Hubbard-U Corrections Research Articles

Ligand Influence on Local Magnetic Moments in Fe-Based Metal–Organic Networks

Planar metal–organic networks are highly promising materials due to their modular nature and wide-ranging possible applications from spintronics up to biosensing. Spin state transitions connect local magnetic properties with structural modifications. In this paper, we report on ab initio calculations for two metal–organic planar networks, the Fe-phthalocyanine (Pc) polymer and its precursor material Fe-tetracyanobenzene (TCNB). The spin-polarized generalized gradient approximation to density functional theory with an explicit treatment of the Hubbard-U correction (SGGA+U) indicates a spin state transition between the well confirmed S = 1 state for Fe-Pc and a local, high-spin S = 2 state at the Fe site for Fe-TCNB. The high-spin state at the Fe site is confirmed by X-ray absorption spectroscopy (XAS) measurements of the Fe-TCNB network on the Au(111) substrate in connection with a multiplet analysis. We propose a possible spin state transition between Fe-TCNB and Fe-Pc by the on-surface synthesis of the l...

Spin Hall torques generated by rare-earth thin films

We report an initial experimental survey of spin-Hall torques generated by the rare-earth metals Gd, Dy, Ho, and Lu, along with comparisons to first-principles calculations of their spin Hall conductivities. Using spin torque ferromagnetic resonance (ST-FMR) measurements and DC-biased ST-FMR, we estimate lower bounds for the spin-Hall torque ratio, $\xi_{SH}$, of $\approx$ 0.04 for Gd, $\approx$ 0.05 for Dy, $\approx$ 0.14 for Ho, and $\approx$ 0.014 for Lu. The variations among these elements are qualitatively consistent with results from first principles (density functional theory, DFT, in the local density approximation with a Hubbard-U correction). The DFT calculations indicate that the spin Hall conductivity is enhanced by the presence of the partially-filled $f$ orbitals in Dy and Ho, which suggests a strategy to further strengthen the contribution of the $f$ orbitals to the spin Hall effect by shifting the electron chemical potential.

Static and Dynamical Properties of heavy actinide Monopnictides of Lutetium.

In this work, density functional theory within the framework of generalized gradient approximation has been used to investigate the structural, elastic, mechanical, and phonon properties of lutetium monopnictides in rock-salt crystal structure. The spin orbit coupling and Hubbard-U corrections are included to correctly predict the essential properties of these compounds. The elastic constants, Young’s modulus E, Poisson’s ratio v, shear modulus G, anisotropy factor A and Pugh’s ratio are computed. We found that all lutetium monopnictides are anisotropic and show brittle character. From the wave velocities along [100], [110] and [111] directions, melting temperature of lutetium monopnictides are predicted. Dynamical stability of these monopnictides has been studied by density functional perturbation theory.

Structural, Magnetic, and Optoelectronic Properties of TbNi5, TbNi3Ti2 and TbNi3V2 Compounds

The spin-polarized, electronic, magnetic, and optical properties of TbNi5, TbNi3Ti2, and TbNi3V2 intermetallic compounds have been calculated by employing the full-potential linear augmented plane waves (FP-LAPW) within the density functional theory (DFT) and implemented in the WIEN2k package. In this approach, the generalized gradient approximation with Hubbard U-correction (GGA + U) was chosen as exchange-correlation potential. The electronic structure such as band structure and density of states have been investigated and compared among them. The frequency dependences of dielectric function, optical absorption, reflectivity, and optical conductivity are determined. The optical spectra are changed due to the substitution of nickel with titanium and vanadium. Total and local magnetic moments of Tb, Ni, Ti, and V are also estimated; it is shown that the total magnetic moment of the three alloys is vigorously contributed by the local magnetic moment of terbium.

First principles calculation on the magnetic, optical properties and oxygen vacancy effect of CexY3−xFe5O12

We report a first principles study on the magnetic and optical properties of Ce substituted yttrium iron garnet (CexY3−xFe5O12) (Ce:YIG) (x = 0.125, 0.25, 0.5, and 1.0). Using density functional theory with Hubbard-U corrections, we demonstrate that Ce3+-Fe3+(tetrahedral) charge transfer is the dominating mechanism of enhanced near infrared absorption in Ce:YIG. In particular, oxygen vacancies are found to be able to stabilize Ce3+ from converting to Ce4+, at the same time reduce two neighboring Fe3+ to Fe2+ which occupy both the octahedral and tetrahedral sites. The formation enthalpy of Ce4+-Fe2+ state is strongly dependent on the distance from the Ce ion to the oxygen vacancy, which is closely related to the local lattice distortion around the Ce ion. This result provides theoretical insight for developing high figure of merit magneto-optical materials for nonreciprocal photonic applications.

Computational and Experimental Investigation of Ti Substitution in Li1(NixMnxCo1-2x-yTiy)O2 for Lithium Ion Batteries.

Aliovalent substitutions in layered transition-metal cathode materials has been demonstrated to improve the energy densities of lithium ion batteries, with the mechanisms underlying such effects incompletely understood. Performance enhancement associated with Ti substitution of Co in the cathode material Li1(NixMnxCo1-2x)O2 were investigated using density functional theory calculations, including Hubbard-U corrections. An examination of the structural and electronic modifications revealed that Ti substitution reduces the structural distortions occurring during delithiation due to the larger cation radius of Ti(4+) relative to Co(3+) and the presence of an electron polaron on Mn cations induced by aliovalent Ti substitution. The structural differences were found to correlate with a decrease in the lithium intercalation voltage at lower lithium concentrations, which is consistent with quasi-equilibrium voltages obtained by integrating data from stepped potential experiments. Further, Ti is found to suppress the formation of a secondary rock salt phase at high voltage. Our results provide insights into how selective substitutions can enhance the performance of cathodes, maximizing the energy density and lifetime of current Li ion batteries.

Density functional theory studies of the electronic and magnetic structure of GdxSr1−xFeO3 perovskite alloys

Electronic structure calculations based on density-functional theory with the local spin density approximation with Hubbard-U corrections (LSDA+U) for GdxSr1−xFeO3 perovskite alloys with gadolinium (Gd) concentrations varying from 0% up to 100% have been performed using the accurate full-potential linearized augmented plane wave method. The structural and magnetic stabilities of GdxSr1−xFeO3 were addressed. LSDA+U calculations show that for gadolinium (Gd) concentration x<0.676, the favored structure is a ferromagnetic cubic phase, while for gadolinium (Gd) concentration x≥0.676, the favored structure is an anti-ferromagnetic orthorhombic phase. The calculated ground-state properties—the equilibrium lattice constant, bulk modulus—are in good agreement with experimental results. Our results show that the holes introduced by Sr substitution in an orthorhombic structure have mainly Fe 3d character, which are responsible for the reduction of the band gap. Through the DOS calculations, all cubic alloys are founded metals except for the concentration x=0.125 where the material adopts the half-metallic character.

Role of Fe and Co in optical conductivity and electronic structure of TbNi4Fe and TbNi4Co

Optical properties of two intermetallic compounds TbNi4Fe and TbNi4Co have been studied employing ellipsometry in a spectral range 0.22–15 μm to reveal their characteristic features in comparison with the parent compound TbNi5. The electronic structure of TbNi4Fe and TbNi4Co was calculated within the LSDA + U method (local spin density approximation with Hubbard U-correction). Based on the calculated electronic structure results, the theoretical optical conductivity was calculated and used to interpret experimental conductivity in the range of interband optical absorption.

Computational Investigation of Electron Small Polarons in α-MoO3

The properties of electron small polarons in α-MoO3 are investigated computationally employing density-functional-theory with Hubbard-U corrections (DFT+U) and hybrid functionals (HSE06). These met...

Ytterbium monopnictides under high pressure and high temperature: Structural and electronic properties studied using local spin density approximation and Hubbard corrections approach

The structural and electronic properties of ytterbium monopnictides YbX (X = N, P) compounds have been investigated using full potential linear augmented plane wave plus local orbitals approach within the framework of density functional theory. The local spin-density approximation along with Hubbard-U corrections and spin–orbit coupling have been used for correct prediction of electronic properties. At ambient conditions ytterbium monopnictides stabilize in NaCl (B1) structure. Also, the equilibrium properties, viz., lattice constants (a), bulk modulus (B0) and its pressure derivative (B0′) and total energy (E0) have been calculated in three other phases namely CsCl (B2), zinc blende (B3) and body centred tetragonal. Under compression, both YbN and YbP undergo first-order structural transition from B1 to B2 phase at 164.0 and 33.7 GPa of pressure, respectively. Obtained results show that YbX is semi-metallic in B1 phase and metallic in B2 phase. Thermal parameters such as Gruneisen parameter, Debye temperature, specific heat, thermal expansion coefficient and entropy have been determined as a function of pressure and temperature. The calculated lattice and other parameters are in good agreement with experimental and other theoretical values reported earlier.

Study of structural and electronic properties of NdSb employing local spin density approximation and Hubbard correction

Full-potential linear augmented plane wave plus local orbital method employing local spin-density approximation along with Hubbard-U corrections and spin–orbit coupling have been used to study electronic, structural and phase transition properties of NdSb. Under compression, they undergo first-order structural transition from B1 to BCT phase. Computed phase transition pressure is 15.1 GPa, this agrees well with their measured value of 15 GPa. Structural properties viz., equilibrium lattice constants, bulk modulus, its pressure derivative and volume collapse are also in closer agreement with experimental data. Present strategy shows significant impact on energy levels of occupied and unoccupied 4f states in electronic structure of NdSb and provides better description of crystal properties of present system.

Theoretical Investigation of the Activity of Cobalt Oxides for the Electrochemical Oxidation of Water

The presence of layered cobalt oxides has been identified experimentally in Co-based anodes under oxygen-evolving conditions. In this work, we report the results of theoretical investigations of the relative stability of layered and spinel bulk phases of Co oxides, as well as the stability of selected surfaces as a function of applied potential and pH. We then study the oxygen evolution reaction (OER) on these surfaces and obtain activity trends at experimentally relevant electro-chemical conditions. Our calculated volume Pourbaix diagram shows that β-CoOOH is the active phase where the OER occurs in alkaline media. We calculate relative surface stabilities and adsorbate coverages of the most stable low-index surfaces of β-CoOOH: (0001), (0112), and (1014). We find that at low applied potentials, the (1014) surface is the most stable, while the (0112) surface is the more stable at higher potentials. Next, we compare the theoretical overpotentials for all three surfaces and find that the (1014) surface is the most active one as characterized by an overpotential of η = 0.48 V. The high activity of the (1014) surface can be attributed to the observation that the resting state of Co in the active site is Co(3+) during the OER, whereas Co is in the Co(4+) state in the less active surfaces. Lastly, we demonstrate that the overpotential of the (1014) surface can be lowered further by surface substitution of Co by Ni. This finding could explain the experimentally observed enhancement in the OER activity of Ni(y)Co(1-y)O(x) thin films with increasing Ni content. All energetics in this work were obtained from density functional theory using the Hubbard-U correction.

Structural, electronic, elastic and thermal properties for curium monopnictides: A first-principles study

An investigation on structural, electronic, elastic and thermal properties of curium monopnictides (CmX; X=N, P, As, Sb and Bi) under ambient and high pressure is conducted using first-principles calculations based on density functional theory (DFT) within generalized gradient approximation (GGA) and local spin density approximation (LSDA) along with Hubbard-U corrections and spin–orbit coupling. Our results demonstrate that there exist a structural phase transition from their ambient NaCl (B1) type structure to CsCl (B2) type structure at 111.80, 36.81, 22.08, 7.52 and 7.14GPa for CmN, CmP, CmAs, CmSb and CmBi respectively using LSDA. The structural parameters like lattice constant (a0), bulk modulus (B) and pressure derivative of bulk modulus (B′) are reported in B1 and B2 phases and also compared with available results. The calculated band structures indicate that B1 phase of all the CmX compounds are metallic for majority spin and semi metallic for minority spin. The LSDA+U strategy shows significant impact on the energy levels of the occupied and unoccupied ‘f’ states in the electronic structure and provides better description of all the compounds. The thermal and mechanical properties are predicted from the calculated values of elastic constants. The ductility of these compounds is determined by calculating the bulk to shear ratio B/GH. Our calculated results are consistent with experimental and other theoretical data. Elastic constants of these compounds are determined for the first time.

Spin/orbit moment imbalance in the near-zero moment ferromagnetic semiconductor SmN

SmN is ferromagnetic below 27 K, and its net magnetic moment of 0.03 Bohr magnetons per formula unit is one of the smallest magnetisations found in any ferromagnetic material. The near-zero moment is a result of the nearly equal and opposing spin and orbital moments in the 6H5/2 ground state of the Sm3+ ion, which leads finally to a nearly complete cancellation for an ion in the SmN ferromagnetic state. Here we explore the spin alignment in this compound with X-ray magnetic circular dichroism at the Sm L2,3 edges. The spectral shapes are in qualitative agreement with computed spectra based on an LSDA+U (local spin density approximation with Hubbard-U corrections) band structure, though there remain differences in detail which we associate with the anomalous branching ratio in rare-earth L edges. The sign of the spectra determine that in a magnetic field the Sm 4f spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4f orbital moment and antiparallel to the spin moment. Further measurements on very thin (1.5 nm) SmN layers embedded in GdN show the opposite alignment due to a strong Gd-Sm exchange, suggesting that the SmN moment might be further reduced by about 0.5 % Gd substitution.

On the chemical state of Co oxide electrocatalysts during alkaline water splitting

Resonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.

Theory study of rare earth (Ce, Pr) doped GaN in electronic structrue and optical property

Using the pseudopotential plane-wave method within the density functional theory as implemented in the Vienna ab-initio simulation package, we investigate the crystal parameters, electronic structures and optical properties of rare earth Ce and Pr doped GaN. The local spin density approximation with Hubbard-U corrections method is used to treat the correlation effect of strongly localized rare-earth 4f electron states. The results show that the crystal parameters increase after doping Ce and Pr in GaN. The Ce impurity introduces defect level in the gap while for Pr the level lies near the valence band maximum, and the defect levels are contributed by Ce and Pr 4f electron states. In addition, the dopings of Ce and Pr give rise to spin polarization and magnetic-order. For GaN:Ce, there appear two new peaks, one is in the low energy region of imaginary dielectric function and the other is in the low energy region of absorption coefficient. These new peaks are probably related to the defect level in the gap. For GaN:Pr, red shifts of the dielectric peak and absorption edge duo to bandgap narrowing are observed.

Electronic Origin of the Surface Reactivity of Transition-Metal-Doped TiO2(110)

We investigate the surface reactivity of doped rutile M-TiO2(110) (M = V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni) using density functional theory (DFT) and Hubbard-U corrected DFT calculations (DFT+U method). The oxygen adsorption bond, used as the surface reactivity measure, is stronger on the doped TiO2 surfaces as compared with that on the undoped TiO2 surface. We relate this increase in reactivity of the doped TiO2 surfaces to the presence of localized surface resonances and surface states in the vicinity of the Fermi level. We find that the center of these localized states on doped TiO2 is a good descriptor for the oxygen adsorption energy. The inclusion of the Hubbard-U correction to DFT barely modifies the oxygen adsorption energy on undoped TiO2, whereas it destabilizes the oxygen adsorption energies on doped TiO2 when compared with results from standard DFT. Nevertheless, we find that the oxygen adsorption energy trends predicted by a standard GGA-DFT functional are reproduced when the Hubbard-U c...

Correlation effects on optical properties of TbOFeAs

Based on local density approximation and Hubbard-U corrections (LDA+U), we study the influence of Coulomb interaction for Tb 4f states on the optical properties of the recently discovered superconductor, such as TbOFeAs. Within the incorporation of onsite Hubbard effect in TbOFeAs, we discuss the electronic structure, as well as the optical spectra and we compared them to LDA calculations. For non-magnetic (NM) configuration, the electronic structure exhibits high density of states, N(EF) in the proximity of Fermi level. With and without the electronic correlation effects, we carried out the calculations for the optical properties such as the optical conductivity, joint density of states (JDOS), optical absorption, the electron energy loss function and reflectivity of TbOFeAs in a large photon energy scale of 30 eV. Despite the absence of a Mott insulator transition, we infer that the electronic correlation effects are prominent in the recently discovered superconductor, like TbOFeAs. We also predict the in-plane anisotropy of plasma frequency that has been evaluated recently in the other ReOFeAs systems.

Importance of Correlation in Determining Electrocatalytic Oxygen Evolution Activity on Cobalt Oxides

Co-based oxides are suitable electrode materials for the electrocatalytic oxygen evolution reaction (OER) with promising activity and stability, in addition to being widely available and relatively cheap. We investigate OER on Co3O4(001) and β-CoOOH (011̅2) surfaces using density functional theory calculations (DFT). We construct surface Pourbaix diagrams and investigate the theoretical overpotential for the elementary steps involved in OER on these surfaces. We show that inclusion of the Hubbard-U correction to DFT (DFT+U) is necessary to recover experimentally observed trends in the activity for the strongly correlated cobalt oxides. We find that the inclusion of the Hubbard-U correction lowers the activity of both Co3O4(001) and β-CoOOH(011̅2) when compared to results from pure DFT. In addition, the Hubbard-U correction shifts the location of Co3O4 and β-CoOOH from the strong binding leg to the weak binding leg of the OER volcano plot. The calculations also suggest that the theoretical overpotentials for Co3O4 and β-CoOOH are very nearly the same. We ascribe this to a similar local coordination environment of the active Co site in Co3O4 and CoOOH under OER conditions.

Investigating the electronic structure of fluorite-structured oxide compounds: comparison of experimental EELS with first principles calculations

Energy loss spectra from fluorite-structured ZrO2, CeO2, and UO2 compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.