Special Issue "Modeling Transition-Metal Systems: Emerging Developments and Applications"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Materials".

Deadline for manuscript submissions: closed (8 February 2021).

Special Issue Editors

Dr. Matteo Cococcioni
E-Mail Website
Guest Editor
The Physics Department, The University of Pavia, Pavia, Italy.
Interests: theory of condensed matter physics; density functional theory; beyond-DFT methods for correlated systems; model Hamiltonians and their numerical solutions; ab initio modeling of transition-metal compounds: ground-state (electronic, magnetic, structural, vibrational) properties and phase transitions; Li– and Na–ion batteries; minerals of the Earth interior; complex oxides; transition-metal-based molecular systems
Dr. Andrea Floris
E-Mail Website
Guest Editor
School of Chemistry, College of Science, University of Lincoln, Lincoln LN6 7TS, UK
Interests: density functional theory (DFT); DFT+U method for strongly correlated systems; DFT+U for phonons; phonons and electron–phonon interaction; superconducting DFT (SCDFT); molecular self-assembly on metallic and insulating surfaces; on-surface synthesis; nanostructured systems

Special Issue Information

Dear Colleagues,

Transition-metal compounds are at the core of several cutting-edge technologies, including, among others, the production, storage, and efficient use of energy, advanced electronics, sensing, actuation, and functionalization. The properties that make these systems appealing, both scientifically and technologically, often stem from the marked localization and strong correlation of d and f valence electrons that, promoting a strong interplay between conduction, magnetic, structural, and chemical properties of the materials, give rise to unconventional electronic ground states and exotic behaviors.

The ab initio modeling of realistic systems (in terms of complexity and size) is crucial to rationalize their behavior and to design novel materials with new/improved functionalities. This is still a challenging task, due to the overwhelming computational costs associated with an accurate description of the many-body electronic wavefunction, and the general difficulty of grasping the effects of electronic correlations through energy functionals of the electronic density.

This Special Issue aims to attract leading researchers in the field of ab initio modeling of strongly correlated materials. The main objectives are to review the beyond-DFT computational approaches used to model correlated materials and to discuss some of the most recent developments; to illustrate significant advances on the calculation of relevant properties for materials characterization and technological applications; and to discuss and clarify some of the most important aspects of the physics of these systems. Particular emphasis will be given to methodological and application-related investigations discussing:

  • The effects of correlation on phase stability and vibrational properties;
  • The interplay among crystal structure, magnetic orders, and conduction properties;
  • Magnetism and electronic conduction in 2D transition-metal and rare-earth compounds;
  • Strong correlation and photo-/electrochemical properties.

Dr. Matteo Cococcioni
Dr. Andrea Floris
Guest Editors

Manuscript Submission Information

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Keywords

  • Ab initio calculations
  • DFT and beyond DFT methods
  • Electronic correlations and degenerate ground states
  • Electronic localization
  • Magnetism
  • Phase stability and transitions
  • Vibrational properties and electron-phonon interactions
  • Conduction properties
  • Metal to insulator (Mott) transitions
  • Defect formation and reactivity

Published Papers (7 papers)

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Research

Jump to: Review

Article
Magnetic Energy Landscape of Dimolybdenum Tetraacetate on a Bulk Insulator Surface
Appl. Sci. 2021, 11(9), 3806; https://doi.org/10.3390/app11093806 - 23 Apr 2021
Viewed by 354
Abstract
The magnetic states and the magnetic anisotropy barrier of a transition metal molecular complex, dimolybdenum tetraacetate, are investigated via density functional theory (DFT). Calculations are performed in the gas phase and on a calcite (10.4) bulk insulating surface, using the Generalized-Gradient Approximation (GGA)-PBE [...] Read more.
The magnetic states and the magnetic anisotropy barrier of a transition metal molecular complex, dimolybdenum tetraacetate, are investigated via density functional theory (DFT). Calculations are performed in the gas phase and on a calcite (10.4) bulk insulating surface, using the Generalized-Gradient Approximation (GGA)-PBE and the Hubbard-corrected DFT + U and DFT + U + V functionals. The molecular complex (denoted MoMo) contains two central metallic molybdenum atoms, embedded in a square cage of acetate groups. Recently, MoMo was observed to form locally regular networks of immobile molecules on calcite (10.4), at room conditions. As this is the first example of a metal-coordinated molecule strongly anchored to an insulator surface at room temperature, we explore here its magnetic properties with the aim to understand whether the system could be assigned features of a single molecule magnet (SMM) and could represent the basis to realize stable magnetic networks on insulators. After an introductory review on SMMs, we show that, while the uncorrected GGA-PBE functional stabilizes MoMo in a nonmagnetic state, the DFT + U and DFT + U + V approaches stabilize an antiferromagnetic ground state and several meta-stable ferromagnetic and ferrimagnetic states. Importantly, the energy landscape of magnetic states remains almost unaltered on the insulating surface. Finally, via a noncollinear magnetic formalism and a newly introduced algorithm, we calculate the magnetic anisotropy barrier, whose value indicates the stability of the molecule’s magnetic moment. Full article
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Article
Extensive Benchmarking of DFT+U Calculations for Predicting Band Gaps
Appl. Sci. 2021, 11(5), 2395; https://doi.org/10.3390/app11052395 - 08 Mar 2021
Cited by 2 | Viewed by 725
Abstract
Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band [...] Read more.
Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band edge states is a computationally tractable approach to improve the accuracy of band gap predictions beyond that of DFT calculations based on (semi)local functionals. At variance with DFT approximations, which are not intended to describe optical band gaps and other excited-state properties, DFT+U can be interpreted as an approximate spectral-potential method when U is determined by imposing the piecewise linearity of the total energy with respect to electronic occupations in the Hubbard manifold (thus removing self-interaction errors in this subspace), thereby providing a (heuristic) justification for using DFT+U to predict band gaps. However, it is still frequent in the literature to determine the Hubbard U parameters semiempirically by tuning their values to reproduce experimental band gaps, which ultimately alters the description of other total-energy characteristics. Here, we present an extensive assessment of DFT+U band gaps computed using self-consistent ab initio U parameters obtained from density-functional perturbation theory to impose the aforementioned piecewise linearity of the total energy. The study is carried out on 20 compounds containing transition-metal or p-block (group III-IV) elements, including oxides, nitrides, sulfides, oxynitrides, and oxysulfides. By comparing DFT+U results obtained using nonorthogonalized and orthogonalized atomic orbitals as Hubbard projectors, we find that the predicted band gaps are extremely sensitive to the type of projector functions and that the orthogonalized projectors give the most accurate band gaps, in satisfactory agreement with experimental data. This work demonstrates that DFT+U may serve as a useful method for high-throughput workflows that require reliable band gap predictions at moderate computational cost. Full article
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Article
Comparative Analysis of DFT+U, ACBN0, and Hybrid Functionals on the Spin Density of YTiO3 and SrRuO3
Appl. Sci. 2021, 11(2), 616; https://doi.org/10.3390/app11020616 - 10 Jan 2021
Viewed by 514
Abstract
We present a quantitative analysis of the theoretical spin density map of two ferromagnetic perovskites, YTiO3 and SrRuO3. We calculated the spin density using the standard density functional theory (DFT)+U method, where the Hubbard U correction is applied to the Ti and Ru ions, and with the pseudo-hybrid ACBN0 method, where the Hubbard U parameters are determined self-consistently. The ACBN0 calculations yielded a large value of the Hubbard U of the oxygen 2p orbitals. We also used the screened hybrid HSE06 functional, which is widely used to describe the electronic structure of oxides. We used the Quantum Theory of Atoms in Molecules (QTAIM) theory and integrated the spin density in the atomic basins instead of projecting on atomic orbitals. This way, our results can be compared to experimental reports as well as to other DFT calculations. Full article
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Article
Electronic Structure Correspondence of Singlet-Triplet Scale Separation in Strained Sr2RuO4
Appl. Sci. 2021, 11(2), 508; https://doi.org/10.3390/app11020508 - 06 Jan 2021
Cited by 1 | Viewed by 615
Abstract
At a temperature of roughly 1 K, Sr2RuO4 undergoes a transition from a normal Fermi liquid to a superconducting phase. Even while the former is relatively simple and well understood, the superconducting state has not even been understood after 25 years of study. More recently, it has been found that critical temperatures can be enhanced by the application of uniaxial strain, up to a critical strain, after which it falls off. In this work, we take an “instability” approach and seek divergences in susceptibilities. This provides an unbiased way to distinguish tendencies to competing ground states. We show that in the unstrained compound, the singlet and triplet instabilities of the normal Fermi liquid phase are closely spaced. Under uniaxial strain, electrons residing on all orbitals contributing to the Fermiology become more coherent, while the electrons of the Ru-dxy character become heavier, and the electrons of the Ru-dxz,yz characters become lighter. In the process, Im χ(q,ω) increases rapidly around q = (0.3,0.3,0)2π/a and q = (0.5,0.25,0)2π/a, while it gets suppressed at all other commensurate vectors, in particular at q = 0, which is essential for spin-triplet superconductivity. We observe that the magnetic anisotropy under strain drops smoothly, which is concomitant with the increment in singlet instability. Thus, the triplet superconducting instability remains the lagging instability of the system, and the singlet instability enhances under strain, leading to a large energy-scale separation between these competing instabilities. However, since this happens even without spin-orbit coupling, we believe it is primarily the enhancement in the spin fluctuation glue around quasi-anti-ferromagnetic vectors that drives the Cooper pairing instead of the magnetic anisotropy. At large strain, an instability to a spin density wave overtakes the superconducting one. The analysis relies on a high-fidelity, ab initio description of the one-particle properties and two-particle susceptibilities, based on the quasiparticle self-consistent GW approximation augmented by dynamical mean field theory. This approach is described and its high fidelity confirmed by comparing to observed one- and two-particle properties. Full article
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Article
All-t2g Electronic Orbital Reconstruction of Monoclinic MoO2 Battery Material
Appl. Sci. 2020, 10(17), 5730; https://doi.org/10.3390/app10175730 - 19 Aug 2020
Viewed by 474
Abstract
Motivated by experiments, we undertake an investigation of electronic structure reconstruction and its link to electrodynamic responses of monoclinic MoO2. Using a combination of LDA band structure with DMFT for the subspace defined by the physically most relevant Mo 4d-bands, we unearth the importance of multi-orbital electron interactions to MoO2 parent compound. Supported by a microscopic description of quantum capacity we identify the implications of many-particle orbital reconstruction to understanding and evaluating voltage-capacity profiles intrinsic to MoO2 battery material. Therein, we underline the importance of the dielectric function and optical conductivity in the characterisation of existing and candidate battery materials. Full article
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Article
Many-Body Effects in FeN4 Center Embedded in Graphene
Appl. Sci. 2020, 10(7), 2542; https://doi.org/10.3390/app10072542 - 07 Apr 2020
Cited by 2 | Viewed by 745
Abstract
We introduce a computational approach to study porphyrin-like transition metal complexes, bridging density functional theory and exact many-body techniques, such as the density matrix renormalization group (DMRG). We first derive a multi-orbital Anderson impurity Hamiltonian starting from first principles considerations that qualitatively reproduce [...] Read more.
We introduce a computational approach to study porphyrin-like transition metal complexes, bridging density functional theory and exact many-body techniques, such as the density matrix renormalization group (DMRG). We first derive a multi-orbital Anderson impurity Hamiltonian starting from first principles considerations that qualitatively reproduce generalized gradient approximation (GGA)+U results when ignoring inter-orbital Coulomb repulsion U and Hund exchange J. An exact canonical transformation is used to reduce the dimensionality of the problem and make it amenable to DMRG calculations, including all many-body terms (both intra- and inter-orbital), which are treated in a numerically exact way. We apply this technique to FeN 4 centers in graphene and show that the inclusion of these terms has dramatic effects: as the iron orbitals become single occupied due to the Coulomb repulsion, the inter-orbital interaction further reduces the occupation, yielding a non-monotonic behavior of the magnetic moment as a function of the interactions, with maximum polarization only in a small window at intermediate values of the parameters. Furthermore, U changes the relative position of the peaks in the density of states, particularly on the iron d z 2 orbital, which is expected to affect the binding of ligands greatly. Full article
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Review

Jump to: Research

Review
Advanced First-Principle Modeling of Relativistic Ruddlesden—Popper Strontium Iridates
Appl. Sci. 2021, 11(6), 2527; https://doi.org/10.3390/app11062527 - 11 Mar 2021
Viewed by 445
Abstract
In this review, we provide a survey of the application of advanced first-principle methods on the theoretical modeling and understanding of novel electronic, optical, and magnetic properties of the spin-orbit coupled Ruddlesden–Popper series of iridates Srn+1IrnO3n+1 (n = 1, 2, and ). After a brief description of the basic aspects of the adopted methods (noncollinear local spin density approximation plus an on-site Coulomb interaction (LSDA+U), constrained random phase approximation (cRPA), GW, and Bethe–Salpeter equation (BSE)), we present and discuss select results. We show that a detailed phase diagrams of the metal–insulator transition and magnetic phase transition can be constructed by inspecting the evolution of electronic and magnetic properties as a function of Hubbard U, spin–orbit coupling (SOC) strength, and dimensionality n, which provide clear evidence for the crucial role played by SOC and U in establishing a relativistic (Dirac) Mott–Hubbard insulating state in Sr2IrO4 and Sr3Ir2O7. To characterize the ground-state phases, we quantify the most relevant energy scales fully ab initio—crystal field energy, Hubbard U, and SOC constant of three compounds—and discuss the quasiparticle band structures in detail by comparing GW and LSDA+U data. We examine the different magnetic ground states of structurally similar n = 1 and n = 2 compounds and clarify that the origin of the in-plane canted antiferromagnetic (AFM) state of Sr2IrO4 arises from competition between isotropic exchange and Dzyaloshinskii–Moriya (DM) interactions whereas the collinear AFM state of Sr3Ir2O7 is due to strong interlayer magnetic coupling. Finally, we report the dimensionality controlled metal–insulator transition across the series by computing their optical transitions and conductivity spectra at the GW+BSE level from the the quasi two-dimensional insulating n = 1 and 2 phases to the three-dimensional metallic n= phase. Full article
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