First-Principles Prediction of Structural,Magnetic, Electronic,and Elastic Properties of Full-Heusler Compounds Co<Subscript>2</Subscript>YIn (Y = Ti,V)

Density functional theory based on the full-potential linearized augmented plane wave (FP-LAPW) method is used to investigate the structural, magnetic, electronic, and elastic properties of Heusler alloys Co₂YIn (Y = Ti, V). It is shown that the calculated spin magnetic moments using the local spin-density approximation (LSDA), generalized gradient approximation (GGA), LSDA + U, and Tran–Blaha (TB)-modified Becke–Johnson (mBJ)-local density approximations (LDA) are in good agreement with the Slater–Pauling rule. The obtained results with LSDA, GGA-PBE, and LSDA + U of the density of states illustrate that both compounds have a metal behavior; however, mBJ-LDA predicts Co₂VIn alloy to be a half metal. The band structure obtained with mBJ-LDA has an indirect band gap along the Γ–X symmetry with energy of 0.4 eV for Co₂VIn, and E _F lies in the middle of the gap; the electrons at the Fermi level are fully spin-polarized. The calculation of elastic properties indicates the stability of these compounds, and they have a ductile behavior. The 3D dependences of Young’s modulus exhibit a strong anisotropic character. The high values of the elastic constant C ₁₁ reflect the strength of the bonding Ti (V)–In.     

Theoretical Study of Electronic and Magnetic Properties of Newly Derived Iron–Pnictide Compound

A theoretical study based on DFT-LDA of the structural, electronic, and magnetic properties of the new substitution CaFe₂B₂ compound derived from BaFe₂As₂ is presented. Through a relaxation calculation, we found that this compound crystallizes in tetragonal and orthorhombic phases. Formation energies, lattice parameters, density of states, and magnetic moments are calculated. The ground state for this compound is nonmagnetic (NM). As for the anti-ferromagnetic state, more formation energy is characterized by large magnetic moment on each Fe atoms for AFM spin configuration. The results are compared with previous calculations and experimental data. The results of electronic and magnetic properties show that the partial and total density of states for NM, FM, and AFM phases are characterized by strong hybridization between Fe and B atoms. The energy band structure indicates the presence of overlapping valence and conduction bands at the Fermi level. The analyses of charge densities show that the type of bonding in the CaFe₂B₂ compound is metallic. An important resemblance with the original compound is observed which leads us to think that this compound is maybe a superconducting material.     

Response Functions of an Artificial Anderson Atom in the Atomic Limit

We consider the spin and pseudospin (charge) response functions of the exactly soluble Anderson atom model. We demonstrate, in particular, that a deviation from the magnetic Curie-law behaviour, appropriate for a free spin one-half, increases with increasing asymmetry and temperature. In general, oscillator strength is transferred from the spin degrees of freedom to the pseudospin modes. We also consider the negative-U Anderson atom and demonstrate that the pseudospin modes are the relevant low-energy excitations in this case. Especially, the roles of the spin and charge excitations are interchanged upon reversal of the intrasite Coulomb repulsion, U.     

First-Principles Study of the Electronic Structure of Heavy Fermion YbNi2

We investigate the electronic properties of YbNi₂ by means of band structure calculations based on the density functional theory within LDA (local density approximation), fully relativistic, and LDA+U schemes. The 4f derived bands are studied within a relativistic framework which yields flat and spin-orbit split bands, and a correlated band method (LDA+U) that includes correlation corrections. In both cases, the 4f bands, which is located roughly 200 meV below the Fermi level (E _F ), hybridize weakly with the dispersive Ni-3d bands. When the fully relativistic scheme is applied, the 4f derived bands split into lower and higher bands due to spin-orbit coupling effects. The 4f electrons are delocalized through the hybridization with conduction electrons, and the hybridization between f and conduction d electrons also plays a important role in YbNi₂. The on-site Coulomb potential is added to the Yb-derived 4f orbitals, the degeneracy between the 4f orbitals would be lifted partially and they are split into three manifolds bands. The Fermi surface splits into three different sheets which are from main the Yb-4f derived bands and Ni-3d bands. Band structure calculations reveal a saddle points existence at the L point in the energy dispersion curve closed to E _F , whereby, we think YbNi₂ might have a superconducting properties. In addition, the quasiparticle mass enhancement inferred by comparing γ to the density of states (DOS) at the Fermi level indicates the effective mass of YbNi₂ enhanced with the fully relativistic results.     

The Peculiarities of Americium Electronic Structure and Magnetic Susceptibility

By implementing a self-consistent procedure that combines the calculations of electronic spectrum within LDA+U+SO approximation with a generalized spin-fluctuation s(p)df-model, we have calculated the densities of states and temperature dependence of magnetic susceptibility of Americium. It was revealed that in the vicinity of the Fermi energy the densities of states of Am d-electrons become comparable to its f-electron densities of states that leads to that both bands make comparable contributions to the spin magnetic susceptibility. It is shown that the unusual transition from Curie-like to Pauli-like magnetic susceptibility, which is observed in Am, is caused by coexistence in Am of electronic states of two types—the localized, responsible for the formation of Curie-like susceptibility and the itinerant, which lead to temperature-independent magnetic susceptibility.     

Spin and Charge Susceptibilities of the Two-Orbital Model within the Cluster Perturbation Theory for Fe-Based Materials

Cluster perturbation theory is used to calculate band structure, spectral functions, Fermi surface, and spin and charge susceptibilities for the two-orbital model of iron pnictides with the on-site multiorbital Hubbard interactions. Susceptibilities are calculated within the approximation combining the cluster perturbation theory for the self-energy corrections and the random-phase approximation (RPA) for the vertex renormalizations. Calculations for the small values of Hubbard repulsion U ≤ 2 eV confirm that the rigid band approximation and RPA for the spin and charge susceptibilities are suitable approaches for the case of weak interactions.     

The Density Functional Study of Structural,Electronic, Magnetic and Thermodynamic Properties of XFeSi (X = Gd,Tb, La) and GdRuSi Compounds

The structural, electronic, magnetic, and thermodynamic properties of XFeSi (X = Gd, Tb, La) and GdRuSi compounds are investigated using density functional theory by the WIEN2k code. Using the first-principle procedure, the Hubbard parameter of Gd and Tb 4f electrons and La 5d electrons of XFeSi (X = Gd, Tb, La) and GdRuSi compounds is calculated. The structural and electronic and magnetic properties of these compounds within GGA and GGA + U approaches in the presence of spin-orbit coupling are calculated and compared. The calculated results indicate that the ferromagnetic phase is the most stable phase of XFeSi (X = Gd, Tb) and GdRuSi compounds and the nonmagnetic phase is the stable phase of LaFeSi. The magnetic moment of GdFeSi, GdRuSi, and TbFeSi compounds is due to Gd and Tb atoms. The calculated electronic band structures of these compounds show that these compounds have metallic behavior. Furthermore, the thermodynamic properties of these compounds using the quasi-harmonic Debye model as a function of temperature and pressure within GGA and GGA + U approaches are investigated.     

Electronic and Magnetic Investigations of Rare-Earth Tm-doped AlGaN Ternary Alloy

Electronic structure and magnetic interaction of substitutional thulium rare earth-doped wurtzite Al_0.5Ga_0.5N ternary alloy have been performed using density-functional theory within local spin-density approximation with Hubbard-U corrections (LSDA+U) approach. The LSDA+U method is applied to the rare earth (RE) 4f states. The calculation of formation energy shows that it is more energetically favorable for a substitutional Tm atom to replace the Al atom than the Ga atom. For AlGaN:Tm, the lattices parameters are expanded due to larger ionic radius of Tm than that of Al atoms. The energy band gap of AlGaN:Tm has direct character and its width becomes small compared with that of AlGaN. The magnetic coupling between Tm ions in the nearest neighbor sites is ferromagnetic. Magnetic interaction of rare earth ion with the host states at the valence and conduction band edges has been investigated and compared to those of GaN:Mn and has been found to be relatively small.     

Influence of Long-Range Coulomb Interaction and On-Site Hubbard Repulsion on the Formation of d-Wave Copper-Pairing in High-T c Cuprates

We develop a diagram technique for the self-consistent treatment of the long-range Coulomb interaction and on-site Hubbard repulsion in the normal and superconducting state of high-T _c cuprates. The resultant analytical expression for the “screened” matrix elements taking into account long-range and on-site repulsion has been derived. In particular, it accounts for processes with and without spin-flip due to an exchange of spin and charge density fluctuations. Furthermore, we derive the expressions for the normal and anomalous self-energy parts near the superconducting transition temperature T _c that takes into account the vertex corrections including crossing diagrams. The contribution of the crossing parts is taken within the ladder approximation (similar to Fluctuation-Exchange approximation) where the role of Hubbard on-site interaction is replaced by the Coulomb matrix element with a spin-flip averaged over the momentum. Finally, the developed scheme allows to analyze the formation of d-wave superconductivity and its stability in presence of the long-range Coulomb repulsion within a self-consistent anisotropic Eliashberg-like approach.     

Magnetic Moments in SmCo5 and SmCo5?x Cu x Films

SmCo₅ is an emerging perpendicular magnetic material for super-high density magnetic recording, due to its large magnetic anisotropy energy. In this paper, the magnetic moments of SmCo_5?x Cu _x have been studied using first principles calculation based on density-functional theory (DFT). Calculations are performed using the pseudopotential plane wave DFT code Vienna ab initio simulation package (VASP) with the projector augmented wave (PAW) method. The local density approximation LDA+U method is used for the calculation of the exchange correlation energy of Sm. The calculation results show that the average Co magnetic moment of SmCo_5?x Cu _x decreases with the increase of Cu doping concentrations, and the influence of the Cu doping on the spin state of Co is greater than that of Sm. The magnetic anisotropy energy of SmCo₅ is analyzed. The electronic density of states and the differential in spin densities of atoms show that the spatial distribution of 4f electron and the 4f?C3d coupling are the controlling factors of the magnetic anisotropy energy of SmCo₅.     

The Coulomb interaction and volume compression effect on spin and orbital magnetic moment of NiO

This calculation focuses on the effect of the Coulomb interaction U and the volume compression (VC) on both spin (m(s)) and orbital (m(o)) magnetic moment of NiO by using the local spin density approximation plus the Coulomb interaction (LSDA + U) method within full potential linear muffin-tin orbital (FP-LMTO). Our calculated results indicated that both spin and orbital magnetic moment strongly correlated to U and VC. The relevant results exhibited the increasing spin and orbital magnetic moments with increasing Coulomb interaction and decreasing volume compression. The interesting behavior appears when volume compression is greater than 70% at which m(s) collapses whereas m(o) linearly decreases. Further increase of volume compression to be at 80% leads to the disappearance of both magnetic moments. The present results exhibit the good correspondence to the experimental data at the normal pressure and U value of 6 eV.     

Insight into Structural,Electronic, Magnetic,Mechanical, and Thermodynamic Properties of Actinide Perovskite BaPuO<Subscript>3</Subscript>

In this paper, we have performed a systematic investigation on structural, magnetic, electronic, mechanical, and thermodynamic properties of BaPuO₃ perovskite within density functional theory (DFT) using generalized gradient approximations (GGA), onsite coulomb repulsion (GGA + U), and modified Becke-Johnson (mBJ). The calculated structural parameters were found in good agreement with the experimental results. A large value of magnetic moment equal to integer value of 4 μ_B was obtained for the compound. The spin-polarized electronic band structure and density of states present 100% of spin polarization at Fermi level, resulting in half-metallic nature for the compound with spin-up states as metallic and spin- down states as a semiconducting. The elastic and mechanical properties have also been predicted. Moreover, we have calculated thermodynamic properties like Debye temperature (??_D), specific heat (C_V), entropy (S), etc. using quasi-harmonic Debye model.     

A First Principle Study of Co2CrZ (Z=In, Sn, Sb) Based on FP-LAPW Method

The structural and electronic properties of Heusler alloys Co₂CrZ (Z=In, Sn, Sb) have been studied using the generalized gradient approximation (GGA) and local spin density approximation (LSDA), respectively. Among the systems under investigation, Co₂CrSb gives 100 % spin polarization at E _F . Co₂CrSb is the most stable Half-Metallic Ferromagnets (HMFs) with an energy gap 0.25 eV at the Fermi level in the spin-down channel. We have also found that the increase in the total magnetic moment as Z goes from In to Sb. The calculated density of states (DOS) and band structures shows the half-metallic ferromagnets (HMF) character of Co₂CrSb.     

Electron-Phonon Interaction in the Presence of Strong Coulomb Repulsion

We consider the Hubbard-Holstein model on a 2D tilted eight-site square lattice at quarter fillings having on-site (λ ₁) and inter-site (λ ₂) electron-phonon (EP) interactions and inter-site Coulomb repulsion (V ). Mobility of electrons decreases, and hence, localization of electrons is favored by λ ₁ as well as inter-site Coulomb repulsion in the non-interacting limit (U/t = 0.0). In the interacting case (U/t = 8.0), an initial delocalization effect observed due to the competition between on-site Coulomb repulsion and EP interaction λ ₁ though V favors localization. For smaller λ ₁, the competition between λ ₂ and V increases the delocalization effect. As λ ₂ increases further, localization occurs. In the interacting case, localization occurs at much higher values of λ ₂ due to the additional delocalization effect of U. On-site (S0) and/or large bipolarons are formed with λ ₁ in the presence of V, and inter-site Coulomb repulsion effectively increases λ ₁. Thus, V favors the formation of S0 bipolarons. However, the formation of S1 bipolarons is suppressed by inter-site Coulomb repulsion. An existence of critical λ ₂ = λ _2c is observed for the formation of on-site (S0) and neighboring-site (S1) bipolarons.     

Features of the electron structure and magnetic susceptibility of δ-plutonium

Density of electron states, exchange-enhanced spin susceptibility, and orbital magnetic susceptibility of plutonium in the region of existence of a stable δ phase have been calculated within a generalized s(p)df model that takes into account both the band motion of electrons and their on-site and intersite interactions. It is shown that splitting of the electron spectrum by fluctuating intrinsic exchange fields leads to the formation of temperature-induced local magnetic moments, temperature-dependent changes in the occupancies of d, f bands, and spin-fluctuation-induced charge fluctuations.     

Band Structure and Quantum Oscillations in YBa2Cu3O7: A Local Spin Density Approximation with the On-Site Coulomb Interaction Study

First-principles calculations have been performed on the Bravais lattice, the density of states (DOS), the band structure (BS) and the Fermi Surface (FS) topology for high-T _c superconductor YBa₂Cu₃O₇. We used the method of the Full-Potential non-orthogonal Local-Orbital Minimum-Basis Band-structure Scheme (FPLO) [in the local spin density approximation with the on-site Coulomb interaction (LSDA+U) and the coherent potential approximation (CPA)] coupled with the XFSF program. Our FPLO Fermi surface consists of four large quasi-2D cylinders centered on the Brillouin zone corners with mostly CuO₂ plane character and two quasi-1D sheets with mostly Cu–O apex (or apical O) chain character. The main feature of this Fermi surface is that each CuO chain-sheet shows up in its center a small tubular pocket with a funnel-like shape (Fermi pockets) with mainly Ba–O apex character which could give rise to the quantum oscillations observed by Doiron-Leyraud et al. (Nature 447: 565, 2007). Our FPLO band structure (BS) shows that three flat bands cross the Fermi level (FL) which suggests that the two Fermi pockets could be produced by the conjunction near the Fermi level of two flat bands arising from the apex oxygen (BaO layer). This is consistent with the density of states (DOS) where the Fermi level is dominated by the two O apex (BaO) peaks (～90 %). Moreover, the strong narrow peak exhibited in the DOS just below the Fermi level (FL) corresponds to the van Hove singularity (VHS) and originates from the hybridization of Cu 3d and apex O2p (BaO) orbitals. In other hand, the calculated value of the bare band-structure Sommerfeld constant γ _0band=10.60 mJ/(mol?K²) corresponds to a very high Fermi level density of states N(E _F )=4.72 states/(eV×cell) and the electron–phonon coupling constant λ is found to be about 1.64 which implies that YBa₂Cu₃O₇ compound is a strong-coupling superconductor and the cyclotron effective mass m ^? is enhanced in comparison with the band mass m _b [m ^?=(1+λ)m _b ]. Also, the discrepancy between our FPLO calculated Debye temperature (274.15 K) and that obtained experimentally (437 K) may originate from a phonon anisotropy likely generated by the antiferromagnetic spin fluctuations. Our results therefore provide evidence of changes in the topology of the Fermi surface materialized by the Ba–O apex small pockets formed in the center of the Cu–O apex Fermi surface sheets.     

First-principles Calculations of Structural,Magnetic Electronic and Optical Properties of Rare-earth Metals TbX (X=N,O, S,Se)

The structural, magnetic, electronic and optical properties of Terbium-based binaries (TbX) (X = N, O, S and Se) in two cubic structures NaCl and CsCl have been investigated. This study is carried out by Full-Potential linearized Muffin-Tin orbitals (FP-LMTO) method in the framework of the functional theory of density (DFT) implemented in the lmtart code. The presence of f-state electrons in these induced high-correlation materials led us to study these systems using local density approximation (LDA) for the paramagnetic state and spin local density approximation (LSDA) for the ferromagnetic state within two cubic structures. We have demonstrated that these binaries are stable in the ferromagnetic state in the cubic phase of NaCl, which allows us to deduce the magnetic moments of these components. From the electronic band structures and density states, we have concluded that TbX (X = N, O, S and Se) are metallic in the NaCl phase. The results obtained in this work show that the theoretical parameters of the ground state, structure of the bands, density of the states (DOS) and optical properties agree well with other available theoretical and experimental data.     

GGA and GGA+U Description of Structural, Magnetic, and Elastic Properties of Rh2MnZ (Z=Ge, Sn, and Pb)

Structural, magnetic, electronic, and elastic properties of Rh₂MnGe, Rh₂MnSn, and Rh₂MnPb Heusler compounds have been calculated using full potential linearized augmented-plane wave plus local orbitals (FP-L/APW+lo) method based on the spin density functional theory, within the generalized gradient approximation (GGA) and (GGA+U) (U is the Hubbard correction). Results are given for the lattice parameters, bulk moduli, spin magnetic moments and elastic constants. We have derived the bulk and the shear moduli, Young’s and Poisson’s ratio for Rh₂MnZ (Z=Ge, Sn, and Pb). The elastic modulus of Rh₂MnGe is predicted to be the highest. Also, we have estimated the Debye temperatures from the average sound velocity. We discuss the electronic structures, total and partial densities of states and local moments and we investigate the pressure effect on the elastic properties by calculating the elastic constants at various volumes.     

LDA + U and GGA + U studies of Al-rich and bulk goethite (α-FeOOH)

The electronic and structural properties of bulk goethite and Al-rich goethite were studied on the basis of spin-polarized DFT at the LDA + U and GGA(PW91) + U levels. Firstly, the periodic model of bulk goethite was optimized varying the value of U_eff. Considering all the results obtained we can conclude that the bulk goethite described at the GGA + U level with U_eff = 6 eV gives us the better agreement with different physical properties. The results of magnetic moments of Fe ions, the DOS analysis and the Bader atomic charges identify goethite as an antiferromagnetic Fe(III) compound. For Al-rich goethite the GGA + U (U_eff = 6 eV) approach was used. The isomorphous substitution of one Fe ion with Al ion produces the reduction of the cell parameters with respect to the bulk goethite. Regarding the magnetic ordering, it was observed that Fe atoms surrounding the Al atom must have the same spin projection, i.e., spin-up or -down. The charge density was changed with the addition of Al ion, producing a depletion where the ion is located and some electron redistribution in the zone of the oxygen atoms surrounding the Al ion. This behavior produces some small magnetization in these O ions.     

Structural and Magnetic Properties of MnTe Phases from Ab Initio Calculations

We present ab initio studies of NiAs, zinc-blende, wurtzite, and rocksalt MnTe with possible magnetic arrangements of Mn atoms using spin-polarized density functional theory and generalized gradient approximation for exchange and correlation. The impact of an empirical on-site Coulomb interaction U on the lattice constants and ground-state energies is especially studied for antiferromagnetic NiAs and zinc-blende phases, for which contradictory results are found. While U=0 eV gives the correct energetic ordering of the crystal structures, the experimental lattice constants can only be reproduced for finite U=2,…,4 eV. The Hubbard U also strongly influences the magnitude of the magnetic interaction parameters and hence the transition temperatures. In order to obtain Néel temperatures in agreement with experiment a value of about U=4 eV is needed. We demonstrate how U affects the magnetic coupling via the localization of the Mn3d orbitals.