arXiv subjects

R. A. Ortiz

Publications and source records attributed to R. A. Ortiz.

3 recordsLinked to original sources

arXiv2025

Oxygen sublattice disorder and valence state modulation in infinite-layer nickelate superlattices

The family of infinite-layer nickelates promises important insights into the mechanism of unconventional superconductivity. Since superconductivity has so far only been observed in epitaxial thin films, heteroepitaxy with the substrate or a capping layer possibly plays an important role. Here, we use soft x-ray spectroscopy to investigate superlattices as a potential approach for a targeted material design of high-temperature superconductors. We observe modulations in valence state and oxygen coordination in topotactically reduced artificial superlattices with repeating interfaces between nickelate layers and layers of materials commonly used as substrates and capping layers. Our results show that depending on the interlayer material metallic conductivity akin to the parent infinite-layer compounds is achieved. Depth-resolved electronic structure measured by resonant x-ray reflectivity reveals a reconstructed ligand field and valence state at the interface, which is confined to one or two unit cells. The central layers show characteristics of monovalent nickel, but linear dichroism analysis reveals considerable disorder in the oxygen removal sites. We observe a quantitative correlation of this disorder with the interlayer material that is important for future modeling and design strategies.

cond-mat.supr-con↗

arXiv2021

Magnetic correlations in infinite-layer nickelates: an experimental and theoretical multi-method study

We report a comprehensive study of magnetic correlations in LaNiO$_{2}$, a parent compound of the recently discovered family of infinite-layer (IL) nickelate superconductors, using multiple experimental and theoretical methods. Our specific heat, muon-spin rotation ($μ$SR), and magnetic susceptibility measurements on polycrystalline LaNiO$_{2}$ show that long-range magnetic order remains absent down to 2 K. Nevertheless, we detect residual entropy in the low-temperature specific heat, which is compatible with a model fit that includes paramagnon excitations. The $μ$SR and low-field static and dynamic magnetic susceptibility measurements indicate the presence of short-range magnetic correlations and glassy spin dynamics, which we attribute to local oxygen non-stoichiometry in the average infinite-layer crystal structure. This glassy behavior can be suppressed in strong external fields, allowing us to extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures, in analogy to doped cuprates that possess robust non-local spin fluctuations. The distinct temperature dependence of the intrinsic susceptibility of LaNiO$_{2}$ can be theoretically understood by a multi-method study of the single-band Hubbard model in which we apply complementary cutting-edge quantum many-body techniques (dynamical mean-field theory, cellular dynamical mean-field theory and the dynamical vertex approximation) to investigate the influence of both short- and long-ranged correlations. Our results suggest a profound analogy between the magnetic correlations in parent (undoped) IL nickelates and doped cuprates.

cond-mat.str-el↗

arXiv2021

A superlattice approach to doping infinite-layer nickelates

The recent observation of superconductivity in infinite-layer Nd$_{1-x}$Sr$_x$NiO$_2$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of LaNiO$_3$ embedded in insulating LaGaO$_3$, and used layer-selective topotactic reactions to reduce the nickelate layers to LaNiO$_{2}$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction and found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition metal oxides.

cond-mat.supr-con↗