Polar Catastrophe Research Articles

The effect of Thole functions on the simulation of ionic liquids with point induced dipoles at various densities

Point-induced dipoles are used to mimic electronic degrees of freedom in molecular dynamics simulations. Ultrashort distance interactions of these induced dipoles are damped by the so-called Thole functions to avoid the "polarization catastrophe." This study aims at the overall impact of Thole functions on structure as well as single-particle and collective dynamics of the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate and thereby extends common studies of the effect of Thole functions on energy minimized geometries.

Estimation of the concentration of charge carriers in a conductive layer between LaAlO3 and SrTiO3 dielectrics

Using the capacitor model by Goetzberger, the concentration of charge carriers on the contact layer LAO’STO is evaluated. The obtained results are shown to be comparable with those obtained within the framework of “polar catastrophe” model. Some possible interpretations of charge carriers, concentration variations, depending on the thickness of deposited layers, are presented as well, both for the “pure” (LAO) and composite (LASTO) ones.

Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3oxide heterostructures

A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment - and hence internal electric fields - within the LaAlO3 layer. Firstly, the Ti 2p core level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Secondly, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased - agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only 300meV between n=2 (insulating interface) and n=6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell by unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n>3. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n=4 and already for n=2, mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.

Tuning the carrier density of LaAlO3/SrTiO3 interfaces by capping La1−xSrxMnO3

We present a systematical study on the electronic transport properties of the insulating LaAlO3 (3 unit cells)/SrTiO3 interfaces capping with thin layers of La1−xSrxMnO3, whose formal polarization is continually tuned by Sr doping. When the Sr doping is lower than 2/3, the LaAlO3/SrTiO3 interfaces show metallic behaviors. The carrier mobility is almost independent on the Sr doping for metallic interface, indicating that the capping layer does not change the density of the oxygen vacancies and the interface intermixing. However, the sheet carrier densities monotonically decrease as increasing Sr doping, which is ascribed to the decrease of the La1−xSrxMnO3 formal polarization. These results strongly support the intrinsic mechanism of the polar catastrophe model and provide a new approach to tailor the interface states of complex oxide heterostructures.

Lattice screening of the polar catastrophe and hidden in-plane polarization in KNbO3/BaTiO3interfaces

We have carried out first-principles simulations, based on density functional theory, to obtain the atomic and electronic structure of (001) BaTiO$_{3}$/KNbO$_{3}$ interfaces in an isolated slab geometry. We tried different types of structures including symmetric and asymmetric configurations, and variations in the thickness of the constituent materials. The spontaneous polarization of the layer-by-layer non neutral material (KNbO$_{3}$) in these interfaces cancels out almost exactly the "built-in" polarization responsible for the electronic reconstruction. As a consequence, the so-called polar catastrophe is quenched and all the simulated interfaces are insulating. A model, based on the modern theory of polarization and basic electrostatics, allows an estimation of the critical thickness for the formation of the two-dimensional electron gas between 42 and 44 KNbO$_{3}$ unit cells. We also demonstrate the presence of an unexpected in-plane polarization in BaTiO$_{3}$ localized at the $p$-type TiO$_2$/KO interface, even under in-plane compressive strains. We expect this in-plane polarization to remain hidden due to angular averaging during quantum fluctuations unless the symmetry is broken with small electric fields.

Effect of polar distortion on electronic structures of (001) LaGaO3/SrTiO3 interface

First-principles calculations of electronic structures of (001) epitaxial LaGaO 3 /SrTiO 3 heterostructures were performed in the framework of density functional theory. The effects of atomic relaxation on electronic characteristics of both n-type (LaO) + /(TiO 2 ) 0 and p-type (GaO 2 ) − /(SrO) 0 interfaces are investigated. It is found that the n-type interface remains metallic, whereas the p-type interface becomes insulating after atomic relaxation. Polar distortion in the LaGaO 3 layers associated with the atomic relaxation strongly screens the intrinsic electric field induced by periodically stacking (LaO) + and (GaO 2 ) − charged atomic layers on SrTiO 3 with charge neutral (001) atomic layers. This relieves the trend to a polar catastrophe and reduces the carrier charge density on the interface. ► The electronic structures of (001) LaGaO 3 /SrTiO 3 interface is calculated. ► The effects of atomic relaxation on electronic characteristics are investigated. ► Polar distortion in LaGaO 3 layers screens the intrinsic electric field. ► Polar distortion relieves the trend to a polar catastrophe. ► These result in reduced interfacial carrier density.

Solvent Boundary Potentials for Hybrid QM/MM Computations Using Classical Drude Oscillators: A Fully Polarizable Model.

Accurate quantum mechanical/molecular mechanical (QM/MM) treatments should account for MM polarization and properly include long-range electrostatic interactions. We report on a development that covers both these aspects. Our approach combines the classical Drude oscillator (DO) model for the electronic polarizability of the MM atoms with the generalized solvent boundary Potential (GSBP) and the solvated macromolecule boundary potential (SMBP). These boundary potentials (BP) are designed to capture the long-range effects of the outer region of a large system on its interior. They employ a finite difference approximation to the Poisson-Boltzmann equation for computing electrostatic interactions and take into account outer-region bulk solvent through a polarizable dielectric continuum (PDC). This approach thus leads to fully polarizable three-layer QM/MM-DO/BP methods. As the mutual responses of each of the subsystems have to be taken into account, we propose efficient schemes to converge the polarization of each layer simultaneously. For molecular dynamics (MD) simulations using GSBP, this is achieved by considering the MM polarizable model as a dynamical degree of freedom, and hence contributions from the boundary potential can be evaluated for a frozen state of polarization at every time step. For geometry optimizations using SMBP, we propose a dual self-consistent field approach for relaxing the Drude oscillators to their ideal positions and converging the QM wave function with the proper boundary potential. The chosen coupling schemes are evaluated with a test system consisting of a glycine molecule in a water ball. Both boundary potentials are capable of properly reproducing the gradients at the inner-region atoms and the Drude oscillators. We show that the effect of the Drude oscillators must be included in all terms of the boundary potentials to obtain accurate results and that the use of a high dielectric constant for the PDC does not lead to a polarization catastrophe of the DO models. Optimum values for some key parameters are discussed. We also address the efficiency of these approaches compared to standard QM/MM-DO calculations without BP. In the SMBP case, computation times can be reduced by around 40% for each step of a geometry optimization, with some variation depending on the chosen QM method. In the GSBP case, the computational advantages of using the boundary potential increase with system size and with the number of MD steps.

Electronic charge redistribution in LaAlO3(001) thin films deposited at SrTiO3(001) substrate: First-principles analysis and the role of stoichiometry

We present a comprehensive first-principles study of the electronic charge redistribution in atomically sharp LaAlO${}_{3}$/SrTiO${}_{3}$(001) heterointerfaces of both $n$ and $p$ types allowing for nonstoichiometric composition. Using two different computational methods within the framework of the density functional theory (linear combination of atomic orbitals and plane waves) we demonstrate that conducting properties of LaAlO${}_{3}$/SrTiO${}_{3}$(001) heterointerfaces strongly depend on termination of LaAlO${}_{3}$(001) surface. We argue that both the ``polar catastrophe'' and the polar distortion scenarios may be realized depending on the interface stoichiometry. Our calculations predict that heterointerfaces with a nonstoichiometric film---either LaO-terminated $n$ type or AlO${}_{2}$-terminated $p$ type---may exhibit the conductivity of $n$ or $p$ type, respectively, independently of LaAlO${}_{3}$(001) film thickness.

Superconducting and Ferromagnetic Phases inSrTiO3/LaAlO3Oxide Interface Structures: Possibility of Finite Momentum Pairing

We introduce a model to explain the observed ferromagnetism and superconductivity in LAO/STO oxide interface structures. Because of the polar catastrophe mechanism, 1/2 charge per unit cell is transferred to the interface layer. We argue that this charge localizes and orders ferromagnetically via exchange with the conduction electrons. Ordinarily, this ferromagnetism would destroy superconductivity, but, due to strong spin-orbit coupling near the interface, the magnetism and superconductivity can coexist by forming a Fulde-Ferrell-Larkin-Ovchinikov-type condensate of Cooper pairs at finite momentum, which is surprisingly robust in the presence of strong disorder.

Polar-discontinuity-retainingA-site intermixing and vacancies at SrTiO3/LaAlO3interfaces

Our anomalous surface x-ray diffraction study of the interface between SrTiO${}_{3}$(001) substrates and thin polar films of LaAlO${}_{3}$ shows La/Sr interdiffusion leading to neutrally charged Sr${}_{1\ensuremath{-}1.5x}$La${}_{x}$O layers. From a thickness of two unit cells, the LaAlO${}_{3}$ film's interior retains its polar stacking sequence, which is compensated by a cationic compositional change of the substrate's terminating TiO${}_{2}$ layer. While such valence-retaining $A$-site intermixing does not resolve the polar catastrophe, it may be a general feature at oxide interfaces influencing the electronic and magnetic properties.

Electronic charges and electric potential at LaAlO3/SrTiO3interfaces studied by core-level photoemission spectroscopy

We studied LaAlO${}_{3}$/SrTiO${}_{3}$ interfaces for varying LaAlO${}_{3}$ thickness by core-level photoemission spectroscopy. In Ti $2p$ spectra for conducting ``$n$-type'' interfaces, Ti${}^{3+}$ signals appeared, which were absent for insulating ``$p$-type'' interfaces. The intensity of the Ti${}^{3+}$ signals was significantly higher than that expected for the transport carrier densities, indicating that part of the carriers at the interfaces are localized. The Ti${}^{3+}$ signals increased with LaAlO${}_{3}$ thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO${}_{3}$ thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects and/or adsorbates providing charges to the interface even below the critical thickness.

First-principles prediction of a two dimensional electron gas at the BiFeO3/SrTiO3 interface

The structural and electronic properties of the BiFeO3(BFO)/SrTiO3(STO) heterostructure are studied using supercell approach by ab-initio density functional theory calculations. Our calculations indicate that a two-dimensional electron gas (2DEG) exists at the n-type (BiO)+ and (TiO2)0 interface, due to the fact that electronic reconstruction is expected to avoid the polar catastrophe. Moreover, the large polarization discontinuity between the paraelectric SrTiO3 and ferroelectric BiFeO3 brings about distinguishable 2DEG behaviors to the n-type BiO/TiO2 interfaces, due to the asymmetric screening charge.

Understanding the mechanism of conductivity at the LaAlO3/SrTiO3(001) interface

The observation of conductivity at (001)-oriented interfaces of the 2 band insulators LaAlO3 and SrTiO3 is both fascinating and potentially useful for next-generation electronics. The paradigm commonly used to explain this phenomenon is an electronic reconstruction resulting from the instability created by forming an interface of polar and nonpolar perovskites, leading to the formation of a two-dimensional electron gas. This explanation has typically been conceptualized within the framework of an atomically abrupt interface. However, a significant and growing body of data now exists which reveals that the interface is not abrupt, and that all four cations diffuse across the interface. Yet, the potential roles of the resulting defects and dopants in alleviating the polar catastrophe and promoting conductivity are rarely considered. The purpose of this prospective is to take an overview of the field from outside the reigning paradigm and consider ways in which dopants and defects might affect the electronic structure.

First-Principles Modeling of Electrostatically Doped Perovskite Systems

Macroscopically, confined electron gases at polar oxide interfaces are rationalized within the simple "polar catastrophe" model. At the microscopic level, however, many other effects such as electric fields, structural distortions and quantum-mechanical interactions enter into play. Here, we show how to bridge the gap between these two length scales, by combining the accuracy of first-principles methods with the conceptual simplicity of model Hamiltonian approaches. To demonstrate our strategy, we address the equilibrium distribution of the compensating free carriers at polar LaAlO(3)/SrTiO(3) interfaces. Remarkably, a model including only calculated bulk properties of SrTiO(3) and no adjustable parameters accurately reproduces our full first-principles results. Our strategy provides a unified description of charge compensation mechanisms in SrTiO(3)-based systems.

LaMnO3 Thin Films Grown by Using Pulsed Laser Deposition and Their Simple Recovery to a Stoichiometric Phase by Annealing

We systematically investigated various physical properties of epitaxial LaMnO{sub 3} thin films fabricated on SrTiO{sub 3} substrate by using pulsed laser deposition. In particular, we observed drastic changes in their properties when the as-grown films were annealed in a reduced-oxygen atmosphere. Whereas the as-grown LaMnO{sub 3} film showed ferromagnetic and semiconducting properties with a small optical band gap, the LaMnO{sub 3} films annealed at temperature higher than 700 C showed antiferromagnetic and insulating properties with an enlarged band gap. The optical features also changed drastically, in that the optical transition peak shifted to a higher energy with additional fine structures. Such changes were made in a direction such that the LaMnO{sub 3} films were more stoichiometric, indicating that the stoichiometric phase could be recovered by simple annealing of the pulsed-laser-deposited films. Finally, possibilities of the polar catastrophe scenario at the interface between LaMnO{sub 3} and SrTiO{sub 3} are briefly discussed.

On the inter ionic potentials of molten salts

Inter ionic pair potentials including the effect of ion polarization have been re-examined after the brief survey of their study. The cause of major defect of the polarizable ion model, the polarization catastrophe , has been examined. New expression of screened ion potentials including ion polarization has been proposed to avoid the catastrophe. The application to molten CuI has been demonstrated as an example

A water potential based on multipole moments trained by machine learning — Reducing maximum energy errors

A potential that strives to represent the Coulomb interaction realistically must include polarization. In our approach, three decisions were made to accomplish this: (i) define an atom according to quantum chemical topology (QCT), (ii) express the interaction between atoms via their multipole moments, and (iii) use machine learning to capture the response of an atomic multipole moment to a change in this atom’s environment. This approach avoids explicit (distributed) polarizabilities and eliminates the problem of polarization catastrophe. Previously, we showed ( Phys. Chem. Chem. Phys. 2009, 11, 6365 ) that a machine learning method called kriging predicted atomic multipole moments more accurately than competing machine learning methods. This was established for the atoms of a central water molecule in water clusters, from the dimer to the hexamer. The prediction errors in all multipole moments were collectively assessed by errors in total interaction energy, for thousands of clusters configurations. Here, we target the maximum errors, with an eye on reducing the worst predictions that the potential may return. We demonstrate proof-of-principle for the water dimer using local kriging.

Polarity-induced oxygen vacancies atLaAlO3∕SrTiO3interfaces

Using first-principles density functional theory calculations, we find a strong position and thickness dependence of the formation energy of oxygen vacancies in LaAlO3|SrTiO3 (LAO|STO) multilayers and interpret this with an analytical capacitor model. Oxygen vacancies are preferentially formed at p-type SrO|AlO2 rather than at n-type LaO|TiO2 interfaces; the excess electrons introduced by the oxygen vacancies reduce their energy by moving to the n-type interface. This asymmetric behavior makes an important contribution to the conducting (insulating) nature of n-type (p-type) interfaces while providing a natural explanation for the failure to detect evidence for the polar catastrophe in the form of core level shifts.

First-principles study of electronic reconstructions ofLaAlO3/SrTiO3heterointerfaces and their variants

We present a first-principles study of the electronic structures and properties of ideal (atomically sharp) LaAlO3/SrTiO3 (001) heterointerfaces and their variants such as a new class of quantum well systems. We demonstrate the insulating-to-metallic transition as a function of the LaAlO3 film thickness in these systems. After the phase transition, we find that conduction electrons are bound to the n-type interface while holes diffuse away from the p-type interface, and we explain this asymmetry in terms of a large hopping matrix element that is unique to the n-type interface. We build a tight-binding model based on these hopping matrix elements to illustrate how the conduction electron gas is bound to the n-type interface. Based on the `polar catastrophe' mechanism, we propose a new class of quantum wells at which we can manually control the spatial extent of the conduction electron gas. In addition, we develop a continuous model to unify the LaAlO3/SrTiO3 interfaces and quantum wells and predict the thickness dependence of sheet carrier densities of these systems. Finally, we study the external field effect on both LaAlO3/SrTiO3 interfaces and quantum well systems. Our systematic study of the electronic reconstruction of LaAlO3/SrTiO3 interfaces may serve as a guide to engineering transition metal oxide heterointerfaces.

Polar catastrophe and the spin-polarized electron gas at the LaMnO3/SrMnO3 interface

From calculations using ab initio density-functional methods, the authors show that analogous to the much-studied polar LaAlO3/SrTiO3 interface, there also forms a two-dimensional electron gas at the LaMnO3/SrMnO3 (100) interface due to the polar catastrophe. The electron gas originates from the accumulation of half an electron per cell to quench the polar catastrophe and from the leakage of Mn(eg) electrons from the LaMnO3 side to the SrMnO3 side. These electrons mediate a ferromagnetic double-exchange interaction between the Mn moments at the interface, causing the interface region to be ferromagnetic and in turn the electrons becoming spin-polarized owing to the Zeeman field of the Mn moments.