Effects Of On-site Coulomb Interaction Research Articles

First-principle computation of some physical properties of half-Heusler compounds for possible thermoelectric applications.

Using the density functional theory (DFT) method, we investigate the properties of LaXSi (X = Pt, Pd) half-Heusler compounds. To ensure the stability of both compounds, we employed two criteria: the Birch-Murnaghan equation of state and the negative formation energy. The evaluation of elastic constants (ECs) plays a crucial role in determining the mechanical stability of both compounds. Specifically, we ensure that the conditions C11 - C12 > 0, C11 > 0, C11 + 2C12 > 0, and B > 0 are satisfied and exhibit mechanical anisotropy and ductility. The analysis of electronic properties clearly indicates that LaPtSi displays metallic behavior in both the spin-up and spin-down states. In the spin-up state of LaPdSi, a band gap is observed, which indicates its characteristic of being a half-metal. A comprehensive investigation of optical properties revealed that these compounds display notable absorption and optical conductivity at higher energy levels. Conversely, they exhibit transparency to incident photons at lower energy levels. Based on the findings, it can be concluded that these compounds are highly suitable for application in high-frequency UV devices. The thermoelectric properties clearly indicate that both materials exhibit high power factors, electrical conductivity, and figures of merit (ZT), suggesting their potential as exceptional thermoelectric materials. The simulations conducted in this study consider the effect of on-site Coulomb interactions by incorporating the Hubbard U term within the GGA + U. Our findings contribute valuable insights that can facilitate further experimental investigations and provide comprehensive validation.

Perovskite R{\bar 3}c phase AgCuF3: multiple Dirac cones, 100% spin polarization and its thermodynamic properties.

Very recently, experimentally synthesized R{\bar 3}c phase LaCuO3 was studied by Zhang, Jiao, Kou, Liao & Du [J. Mater. Chem. C (2018), 6, 6132-6137], and they found that this material exhibits multiple Dirac cones in its non-spin-polarized electronic structure. Motivated by this study, the focus here is on a new R{\bar 3}c phase material, AgCuF3, which has a combination of multiple Dirac cones and 100% spin polarization properties. Compared to the non-spin-polarized system LaCuO3, the spin-polarized Dirac behavior in AgCuF3 is intrinsic. The effects of on-site Coulomb interaction, uniform strain and spin-orbit coupling were added to examine the stability of its multiple Dirac cones and half-metallic behavior. Moreover, the thermodynamic properties under different temperatures and pressures were investigated, including the normalized volume, thermal volume expansion coefficient, heat capacity at constant volume and Debye temperature. The thermal stability and the phase stability of this material were also studied via ab initio molecular dynamic simulations and the formation energy of the material, respectively.

Half-metallic ferromagnetism and metal–insulator transition in Sn-doped SrRuO3 perovskite oxides

We investigate the electronic and magnetic properties of SrRu1-xSnxO3 by carrying out density-functional-theory calculations to show that a half-metallic ferromagnetic ground state emerges for the Sn doping of x≳0.5. To examine the effect of on-site Coulomb interactions for the Ru d orbitals, which was suggested to enhance the half-metallicity in SrRuO3, we employed both the local spin-density approximation (LSDA) as well as the LSDA + U method. For all the possible configurations of Sn doping for x=1/8,1/4,1/2,5/8,3/4, and 7/8 within the 2×2×2 unit cell, we monitor the Ru t2g bandwidth as well as the valence band maximum in the majority-spin channel and demonstrate that the Ru d electron hopping is blocked by the Sn-substituted sites so that the Ru t2g bandwidth becomes reduced as the doping x increases. For x<0.5, the valence band maximum still touches the Fermi level so that SrRu1-xSnxO3 remains as a usual ferromagnetic metal. A further reduction of the Ru t2g bandwidth for the range of 0.5≲x≲0.7 turns it into a half-metallic ferromagnet. As for x>0.7, the Ru t2g bandwidth gets so narrow that even a small on-site Coulomb interaction, e.g., Ueff=1.0 eV induces a band-gap, which indeed corresponds to a gap of the Ru impurity bands in the SrSnO3 oxide semiconductor.

Effects of on-site Coulomb interaction (U) on the structural and electronic properties of half-metallic ferromagnetic orthorhombic Pr0.75Na0.25MnO3 manganite: a LDA + U calculation and experimental study

In this study, the structural, electronic, and half-metallic properties of monovalent-doped Pr0.75Na0.25MnO3 manganite were investigated via density functional theory (DFT) with local density approximation (LDA) and local density approximation plus Hubbard U parameter (LDA + U). The effect of on-site Coulomb interaction on the structural and electronic properties of REMnO3 (RE = Pr, Nd, La) manganites, which consist of strongly interacting RE 4f and transition metal Mn 3d electrons, were studied. The strong Coulomb repulsion between electrons was corrected using the Hubbard parameter U, which ranged from 2 to 6 eV for Mn 3d and was set at 6 eV for Pr 4f. The calculated results showed that the structural parameters, charge and spin densities, and the total and partial densities of states of Pr0.75Na0.25MnO3 manganite were sensitive to the change in U value. The crystal structure and energy band gap of Pr0.75Na0.25MnO3 manganite were calculated using LDA + U with the optimized U value of 2 eV for Mn 3d. The results calculated with a U value of 2 eV showed better agreement with the experimental data compared with those calculated with U values of 4 eV and 6 eV. Interestingly, the LDA + U-based calculations revealed that Pr0.75Na0.25MnO3 manganite has a half-metallic ferromagnetic character. The majority-spin electrons exhibited metallic behavior, whereas the minority-spin electrons exhibited insulating gaps of 2.50, 3.28, and 4.20 eV for the U values of 2, 4, and 6 eV, respectively. The calculated band structure and density of states indicated that the on-site Coulomb repulsion U significantly influenced the hybridization of O 2p with Mn 3d orbitals at the valence and conduction bands.

Effect of on-site Coulomb interaction on electronic and transport properties of 100% spin polarized CoMnVAs

The structural, electronic, magnetic and transport properties of a new quaternary Heusler alloy CoMnVAs have been investigated by employing generalized gradient approximation (GGA), modified Becke-Johnson (mBJ) and GGA with Hubbard U correction (GGA+U). The alloy is energetically more stable in ferromagnetic Y1 type structure. Elastic parameters reveal high anisotropy and ductile nature of the material. CoMnVAs shows half-metallic ferromagnet character with 100% spin polarization at Fermi level with band gap of 0.55eV in the minority spin state. The alloy also possesses high electrical conductivity and Seebeck coefficients with 15μVK−1 at room temperature, achieving a figure of merit of 0.65 at high temperatures. The high degree of ductility, 100% spin polarization and large Seebeck coefficient, makes it an attractive candidate to be used in spin voltage generators and thermoelectric materials.

Effect of on-site Coulomb interaction (U) on the electronic and magnetic properties of Fe2MnSi, Fe2MnAl and Co2MnGe

The electronic band structures, density of states' plots and magnetic moments of Fe2MnSi, Fe2MnAl, and Co2MnGe are studied by using the first principles calculation. The FM solutions using LSDA without U show the presence of half-metallic ferromagnetic (HFM) ground state in Fe2MnSi, whereas the ground state of Fe2MnAl is found to be metallic. In both compounds the maximum contribution to the total magnetic moment is from the Mn atom, while the Fe atom contributes very less. The electronic structures and magnetic moments of Fe-based compounds are affected significantly by U under around-the-mean-field (AMF) double counting scheme, whereas its effect is very less on Co2MnGe. The magnetic moment of Fe atom in Fe2MnSi (Fe2MnAl) increased by ∼70% (∼75%) and in Mn atom it decreases by ∼50% (∼70%) when the value of U is increased from 1 to 5eV. Hund's like exchange interactions are increasing in Fe atom while decreasing in Mn atom with increase in U. The Fe and Mn moments are ferromagnetically coupled in Fe2MnSi for all values of U, whereas in Fe2MnAl they are coupled antiferromagnetically below U=2eV and ferromagnetically above it. Above U=2eV the metallic ground state of Fe2MnAl changes to semiconducting ground state and the ferromagnetic coupling between Fe and Mn atoms appears to be responsible for this. This shows that the validity of AFM double counting scheme is not robust for the entire range of U in the Fe2MnAl compound.

Ground State Electronic and Magnetic Properties of LaCrO&lt;sub&gt;3&lt;/sub&gt; System

First principle calculations are carried out to investigate the ground state electronic and magnetic properties of G-type antiferromagnetic (AFM) LaCrO3 compound in orthorhombic phase. The orthorhombic LaCrO3, a distorted GdFeO3 – type perovskite exhibits AFM transition at ~290 K. Additionally; this system shows weak ferromagnetism at room temperature, in conjugation with its semiconducting properties. In this paper, we will discuss the onset of weak ferromagnetism and electronic properties of LaCrO3, implementing generalized gradient approximation for exchange correlation potential. We present results for electronic structure and density of states, including the effect of onsite Coulomb interaction and non-collinear arrangement of Cr3+ magnetic ions in G-type AFM geometry.

Band gap of β-PtO2 from first-principles

We studied the band gap of β-PtO2 using first-principles calculations based on density functional theory (DFT). The results are obtained within the framework of the generalized gradient approximation (GGA), GGA+U, GW, and the hybrid functional methods. For the different types of calculations, the calculated band gap increases from ∼0.46 eV to 1.80 eV. In particular, the band gap by GW (conventional and self-consistent) calculation shows a tendency of converging to ∼1.25 ± 0.05 eV. The effect of on-site Coulomb interaction on the bonding characteristics is also analyzed.

First-principles investigation of electronic, vibrational, elastic, and structural properties of ThN and UN up to 100 GPa

We have investigated the electronic properties, phonon dispersion relations, elastic constants, structural phase transitions, and pressure-volume equations-of-state of thorium (Th) and uranium (U) mononitrides (ThN and UN) under pressure (0\ensuremath{-}100 GPa) using pseudopotential density functional theoretical methods. The generalized gradient approximation (GGA) is found to describe the ground-state and high-pressure experimental data much better than the local density approximation (LDA) for both compounds. ThN shows acoustic mode phonon softening along the \ensuremath{\Gamma}\ensuremath{-}X direction of the Brillouin zone in the NaCl phase under pressure, followed by a transition to a CsCl structure at 72.5 GPa. Detailed electronic structure analysis revealed an electronic topological transition under pressure that could be responsible for acoustic mode phonon softening and structural phase transition. Unlike ThN, UN shows a structural phase transition from an NaCl to R-3m structure at a much smaller pressure (18 GPa), and the calculated ${C}_{44}$ shear elastic constant decreases with pressure and becomes negative at 15 GPa. A Peierls-like distortion, due to $f$ states, is found responsible for elastic instability and structural phase transition. Our results are in reasonably good agreement with available experimental data. We have also tested the effect of on-site Coulomb interactions on a few ground-state properties and on the phase-transition behavior of UN.

Adsorption of Ge nanowire with 3 d transition metals: A density-functional theory study

Using density-functional theory calculations, we investigate the energetic, electronic and magnetic properties of 3 d transition metals adsorption on hydrogen-passivated germanium nanowire along [0 0 1] direction. It is found that most of the adatoms prefer to bind on the HH site, while the Sc adatom prefers to the Top site. The atoms of good conducting metals, such as Cu and Zn form weak bonding with the wire, however, those such as Ti, V, Fe, Co, and Ni have relative larger binding energies. In addition, various types of wires can be obtained depending on the adatom species, including nonmagnetic metals (Sc or Cu adsorption) and semiconductors (Ni or Zn adsorption), weak-ferromagnetic metals (Ti or V adsorption), ferromagnetic semiconductor (Cr adsorption), and more interesting the ferromagnetic half-metals (Mn, Fe or Co adsorption) with 100% spin-polarization at the Fermi level. These ferromagnetic wires should have potential applications in the fields of quasi-one-dimensional spintronics devices. Detail analysis of the density of states indicates that the ferromagnetism originates mainly from spin-split of the TM-3 d states. Furthermore, the effect of on-site Coulomb interaction on the stability of the three ferromagnetic half-metallic wires is also examined, and it is found that the half-metallic ground state of Mn- or Co-adsorbed wire is more robust than that of Fe-adsorbed wire.

Effect of on-site Coulomb interactions on the electronic structure and magnetic property of Gd 2 cluster

To understand the intriguing magnetic property of the Gd 2 cluster, we performed first-principles electronic structure calculations. A reduced coordination drives the cluster to have different electronic and magnetic properties from its bulk counterpart. The on-site Coulomb interaction, U, is found to be important for the moment enhancement, whereas its magnetic ground state does not change as a function of U. The orbital-decomposed magnetic moments show that 6 s and 5 d electron contributions become significant. From fully relativistic calculations we found that the effect of spin–orbit coupling on the magnetic moment is not significant and the orbital moment remains small.

Competition between structural distortion and magnetic moment formation in fullerene C20

We investigate the effect of on-site Coulomb interactions on the structural and magnetic ground state of C20 fullerene based on density-functional-theory calculations within the local density approximation (LDA) plus on-site Coulomb corrections (LDA+U). The total energies of the high symmetry (Ih) and the distorted (D3d) structures of C20 are calculated for different spin configurations. The ground state configurations are found to depend on the forms of exchange-correlation potentials and the on-site Coulomb interaction parameter U, reflecting the subtle nature of the competition between Jahn–Teller distortion and magnetic instability in the C20 fullerene. While the nonmagnetic state of the distorted D3d structure is robust for small U, a magnetic ground state of the undistorted Ih structure emerges for U larger than 4 eV when the LDA exchange-correlation potential is employed.

Phonon frequency shift and effect of correlation on the electron-phonon interaction in heavy fermion systems

The electron-phonon interaction in the periodic Anderson model (PAM) is considered. The PAM incorporates the effect of onsite Coulomb interaction (U) between f-electrons. The influence of Coulomb correlation U on the phonon response of the system is studied by evaluating the phonon spectral function for various parameters of the model. The numerical evaluation of the spectral function is carried out in the long wavelength limit at finite temperatures keeping only linear terms in U. The observed behaviour is found to agree well with the general features obtained experimentally for some heavy fermion (HF) systems.

Molecular Magnets Based on Organic Charge Transfer Complexes

Abstract Organic charge transfer complexes form a large variety of electronic structures from insulating state to metallic state in low-dimensional frameworks, which are governed by the competition between the transfer integral and the on-site Coulomb interaction. In the insulating regime, the generation of localized magnetic moments of π-electron origin provides low-dimensional magnetic systems having the features of quantum spins. The increase in the transfer integral which works to suppress the effect of on-site Coulomb interaction makes π-electrons delocalized, resulting in the appearance of the systems with the coexistence of magnetism and electron transport as materials classified around the Mott–Hubbard boundary. The introduction of localized magnetic moments of d-electrons in the complexes gives rise to the interaction between the π-electrons and the magnetic moments of d-electrons. Localized magnetic moments of d-electrons embedded in insulating complexes are expected to interact with each other through superexchange interaction mediated by the organic π-electron system, while localized d-spins in metallic complexes are coupled to each other via the π–d interaction, leading to the generation of molecule-based metal magnets. The studies of organic complex-based molecular magnets based on TTF-type donor molecules performed in our group are reviewed.

Phase Diagram of Two-Dimensional Organic Conductors: (BEDT-TTF)2X

The effects of on-site Coulomb interaction in two-dimensional organic conductors, κ-(BEDT-TTF) 2 X, α-(BEDT-TTF) 2 I 3 and (BEDT-TTF) 2 MHg(SCN) 4 , have been studied within the Hartree-Fock approximation. If the strength of Coulomb interaction exceeds some critical value, various kinds of antiferromagnetic states appear and these phases have been classified on a phase diagram with two key parameters, which leads to a unified view on the ground states of this unique family.

Interrelationship among Electronic States ofα-(ET)2I3, (ET)2MHg(SCN)4and κ -(ET)2X

Interrelationship among electronic structures of α -(ET) 2 I 3 , (ET) 2 MHg(SCN) 4 and κ -(ET) 2 X is studied theoretically. The effects of on-site Coulomb interaction are investigated at absolute zero within the Hartree-Fock approximation. If Coulomb interaction exceeds some critical value, various kinds of antiferromagnetic states emerge, which depend crucially on the transfer integrals between ET molecules in a unit cell. The above three types of materials are classified on the axes of two transfer integrals, leading to a unified view of the compounds.

Variational theory for a finite-U periodic Anderson model: Application to heavy-electron materials.

We exhibit a variational wave function for the orbitally nondegenerate Anderson lattice model, which incorporates the effects of onsite Coulomb interaction to deal with essentially metallic systems. The average occupation in the correlated orbitals, the renormalized hybridization matrix element, and the mass enhancement are calculated as a function of Coulomb interaction U and the hybridization matrix element V. Our results for the U\ensuremath{\rightarrow}\ensuremath{\infty} limit are in agreement with the existing results for the infinite-U problem. We show that the infinite-U approximation is a good approximation for a class of materials with U&gt;${\mathit{U}}_{\mathit{c}}$ and V${\mathit{V}}_{\mathit{c}}$. The calculation of the effective mass ${\mathit{m}}^{\mathrm{*}}$ in the heavy-fermion regime shows that it becomes large mainly because of the small hybridization of f electrons with the conduction band rather than because of large Coulomb correlations. The advantages of our approach are briefly discussed.