibit distinct surface reconstructions, including a new 2 × 2 surface reconstruction in PdCrO 2 that has never been previously reported for any delafossite material.Utilizing first-principles density functional theory (DFT) calculations, we demonstrate that surface reconstructions induced by excess oxygen significantly reduce the surface energy for A terminations, while BO 2 terminations exhibit relative stability compared to A terminations.In order to show a comprehensive comparison of the metallic delafossites -PtCoO 2 , PdCoO 2 , and PdCrO 2 -We present side-by-side ARPES, low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), and first-principles calculations of these materials in the subsequent figures.
Metallic delafossite thin films of PtCoO 2 , PdCoO 2 , and PdCrO 2 are synthesized in a Veeco Gen10 MBE system on (001) sapphire substrates.The atomic structure of delafossites, exemplified by PtCoO 2 , is illustrated in Fig. 1(a).The growth process is described in detail in the supplementary material.Figure 1(b) presents the results of PtCoO 2 films' growth in an adsorption-controlled regime, highlighting the narrow growth window for achieving single-phase PtCoO 2 films.8][19][20][21][22][23][24][25] The pure-phase nature of the PtCoO 2 films is confirmed by θ-2θ x-ray diffraction scans in Fig. 1(d).Resistivity vs temperature measurements of a 13.3-nm-thick PtCoO 2 film, conducted using a Quantum Design Physical Property Measurement System (PPMS) employing a fourpoint van der Pauw geometry, are shown in Fig. 1(e).As the 4-nm-thick PdCoO 2 buffer layer is quite flat as shown in the supplementary material (Fig. S1), its resistance has been subtracted out of the resistance measurement of the PtCoO 2 film (assuming a simple parallel resistance model).The resistivity of the PtCoO 2 film is subsequently calculated using the average thickness of the PtCoO 2 film.The residual resistivity ratio (RRR = ρ 300K /ρ 4K ) of this PtCoO 2 film is 1.91 in its as-grown state (i.e., without any ex situ post anneal).For comparison, the RRR of a PtCoO 2 single crystal is 52.5. 3 The resistivity of our PtCoO 2 film shows a similar linear temperature dependence behavior at high temperatures to that observed in the single crystals.The resistivity comparison between the film and the single crystal is shown in the supplementary material (Fig. S1).The microstructure of the PtCoO 2 film is investigated by highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images in Fig. 1(f) and electron energy-loss spectroscopy (EELS) maps found in the supplementary material (Fig. S4).Details on the growth method, as well as the structural and electrical characterization of PdCrO 2 films, can be found in the supplementary material (Figs.S2 and S3).All PtCoO 2 films investigated in this study exhibit a mixed termination of platinum and CoO 2 , a result of the adsorption-controlled method utilized to produce them. 24The palladium termination of the PdCoO 2 film and the CrO 2 termination of the PdCrO 2 films were achieved via the shutter-controlled growth method employed for their growths, as described in prior work. 22,24Note that not all delafossite films with the described terminations exhibit surface reconstructions.For example, no reconstruction features are present in the Pd-and CoO 2 -terminated PdCoO 2 films described in Ref. 24.This points to a multitude of routes to compensate for the polar surface charge in these materials and a delicate balance of energetics.The precise conditions for stabilizing the different reconstructions observed in these delafossite films, thus, require further study.
ARPES measurements were employed to investigate the bulk states and surface reconstructions of metallic delafossite films.ARPES measurements were conducted using lab-based ARPES systems, which consist of a Scienta Omicron VUV 5000 emitting He-I photons at 21.2 eV and He-II photons at 40.8 eV, and a Fermion Instruments BL1200s discharge lamp for neon-I photons at 16.85 eV. 26The detection of emitted electrons was performed using a VG Scienta R4000 electron analyzer.Prior to ARPES measurements, all films were exposed to air and then underwent re-annealing in ozone at a background partial pressure of 5 × 10 −6 Torr at 480 ○ C for 5 min.
In Figs.2(a), 2(c), and 2(e), we compare the Fermi surfaces of PtCoO 2 , PdCoO 2 , and PdCrO 2 films, respectively.Specifically, Fig. 2(a) highlights a hexagonal pocket, denoted as the α band.This intense band is centered at the Γ point and represents the platinum-driven band of PtCoO 2 , consistent with observation from previous studies on PtCoO 2 single crystals. 2Interestingly, in addition to the bulk α band, additional band features labeled as α ′ bands are observed both at EF and 150 meV below it, as shown in Figs.2(a) and 2(b).These α ′ bands occupy a similar momentum position to those of the reconstructed bands in PdCrO 2 at low temperatures. 11,13hile the known additional bands in the PdCrO 2 case arise due to In a palladium-terminated PdCoO 2 film, analogous new band features referred to as β ′ bands were found as shown in Figs.2(c) and 2(d).The PdCoO 2 bulk state, denoted as the β band, primarily exhibits palladium character according to prior work on PdCoO 2 single crystals. 14,27,28It is important to note that the β ′ bands in the PdCoO 2 film differ from the symmetry of surface states attributed to palladium and CoO 2 layers, as discussed in previous work. 14,24,29hus, the presence of the α ′ bands in PtCoO 2 films and the β ′ bands in PdCoO 2 films is new territory compared to prior work.
The band structure of the PdCrO 2 film is shown in Figs.2(e) and 2(f).Although the band structures of PdCoO 2 and PtCoO 2 are similar, that of PdCrO 2 exhibits distinctive behavior.This band structure exhibits a hexagonal pocket β band centered at Γ point, representing the primary palladium character of the bulk state, in line with previous reports on PdCrO 2 single crystals. 11,13The bulk bands observed in PtCoO 2 , PdCoO 2 , and PdCrO 2 films are consistent with our independent DFT calculations (see Fig. S11 in the supplementary material). 28Nonetheless, the PdCrO 2 film also presents additional band features, denoted as β ′′ bands, which have not been reported in any prior delafossite materials.Note that the data for the β ′′ band in our PdCrO 2 film were obtained above the AFM transition temperature, suggesting that this band's presence is not associated with the AFM order in PdCrO 2 .Nevertheless, the β ′′ bands are also present in the ARPES result of this PdCrO 2 film taken at 6 K, as shown in the supplementary material (Fig. S5).Moreover, the β ′′ band exhibits distinct dispersion compared to the reconstructed bands driven by AFM order in PdCrO 2 single crystals.In addition, the β ′′ bands are also present in a thinner PdCrO 2 film (4 nm thickness) as shown in Fig. S8 in the supplementary material.Nonetheless, not all the PdCrO 2 films with CrO 2 termination exhibit the β ′′ bands.Figure S9 in the supplementary material shows the dispersions at the M point of the PtCoO 2 and PdCrO 2 films.
We further compare the dispersion of these additional bands in the metallic delafossite films along the Γ-K direction, as illustrated in Fig. 3. Figures 3(a), 3(d), and 3(g) display the momentum dispersion curves at EF of the PtCoO 2 , PdCoO 2 , and PdCrO 2 films, respectively.The high-intensity α band in Fig. 3(a) and the highly dispersive β bands observed in Figs.3(d) and 3(g) correspond to the platinum and palladium bulk state in PtCoO 2 , PdCoO 2 , and PdCrO 2 films, as previously shown in Figs.2(a), 2(c), and 2(e).The α ′ band in the PtCoO 2 film and the β ′ band in the PdCoO 2 film exhibit similar dispersion, as illustrated in Figs.3(c) and 3(f).Conversely, the β ′′ band in the PdCrO 2 film displays a distinct kF position compared to the α ′ band in the PtCoO 2 film and the β ′ band in the PdCrO 2 film, as shown in Fig. 3(i).In addition, Fig. 3(a) reveals the subtle presence of an additional γ band in the PtCoO 2 film, a feature also shown in Fig. 2(a).This γ band could potentially be associated with a platinum surface state, similar to the palladium surface state observed in PdCoO 2 single crystals. 14Given the analogous behavior of the α ′ band in the PtCoO 2 film and the β ′ band in the PdCoO 2 film, our analysis now focuses on the comparison between the PtCoO 2 and PdCrO 2 films.
To investigate the surface reconstructions of the PtCoO 2 film and the PdCrO 2 films, we analyzed their LEED images displayed in Figs.4(a A detailed analysis of the Fermi surface measured on the PtCoO 2 film is performed by extracting the bulk α band at EF in Fig. 2 Our findings provide conclusive evidence for the primary √ 3 × √ 3 surface reconstruction in the PtCoO 2 film, which is also observed in the PdCoO 2 film.In addition, the main 2 × 2 reconstruction has been firmly identified in the PdCrO 2 film.Evidence of these reconstructions is consistently detected by the ARPES, LEED, and RHEED measurements on our films.Nonetheless, such specific reconstructions have not been previously reported in delafossite materials, which are usually prepared by cleavage of single crystals, prompting further investigation into their origin.Figure 4 elucidates the consistency between the RHEED data and the reconstructions observed in LEED results.We further conduct a comparative analysis of the RHEED images of the PdCoO 2 film [in Figs.2(c) and 2(d)] at various stages of growth, enabling real-time monitoring during the growth process.These RHEED images, presented in Fig. S10 in the supplementary material, reveal the emergence of one-thirdorder peaks during the annealing of the as-grown PdCoO 2 film in distilled ozone and persist throughout the cooling process.Remarkably, these one-third-order peaks in the PdCoO 2 film mirror those seen in the PtCoO 2 film [Fig.4(b)], corresponding to the . This observation suggests a potential link between the √ 3 × √ 3 reconstruction and excess oxygen on the film surface, implying that excess oxygen on the surface may mitigate the polar surface charge by reducing the surface energy.Note that the PdCoO 2 film possesses a full layer of palladium termination, introducing a positive charge.The additional oxygen helps to neutralize this polar surface charge, thus lowering the surface energy.Addressing the challenges of polar surfaces can be approached in various ways; for instance, when the PtCoO 2 film is grown by co-deposition, the result is a mixed termination by platinum and CoO 2 .It is important to mention that all films are subjected to annealing in distilled ozone to provide fresh surfaces prior to the ARPES measurement; thus, there are multiple steps during growth and annealing where excess oxygen is supplied and could, thus, attach to the film surface should it be energetically favorable to do so.
Given the limitations of STEM to clearly resolve surface atoms, we next discuss the potential surface scenarios compatible with √ 3 + n/3 oxygen), where n equals 1 or 2. Similarly, for the 2 × 2 reconstruction, we explore the possibility of one oxygen vacancy (2 × 2 − 1/4 oxygen), an excess of A atoms on the BO 2 -terminated surface (2 × 2 + n/4 A-site), and an excess of oxygen atoms on an A-terminated surface (2 × 2 + n/4 oxygen), where n is 1, 2, or 3.In addition, we have calculated the pristine surfaces, denoted as BO 2 -and A-site termination in Fig. 5.For each scenario, we search for the lowest energy surface atomic configuration and then calculate its surface energy relative to the pristine BO 2 -terminated surface.The results are illustrated in Fig. 5, with computational details available in the supplementary material (Fig. S12).
For all three compounds studied, the surface energies associated with creating an oxygen vacancy or adding excess A-site atom(s) on BO 2 -terminated surfaces are found to be positive, often exceeding 0.5 eV per unit cell area (panels 1 and 3 in Fig. 5).This indicates that both removing oxygen from and adding A atom(s) onto the BO 2 -terminated surface are energetically costly processes.Conversely, the introduction of excess oxygen atoms on A-terminated surfaces significantly lowers the surface energy, as shown in panel 5 in Fig. 5.In certain cases, this adjustment leads to negative surface energies when compared to the BO 2 -terminated surface, suggesting that adding oxygen to A-terminated surfaces is energetically favorable and could lead to more stable surface configurations.
The above results further support the hypothesis that the reconstructions observed in PtCoO 2 (mixed termination) and PdCoO 2 (palladium termination) films could be attributed to excess oxygen, which effectively reduces the surface energy.In our ARPES and LEED results, the √ 3 × √ 3 reconstruction is manifest on both PtCoO 2 and PdCoO 2 films.Nonetheless, our calculations indicate that scenarios leading to the √ 3 × √ 3 reconstruction do not consistently exhibit the lowest surface energy.This discrepancy might originate from the limitation of DFT in accurately determining surface energies or could be due to additional complexities present on the experimental film surfaces that are not fully accounted for in the DFT models.For instance, the mixed termination observed in PtCoO 2 could alter the energy landscape in ways not captured by our DFT simulations.Therefore, while our DFT calculations align with our ARPES and LEED findings in suggesting that excess oxygen contributes to the observed reconstructions, they do not conclusively determine the size of the reconstructed supercell.In the case of PdCrO 2 films grown with CrO 2 terminations, scenarios involving excess oxygen are not applicable.This leaves the 2 × 2 reconstruction with 1/4 A-site addition as a scenario with relatively low surface energy, aligning with our experimental observations and presenting a possible explanation for the surface structure of PdCrO 2 films.
In conclusion, we have successfully synthesized high-quality PtCoO 2 , PdCoO 2 , and PdCrO 2 films by MBE, followed by a comprehensive investigation of their surface reconstructions using a combination of ARPES, LEED, and RHEED.Our investigations have unveiled the presence of surface reconstructions in these films, including a √ 3 × √ 3 reconstruction in PtCoO 2 films with mixed platinum-CoO 2 terminations and in palladium-terminated PdCoO 2 film, as well as a distinctive 2 × 2 reconstruction is seen in CrO 2terminated PdCrO 2 films.These findings, which are corroborated by ARPES, LEED, and RHEED analyses, highlight surface reconstructions in these films that have not been previously reported in delafossite single crystals.These new reconstructions are likely closely linked to the polar terminations of the as-grown thin films, where the additional atoms on pristine surfaces could potentially lower their surface energy.DFT calculations shed light on the relative instability of surfaces terminated with A (A = Pt, Pd) atoms compared to those terminated with BO 2 (B = Co, Cr), as evidenced by their higher formation energies.Nonetheless, the presence of excess oxygen can significantly mitigate these energies, leading to reconstructed configurations.Our results show general consistency (with some discrepancies) between the calculated and experimentally observed sizes of the reconstructed supercells.By elucidating the distinct surface reconstructions from the surface states associated with different terminations in metallic delafossites, as well as from the electronic reconstructions driven by magnetic order, our work not only contributes to the understanding of surface phenomena in delafossite materials but also sets the stage for further exploration of highly two-dimensional studies in this intriguing class of materials.
See the supplementary material for a description of how the PtCoO 2 and PdCrO 2 films are grown and analyzed, additional characterization by TEM, RHEED, and ARPES, and details on the method of DFT calculations.(*Electronic mail: qisong@cornell.edu)

A. Film growth
PtCoO 2 , PdCoO 2 , and PdCrO 2 films were all grown on (001) sapphire substrates, which had been annealed to 1000 • C for 6 hours prior to film growths.During the deposition, distilled ozone (a mixture of approximately 80% ozone and 20% oxygen) was introduced at a background partial pressure of 5 × 10 −6 to 8.5 × 10 −6 Torr during all of the growths.The fluxes of platinum, palladium, cobalt, and chromium, evaporated from MBE effusion cells, were adjusted to achieve a flux of approximately 1 × 10 13 atoms cm 2 s −1 , as determined by a quartz crystal microbalance.This initial flux calibration was further refined by measuring the thickness of a platinum calibration film grown on a (111) (ZrO 2 ) 0.905 (Y 2 O 3 ) 0.09 substrate by x-ray reflectivity (XRR); 1 the same was done for palladium.The Co 3 O 4 calibration film was grown on a (100) MgAl 2 O 4 substrate and measured by XRR; the Cr 2 O 3 calibration film was grown on a (001) sapphire substrate and measured by XRR. 1 For the PtCoO 2 film growth, the substrates were heated to temperatures ranging from 500 • C to 540 • C, as monitored by an optical pyrometer operating at a wavelength of 980 nm.To obtain single-phase epitaxial PtCoO 2 films, it was crucial to use an epitaxial PdCoO 2 film as a buffer layer on a (001) Al 2 O 3 substrate, which promoted the nucleation and epitaxial overgrowth of the PtCoO 2 film.For all of the films in this study, a four or seven formula-unit-thick buffer layer of PdCoO 2 on the (001) Al 2 O 3 substrates was employed.Subsequent codeposition of platinum, cobalt, and ozone was carried out under conditions where the excess platinum supplied would desorb as PtO (g), following a similar concept of the absorption-controlled growth conditions utilized for the growth of PdCoO 2 . 2 .The ratio of Pt:Co in this codeposited flux ranged from 1.3 to 1.4.Additional details of PdCoO 2 growth are described in the supplementary material of Ref. 2.
For the PdCrO 2 growth, the substrates were heated to around 600 • C. A 12-nm-thick CuCrO 2 film is employed as a buffer layer to stabilize the delafossite structure of PdCrO 2 under a background pressure of 1 × 10 −6 Torr of distilled ozone.With a continuous flux of distilled ozone giving rise to a background pressure of 5 × 10 −6 Torr, platinum and chromium shutters were actuated to supply monolayer doses of platinum and chromium following the sequence of atomic layers along the c-axis of the crystal structure of PdCrO 2 .The first seven formula-unite-thick layers of PdCrO 2 are grown with a Pd:Cr ratio of 1:1.After the deposition of seven formula-unit-thick layers, 20% excess palladium is supplied in each shuttered dose, i.e., a Pd:Cr ratio of 1.2:1.The excess palladium supplied in each shuttered dose is to make up for the evaporation of palladium oxide at the relatively high substrate temperature and ozone pressure used, similar to the growth of PdCoO 2 films by a shutter-controlled method.
, where t PtCoO2 is the average thickness of the PtCoO 2 film and R Total is the resistance measured for the PtCoO 2 /PdCoO 2 bilayer.Temperature-dependent resistivity measurements of the PdCrO 2 /CuCrO 2 bilayer films with varying (average) thickness of the PdCrO 2 film.The resistivities plotted ignore the effect of the CuCrO 2 buffer layer as the in-plane resistivity ( ρ 11 ) of (001) CuCrO 2 is over 7 orders of magnitude higher than ρ 11 of PdCrO 2 at room temperature and this ratio grows larger as the temperature is lowered because CuCrO 2 is a semiconductor whereas PdCrO 2 is a metal. 6,7milar to PdCoO 2 films, 8 the ultra-thin PdCrO 2 films show insulating behavior.Note due to island growth, electrical percolation influences the electrical transport properties.Among the films with an average thickness of 2.     During the subsequent cool-down to room temperature, strong quarter-order peaks show up, persisting alongside the one-third-order peaks.Note in some ARPES measurements of other PdCoO 2 films lacking the presence of one-third-order peaks in RHEED, the reconstructed band is not observed. 2

E. DFT
First-principles calculations were performed using the Vienna Ab initio Simulation Package (VASP) with PBEsol exchange-correlation functional and projector augmented-wave pseudopotentials.We applied a kinetic energy cutoff of 520 eV and a k-mesh of 12 × 12 × 2 for the bulk conventional cell.We employed the method of Methfessel-Paxton of order 1 to describe the partial occupancy at E F , with a smearing value of 0.1 eV.Symmetric slabs were used for band structure and Fermi surface calculations, see Figs.S12(a We also calculate the surface magnetization for the A-site terminated surfaces.The ground state of the A-site terminated surfaces are ferromagnetic with a small magnetization on the surface A atom 0.4 µB, in agreement with the experiments. 11The surface 120 • AFM state is 20 meV per u.c.area higher in energy than the surface ferromagnetic ground state -showing that the effect of surface magnetization is small compared to the surface energy differences between different vacancy scenarios.Therefore, for the calculation of surface energy, we used non-spin-polarized calculations for PtCoO 2 and PdCoO 2 , and spin-polarized calculations for PdCrO 2 , assuming a fer-romagnetic configuration.The results for the three compounds were tested against results obtained from spin-polarized calculations with Hubbard U = 4 eV, and no qualitative changes were found, confirming a small effect of magnetic configuration on the structural properties and surface energy for PdCrO 2 . The relative surface formation energy E ∆surf was computed by To reduce the computational cost, we applied an asymmetric slab to compute E surf and E ref , as illustrated in Fig. S12(c).We employed BO 2 -terminated bottom and various surface terminations with different reconstructions.Only atoms near the top surface were relaxed.A dipole correction was applied to eliminate the energy due to the artificial electric field resulting from the asymmetric slab 12 .In addition, we extrapolated the surface energy to infinite vacuum width to eliminate the interactions between periodic images.For the situation with A-excess on a BO 2 -termination or oxygen excess on the A-termination, the original locations of excess A site or oxygen atoms with vertical A-O bonds tend not to have the lowest energy.Therefore, for each situation, we compared the energies of all potential surface binding sites for an excess of A site or oxygen after surface relaxation to determine the ground state structure and energy.

FIG. 1 .
FIG. 1. Structural and electrical characterizations of PtCoO 2 thin films grown on (001) Al 2 O 3 substrates by MBE.(a) Crystal structure of PtCoO 2 , with (100) and (110) planes illustrated on the atomic (001) plane by the black lines.(b) Diagram showing the phases obtained as a function of substrate temperature and Pt:Co ratios during film growth by co-deposition on (001) Al 2 O 3 substrates.Red circles indicate phase-pure PtCoO 2 films, blue squares indicate PtCoO 2 films in which Co 3 O 4 exists as a second phase, yellow triangles indicate conditions under which platinum exists as a second phase, and purple diamonds indicate where Co 3 O 4 and platinum impurity phases also exist in the PtCoO 2 films.All PtCoO 2 films described here are deposited onto 4-nm-thick PdCoO 2 films.(c) Atomic force microscopy image of a 13.3-nm-thick PtCoO 2 film deposited on top of a 4.0-nm-thick PdCoO 2 buffer layer.(d) X-ray diffraction of the same PtCoO 2 thin film characterized in (c).* denotes the 006 peak of the (001) Al 2 O 3 substrate.(e) Temperature-dependent resistivity measurements of the same PtCoO 2 film characterized in (c).(f) STEM-HAADF image of a PtCoO 2 film grown on a 4-nm-thick buffer layer of PdCoO 2 grown on a (001) Al 2 O 3 substrate, viewed along the [201] zone axis of PtCoO 2 .The film shows high crystalline quality with abrupt interfaces.(g) A higher magnification view of the region outlined by the orange box in (f).The inset of (g) displays an overlay with the structural model where platinum gives the strongest contrast, followed by cobalt due to its lower atomic number, whereas oxygen is invisible in the STEM-HAADF image.

FIG. 2 .FIG. 3 .
FIG. 2. Photoemission intensity maps, collected using 21.2 eV photons, at E F for (a) the 17.3-nm-thick PtCoO 2 film (including 4-nm-thick PdCoO 2 buffer layer), (c) a 18.3nm-thick PdCoO 2 film, and (e) a 14.1-nm-thick PdCrO 2 film.(b), (d), and (f) represent the same maps as (a), (c), and (e), respectively, but are taken at 150 meV below E F .The data of the PtCoO 2 and PdCoO 2 films were obtained at a temperature of 6 K, while the PdCrO 2 film data were taken at 50 K, i.e., above the AFM transition temperature (37 K) observed in PdCrO 2 single crystals.Data of the PdCrO 2 film collected at 6 K are shown in Fig. S5 in the supplementary material.

FIG. 4 .
FIG. 4. Surface reconstructions observed via LEED, RHEED, and ARPES.(a) LEED image of the PtCoO 2 film, where red solid diamonds correspond to the 1 × 1 reciprocal lattice.The green diamonds highlight the √ 3 × √ 3 reconstruction, as shown in the hexagonal green zone.The yellow hexagons indicate the observed 4 × 4 reconstruction in LEED, signified by the zone covered by dashed yellow lines.The black arrow corresponds to the [100] electron-beam direction of the RHEED image shown in (b).The white dashed line denotes the direction along which the RHEED image is captured.(b) RHEED pattern of the PtCoO 2 in the (100) * reciprocal lattice plane, corresponding to the white dashed line in (a).The dashed line indicates where the intensity data are extracted.The symbols on the dashed line match those on the white dashed line in (a), precisely aligning with the peaks observed in the RHEED image.(c) and (d) The same information as (a) and (b), respectively, but for the PdCrO 2 film.In (c), the red solid diamonds correspond to the 1 × 1 reciprocal lattice of the PdCrO 2 film, while the blue hexagons illustrate the 2 × 2 reconstruction shown in the blue hexagonal zone.A weak √ 3 × √ 3 reconstruction is indicated by the green diamonds in the dashed hexagonal green zones.(e) The Fermi surface of the PtCoO 2 film, where the central hexagonal pocket made up of green dots is extracted from where the bulk state α band crosses E F .The surrounding hexagonal pockets are multiplied by the √ 3 × √ 3 folding.The red arrows indicate one of the nesting directions.(f) The same as (e), but for the PdCrO 2 film with the 2 × 2 reconstruction.

√ 3 .
) and 4(c).In the composite LEED image of the PtCoO 2 film [Fig.4(a)], the reciprocal lattice of PtCoO 2 is marked by red solid diamonds, as shown by the red solid hexagonal zone in the center.A distinct √ 3 × √ 3 reconstruction with a 30 ○ rotation is indicated by the green diamonds along with a less pronounced 4 × 4 reconstruction signified by the yellow hexagons.The √ 3 × √ 3 reconstruction with a 30 ○ rotation is simplistically denoted as √ 3 × The green zones (solid lines) and yellow zones (dashed lines) denote the pronounced √ 3 × √ 3 and the subtle 4 × 4 reconstructed orientations, respectively.Importantly, these reconstruction patterns observed in the PtCoO 2 LEED image correspond precisely with those observed in the RHEED image along the [100] direction in reciprocal space, as evidenced by comparing the white dashed line in Fig. 4(a) with the peak positions in the RHEED image [Fig.4(b)].Similarly, the LEED image for the PdCrO 2 film [Fig.4(c)] displays the reciprocal lattice of PdCrO 2 denoted by red solid diamonds, with a strong 2 × 2 reconstruction denoted by blue hexagons and a weak √ 3 × √ 3 reconstruction represented by green diamonds.The blue zones (solid lines) and green zones (dashed lines) represent the strong 2 × 2 and the weak √ 3 × √ 3 reconstructed orientations, respectively.Once again, the reconstructions identified in the LEED images are seen to be consistent with the peaks observed in the RHEED image along the [100] direction, as indicated in Figs.4(c) and 4(d), ensuring a consistent interpretation of the surface reconstructions across different imaging techniques.
(a) and folding it with the √ 3 × √ 3 reconstruction.The corresponding results are presented in Fig. 4(e), where we observed a striking match between the additional band features at EF (α ′ band) and the folded band with the √ 3 × √ 3 reconstruction.Despite its presence in both the LEED and RHEED analyses of the PtCoO 2 film, we did not observe any additional 4 × 4 reconstruction at EF.It is worth noting that the intensity of the 4 × 4 reconstruction is considerably weaker compared to the √ 3 × √ 3 reconstruction, which may account for its limited visibility.Similarly, we folded the bulk band of the PdCrO 2 film at EF, which revealed an almost exact correspondence between the additional β ′ band and the band folded with the 2 × 2 reconstruction, as presented in Fig. 4(f).Interestingly, the band structure of the PdCrO 2 film at EF did not exhibit the √ 3 × √ 3 reconstruction suggested by the LEED result.This discrepancy could be attributed to the relatively weak √ 3 × √ 3 reconstruction compared to the 2 × 2 reconstruction, as evidenced in the LEED analysis of the PdCrO 2 film presented in Fig. 4(f).

FIG. 5 .
FIG. 5. Surface energy analysis via first-principles DFT calculations for PtCoO 2 , PdCoO 2 , and PdCrO 2 .The surface energies for different scenarios are compared against the BO 2 terminated surface in panel 2, i.e., oxygen vacancy, A excess, and oxygen excess, which modify the pristine A or BO 2 terminated surfaces and lead to √ 3 × √ 3 and 2 × 2 reconstruction.The vertical arrows point to the surface configurations with the lowest surface energy."BO 2 -term" stands for films terminated by a CoO 2 layer or CrO 2 layer, and "A-term" represents films terminated by a palladium or platinum layer.We used non-spin-polarized calculations for PtCoO 2 and PdCoO 2 , and spin-polarized calculations for PdCrO 2 , assuming a ferromagnetic configuration.

√ 3 × √ 3 and 2 × 3 +
2 reconstructions through DFT calculations.We have chosen to omit scenarios leading to the weak 4 × 4 reconstruction observed in PtCoO 2 due to the complexity introduced by their large supercell sizes.For the oxygen), an excess of A-site atoms on a BO 2 -terminated surface ( n/3 A-site), and an excess of oxygen atoms on an A-terminated surface (√ 3 × This paper was primarily supported by the U.S. Department of Energy, Office of Basic Sciences, Division of Materials Science and Engineering under Award No. DE-SC0002334.This research was funded in part by the Gordon and Betty Moore Foundation's EPiQS Initiative (Grant Nos.GBMF3850 and GBMF9073 to Cornell University).This paper made use of the Cornell Center for Materials Research shared facilities, which are supported through the NSF Materials Research Science and Engineering Centers Program (Grant No. DMR-1719875).B.D.F., M.R.B., and B.P. acknowledged support from the National Science Foundation Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM) under Cooperative Agreement No. DMR-2039380.This paper also made use of the Cornell Energy Systems Institute Shared Facilities partly sponsored by the NSF (Grant No. MRI DMR-1338010) and the Kavli Institute at Cornell.Substrate preparation was performed, in part, at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the NSF (Grant No. NNCI-2025233).P.K. acknowledged support from the European Research Council (through the QUESTDO Project, 714193) and The Leverhulme Trust (Grant No. RPG-2023-256).

2
FIG. S1: (a) Resistivity comparison between the 13.3 nm PtCoO 2 film (after subtraction of the contribution from the 4-nm-thick PdCoO 2 buffer layer) and a PtCoO 2 single crystal (from Ref. 3).Due to the rough surface of the PtCoO 2 shown in Fig. 1(c), the approximate resistivity of the PtCoO 2 film is what is plotted in (a).The residual resistivity ratio (RRR = ρ 300K /ρ 4K ) of the single crystal is from Ref. 4. (b) Derivative of the film resistivity with respect to temperature as a function of the temperature for the same PtCoO 2 film, presenting linear temperature dependence at high temperatures.The calculation used a Savitzky-Golay smooth filter. 5(c) Temperature-dependent resistivity measurements of a 4.0-nm-thick PdCoO 2 film.(d) Atomic force microscopy images of the PdCoO 2 film characterized in (c).The resistance subtraction to estimate the resistance of the PtCoO 2 film (R PtCoO2 ) from which the resistivity of the PdCoO 2 film shown in Fig. 1(e) is calculated involved the following formula: 1 FIG. S2: Atomic force microscopy images of (a)-(d) varying thicknesses of CuCrO 2 films and (e)-(h) varying thicknesses of PdCrO 2 films grown on top of 12-nm-thick CuCrO 2 films.Due to island growth, the ultra-thin CuCrO 2 films do not cover the entire substrate; similarly, the ultra-thin PdCrO 2 films also exhibit island growth on and between the underlying CuCrO 2 islands.The thickness given refers to the average thickness of the films.
FIG. S3: Structural and electrical characterization of PdCrO 2 thin films grown on 12-nm-thick (average thickness) CuCrO 2 buffer layers on (001) Al 2 O 3 substrates by MBE.(a) FIG. S4: EELS elemental mapping confirms a homogeneous elemental distribution in the alternating Pd + and CoO 2− with sharp interfaces between substrate, buffer layer, and thin film.
FIG. S6: Photoemission intensity maps of a PtCoO 2 film at photon energies of (a) 16.7 eV and (b) 21.2 eV, respectively, revealing the presence of the √ 3 × √ 3 reconstruction at both photon energies.
FIG. S7: Photoemission intensity maps of PtCoO 2 and PdCoO 2 films were obtained using a photon energy of 40.4 eV (Helium II).At this energy, the reconstructed bands are not visible.A comparison experiment was conducted by initially measuring the ARPES using a 40.4 eV photon energy and subsequently with a 21.2 eV photon energy.Interestingly, the reconstruction is only observed in the data acquired using the 21.2 eV and 16.7 eV photon energies, as shown in Fig.S6.This energy dependence in the reconstruction bands could potentially be attributed to the relevant photoemission cross section.
FIG. S8: Photoemission intensity maps of a 4-nm-thick PdCrO 2 film obtained using a photon energy of 21.2 eV.The 2 × 2 reconstruction feature is present both below and above T N .
FIG. S9: Photoemission intensity distributions along the K-M-K direction of PtCoO 2 andPdCrO 2 films obtained using a photon energy of 21.2 eV.

)
FIG. S11: Band-structure calculations were performed for PtCoO 2 , PdCoO 2 , and PdCrO 2 with different terminations.Inserts are the corresponding Fermi surfaces.The green, blue, and orange dots highlight the projection onto the surface A-site, B-site, and O atoms, respectively, whichwere obtained from DFT using the post-processing tool VASPKIT.9  .
S1) where E surf is the energy of the √ 3 × √ 3 or 2 × 2 reconstructed surface.E ref is the energy of the reference structure, and here we chose the clean BO 2 -terminated surface as our reference, since here only the relative energies of the configurations matter.N × µ atom is the chemical potential of the N atoms added or removed from the reference structure.The chemical potential for Pt/Pd was estimated from the DFT energy of bcc Pt/Pd metal.For oxygen, we used half the DFT energy of a single O 2 molecule in a large box.The results are µ O = −4.688,µ Pt = −6.965,µ Pd = −5.692 in units of eV.