Hubbard Energy Research Articles

Impact of Cr–O hybridization in ACrO3 (A = La, Y): A theoretical investigation

Electronic properties of spin polarized antiferromagnetic ACrO3 (A = La, Y) are explored with Hubbard model using density functional theory (DFT). These two isostructural systems are investigated using the different Hubbard energy and analyzed the hybridization of chromium 3d orbitals and oxygen 2p orbitals and the change in energy bandgaps against the Hubbard energy. The bond length and bond angle affect significantly the orbital contributions of Cr-3d and O-2p electrons for both the system. We noticed that the Cr–O hybridization affects the orbital degeneracy and is substantiated with partial density of states. These results emphasize the contribution of Hubbard energy in correlated electron systems.

Onset of Antiferromagnetic Insulator‐to‐Ferromagnetic Half Metal‐to‐Antiferromagnetic Metal Transitions in La1−xCaxCrO3 and their Microscopic Origin

The magnetic ground state of LaCrO3 and CaCrO3 solid solution is investigated in the complete miscibility range, that is, La1−xCaxCrO3 with 0 ≤ x ≤ 1 at the interval of x = 0.25 to understand the magnetic phase transitions with Ca substitutional fraction. The density functional theory is used with Hubbard correction to account for Cr exchange interaction. The isostructural La1−xCaxCrO3 system exhibits antiferromagnetic insulator (x = 0, i.e., LaCrO3) to ferromagnetic half metal (x = 0.75, i.e., La0.25Ca0.75CrO3) to antiferromagnetic metal (x = 1, i.e., CaCrO3) phase transitions. Cr changes its oxidation state from 3+ in LaCrO3 to 4+ in CaCrO3, showing both 3+ and 4+ states for intermediate compositions. Interestingly, the introduction of Hubbard energy U and Ca substitution affects Cr–O hybridization, as observed in partial density of states. A compositional phase diagram with magnetic ground state is presented for the La1−xCaxCrO3 (0 ≤ x ≤ 1) system.

Electronic structure and resonant inelastic x-ray scattering in GaTa4Se8

We have investigated the electronic structure of the lacunar spinel compound GaTa4Se8 within the density-functional theory (DFT) using the generalized gradient approximation while taking into account strong Coulomb correlations (GGA+U) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. GaTa4Se8 can be classified as a Mott insulator since it was expected to be metallic from GGA band structure calculations in contradiction with the resistivity and optical measurements. Strong SOC splits the t2g-type manifold into a lower jeff = 3/2 quartet and an upper jeff = 1/2 doublet. The empty jeff = 1/2 band is well separated from the jeff = 3/2 one and almost completely given by linear combinations of d5/2 states. The partly filled jeff = 3/2 band is dominated by d3/2 states with some weight of d5/2 ones and consists of two groups of bands separated by an energy gap. The low energy jeff = 3/2 band situated at the Fermi level is very narrow, therefore even a small Hubbard U opens the energy gap. We have investigated theoretically the resonant inelastic x-ray scattering (RIXS) spectra at the Ta L2,3 edges. The calculated results are in good agreement with experimental data. We have found that the best agreement between the theory and experiment in the value of the Hubbard energy gap and RIXS spectra at the Ta L2,3 edges can be achieved for Ueff = 1.0 eV. The experimentally measured RIXS spectrum of GaTa4Se8 at the Ta L3 edge in addition to the elastic scattering peak at 0 eV possesses a sharp feature below 1 eV corresponding to transitions within the Ta t2g levels. The excitation located from 1 eV to 2 eV is due to t2g→eg transitions. The third wide very intensive structure from 2.8 eV up to 12 eV is almost completely derived from ligand-to-ligand charge transfer excitations. Such a situation is quite unique for the RIXS spectra in transition 5d metal systems. The origin of this dominant contribution as well as the electronic correlation in GaTa4Se8 is a consequence of its peculiar crystal structure which forms tetrahedral Ta4 clusters with very small intracluster Ta–Ta and Ta–Se distances and rather large distances between the Ta4 clusters. It results in the formation of localized electronic states and extremely strong Ta 5d - Se 4p hybridization which creates 5dSe states with high density of states. Such hybridization is much stronger in comparison with the corresponding 5d - O 2p hybridization in 5d transition metal oxides due to more extended Se 4p wave functions in comparison with oxygen 2p ones.

Strongly bound Wannier-Mott exciton in pristine (LaO)MnAs and origin of ferrimagnetism in F-doped (LaO)MnAs.

We study the electronic, magnetic, and optical properties of (LaO1-xFx)MnAs (x = 0, 0.0625, 0.125, 0.25) systems, calculated using the generalized gradient approximation (GGA) corrected by Hubbard energy (U) = 1 eV. For x = 0, this system shows equal bandgap (Eg) values for spin-up and spin-down of 0.826 eV, with antiferromagnetic (AFM) properties and local magnetic moment in the Mn site of 3.86 μB per Mn. By doping F with x = 0.0625, the spin-up and spin-down Eg values decrease to 0.778 and 0.798 eV, respectively. This system, along with antiferromagnetic properties, also has a local magnetic moment in the Mn site of 3.83 μB per Mn. Increasing doping F to x = 0.125 induces increases of Eg to 0.827 and 0.839 eV for spin-up and spin-down. However, the AFM remains, where μMn slightly decreases to 3.81 μB per Mn. Furthermore, the excess electron from the F ion induces the Fermi level to move toward the conduction band and changes the bandgap type from indirect bandgap (Γ → M) to direct bandgap (Γ → Γ). Increasing x to 25% induces the decrease of spin-up and spin-down Eg to 0.488 and 0.465 eV, respectively. This system shows that the AFM changes to ferrimagnetism (FIM) for x = 25%, with a total magnetic moment of 0.78 μB per cell, which is mostly contributed by Mn 3d and As 4p local magnetic moments. The change from AFM to FIM behavior results from competition between superexchange AFM ordering and Stoner's exchange ferromagnetic ordering. Pristine (LaO)MnAs exhibits high excitonic binding energy (∼146.5 meV) due to a flat band structure. Our study shows that doping F in the (LaO)MnAs system significantly modifies the electronic, magnetic, and optical properties for novel advanced device applications.

Role of surface engineering of hybrid structure for high performance quantum dots based photoelectrochemical hydrogen generation

We report the synthesis of a TiO2 hybrid structure, consisting of a combination of nanorods and nanoparticles, subsequently treated with hydrazine to enhance the performance of photoelectrochemical (PEC) hydrogen (H2) generation. The optimized TiO2 hybrid photoanode sensitized with Quantum dots (QDs), yields a saturated photocurrent density of 4.25 mA cm−2 (at 0.8 V vs RHE), which is 172% higher than that of the reference sample. The optimized hybrid photoanode treated with hydrazine exhibits an additional 28% increase in the saturated photocurrent density, reaching 5.43 mA cm−2 with CdS QDs, and 8.12 mA cm−2 with CdS/CdSe QDs (at 0.8 V vs RHE), while maintaining 80% of the initial value of photocurrent density, after 2 h of continuous one sun illumination (AM 1.5 G, 100 mW cm−2). We used Density Functional Theory with Hubbard energy correction (DFT + U) calculations to describe the mechanism that underpins this significant improvement. DFT + U results highlighted that the concentration of the hydrazine treatment plays a crucial role and affects the sites (surface or interstitial) where nitrogen may be present. This eventually affects the recombination centers within the hybrid photoanode. Thus, the results of this work define a promising strategy to optimize the morphology of the hybrid photoanodes via hydrazine surface engineering to fabricate efficient and stable PEC water splitting devices for H2 generation.

Measuring Density Functional Parameters from Electron Diffraction Patterns.

We integrate density functional theory (DFT) into quantitative convergent-beam electron diffraction (QCBED) to create a synergy between experiment and theory called QCBED-DFT. This synergy resides entirely in the electron density which, in real materials, gives rise to the experimental CBED patterns used by QCBED-DFT to refine DFT model parameters. We use it to measure the Hubbard energy U for two strongly correlated electron systems, NiO and CeB_{6} (U=7.4±0.6 eV for d orbitals in NiO and U=3.0±0.6 eV for f orbitals in CeB_{6}), and the boron position parameter x for CeB_{6} (x=0.1992±0.0003). In verifying our measurements, we demonstrate an accuracy test for any modeled electron density.

Revisiting the Structural, Electronic, and Magnetic Properties of (LaO)MnAs: Effect of Hubbard Correction and Origin of Mott-Insulating Behavior.

We study the structural, electronic, and magnetic properties of the antiferromagnetic-layered oxyarsenide (LaO)MnAs system from the first-principle calculation. The increasing Hubbard energy (U) in the Mn 3d orbital induces the increasing local-symmetry distortions (LSDs) in MnAs4 and OLa4 tetrahedra. The LSD in MnAs4 tetrahedra is possibly promoted by the second-order Jahn–Teller effect in the Mn 3d orbital. Furthermore, the increasing U also escalates the bandgap (Eg) and the magnetic moment of Mn (μMn). The value of U = 1 eV is the most appropriate by considering the structural properties. This value leads to Eg and μMn of 0.834 eV and 4.31 μB, respectively. The calculated μMn is lower than the theoretical value for the high-spin state of Mn 3d (5 μB) due to the hybridization between Mn 3d and As 4p states. However, dxy states are localized and show the weakest hybridization with valence As 4p states. The Mott-insulating behavior in the system is characterized by the Eg transition between the valence and conduction dzx/dzy states. This work shows new physical insights for advanced functional device applications, such as spintronics.

Microscopic theory of organic magnetoresistance based on kinetic equations for quantum spin correlations

The correlation kinetic equation approach is developed that allows describing spin correlations in a material with hopping transport. The quantum nature of spin is taken into account. The approach is applied to the problem of the bipolaron mechanism of organic magnetoresistance (OMAR) in the limit of large Hubbard energy and small applied electric field. The spin relaxation that is important to magnetoresistance is considered to be due to hyperfine interaction with atomic nuclei. It is shown that the lineshape of magnetoresistance depends on short-range transport properties. Different model systems with identical hyperfine interaction but different statistics of electron hops lead to different lineshapes of magnetoresistance including the two empirical laws $H^2/(H^2 + H_0^2)$ and $H^2/(|H| + H_0)^2$ that are commonly used to fit experimental results.

Electron correlation effects and magneto-optical properties of yttrium iron garnet

The electronic structure, magnetic, optical and magneto-optical properties of yttrium iron garnet (YIG) were investigated by using first-principles calculations with Hubbard energy correction for the treatment of the strong electron correlation. The effective on-site Coulomb interaction parameters Ueff for YIG were determined from the constrained density functional theory within a linear response theory. Our results find a hybridization between the O-2p and Fe-3d orbitals. We also observe the crucial role of the electron correlation of the Fe-3d orbitals according to the magneto-optical properties of YIG.

On the Phase Diagram of the 1D Hubbard Model

The conditions for the emergence of inhomogeneous states are investigated from analysis of the equation of state. It is assumed that Hubbard energy U > t, i.e., corresponds to the conditions of strong electron–electron correlations. The magnetic phase diagram is obtained in a wide range of electron concentrations corresponding to the lower Hubbard band.

Tuning spin state of Ru4+ ion and Jahn-Teller distortion in cubic SrRuO3 system by controlling in-plane strain: A first-principle study

We report the electronic and magnetic properties of the ferromagnetic metallic SrRuO3 system using the plane-wave method within the GGA. The initial evaluation of the effect of effective Hubbard energy (Ueff = U − J) on crystal structure reveals the fully-optimized lattice parameters of a = 4.0496 and 4.0615 Å for Ueff = 0 and 2.9 eV, respectively. Then, we control the in-plane strain (εxx) in the range of εxx = −0.2 to −2.0% within the GGA + U. For all εxx values, the metallic behavior comes from the spin-up Ru 4d-eg and spin-down Ru 4d-t2g states overlapping the Fermi energy level. We find that the increment of the absolute value of εxx (|εxx|) enhances the local magnetic moment of Ru4+ ion, which exhibits the mixed low-spin and high-spin states. Interestingly, the increment of |εxx| enhances the Jahn-Teller distortion, the high-spin state contribution, and the exchange splitting between spin-up and spin-down states in Ru4+ 4d orbital. Additionally, the increment of |εxx| also increases and decreases the local magnetic moments of xy-plane and z-axis O2- ions, respectively. Our result highlights the significant role of εxx in tuning the spin state and Jahn-Teller distortion of Ru4+ ion in the system, which is essential for novel magnetic device applications.

Studi Komputasi Sifat Elektronik dan Sifat Optik Fotoelektroda Titanium Dioksida (TiO2) pada Fasa Anatase dan Rutile

This research focuses on photoelectric Titanium Dioxide (TiO2) as a tool for Sensitive Solar Cell Dyes (DSSC). Anatase and rutile phase. some of the things studied were: crystal structure, band structure, structural structure, photoelectric optical properties of TiO2. The method used in this study is a calculation using the DFT theory and quantum expresso software. The TiO2 cell unit has the optimal shape for the pure antase TiO2 model which has a parameter a = b 3.785 units, c = 9.514 units, α = β = γ = 90O. The average T-O bond length is 1,966 barrels. This phase has a triclinic lattice type with group space P1 (1) which has a volume unit of 136.2991 A3. The Rutile phase has a parameter a = b = 4.5593 with c = 2,958 stars with α = β = γ = 90O. The average T-O bond length is 2.77 barrels. This phase has a Triclinic lattice type with a space group P1 (1) with a volume unit of 52.44233 A3. The anatase phase of TiO2 has indirect characteristics, namely M and Г. coordinate. Based on the calculation of the large bandgap obtained at 3.2 eV. This is in accordance with the data experiment. The energy of the band tape must be greater than the minimum energy needed for the redox reaction in the dye. While the characteristics of the ribbon emphasize the value of the absorption coefficient in light and the rate of electron recombination. Materials with direct band gab have high combination coefficients but with high combination levels. This greatly influences the DSSC efficiency value. TIO2 Rutile phase has a direct property of 3.5 eV for potential Hubbard energy U + 7. This value is closed to the experiment result.

The structure, sintering process, and chemical durability of Ce0.5Gd0.5PO4 ceramics

In this work, Ce0.5Gd0.5PO4 was designed by first principles calculations to investigate the effect of 50% doped Gd on its structural properties. The Ce0.5Gd0.5PO4 ceramics were then prepared via hot-press sintering (HPS) and pressureless sintering (PLS). Their chemical durability was studied to evaluate the leaching resistance of Ce and Gd under leachates along with an examination of the effect of sintering on chemical durability. The results show that the local-density approximation (LDA) plus Hubbard energy (U) is a suitable method to describe the strong spin-orbit coupling of Ce and Gd. The lattice parameters of Ce0.5Gd0.5PO4 match well with experimental values (1.39% error). The structural analysis showed that the lower symmetry of the [GdO9] polyhedron corresponds to its lower defect stability when Gd occupies the Ce sites. For studies of chemical durability, the leach rates of Gd are relatively higher than those of Ce in pH = 3–11 leachates, showing that the [GdO9] polyhedrons with lower symmetry are more easily to be destroyed under leachates. While both Ce0.5Gd0.5PO4 ceramics prepared by HPS (1150 °C) and PLS (1450 °C) had high chemical durability in all leachates, the ceramics prepared by PLS had lower leach rates in pH = 3 leachates. This shows that PLS is a better method to prepare Ce0.5Gd0.5PO4 ceramics.

Gutzwiller projection for exclusion of holes: Application to strongly correlated ionic Hubbard model and binary alloys

We consider strongly correlated limit of variants of the Hubbard model (HM) in which on parts of the system it is energetically favourable to project out doublons from the low energy Hilbert space while on other sites of the system it is favourable to project out holes while still allowing for doublons. As an effect the low energy Hilbert space itself varies with sites of the system. Though the formalism is well developed for the case of doublon projection in the literature, case of hole projection has not been explored in detail so far. We derive basic framework by defining creation and annihilation operators for electrons in a restricted Hilbert space where holes are projected out but which still allows for doublons. We generalise the idea of Gutzwiller approximation for case of hole projection which has been done in literature for the case of doublon projection. To be specific, we provide detailed analysis of strongly correlated limit of the ionic Hubbard model (IHM) which has a staggered potential $\Delta$ on two sublattices of a bipartite lattice and the correlated binary alloys which have binary disorder $\pm V/2$ randomly distributed on sites of the lattice. In both the cases, for $\Delta \sim U \gg t$ and for $V\sim U \gg t$, where $U$ is the Hubbard energy cost for having a doublon at a site, there are sites on which doublons are allowed while holes are the maximum energy states. We do a systematic generalization of similarity transformation for both these cases and obtain the effective low energy Hamiltonian. We further derive Gutzwiller approximation factors which provide renormalization of various terms in the effective low energy Hamiltonian due to the Gutzwiller projection operators, excluding holes on some sites and doublons on the remaining sites.

Magnetoresistance in organic semiconductors: Including pair correlations in the kinetic equations for hopping transport

We derive the kinetic equations for polaron hopping in organics that explicitly take into account the double occupation possibility and pair intersite correlations. The equations include simplified phenomenological spin dynamics and provide a self-consistent framework for the description of the bipolaron mechanism of the organic magnetoresistance. At low applied voltages the equations can be reduced to effective resistor network that generalizes the Miller-Abrahams network and includes the effect of spin relaxation on the system resistivity. Our theory discloses the close relationship between the organic magnetoresistance and the intersite correlations. Moreover, in the absence of correlations, as in ordered system with zero Hubbard energy, the magnetoresistance vanishes.

Mechanistic insights into water adsorption and dissociation on amorphous TiO2-based catalysts

Despite having defects, amorphous titanium dioxide () have attracted significant scientific attention recently. Pristine, as well as various doped catalysts, have been proposed as the potential photocatalysts for hydrogen production. Taking one step further, in this work, the author investigated the molecular and dissociative adsorption of water on the surfaces of pristine and doped catalysts by using density functional theory with Hubbard energy correction (DFT+U). The adsorption energy calculations indicate that even though there is a relatively higher spatial distance between the adsorbed water molecule and the surface, pristine surface is capable of anchoring molecule more strongly than the doped as well as the rutile (1 1 0) surface. Further, it was found that unlike water dissociation on crystalline surfaces, water on pristine catalyst experience the dissociation barrier. However, this barrier reduces significantly when is doped with , providing an alternative route for the development of an inexpensive and more abundant catalyst for water splitting.

Electronic structures and ferroelectric properties of Ba-doped ZnO

Wurtzite ZnO has long been considered to be a promising candidate material for photovoltaic application due to its high power conversion efficiency. More interestingly and very recently, some research results suggested that the ferroelectric property of the photovoltaic material introduced by chemical elements doping can promote its power conversion efficiency significantly. Therefore, in order to understand the effect of Ba doping on the electronic structure and the ferroelectric properties of ZnO and to reveal the potentially optoelectronic properties of Zn1-xBaxO, the energy band structure, the density of states, and the polarizability and the relative dielectric constant of the bulk Ba-doped ZnO supercell system, in which the Zn atoms are partly and uniformly substituted by the Ba atoms, are investigated by using the first-principles method based on the density functional theory and other physical theory. The norm-conserving pseudopotentials and the plane-wave basis set with a cut-off energy of 600 eV are used in the calculation. The generalized gradient approximation refined by Perdew and Zunger (GGA-PBE), the local density approximation (LDA) and the local density approximation added Hubbard energy (LDA+U) are employed for determining the exchange-correlation energy respectively. Brillouin zone is set to be within 4×4×5K point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 2.0×10-6 eV/atom. Additionally, in order to obtain a stable and accurate calculation result, the cell structure is optimized prior to calculation. The calculated results suggest that the bulk Ba-doped ZnO semiconductor system is still a semiconductor with a direct wide band gap. The band gap of Zn1-xBaxO increases gradually with Ba atom doping percentage increasing from 12.5% to 87.5%. Consequently, the ferroelectric polarization properties and the dielectric properties of the bulk Ba-doped wurtzite ZnO materials are tailored by doping Ba atoms. It indicates that the polarizability of Zn1-xBaxO system increases with Ba doping atomic percentage increasing, especially, the polarizability reaches to a maximum when the atomic percentage of doping is 75%. Meanwhile, the relative dielectric constant inversely decreases with Ba atomic percentage increasing. This is attributed to the effective contribution of Ba atoms to the density of state at the bottom of the valence band. The diagonalized components of polarizability imply that there are possible micro-domains in the supercell while applying externally electric field to it. And the supercell presents a nearly isotropic polarizability macroscopically due to the strong interaction among the electric dipole moments existing in the different domains.

Improved electronic structure and magnetic exchange interactions in transition metal oxides

We discuss the application of the Agapito Curtarolo and Buongiorno Nardelli (ACBN0) pseudo-hybrid Hubbard density functional to several transition metal oxides. For simple binary metal oxides, ACBN0 is found to be a fast, reasonably accurate and parameter-free alternative to traditional DFT + U and hybrid exact exchange methods. In ACBN0, the Hubbard energy of DFT + U is calculated via the direct evaluation of the local Coulomb and exchange integrals in which the screening of the bare Coulomb potential is accounted for by a renormalization of the density matrix. We demonstrate the success of the ACBN0 approach for the electronic properties of a series technologically relevant mono-oxides (MnO, CoO, NiO, FeO, both at equilibrium and under pressure). We also present results on two mixed valence compounds, Co3O4 and Mn3O4. Our results for these binary oxides and all the materials we have investigated, obtained at the computational cost of a standard LDA/PBE calculation, are in excellent agreement with hybrid functionals, the GW approximation and experimental measurements.

Driving force of the orbital-relevant electronic nematicity in Fe-based superconductors

The electronic nematic responses in Fe-based superconductors have been observed ubiquitously in various experimental probes. One novel nematic character is the d-wave bond orbital-relevant nematic charge order which was firstly proposed by symmetry analysis and then conformed by angle-resolved photoemission spectroscopy. In this paper, we present a mechanism that the driving force of the orbital-relevant nematic charge order is the reduction of the large Hubbard energy in the particle-hole charge channel by virtual hopping processes. This is one scenario from strong-coupling consideration. The same virtual hopping processes can lead to a super-exchange interaction for the spin magnetic order in the particle-hole spin channel and a pairing interaction for the superconducting order in the particle-particle channel. Thus the electronic nematic order, the spin magnetic order and the pairing superconducting order are intrinsically entangled and they can all stem from the same microscopic virtual hopping processes in reduction of the Hubbard energy. The electronic nematicity, the spin magnetism and the pairing superconductivity in unconventional superconductors are proposed to be unified within this mechanism.

Correlated disordered electrons and organic magnetoconductance

The purpose of this paper is to explore the magnetoconductance (MC) effect in an organic semiconductor. By taking into account the external magnetic field and the hyperfine interaction, we determine the magnetoconductance (MC) from a numerical solution by using a steady-state rate equation for correlated electrons in Gaussian disordered systems. The ingredient of this model is that at thermal equilibrium, the spins of the polarons relax in the same direction as the local magnetic fields. We have investigated the MC dependence of the external magnetic field, the hyperfine interaction, the carrier densities, the intra-site coulomb energy, and the applied electrical field. We show that the interplay between the Hubbard energy and the charge-carrier density leads to positive and negative magnetoconductance. The theoretical calculations are in qualitative agreement with the experimental results.