Transition-metal oxide heterostructures are attractive for functional device applications because electronic and magnetic phases that are inaccessible in the bulk can be stabilized by epitaxial strain, confinement, charge doping, or interface effects.
We use resonant elastic x-ray scattering to investigate ordering phenomena of spin, charge and orbitals in nanoscale complex oxide multilayers....
Perovskite rare-earth nickelates, $R$NiO$_3$, display a rich and only partially understood phase diagram, where all compounds with R from Pr to Lu undergo a metal-insulator transition (MIT) that is accompanied by a structural distortion. We use density functional theory (DFT) and its extensions (DFT+U, DFT+DMFT), combined with symmetry-based distortion mode analysis to explore the interplay...
RNiO$_3$ perovskites (R = rare earth, Tl$^{3+}$, Pb$^{3+}$ or Bi$^{3+}$) display a metal to insulator transition (MIT) at temperatures T$_{MIT}$ than increase by decreasing the lanthanide ionic radius. The electronic localization is associated to a charge disproportionation of the type 2Ni$^{3+}$ = Ni$^{3+d}$ + Ni$^{3-d}$ and with subtle structural distortions related to the...
The perovskite rare-earth nickelates RNiO$_3$ are a family of transition metal oxides with strong electron correlations. They exhibit antiferromagnetic order with four Ni spins per period below a composition-dependent Néel temperature $T_\text{N}$. Despite its high relevance to theoretical claims of ferroelectricity in the magnetically ordered phase, the exact arrangement of moments is still...
Rare-earth nickelates, RNiO$_{3}$, represent an intriguing example of materials with a highly tunable metal-insulator transition (MIT) and high potential for heterostructure engineering. The $T$-dependent paramagnetic MIT observed both in the bulk and ultrathin films of RNiO$_{3}$ is almost always accompanied by a subtle crystal-structure transformation, resulting in disproportionation of...
We used reflectometry and ellipsometry to investigate electronic properties of rare-earth nickelate films. We study the temperature dependence of the optical conductivity as chemical composition and strain varies close to charge and magnetic ordering temperatures.
The optical spectra show a characteristic two-peak structure when the material switches from metal to insulator that DMFT...
Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report measurements of the electronic structure of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial thin 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...
TbMnO$_3$ is a well-studied low-temperature multiferroic. Below 41 K the magnetic system orders antiferromagnetically into a spin density wave. Upon further cooling, below 27 K, the magnetic order changes to cycloidal and a ferroelectric polarization arises.
We present our results on photoinduced demagnetization dynamics in TbMnO$_3$, following excitation at 1.55 eV and 3 eV photon energies....
Complex oxide thin films and heterostructures exhibit a wide variety of interesting functionalities that are generally determined by numerous factors.
Here, we investigate the effects of epitaxial strain, dimensional confinement, as well as interface and surface effects on the electronic properties of the correlated metal CaVO$_3$, using a combination of density functional theory (DFT) and...
The control of the oxygen vacancies content in magnetoelectric oxides opens new routes to induce unexpected properties in strained thin films. Here we show experimental evidence for the increase of the oxygen-vacancy content upon increasing tensile epitaxial strain in multiferroic SrMnO3 thin films. Furthermore, a novel dependence of the oxygen stoichiometry on the film thickness has been...
Multiferroism can originate from the breaking of inversion symmetry caused by magnetic-spiral order. It usually arises due to competing magnetic exchange interactions that differ by their range and sign, and thus occurs at low temperatures. I present a mechanism that works at much higher temperatures. It relies on frustrating bonds randomly introduced along a single crystallographic direction,...
The sensitivity of the Jeff = 1/2 state of Sr2IrO4 to local coordination and structural distortions suggests epitaxial strain as an ideal route for manipulating this exotic ground state. As recently demonstrated, oxygen K-edge RIXS is capable to capture magnetic excitations in 5d-oxides [1]. From O K-edge RIXS on roughly 20-nm thick Sr2IrO4 films we observed the low-energy elementary...
In 5-d transition metal oxides, spin-orbit coupling becomes sizable and its interplay with e.g. the Coulomb interaction has led to the observation of exciting novel ground states. We report here on NaOsO3, which has been proposed to be the first realization of a Slater insulator i.e. a system in which a metal-insulator transition is driven only by antiferromagnetic correlation. We employed...
Like parent compounds of other unconventional and/or high-$T_c$ superconductor families, BaBiO$_3$ is an unexpected insulator that becomes superconducting (> 30 K) when doped. But bismuth oxides also offer an interesting contrast to other HTSCs, as they have no magnetic order and very weak electron correlations. I will summarize our efforts to revisit these decades-old materials with modern...
We propose a general scheme to probe the compatibility of arbitrary pairing states with a given normal state Hamiltonian by the introduction of a concept called "superconducting fitness". This quantity gives a direct measure of the suppression of the superconducting critical temperature in the presence of key symmetry-breaking fields, even in complex multi-band systems. In the light of this...
Ca$_2$RuO$_4$ is an archetypal example for multi-band Mott physics including spin-orbit and Hund's coupling. For decades, the mechanism underlying its Mott insulating state has remained elusive. This talk will present the complete low-energy ruthenium band structure as observed by ARPES in the paramagnetic insulating state of Ca$_2$RuO$_4$ . These results suggest that Ca$_2$RuO$_4$ is a...
We measured, by angle-resolved photoemission spectroscopy, the electronic structure of LiCu2O2, a mixed-valence cuprate where planes of Cu(I) ions are sandwiched between layers containing one-dimensional edge-sharing Cu(II) chains. We find that the Cu(I)- and Cu(II)-derived electronic states form separate electronic subsystems. The valence band, of the Cu(I) character, disperses within the...
DFT+U is a simple and powerful tool to model systems containing partially-filled manifolds of localized states. Conceptual and practical methods to determine U ab initio have been introduced long ago, based either on the constrained random-phase approximation or on linear-response theory. However, these approaches are often overlooked due to their cost or complexity. Here, we introduce a...
