Electronic structure of buried LaNiO3 layers in (111)-oriented LaNiO3/LaMnO3 superlattices probed by soft x-ray ARPES

Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report electronic structure measurements of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate layers present a similar electronic structure. However, as the thickness of the LaNiO3 is reduced the superlattices become insulating. These heterostructures do not show a marked redistribution of spectral weight in momentum space but exhibit a pseudogap of 50 meV.

Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report electronic structure measurements of (111)oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate layers present a similar electronic structure. However, as the thickness of the LaNiO3 is reduced the superlattices become insulating. These heterostructures do not show a marked redistribution of spectral weight in momentum space but exhibit a pseudogap of ≈ 50 meV.

Main Text
With the advancement of synthesis techniques it is possible nowadays to control the growth of transition metal oxide (TMO) epitaxial heterostructures at the unit cell level. The realization of such heterostructures have resulted in the discovery of fascinating phenomena as a consequence of the confinement of strongly correlated electrons in ultrathin high quality layers. [1][2][3] Since these thin layers are embedded in heterostructures, interfacial phenomena such as charge transfer and magnetic coupling play an important role together with dimensionality in determining the ground state of the system. [4][5][6][7][8] The recent development of combined angle-resolved photoemission spectroscopy (ARPES) and oxide heterostructures growth setups has allowed the electronic structure of TMO heterostructures to be probed directly.
However, the small electron escape depth in VUV-ARPES experiments have limited most studies to the top layer of oxide thin films. [2,[9][10][11] Most members of the nickel based perovskites oxides RNiO3, (R is a trivalent rare earth) exhibit a metal-to-insulator transition as a function of temperature. [12][13][14][15] In addition, a transition from a paramagnetic to an antiferromagnetic ground state is observed in these compounds. The critical temperature for both transitions is a function of the Ni-O-Ni bond angle and can be controlled by changing the R ion size. The least distorted member of the nickelates family, LaNiO3 (LNO), constitutes an exception since it remains rhombohedral, metallic and paramagnetic at all temperatures. Despite presenting this rather simple bulk phase diagram, studies of thin LNO films have surged motivated by the possibility of tailoring their properties by epitaxial strain, [16,17] thickness control, [18][19][20] and particularly by using them as a building block in epitaxial oxide heterostructures and devices [3,[21][22][23][24][25][26][27][28][29]. It has been shown by transport experiments that while thick (001)-LNO films grown on different substrates are metallic, as the thickness is reduced a metal to insulator transition is observed. [9,18] A similar dimensionality induced metal-insulator transition accompanied by the stabilization of an antiferromagnetic ground state was found in LAO/LNO superlattices. [3] Photoemission spectroscopy experiments have consistently reported a loss of spectral weight at the Fermi level (EF) accompanying the thickness driven metal-insulator transition, however, there is no consensus of the underlying physical mechanisms behind this observation. [9,[30][31][32] ARPES experiments showed that the 3D electronic structure of (001)-oriented LNO thin films remains intact down to 15 Å while for thinner films the destruction of Fermi liquid like quasiparticles [9] and the appearance of 1D Fermi surface (FS) nesting [32] was reported.
Interfacial and confinement effects in heterostructures grown along the perovskite -(111) orientation are much less studied [24,29,33,34]. Interestingly, in a thin film with a thickness of two monolayers (ML) of LNO grown along this orientation the Ni atoms form a buckled honeycomb lattice, such heterostructure was predicted to host novel topological phases which are yet to be found experimentally. [35][36][37][38] The recent realization of high quality heterostructures that combine LNO with thin insulating ferromagnetic LaMnO3 (LMO) layers along the (111) direction and exhibit unexpected phenomena is an example of the novel physics found in this type of heterostructures.
Transport experiments reveal a metal to insulator transition in (111)-LNO/LMO superlattices as the LNO thickness is reduced. The magnetic properties of the insulating phase are rather surprising. When the samples are cooled down with an applied magnetic field they display negative exchange bias (EB). [24] The sign of the EB can be reversed as a function of temperature for heterostructures with an LNO thickness of 7 monolayers (MLs), defined as the distance between consecutive Ni atoms of ~ 2.19 Å. This behavior was interpreted based on the emergence of an AF spiral with a (1/4, 1/4, 1/4) wave vector in the nickelate layer that couples the ferromagnetic LMO layers. [28] Here we employ soft x-ray ARPES (SX-ARPES) to study the electronic structure We have employed SX-ARPES to investigate the electronic structure of a LNO thin film and superlattices grown on STO-(111) oriented substrates. The experiments have been carried out at the SX-ARPES end station of the ADRESS beamline of the Swiss Light Source, (Switzerland). [39]. We used p-polarization of incident X-rays unless stated otherwise and the sample temperature was set to T=12 K. All samples were grown ex-situ by off-axis sputter deposition as described in Ref. [24] and exposed to air prior to the ARPES experiments. We first examined LNO15, a thin film with a thickness of 15 MLs. Then, we proceeded to study the evolution of the electronic  1 -2] direction obtained at different Γ points are shown in Figs. 2k and 2l. We observe that the band derived from the eg states of Ni which forms the electron pocket extends up to ~ 170 meV. A similar bandwidth was observed for thin films grown along the (001) direction indicating that distortions imposed by the substrate do not play a major part in determining this parameter. [44] We then turn our attention to the k-space cut through the R point at the BZ corner. In this measurement shown in Fig. 2b the electron pocket is absent as expected while large Fermi surface contours corresponding to cuts through the center of the bulk hole pockets are found at the locations predicted by the model in Fig. 2e. However the correspondence between calculation and data is not entirely satisfactory. The small hole contours predicted by the model are not resolved clearly in the experiment and the shape of the large hole contours differs slightly from the model. These subtle discrepancies could arise from the use of a simplified cubic unit cell in our tight binding model and/or from more intricate structural or electronic effects at the LNO/LMO interfaces. In order to distinguish these scenarios, we compare in Fig. 2g-2h our tight-binding model to a bulk DFT calculation that includes a trigonal distortion due to strain in the film and the bulk rotations of the NiO6 octahedron. We first notice that the DFT-FS obtained for the Γ and R point are identical. This is due to the presence of backfolded bands, which are generally weak in ARPES. [45] In order to highlight the fundamental bands, we overlay the tight-binding model with a single atom in the basis. Interestingly we see that the hexagonal-like central hole pocket in Intriguingly though, the k-space map obtained by integrating spectra over a narrow window around E -EF = 50 meV does not show a marked redistribution of spectral weight as it is characteristic for nesting driven itinerant spin-density wave phases. [48,49] While these findings do not exclude AF ordering in LNO as it was invoked in Ref. 28 to explain the exchange bias observed in superlattices with 7 MLs LNO, [24,28] they suggest that the insulating behavior of thin LNO layers in LNO/LMO superlattices is triggered by a gradual loss of quasiparticle coherence, rather than a well-defined phase transition to a magnetically ordered state.
We further investigated the loss of spectral weight at the Fermi level by analyzing the angle integrated spectra of the three mentioned heterostructures. In the valence band spectra of the superlattices shown in Fig. 4a there is a distinct peak ~2.3 eV below the Fermi level that is absent in the thin films and corresponds to the top LMO layer. In the near-EF region we identify two peaks located ~0.1 and 1 eV below EF. The optical gap of LMO has been reported between 0.6 up to 1 eV and previous XPS measurements revealed the onset of the density of states at 0.5 eV below the Fermi level. [50][51][52][53] Hence, we conclude that the peak located 0.1 eV below EF corresponds to eg states in Ni with the feature at 1 eV likely having contributions from both LMO and LNO. Figure 4b depicts Fig. 2c).

Absence of sample charging in [LNO7/LMO5]6 spectra.
Since the [LNO7/LMO5]6 superlattice is insulating at low temperatures, we took special precautions to test for the possibility of sample charging. In Fig. S2 we show angle integrated spectra obtained for different sizes of the exit slit and thus different photon fluxes on the sample. As observed in the figure the spectra are basically coincident for different exit slit sizes which confirms the absence of sample charging.
In the same figure we show a reference angle integrated spectrum measured on polycrystalline Au at the same temperature and photon energy to illustrate the opening of a pseudogap.

Density functional theory
The DFT Fermi surfaces were obtained using Vienna Ab-initio Simulation Package (VASP) [1,2] within Generalized Gradient Approximation (PBE parametrization [3]). The plane-wave cut-off energy was set to 600 eV, a k-mesh of 16x16x16 points for the rhombohedral and of 20x20x20 points for the cubic structures

Spherical vs. plane cuts
In Figure S4 we show the difference between a FS obtained as cuts of the 3D electronic structure of LNO with a plane passing through the Γ point (a,c) and a spherical k-space contour probed by our SX-ARPES experiment (b,d). In the manuscript we show plane cuts since this is easier to interpret and describing the curved cuts will confuse the reader. We notice however that some of the distortions observed in the measurements can be explained based on these maps.