Toward functionalized ultrathin oxide films: the impact of surface apical oxygen
J. Gabel, M. Pickem, P. Scheiderer, L. Dudy, B. Leikert, M. Fuchs, M. Stübinger, M. Schmitt, J. Küspert, G. Sangiovanni, J. M. Tomczak, K. Held, T.-L. Lee, R. Claessen, M. Sing
Adv. Electron. Mater.  2021, 2101006 (2021)
arXiv:2202.10778

Thin films of transition metal oxides open up a gateway to nanoscale electronic devices with novel electronic functionalities. While such films are commonly prepared in an oxygen atmosphere, they are typically considered to be ideally terminated with the stoichiometric composition. Using the prototypical correlated metal SrVO3 as an example, it is demonstrated that this idealized description overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not explicitly exposed to air, are shown to severely affect the intrinsic electronic structure of a transition metal oxide film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in the thin films. These findings highlight that it is important to take into account surface apical oxygen and the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of transition metal oxides. 

Zoology of spin and orbital fluctuations in ultrathin oxide films
M. Pickem, J. Kaufmann, K. Held, J. M. Tomczak
Phys. Rev. B 104, 024307 (2021)
arXiv:2201.07058

Many metallic transition-metal oxides turn insulating when grown as films that are only a few unit cells thick. The microscopic origins of these thickness-induced metal-to-insulator transitions, however, remain in dispute. Here, we simulate the extreme case of a monolayer of an inconspicuous correlated metal—the strontium vanadate SrVO3—deposited on a SrTiO3 substrate. Crucially, our system can have a termination to vacuum consisting of either a SrO or a VO2 top layer. While we find that both lead to Mott insulating behavior at nominal stoichiometry, the phase diagram emerging upon doping—chemically or through an applied gate voltage—is qualitatively different. Indeed, our many-body calculations reveal a cornucopia of nonlocal fluctuations associated with (in)commensurate antiferromagnetic, ferromagnetic, and stripe and checkerboard orbital ordering instabilities. Identifying that the two geometries yield crystal-field splittings of opposite signs, we elucidate the ensuing phases through the lens of the orbital degrees of freedom. Quite generally, our work highlights that interface and surface reconstruction and the deformation or severing of coordination polyhedra in ultrathin films drive rich properties that are radically different from the material's bulk physics. 

Particle-hole asymmetric lifetimes promoted by spin and orbital fluctuations in SrVO3 monolayers
M. Pickem, J. M. Tomczak, K. Held
Phys. Rev. Research 4, 033253 (2022)
arXiv:2008.12227 

The two-dimensional nature of engineered transition-metal ultra-thin oxide films offers a large playground of yet to be fully understood physics. Here, we study pristine SrVO3 monolayers that have recently been predicted to display a variety of magnetic and orbital orders. Above all ordering temperatures, we find that the associated non-local fluctuations lead to a momentum differentiation in the self-energy, particularly in the scattering rate. In the one-band 2D Hubbard model, momentum-selectivity on the Fermi surface ("k=kF") is known to lead to pseudogap physics. Here instead, in the multi-orbital case, we evidence a differentiation between momenta on the occupied ("k<kF") and the unoccupied side ("k>kF") of the Fermi surface. Our work, based on the dynamical vertex approximation, complements the understanding of spectral signatures of non-local fluctuations, calls to (re)examine other ultra-thin oxide films and interfaces with methods beyond dynamical mean-field theory, and may point to correlation-enhanced thermoelectric effects. 

Anisotropy of electronic correlations: On the applicability of local theories to layered materials
B. Klebel-Knobloch, T. Schäfer, A. Toschi, J. M. Tomczak
Phys. Rev. B. 103, 045121 (2021)
arXiv:2005.01369

Besides the chemical constituents, it is the lattice geometry that controls the most important material properties. In many interesting compounds, the arrangement of elements leads to pronounced anisotropies, which reflect into a varying degree of quasi-two-dimensionality of their low-energy excitations. Here we start by classifying important families of correlated materials according to a simple measure for the tetragonal anisotropy of their ab initio electronic (band) structure. Second, we investigate the impact of a progressively larger anisotropy in driving the nonlocality of many-body effects. To this end, we tune the Hubbard model from isotropic cubic in three dimensions to the two-dimensional limit and analyze it using the dynamical vertex approximation. For sufficiently isotropic hoppings, we find the low-energy self-energy to be well separable into a static nonlocal and a dynamical local contribution. While the latter could potentially be obtained from dynamical mean-field approaches, we find the former to be nonnegligible in all cases. Further, by increasing the model's anisotropy, we quantify the degree of quasi-two-dimensionality which causes this “space-time separation” to break down. Our systematic analysis improves the general understanding of electronic correlations in anisotropic or layered materials and heterostructures and provides useful guidance for future realistic studies.