Tunable correlated-electron phases in (111) LaAlO_(3)/SrTiO_(3) band insulator heterostructures

Density functional theory calculations reveal the existence of different correlated-electron ground states in (111)-oriented n-type LaAlO_(3)/SrTiO_(3) symmetric superlattices. They can be tuned by selecting the SrTiO3 thickness, and range from a trivial metal for thick SrTiO_(3) slabs to a Mott-type antiferromagnet in the ultrathin limit. An itinerant ferromagnet and a half-metal phase are also stable in the intermediate region. This remarkable property is a distinct characteristic of (111) perovskite heterostructures and originates from the combined effect of polar discontinuity at the interface, trigonal lattice symmetry, and quantum confinement. While the polar discontinuity promotes the filling of the empty d states of the SrTiO_(3) with one electron, the trigonal symmetry dictates that the wave function of the occupied bands spreads over the entire SrTiO3 slab. Thus, the electron density can be chosen by selecting the number of SrTiO_(3) layers. For high densities, symmetry breaking and on-site Coulomb interaction drive the occurrence of correlated-electron ground states. Our results show that low dimensionality can lead to unconventional behavior of oxide heterostructures formed by electronically fairly simple nonmagnetic band insulators, and can open perspectives for the use of LaAlO_(3)/SrTiO_(3) superlattices grown along the [111] direction to explore quantum phase transitions

BinPo: An open-source code to compute the band structure of two-dimensional electron systems

We introduce BinPo, an open-source Python code to compute electronic properties of two-dimensional electron systems. A bulk tight binding Hamiltonian is constructed from relativistic density functional theory calculations represented in the basis of maximally localized Wannier functions. BinPo has a Schrödinger-Poisson solver, integrating an electric field-dependent relative permittivity to obtain self-consistently the confining electrostatic potential energy term in the derived tight binding slab system. The band structure, energy slices, and other properties, along with different projections and orientations can be computed. High resolution and publishable figures of the simulations can be generated. In BinPo, priority has been given to ease-of-use, efficiency, readability and modularity, therefore becoming suitable to produce reliable electronic structures simulations at low computational cost. Along with the code itself, we provide files from first-principles calculations for some materials, instructions of use, and detailed examples of its wide range of capabilities. The code was developed with a focus on the ABO3 perovskite structure-based systems, such as SrTiO3 and KTaO3, because of their increasing impact in the materials community. Some features, such as the projection onto orbital states, are restricted to calculations using the relevant orbitals for this family of materials, yet it is possible to include more elements in the basis for the band structure determination of other systems. The use of a relativistic approach allows for the inspection of the role of spin-orbit coupling and the resulting Rashba effect on the systems. We detail the approaches used in the code, so that it can be further exploited and adapted to other problems, such as adding new materials and functionalities which can strength the initial code scopes

SmartFIS: utilizando los teléfonos móviles en el aprendizaje de la Física

El Objetivo General del Proyecto de Innovación “SmartFis” se centraba en facilitar el aprendizaje de los contenidos propios de las múltiples asignaturas impartidas en el Laboratorio de Física General de la Facultad de Ciencias Físicas, en varias titulaciones, mediante la utilización de nuevos recursos didácticos, desarrollando nuevas prácticas de laboratorio basadas en el uso de smartphones, nuevos métodos docentes de laboratorio, y nuevos recursos en el Campus Virtual UCM

Ferroionic inversion of spin polarization in a spin-memristor

Magnetoelectric coupling in artificial multiferroic interfaces can be drastically affected by the switching of oxygen vacancies and by the inversion of the ferroelectric polarization. Disentangling both effects is of major importance toward exploiting these effects in practical spintronic or spinorbitronic devices. We report on the independent control of ferroelectric and oxygen vacancy switching in multiferroic tunnel junctions with a La_(0.7)Sr_(0.3)MnO_3 bottom electrode, a BaTiO_3 ferroelectric barrier, and a Ni top electrode. We show that the concurrence of interface oxidation and ferroelectric switching allows for the controlled inversion of the interface spin polarization. Moreover, we show the possibility of a spin-memristor where the controlled oxidation of the interface allows for a continuum of memresistance states in the tunneling magnetoresistance. These results signal interesting new avenues toward neuromorphic devices where, as in practical neurons, the electronic response is controlled by electrochemical degrees of freedom

Controlled sign reversal of electroresistance in oxide tunnel junctions by electrochemical-ferroelectric coupling

The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupling between ferroelectric and electrochemical states has remained unexplored. Here, we make use of the dynamic formation of the oxygen vacancy profile in the nanometerthick barrier of a ferroelectric tunnel junction to demonstrate the interplay between electrochemical and ferroelectric degrees of freedom at an oxide interface. We fabricate ferroelectric tunnel junctions with a La_0.7Sr_0.3MnO_3 bottom electrode and BaTiO_3 ferroelectric barrier. We use poling strategies to promote the generation and transport of oxygen vacancies at the metallic top electrode. Generated oxygen vacancies control the stability of the ferroelectric polarization and modify its coercive fields. The ferroelectric polarization, in turn, controls the ionization of oxygen vacancies well above the limits of thermodynamic equilibrium, triggering the build up of a Schottky barrier at the interface which can be turned on and off with ferroelectric switching. This interplay between electronic and electrochemical degrees of freedom yields very large values of the electroresistance (more than 10^6% at low temperatures) and enables a controlled switching between clockwise and counterclockwise switching modes in the same junction (and consequently, a change of the sign of the electroresistance). The strong coupling found between electrochemical and electronic degrees of freedom sheds light on the growing debate between resistive and ferroelectric switching in ferroelectric tunnel junctions, and moreover, can be the source of novel concepts in memory devices and neuromorphie computing

Ab initio study of decohesion properties in oxide/metal systems

Several studies of the decohesion properties of various oxide/metal systems have been performed recently by ab initio calculations. However, the use of different computational methods, which involve diverse approximations, energy functionals, or calculation conditions, makes the identification of general trends difficult. In the present work, a broad range of interfaces between an ionic oxide (Al_(2)O_(3), ZrO_(2), HfO_(2), and MgO) and a metal [either transition metal (TM) or Na], has been investigated systematically in order to find correlations among the work of separation (Wsep) and the intrinsic properties of the interface, such as the crystal structure, the strain conditions, or the electronic properties of both constituents. Our main result is that the calculated Wsep adjusts very accurately to a parabolic dependence on the summed surface energies of the metal and the oxide, regardless of the oxide and metal components, crystal lattices, interface orientations, and atomic terminations. Furthermore, Wsep is mostly determined by the surface energies although for interfaces involving nonpolar oxide surfaces the contribution of the interfacial energy is not negligible. The strongest adhesion is found for interfaces formed by polar surfaces and bcc TM, e.g., the Wsep of ZrO_(2)(001)_(O)/TM interfaces changes almost by a factor of 2 depending on whether the TM has bcc or fcc structure. In addition, a correlation between the strain conditions of the equilibrium interface structure and the adhesion properties has been obtained. Finally, in order to predict metal/oxide systems whose mechanical properties are reinforced by the plastic deformation of the metal, we examine the expected behavior of the system beyond the elastic regime in the light of the calculated adherence at the interface. The comparison with the scarcely available experimental data provides good agreement for both the Wsep and the qualitative prediction of mechanical reinforcement

Tight-binding approach to penta-graphene

We introduce an effective tight-binding model to discuss penta-graphene and present an analytical solution. This model only involves the π-orbitals of the sp2-hybridized carbon atoms and reproduces the two highest valence bands. By introducing energy-dependent hopping elements, originating from the elimination of the sp3-hybridized carbon atoms, also the two lowest conduction bands can be well approximated - but only after the inclusion of a Hubbard onsite interaction as well as of assisted hopping terms. The eigenfunctions can be approximated analytically for the effective model without energy-dependent hopping elements and the optical absorption is discussed. We find large isotropic absorption ranging from 7.5% up to 24% for transitions at the Γ-point

Interfacial geometry dependence of the iron magnetic moment: the case of MgO/Fe/MgO

The prediction and experimental demonstration of a very large magnetoresistance in Fe/MgO/Fe tunnel junctions have led to intense study of related systems in the last decade. In the present paper, we concentrate on the role of interface coordination, Fe thickness, and magnetization in the MgO/Fe/MgO mirror. By first-principles analysis, it is shown that the iron magnetic moment can rise up to 4 μB, accounting for observed deviation of the Fe atoms in the vicinity of MgO interfaces. The origin is attributed to site preference predicted by our calculations, namely, that, unlike the case of Fe atoms in the monolayer range sitting just above the oxygen atoms of the MgO(001) substrate, the charge transfer induced by the O p-d Fe interaction leads to a structural distortion that stabilizes the Mg at the very first deposition stages of the capping layer, facing Fe sites

Structural, electronic and magnetic properties of the surfaces of tetragonal and cubic HfO_(2)

We present ab initio density-functional theory (DFT) calculations of the structure and stability of the monoclinic (m), tetragonal (t) and cubic (c) phases of HfO_(2) and of the stability and the structural, electronic, and magnetic properties of the polar (001) surface of t-HfO_(2) and the (100) and (111) surfaces of c-HfO_(2). We show that on all three surfaces, a termination by Hf leads to a metallic and non-magnetic surface, while surfaces covered by a full monolayer of O are predicted to be half-metallic and ferromagnetic, the magnetisms being induced by the Coulomb repulsion between p-holes in the O-2p valence band. In contrast, the partially reduced surfaces terminated by half a monolayer of oxygen are found to be insulating and non-magnetic. Ab initio statistical mechanics in combination with the DFT total-energy calculations show that the partially reduced surfaces are stable over the entire range of admissible values of the chemical potential of oxygen. Investigations of the formation of Hf vacancies on the Hf- and O-terminated surfaces of tetragonal HfO_(2) demonstrate that under oxidizing conditions, the formation of Hf subsurface vacancies is energetically favored on the partially reduced O-terminated surface. The formation of Hf vacancies causes the creation of holes in the O-2p valence band and of magnetic moments on the surrounding O atoms. That the formation of near-surface Hf vacancies on the O-terminated surface is energetically favored is in contrast to a high formation energy for neutral Hf vacancies in bulk HfO2 and suggests a cooperative mechanism between surface- and vacancy-formation. We discuss our findings in relation to recent reports on ferromagnetism in ultrathin HfO_(2) films and other models for the formation of p-wave ferromagnetism

Multiorbital structure of the two-dimensional electron gas in LaAlO_(3)/SrTiO_(3) heterostructures: the formation of a d(xy) ferromagnetic sheet

We demonstrate the formation of a ferromagnetic two-dimensional d_(x)y electron sheet strictly confined to the TiO_(2) interface layer in LaAlO_(3)/SrTiO_(3) heterostructures. Based on first-principles density functional calculations we show that the complex subband structure of the two-dimensional electron gas (2DEG) generated at the LaO/TiO_(2) (001) interface is universal, and almost independent of the SrTiO_(3) thickness. It is composed of a ladder of d_(x)y states of light electrons and only one degenerate d_(xz,yz) heavier subband. All the states are spin polarized although the exchange splitting is only significant for the lowest energy d_(xy) subband, which leads to magnetic moments ferromagnetically coupled and localized at the interface. The SrTiO_(3) ferroelectric-like lattice distortions determine the subband occupation and therefore their orbital character, exchange splitting, and charge density profile. The complex structure of the 2DEG can explain the coexistence in the same sample of superconductivity and magnetism