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openaire: EC/H2020/721642/EU//SOLUTION | openaire: EC/H2020/676580/EU//NoMaD | openaire: EC/H2020/740233/EU//TEC1p Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I "Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications", which took place within the 2018 European Materials Research Society fall meeting.

Heterostructures of transition metal oxides (TMO) perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial 2D electron system (2DES) discovered at the LAO3/STO3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the dependence of the electronic structure on the width of the confining potential using soft X-ray ARPES combined with ab-initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3d t2g bands and, interestingly, redistributes the charge between the dxy and dxz/dyz bands.

Interfaces between complex oxides constitute a unique playground for 2D electron systems (2DES), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO3/SrTiO3 interface is one of the most studied in this regard, and its origin is determined by both the presence of a polar field in LaAlO3 and the insurgence of point defects, such as oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decreased electronic mobility. In this work we use an amorphous WO3 overlayer to control the defect formation and obtain an increased electron mobility and effective mass in WO3/LaAlO3/SrTiO3 heterostructures. The studied system shows a sharp insulator-to-metal transition as a function of both LaAlO3 and WO3 layer thickness. Low-temperature magnetotransport reveals a strong magnetoresistance reaching 900% at 10 T and 1.5 K, the presence of multiple conduction channels with carrier mobility up to 80 000 cm2/Vs and an unusually high effective mass of 5.6 me. The amorphous character of the WO3 overlayer makes this a versatile approach for defect control at oxide interfaces, which could be applied to other heterestrostures disregarding the constraints imposed by crystal symmetry.

Interplay of spin, charge, orbital and lattice degrees of freedom in oxide heterostructures results in a plethora of fascinating properties, which can be exploited in new generations of electronic devices with enhanced functionalities. The paradigm example is the interface between the two band insulators LaAlO3 and SrTiO3 that hosts a two-dimensional electron system. Apart from the mobile charge carriers, this system exhibits a range of intriguing properties such as field effect, superconductivity and ferromagnetism, whose fundamental origins are still debated. Here we use soft-X-ray angle-resolved photoelectron spectroscopy to penetrate through the LaAlO3 overlayer and access charge carriers at the buried interface. The experimental spectral function directly identifies the interface charge carriers as large polarons, emerging from coupling of charge and lattice degrees of freedom, and involving two phonons of different energy and thermal activity. This phenomenon fundamentally limits the carrier mobility and explains its puzzling drop at high temperatures., Nature Communications, 7, ISSN:2041-1723

The question of optical bandgap anisotropy in the monoclinic semiconductor $��$-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk $��$-Ga2O3 was found to be a function of light polarization and crystal orientation, with the lowest onset occurring at polarization in the ac crystal plane around 4.5-4.6 eV; polarization along b unambiguously shifts the onset up by 0.2 eV. The theoretical analysis clearly indicates that the shift of the b onset is due to a suppression of the transition matrix elements of the three top valence bands at $��$ point., 11 pages, 5 figures

We report the electronic properties of the quadruple perovskite CaCu3Ti4O12 as obtained via several density-functional based methods and propose a new interpretation of optical experiments to the effect that four distinct transitions (centered around 0.7, 1.5, 2.5, and 3.5 eV) contribute to the spectrum. The comparison with experiment is satisfactory, especially after we account for the effects of spin disorder, which does not close the fundamental gap but suppresses the transition intensity. We find that some of the methods we employ tend to overestimate considerably the gaps for standard values of the respective adjustable parameters.

We present the first ab-initio band-theory-based description of spin-compensated polarons (known as Zhang-Rice singlets) in a hole-doped cuprate, specifically one-dimensional Ca_{2+x} Y_{2-x} Cu_5 O_10. Zhang-Rice singlets are many-particle configurations relevant to the exotic behavior of hole-doped cuprates, stemming from spontaneous charge localization. They appear in our case-study material above a threshold doping, successively turning the insulating undoped antiferromagnet into a gap insulator, a singlet-rich metallic paramagnet, and finally, a singlet-saturated insulating diamagnet., Comment: 4 pages, 6 figures, submitted to PRL

We describe from advanced first principles calculations the energetics of oxygen doping and its relation to insulator-metal transitions in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We find a strong tendency of doping oxygens to order into non-magnetic Cu$^{1+}$O$_x$ chains at any $x$. Ordering produces one-dimensional metallic bands, while configurations with non-aligned oxygens are insulating. The Cu$^{2+}$O$_2$ planes remain insulating and antiferromagnetic up to a threshold between $x$=0.25 and 0.5, above which a paramagnetic normal-metal state prevails. The in-plane antiferro-paramagnetic competition depends on $x$, but only weakly on the ordering state of the chains., Comment: 4 pages, 6 figures, 2 tables

The Mn-terminated (001) surface of the stable anti-ferromagnetic insulating phase of cubic perovskite CaMnO$_3$ is found to undergo a magnetic reconstruction consisting on a spin-flip process at surface: each Mn spin at the surface flips to pair with that of Mn in the subsurface layer. In spite of very little Mn-O charge transfer at surface, the surface behavior is driven by the $e_g$ states due to $d_{xy}$ $\to$ $d_{z^2}$ charge redistribution. These results, based on local spin density theory, give a double exchange like coupling that is driven by $e_g$ character, not additional charge, and may have relevance to CMR materials., Comment: 4 pages, 5 figures reference added Fig. 3 modified. Caption of Fig. 5 modified

We present ab initio total energy and stress calculations for the unreconstructed and (2x1)-missing-row reconstructed Ir (110) and Rh (110) surfaces. We use a model based on ab initio results to show that the (Nx1) reconstruction is a faceting transition to a long-wavelength--corrugated (111)-like surface, and find the 3x1 structure to be the most stable for Ir in accordance with experiment. We then use the stress density to analyze the stress increase upon reconstruction, and find it to be due to a changed balance of tensile and compressive contributions in the near-surface region., RevTeX 4 pages, 3 figures

Relaxations at nonpolar surfaces of III-V compounds result from a competition between dehybridization and charge transfer. First principles calculations for the (110) and (10$\bar{1}$0) faces of zincblende and wurtzite AlN, GaN and InN reveal an anomalous behavior as compared with ordinary III-V semiconductors. Additional calculations for GaAs and ZnO suggest close analogies with the latter. We interpret our results in terms of the larger ionicity (charge asymmetry) and bonding strength (cohesive energy) in the nitrides with respect to other III-V compounds, both essentially due to the strong valence potential and absence of $p$ core states in the lighter anion. The same interpretation applies to Zn II-VI compounds., Comment: RevTeX 7 pages, 8 figures included; also available at http://kalix.dsf.unica.it/preprints/; improved after review

RESUMEN: El objetivo principal de la tesis de D. Mauro Antonio Pereira Gonçalves ha sido el estudio de las propiedades topológicas no-triviales en sistemas ferroeléctricos mediante simulaciones atomísticas con modelos efectivos de segundos-principios (descritos en el capítulo 2 de la memoria). Desde hace más de una década se conoce la existencia de texturas magnéticas con una topología no trivial (skyrmiones, merones, hopfiones, etc). Estas estructuras poseen unas propiedades muy llamativas: están protegidas por topología (cuesta una energía destruirlas), y se pueden mover fácilmente con corrientes eléctricas externas lo que las hace candidatas ideales para la fabricación de memorias tipo “pista de carreras” (“race-track memories”). La fuerza motriz para que aparezcan estas fases no triviales es la presencia de interacciones relativistas quirales tipo Dzyaloshinskii-Moriya, presentes en láminas o interfases magnéticas en la que hay ausencia o ruptura de la simetría de inversión. Como esta interacción no tiene análogo en sistemas ferroeléctricos, el pensamiento general era que no se podían estabilizar estas texturas con topología no trivial en sistemas polares, caracterizados por la presencia de una polarización espontánea e invertible. Solo en los últimos años se habían mostrados indicios de su existencia en compuestos complejos (nanocolumnas de un material ferroeléctrico de pequeño diámetro embebidas en una matriz dieléctrica), difíciles de caracterizar experimentalmente. Sin embargo, en la presente tesis se predice teóricamente la existencia de skyrmiones en dos sistemas ferroeléctricos distintos, mucho más sencillos que los anteriores y, por lo tanto, experimentalmente verificables: • En el caso de un único material (PbTiO3) en el que se puedan escribir nanodominios ferroeléctricos tipo columna con una polarización opuesta a la de la matriz circundante (Capítulo 3 de la memoria). La rotación de la polarización en la pared de dominio entre las dos polarizaciones es la clave para obtener una estructura con una topología no trivial tipo skyrmion. Este hallazgo constituyó la primera predicción de un skyrmion eléctrico en un único material sencillo. Además de predecir su estabilización, en la tesis se dan las claves para poder modificarlo de manera controlada mediante campos eléctricos, tensiones impuestas por un substrato o por efectos térmicos (temperatura) (Capítulo 4 de la memoria). Se observan distinto tipos de transiciones, tanto skyrmion-skyrmion, como transiciones topológicas entre skyrmiones y sistemas dipolares triviales. Determinadas funciones respuesta (susceptibilidad) divergen en los puntos de la transición. • En el caso de superredes ferroeléctrico/dieléctrico en las que se intercalan capas de PbTiO3 y SrTiO3 (Capítulo 5 de la memoria). Dependiendo de la tensión epitaxial pueden estabilizarse estructuras tipo vórtice o skyrmiones. En la presente tesis se detallan las condiciones para obtener estas últimas, se procede al análisis riguroso de las propiedades topológicas, y sus propiedades funcionales (susceptibilidades y constantes dieléctricas). Este estudio se ha realizado en colaboración con grupos experimentales de la Universidad de California-Berkeley y la Universidad de Cornell. La constante retroalimentación entre teoría y experimento ha dado lugar al primer trabajo en el que se observa la estabilización de skyrmiones polares a temperatura ambiente en materiales ferroeléctricos, siendo los cálculos realizados en la presente tesis, una parte integral de los mismos. ABSTRACT: The main goal of this thesis was to explore the possibility that ferroelectric materials, characterized by a spontaneous and switchable electric polarization, may present topologically non-trivial structures akin to the skyrmions that occur in their ferromagnetic counterparts. The main tool used in the investigation was atomistic simulation based of first-principles effective models (“second-principles methods”), applied to two model systems: ferroelectric PbTiO3 and ferroelectric/paraelectric superlattices made of PbTiO3 and SrTiO3. More precisely, the simulations were used to analyze multidomain configurations in these compounds, motivated by previous reports that they may present non-trivial structural features. The main finding of the thesis is that, indeed, a simple multidomain configuration in PbTiO3 - namely, a columnar nanodomain with polarization opposed to that of its surrounding matrix – is sufficient to generate a dipole texture - associated to the rotation of the polarization at the domain wall between nanodomain and matrix - with the topology of a skyrmion. This constitutes the first prediction of an electric skyrmion in a simple ferroelectric material. Further, it is shown that the properties and topology of this skyrmion can be tuned by external electric and elastic fields, as well as by temperature, obtaining novel effects such as topological and iso-topological phase transitions. Finally, the investigation of the PbTiO3/SrTiO3 superlattices reveals that the skyrmion structures can be obtained as the ground state solution for such systems. This latter study was developed in the context of a collaboration with experimental groups at UC Berkeley and elsewhere, which led to the first experimental confirmation of electric skyrmions. Hence, in conclusion, the theoretical work in this thesis has been an integral part of the discovery of electric skyrmions in ferroelectric materials.

Density functional calculations have been tremendously useful in understanding the microscopic origin of multiferroicity and in quantifying relevant properties in many multiferroics and magnetoelectrics. Here, we focus on a relatively new and promising compound, PbNiO3. The structural, electronic and magnetic properties of its two polymorphs, i.e. the orthorhombic structure with space group Pnma and the rhombohedral LiNbO3-type structure with space group R3c have been studied by using density functional calculations within DFT + U and hybrid functional schemes. Our data convey an accurate description of the pressure-induced phase transition from the rhombohedral to orthorhombic phase at a predicted critical pressure of 5 GPa in agreement with the measured value of 3 GPa. Both phases show the G-type antiferromagnetic configuration as a magnetic ground state, but differ in the spatial anisotropy associated with nearest-neighbor exchange couplings, which is strongly weakened in the rhombohedral LiNbO3-type phase. The predicted large ferroelectric polarization of the rhombohedral phase (Hao et al 2012 Phys. Rev. B 014116) has been re-explored and analyzed in detail using partial density of states, Born effective charge tensors, charge density difference, electron localization function analysis and distortion mode analysis. The asymmetric bonding between the Pb 6s and O 2p orbitals along the [111]-direction is responsible for the polar cationic displacement, giving rise to a predicted large ferroelectric polarization as high as ∼ 100 μC cm-2. © 2014 IOP Publishing and Deutsche Physikalische Gesellschaft.