Growth of PdCoO 2 films with controlled termination by molecular-beam epitaxy and determination of their electronic structure by angle-resolved photoemission spectroscopy

Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO 2 . Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO 2 have been made by replacing cobalt with iron in PdCoO 2 films grown

at the Fermi level with palladium character forms a reconstruction driven by the AFM order from the adjacent CrO 2 layer. [10][11][12][13][14] Comparing AFM PdCrO 2 with nonmagnetic PdCoO 2 , the spins from Cr 3+ interacting inside the CrO 2 layer with the palladium monolayers on either side of the CrO 2 layer play a critical role in the magnetic state of PdCrO 2 . 13 Angle-resolved photoemission spectroscopy (ARPES) is the premiere experimental tool to directly observe electronic structure in quantum materials. The combination of oxide molecularbeam epitaxy (MBE) with ARPES allows us to customize and study the electronic structure of correlated oxides. This setup has enabled understanding of how strain, 15,16 thickness, [17][18][19] hetertostructuring, 20,21 interfaces, 22,23 and terminations [24][25][26][27][28][29] can influence the electronic structure of thin films.
Due to the limited size of delafossite single crystals, the desire to explore the potential of metallic delafossites in electronic 30 and spintronic devices, 31 together with the exciting opportunities that can be explored in delafossite heterostructures, metallic delafossites are being grown in thin-film form by reactive sputtering, 32 pulsed-laser deposition (PLD), 30,[33][34][35][36] and MBE. 37,38 Unfortunately, the transport properties of these films (so far) pale in comparison to the single crystals. Differences between the Fermi surface of the so-called Pd-terminated PdCoO 2 with CoO 2 -terminated PdCoO 2 have recently been reported for epitaxial films of PdCoO 2 grown by PLD. 39 The claims of this prior study would be strengthened by improved evidence of surface termination control.
In this work, we describe an improved synthetic strategy for the growth of PdCoO 2 films with control of surface termination by MBE. Harnessing the ultra-high vacuum connection between our MBE and ARPES, we then study the electronic structure of Pd-terminated and CoO 2 -terminated PdCoO 2 . We find that our PdCoO 2 films exhibit similar bulk bands derived from palladium states but weak surface states compared to those in PdCoO 2 single crystals. Having succeeded in engineering the surface termination in PdCoO 2 over large areas, we then progress to investigate terminations of PdCoO 2 where we deliberately add unpaired electrons and study the resulting electronic structure by ARPES. Although we are able to epitaxially stabilize a variety of surface terminations involving iron substituting for cobalt in PdCoO 2 , we do not see evidence of magnetic order.
Building upon our previous work, 38 thin films of PdCoO 2 were synthesized by MBE in a Veeco GEN10 MBE system on (001) sapphire substrates. Details of the film growth are provided in the supplementary material. Figure 1(a) shows the shutter timing diagram used to supply fluxes of the individual molecular beams to the substrate to form PdCoO 2 . After growth, films were cooled down to 300 ○ C in the same ozone background pressure (around 5 × 10 −6 Torr) in which they were grown and transferred under ultra-high vacuum conditions into an adjacent ARPES chamber. The reflection high-energy electron diffraction (RHEED) patterns acquired after deposition and the x-ray diffraction θ-2θ scans indicate the growth of single-phase PdCoO 2 films as shown in Figs. 1(b) and 1(c). The structure was characterized by a Panalytical Empyrean x-ray diffractometer utilizing Cu Kα 1 radiation. Electrical transport measured by a Quantum Design Physical Property Measurement System (PPMS) using a four-point van der Pauw geometry is shown in Fig. 1(d). The residual resistivity ratio (RRR = ρ 300K /ρ 4K ) of this PdCoO 2 sample with a thickness of 52.1 nm is 3.3 in its as-grown state (i.e., no ex − situ postanneal). For comparison, the highest RRR previously achieved by MBE for a film in its as-grown state was 2.2 for a 10 nm thick PdCoO 2 film. 38 After an ex − situ anneal, films grown by MBE can reach a RRR of around 8 for a 50 nm thick PdCoO 2 film. 37 While these are the highest reported RRR values for films, PdCoO 2 single crystals can exhibit RRR as high as 400. 1 The resistivity of epitaxial PdCoO 2 films and single crystals are comparable at room temperature;

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scitation.org/journal/apm the huge difference in resistivity emerges upon cooling. One likely defect responsible for this difference is the in-plane rotation twins present in all epitaxial delafossite films to date. The presence of 180 ○ in-plane rotation twins in PdCoO 2 film grown on (001) Al 2 O 3 substrates manifest in the x-ray ϕ scan and scanning transmission electron microscopy images shown in Ref. 38, as well as the atomic force microscopy images shown in the supplementary material (Fig. S3).
In − situ ARPES measurements are performed to study the effects of termination on the electronic structure of the PdCoO 2 films. Our lab-based ARPES system photoexcites electrons with a Scienta omicron VUV 5000 helium discharge lamp using He-I photons at 21.2 eV and He-II photons at 40.4 eV. The emitted electrons are detected with a VG Scienta R4000 electron analyzer. The ARPES is vacuum-connected to the MBE growth chamber via an ultra-high vacuum transfer chamber.
We first compare the Fermi surface of Pd-terminated and CoO 2 -terminated PdCoO 2 films in Figs. 2(a) and 2(b), respectively. The sharp hexagonal pocket centered at Γ [illustrated in red in Figs. 2(a) and 2(b)] is observed in both Pd-and CoO 2 -terminated PdCoO 2 films. Two smaller hexagonal pockets inside the bulk state pocket illustrated in green and blue are observed in the CoO 2terminated PdCoO 2 film, in agreement with previous reports of splitting of the CoO 2 surface state driven by spin-orbit coupling. 40,41 For Pd-terminated PdCoO 2 , we do not observe pronounced palladium surface states at EF. Below EF, however, there is some spectral weight possibly from the palladium surface state as described in Ref. 41. At He-II photon energy (40.4 eV), we observe stronger spectral weight below EF, as illustrated in Fig. S5 of the supplementary material. This intensity below EF might be related to the palladium surface state, but at this higher energy, still no palladium surface state appears at EF. In Figs. 3(a) and 3(c), we further compare the dispersion cut along the Γ-K direction of Pd-and CoO 2 -terminated PdCoO 2 , respectively. The fitted Fermi velocities (vFs) of the PdCoO 2 bulk state of Pd-and CoO 2 -terminated films are all around 4.5 eV Å, as shown in Table I, in agreement with previous results measured on PdCoO 2 single crystals by ARPES. 9,13 Despite the invisible palladium surface state at EF, two spin-split surface states from the CoO 2 termination show up at EF. These are indicated by blue and green circles in Fig. 3(c). From the momentum dispersion curves (MDCs) comparison of Pd-and CoO 2 -terminated samples in Fig. 3(e), it is easier to observe the CoO 2 surface states at EF (indicated by the blue arrows) and very little of the palladium surface state is observed below EF (indicated by the red arrow). The Fermi velocities of the CoO 2 surface states are 0.75 eV Å (blue) and 0.5 eV Å (green), respectively. This is roughly 10% of that of the bulk state, in agreement with the previous study of PdCoO 2 single crystals. 9,42 Dispersion along the K-M-K direction of Pd-and CoO 2 -terminated PdCoO 2 films are shown in Figs. 3(f) and 3(h). Both terminations of the PdCoO 2 films show a split band at the M point 0.75 eV below EF and 1.75 eV below EF, as observed in PdCoO 2 single crystals. 41 Interestingly, a hole band below EF at the M point driven by the palladium surface state (as shown in Ref. 41) is not seen in our Pd-terminated films.
Comparing the electronic structure observed for our epitaxial PdCoO 2 thin films to that reported for PdCoO 2 single crystals with different terminations, our PdCoO 2 films have similar bulk state features to those of PdCoO 2 single crystals, but the surface states of our PdCoO 2 films are weaker or even disappear at EF. Note that the alternating layers of Pd + and CoO − 2 along the c-axis of PdCoO 2 are not charge neutral. Doping of the surface by electrons arising from electronic reconstruction (i.e., no structural surface reconstruction) would generate the surface states observed on bulk single crystals.  Ways in which the surfaces of our films differ from the single crystals provide routes to different or no surface states. For ARPES measurements of cleaved (001)-oriented PdCoO 2 single crystals, the polar surface exposed after cleaving may drive an electronic reconstruction of the surface or alternatively a mixture of termination regions, some of which are terminated by palladium and some of which are terminated by CoO 2 , to alleviate the polar surface charge. To synthesize PdCoO 2 films, we use shuttered MBE growth to provide a full layer of palladium or CoO 2 as the terminating surface. This difference in the surface reconstruction structure of epitaxial PdCoO 2 films might result in a different electronic reconstruction from that  38 Furthermore, our films might contain oxygen vacancies to neutralize the surface polar effect. These may also play a role in the differences observed in the surface states between PdCoO 2 single crystals and our epitaxial films. In particular, the palladium surface state is very reactive; it can be essentially destroyed by temperature cycling of ARPES measurements. 42 Prior ARPES results of a PdCoO 2 film grown by PLD, 39 where the palladium termination is confirmed by the absence of a CoO 2 surface state in the electronic structure, does not show the PdCoO 2 bulk state. In contrast, our Pd-terminated films show a strong PdCoO 2 bulk state without a CoO 2 surface state, but are missing the palladium surface state. One possible reason for this is the difference in sample quality, particularly of the sample surface and the ability of MBE to control the termination ARTICLE scitation.org/journal/apm of the PdCoO 2 film. We note that our MBE and ARPES measurements are made immediately following film growth and that the ultra-high vacuum connection between our MBE and ARPES systems enables us to investigate the electronic structure of the pristine growth surface.
With the goal of introducing magnetic order into the surface of PdCoO 2 , we attempt to replace the cobalt in the CoO 2 surface termination with a different transition metal. The low spin state (S = 0) of the d 6 electron configuration of Co 3+ in octahedral coordination underlies the lack of magnetic order in PdCoO 2 . In contrast, PdCrO 2 is known to order antiferromagnetically at 37 K due to the unpaired spins (S = 3/2) arising from Cr 3+ with its d 3 electron configuration. 10,11 Considering what other transition metals are stable in the 3+ oxidation state under the highly oxidizing conditions needed to stabilize PdCoO 2 led us to attempt to substitute Fe 3+ for Co 3+ . Other known iron-containing delafossites, such as the semiconductors AgFeO 2 and CuFeO 2 with bandgap larger than 1 eV, are known to exhibit magnetic phase transitions. [43][44][45][46] Combining magnetic FeO 2 with conducting palladium might create a new metallic delafossite with interesting magnetic properties. Note that PdFeO 2 is neither a known compound nor has it been suggested to form in the delafossite structure by prior crystal chemistry based suggestions of delafossites 47,48 nor first-principles suggestions for new delafossites. 49,50 Our attempts to terminate PdCoO 2 with a monolayer of FeO 2 were successful. To do so, we used epitaxial stabilization, a process in which lattice misfit strain energies and interfacial energies are exploited to favor a desired metastable phase over the equilibrium phase. 51 Other than the dip shown in dρ/dT at low temperature, which is different from other known delafossites, no pronounced magnetic order is observed by magnetic susceptibility measurement down to 3 K. This behavior is in contrast to what is seen for PdCrO 2 . Turning to the electronic structure of the PdFexCo 1−x O 2 films revealed by ARPES, a similar bulk band to PdCoO 2 is observed. No reconstruction appears at the Fermi surface nor is any temperature dependence of the electronic structure of PdFexCo 1−x O 2 seen.
We added a full monolayer of FeO 2 to a Pd-terminated PdCoO 2 film in an attempt to vary the termination of PdCoO 2 by introducing unpaired electrons (spin) from Fe 3+ . As ARPES is a surface sensitive measurement, if the unpaired electrons from Fe 3+ with its d 5 configuration in the surface FeO 2 layer interact within the FeO 2 layer and with the adjacent palladium layer like the CrO 2 layer does in the AFM metal PdCrO 2 , 13 we expect to see distinct features arise in the Fermi surface of the FeO 2 -terminated PdCoO 2 film. The well crystallized FeO 2 termination determined by RHEED is shown in Fig. 5(a). Unfortunately, no difference is observed in the ARPES other than the similar PdCoO 2 bulk state appearing at the Fermi surface in the FeO 2 -terminated PdCoO 2 film, as shown in Figs. 2(c) and 2(g). No reconstruction of the Fermi surface was seen at low temperature. The bulk band was also free of any temperature-dependent feature when we analyzed the MDCs in the Γ-K direction shown in

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scitation.org/journal/apm film does not appear to create a spin interaction with the underlying palladium layer. On the other hand, PdFeO 2 is a metastable phase and we can only stabilize one formula unit of PdFeO 2 . It is possible that the FeO 2 termination is insulating due to oxygen vacancies to neutralize its otherwise polar surface and preventing it from contributing to the electronic structure. The photoemission intensity data we collect include the film beneath the FeO 2 layer, which is PdCoO 2 itself. In addition to replacing the entire CoO 2 monolayer with an FeO 2 monolayer, we also investigated the partial replacement of cobalt with iron hoping that the presence of iron in multiple layers of the Pd(Co,Fe)O 2 structure would enhance the chance of spin interaction between the Fe 3+ and the adjacent palladium layer.  Table I, and within experimental error, they all have the same vF value as that of PdCoO 2 single crystals. 41  Further characterization of the PdFexCo 1−x O 2 films is shown in Fig. 5. The maximum percentage of iron that we are able to incorporate into epitaxial PdFexCo 1−x O 2 films while retaining a single phase is x = 20%. RHEED of a single-phase, 13 nm thick PdFe 0.2 Co 0.8 O 2 film is shown in Fig. 5(a). The fringes in the x-ray diffraction θ-2θ scans of the PdFexCo 1−x O 2 films indicate the high structural quality of these films. Electrical transport measurement on the PdFexCo 1−x O 2 films is shown in Fig. 5(d). Note that the PdCoO 2 film shown in this comparison has a thickness of 13 nm, a quarter of the thickness of the PdCoO 2 film in Fig. 1(d). The upturn in resistivity of the pure PdCoO 2 film (0% Fe) seen in Fig. 5(d) below 20 K likely originates from localization in the ultrathin film. As more cobalt is replaced by iron, the absolute resistivity of the iron-doped PdCoO 2 film keeps increasing. Interestingly, instead of showing an upturn at low temperature like is seen in the pure PdCoO 2 film, the Fe-doped PdCoO 2 films show a drop at low temperature in electrical resistivity. Moreover, as the iron content (x) of the PdFexCo 1−x O 2 film is increased, a more pronounced drop in resistivity is seen. Derivatives of the resistivity as a function of temperature of these PdFexCo 1−x O 2 films are shown in Fig. 5(e). Strikingly, a dip at about 20 K is observed in the temperature derivatives of all iron-doped

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scitation.org/journal/apm PdCoO 2 films, which is opposite to the dρ/dT in PdCrO 2 where a peak shows up at TN driven by AFM order. 10 The amplitude of the dip observed for PdFexCo 1−x O 2 increases with larger iron concentration.
A comparison of the temperature dependence of the Hall coefficient (RH) between a PdCoO 2 film and a PdFe 0.17 Co 0.83 O 2 film is shown in Fig. 6(a). The RH measurements are consistent with electrons acting as the carriers in both PdCoO 2 and PdFe 0.17 Co 0.83 O 2 films. The magnitude of RH in the PdCoO 2 film is in agreement with prior reports from PdCoO 2 single crystals. 57 In contrast to Ref. 39, we do not observe an anomalous Hall effect in our PdCoO 2 films at low temperature. Hall resistivities of the PdCoO 2 film and the PdFe 0.17 Co 0.83 O 2 film are shown in Fig. S9 of the supplementary material. The PdFe 0.17 Co 0.83 O 2 film exhibits a larger RH than does the pure PdCoO 2 film, which could be a result of carriers being trapped by iron-induced structural defects. For the PdCoO 2 film, the temperature dependence of RH at low temperature becomes flat while for the iron-doped PdCoO 2 film the RH starts increasing below 20 K, which is the same temperature at which the change in dρ/dT is observed in Fig. 5. One possibility for the observed resistivity anomaly at low temperature is that it is driven by iron disorder, since it is independent of the iron concentration. One scenario explaining why the RH difference between of PdCoO 2 film and the PdFe 0.17 Co 0.83 O 2 film does not reflect on the band structure is that the electrons from iron do not interact with the electrons from palladium. Instead, iron clusters just trap the electrons from the PdCoO 2 . Within this scenario, it is possible that iron disorder clusters in PdFe 0.17 Co 0.83 O 2 films are revealed by AFM in the supplementary material (Fig. S3). In Figs. 6(c) and 6(d), the magnetoresistance of the same PdCoO 2 film and PdFe 0.17 Co 0.83 O 2 film shows distinct magnetic dependences. The overall scale of magnetoresistance in the PdCoO 2 film is much smaller than that observed in PdCoO 2 single crystals. 58 The temperature dependence of the magnetoresistance of the PdFe 0.17 Co 0.83 O 2 film shows weak-localization-like behavior, which may arise from the magnetic disorder resulting from the addition of iron. The temperature dependence of the magnetic susceptibility of the PdFe 0.17 Co 0.83 O 2 thin film shows no transition or difference between the zero-field-cooled (ZFC) and field-cooled (FC) curves as shown in the supplementary material (Fig. S10). The observed behavior is in contrast to the splitting that is expected when AFM order is observed, such as in PdCrO 2 . 11 Thus, the replacement of cobalt by iron does not appear to result in any spin order. Instead, only signs of magnetic disorder are seen.
In summary, we have synthesized high-quality PdCoO 2 films by MBE and harnessed the atomic layer control afforded by MBE to tune the termination of these films to study the resulting electronic structure by ARPES. On comparing the Pd-terminated and CoO 2 -terminated PdCoO 2 films with those of PdCoO 2 single crystals, though the resistivity of our PdCoO 2 films are far higher than that of single crystals at low temperature, we find the PdCoO 2 bulk states in our films show features similar to those of PdCoO 2 single crystals, while the palladium surface state and CoO 2 surface state are not as strong as those of the PdCoO 2 single crystals. This difference might arise due to different electronic reconstructions. We also studied FeO 2 -terminated PdCoO 2 films and find that the only remaining PdCoO 2 bulk state in the electronic structure is similar to that of PdCoO 2 . In addition, we have successfully synthesized PdFexCo 1−x O 2 films. From the electric transport measurements, the addition of iron further increases the resistivity of PdCoO 2 films at room temperature. Meanwhile, we see different behavior at low

Conflict of Interest
The authors have no conflicts to disclose.

DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request. Building upon our previous work, 1 thin films of PdCoO 2 were synthesized by reactive oxide molecular-beam epitaxy (MBE) in a Veeco GEN-10 MBE system on (001) sapphire substrates.
The substrates were heated to temperatures in the 500 • C to 580 • C range as determined by a thermocouple close to, but not in contact with the substrate heater. A mixture of approximately 80% ozone and 20% oxygen at a background pressure of 5 × 10 −6 to 8.5 × 10 −6 Torr was used during deposition. The fluxes of palladium, cobalt, and iron evaporated from MBE effusion cells were each set to provide a flux of around 1 × 10 13 atoms·cm 2 /sec using a quartz crystal microbalance.
This initial calibration was then refined by measuring the thickness of a palladium calibration Epitaxial PdCoO 2 films were grown in two different ways. In the first method, the palladium and cobalt shutters were actuated to supply monolayer doses of palladium and cobalt following the sequence of atomic layers along the c-axis of the crystal structure of PdCoO 2 , as well as the film termination layer. We refer to this method as "shutter-controlled growth," and results from this method are shown in Fig. S1. Under a continuous ozone flux, growth is initiated with the CoO 2 monolayer. The first fourteen alternating monolayers (containing seven formula units PdCoO 2 ) are grown assuming unity sticking coefficients of cobalt and palladium, i.e., a Pd:Co ratio of 1:1.
After the deposition of seven formula units, 20% excess palladium is supplied in each shuttered dose, i.e., a Pd:Co ratio of 1.2:1. 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. 3 The flux of iron was adjusted according to the desired film composition assuming that the sticking coefficient of iron is unity.
The second method we have used to grow single-phase epitaxial PdCoO 2 films involves the codeposition of cobalt, palladium, and ozone under conditions that the excess palladium supplied desorbs as PdO (g). We refer this method as "absorption-controlled growth," and results using this method are shown in Fig. S2.     Fig. 3(j) of a Pd-terminated PdCoO 2 film and its second derivative with respect to energy. The Pd-terminated surface state is more pronounced when measured at this photon energy.
(e),(f) are the same as (d), but taken from a CoO 2 -terminated PdCoO 2 film and a Fe 0. 17 Table S1. We use linear fitting in the main text. Intensity (arb. units)