Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe₂As₂, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe₂As₂ in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe₂As₂ illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.     

High pressure phase transition and band structures of different phases in CeO2

We report a detailed theoretical calculation of the electronic band structure of CeO₂ in cubic and orthorhombic phases under pressure using a tight-binding linear muffin-tin orbital method (TB-LMTO) within local density approximation (LDA). The compressibility behavior of this compound was discussed in the light of the changes occurring in the electronic structure. Apart from the electronic band structure and structural stability calculations, the density of states (DOS) and Fermi energies (E_f) at various pressures are calculated. The calculated lattice parameter, transition pressure, bulk modulus and the pressure-volume relation are found out to be in good agreement with experimental results.     

Crystal structure, electronic and transport properties of AgSbSe2 and AgSbTe2

Undoped and p- and n-doped AgSbX₂ (X=Se and Te) materials were synthesized by direct fusion technique. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured as a function of temperature in the range from 300 to 600 K.To enlighten electron transport behaviours observed in AgSbSe₂ and AgSbTe₂ compounds, electronic structure calculations have been performed by the Korringa-Kohn-Rostoker method as well as KKR with coherent potential approximation (KKR-CPA) for ordered (hypothetical AgX and SbX as well as AgSbX₂ approximates) and disordered systems (Ag_1−xSb_xX), respectively. The calculated density of states in the considered structural cases shows apparent tendencies to opening the energy gap near the Fermi level for the stoichiometric AgSbX₂ compositions, but a small overlap between valence and conduction bands is still present. Such electronic structure behaviour well agrees with the semimetallic properties of the analyzed samples.     

On the possibility of a phonon mediated transport anomaly in MgB2 under compression

Results of electrical resistance measurements on MgB₂ at ambient temperature up to 25 GPa are presented. An abrupt reduction of nearly 30% in resistance around 18 GPa is observed. Band structure calculations in the presence of a frozen-in distortion of the E_2g phonon mode reveal that one of the closed Fermi sheets corresponding to the σ-band opens along the Γ-A direction at this pressure. It is suggested that the anomaly observed in the resistance is due to this phonon mediated electronic topological transition (ETT).     

Near EF electronic structure of heavily boron-doped superconducting diamond

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (T_c=7.2 K) in order to study the electronic structure near the Fermi level (E_F). Careful determination of measured momentum space that across Γ point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that E_F is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at E_F were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations.     

First-principles study of properties of Mn2ZnMg alloy

We investigate the electronic structures and magnetic properties of Mn₂ZnMg compound with Hg₂CuTi-type structure using first-principles full-potential local orbital minimum basis calculations. Based on the analysis on the electronic structures, it is demonstrated that the compound is half-metallic antiferromagnet and the compound is favorable to form Hg₂CuTi-type structure instead of the conventional L2₁ one. The complicated hybridization among the p and d states dominates mainly the origin of the gap. The Fermi level (E_F) shifts slightly with the lattice parameter changed. Spin-orbit coupling hardly reduces the degree of spin polarization of the density of states at the Fermi level.     

Electronic structure and magnetism of FeGe2

The electronic band structure of FeGe₂ has been calculated using the self-consistent full potential non-orthogonal local orbital minimum basis scheme based on the density functional theory. In the band structure of FeSn₂, Fe 3d and Sn 5p states play important roles near the Fermi level. Our calculations show that large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.     

High pressure phase transition in metallic LaB6: Raman and X-ray diffraction studies

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB₆ have been carried out upto the pressures of about 20 GPa. The subtle phase transition around 10 GPa indicated in Raman measurements is confirmed by ADXRD experiments to be a structural change from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full potential linear augmented plane wave method carried out as a function of pressure show that this transition is driven by the interception of Fermi level by electronic band minimum around the transition pressure.     

Th3Ni2B2N3 a possible new superconducting compound: insights from electronic band structure

La₃Ni₂B₂N₃, which is similar to YNi₂B₂C and related borocarbides, was earlier studied by the electronic structure calculations [D.J. Singh, W.E. Pickett, Phys. Rev. B 51 (1995) 8668.], and was predicted to be a 3-D metal. In search of new compounds of the borocarbide and related families to get higher T_C, we have studied the compound Th₃Ni₂B₂N₃, by the first principles full potential electronic structure calculations by the linear augmented plane wave method. We get similar band structure for Th₃Ni₂B₂N₃ as found for La₃Ni₂B₂N₃, and the various atom-split component density of states show similar properties. The total electron density of states at Fermi level has been increased to about 92 states per Ry per f.u. as compared to 57 states per Ry per f.u. in La₃Ni₂B₂N₃. The main increase is due to the increased hybridization of the 5f states as seen by the more prominent low energy tail in the Th-component density of states. Based on this enhancement we predict Th₃Ni₂B₂N₃ to be a high temperature superconductor with a T_c in excess of 30 K.     

First-principles study of the structural, electronic, and magnetic properties of InCCo3 and InNCo3

Spin-polarized calculations were performed to investigate the structural, elastic, electronic, and magnetic properties of InCCo₃ and InNCo₃. The deviation of our calculated lattice parameters and equilibrium volume from experimental results is less than 0.8% and 2.5%, respectively. The obtained values of elasticity moduli C_ij, bulk modulus B, and shear modulus G are discussed. From the calculated band structure and the total and partial densities of states, we have concluded that these compounds are electrical conductors; moreover, they are bonded by a mixture of covalent, ionic, and metallic bonds. Our calculations show that InCCo₃ has nonmagnetic properties, while InNCo₃ could have a magnetic behaviour, with an average magnetic moment about 0.54 μ_B/atom, which is mostly derived from d electrons of the cobalt atoms in the energy range from −5 eV up to the Fermi level.     

Electronic structure calculations of transparent conductor Cd3TeO6: Optical properties and the effect of In-substitution

The electronic structure of Cd₃TeO₆ has been studied in the terms of first-principles calculations based on the density functional theory in order to investigate their optical properties and In-substitution effects. It was found that the highly dispersed bottom of the conduction band formed from Cd-s orbitals is the origin of the high transparency and conductivity. Cd₃TeO₆ exhibited optical anisotropy in its main crystal axes, and the c-axis showed the most suitable crystal growth direction for obtaining a wide transparent region. A pronounced shift of the absorption edge was effectively observed by the In-substitution, reflecting the domination of the In-5s level in the conduction band near the Fermi level.     

High spin polarization in ordered Cr3Co with the DO3 structure: A first-principles study

The electronic structure of the highly ordered alloy Cr₃Co with the DO₃ structure has been studied by FLAPW calculations. It is found that the ferrimagnetic state is stable and that the equilibrium lattice constant of Cr₃Co equals 5.77 Å. A large peak in majority spin density of states (DOS) and an energy gap in minority spin DOS are observed at the Fermi level, which results in a high spin polarization of 90% in the ordered alloy Cr₃Co. The total magnetic moment of Cr₃Co is 3.12μ_B, which is close to the ideal value of 3μ_B derived from the Slater-Pauling curve. An antiparallel alignment between the moments on the Cr (A, C) sites and the Cr (B) sites is observed. Finally, the effect of lattice distortion on the electronic structure and on magnetic properties of Cr₃Co compound is studied. A spin polarization higher than 80% can be obtained between 5.55 and 5.90 Å. With increasing lattice constant, the magnetic moments on the (A, C) sites increase and the moments on the (B, D) sites decrease. They compensate each other and make the total magnetic moment change only slightly.     

Electronic structure and structural stability of LaAl2 and LaAl3—A comparative study

The large structural stability regime of LaAl₂ and LaAl₃ as a function of pressure is investigated by the band structure calculations using the FP-LAPW method. An earlier experimental study has revealed that there is no structural phase transition at ∼35 and ∼30 GPa for LaAl₂ and LaAl₃, respectively. Our calculations indicate that in the density of states curve of LaAl₂, the Fermi level (E_F) lies in a slope between bonding maxima and antibonding minima. At high pressures the E_F moves slightly towards the valley, but this shifting does not affect its structural stability. In LaAl₃, the E_F falls in a flat region in the density of states and does not move even up to 33 GPa. The band dispersion curves for both the compounds show movement of bands under the influence of pressure. Some of them cross the Fermi level leading to so called Lifshitz transitions. However, it is seen that these electronic changes do not manifest into any volume anomaly in LaAl₃ under pressure. Our study clearly shows that the density of states behavior for LaAl₂ and LaAl₃ satisfies the Yamashita-Asano criterion for structural stability. The theoretical equations of state, bulk modulus and its pressure derivative values are compared with the experimental values.     

First-principles study of quasi-one-dimensional β-Na0.33V2O5

The electronic structure of β-Na_0.33V₂O₅ has been evaluated using the first-principle density functional theory approach. All energy bands near the Fermi surface (FS) disperse principally along the b-axis direction indicating the quasi-one-dimensionality of this system. The theoretical simulation of the optical property yields reasonable explanations for the notable features revealed in the measurements of the optical spectroscopy. Superconductivity appearing under a pressure of 8 GPa has been discussed in connection with the pressure-induced structural and bands alternations. It is suggested that the strong interchain coupling could play a key role in the appearance of superconductivity. The electron correlation effects on the electronic structure have also been calculated and discussed in comparison with photoemission data.     

Electronic structure and anomalous photoemission line-shape of quasi-2D oxide η-Mo4O11

The η-Mo₄O₁₁ compound is a layered two-dimensional (2D) metallic system whose reduced dimensionality originates non-linear properties as charge density wave (CDW) instabilities. We report on synchrotron radiation angle resolved photoemission spectroscopy (ARPES) measurements in order to obtain a detailed picture of the electronic structure of this material. The symmetry of the states near the Fermi level (E_F) has been discussed in relation to the photoemission symmetry selections rules. Our results are in excellent agreement with previous tight-binding calculations and support the hidden nesting concept proposed to explain the CDW instabilities exhibited by this family of compounds. In addition, a very peculiar photoemission line-shape has been found with the presence of localized non-dispersive states. Some possible explanations are discussed.     

Crystal structure, electronic and elastic properties for novel Hf3AlN and Zr3AlN ceramics explored by first principles studies

Investigations into crystal structure, electronic and elastic properties of M₃AlN (M=Hf, Zr) had been conducted by plane-wave pseudopotential calculations. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of these two compounds. The charge density distributions and density of states indicate that there exist relatively soft Al-M and strong N-M covalent bonds, which might be contributed to layered chemical bonding character of M₃AlN. By analyzing Cauchy pressure and the bulk modulus to C₄₄ ratio, Hf₃AlN was predicted to be more ductile than Zr₃AlN.     

Physical properties of the cubic spinel LiMn2O4

We have performed an ab initio study of structural, electronic, magnetic, vibrational and thermal properties of the cubic spinel LiMn₂O₄ by employing the density functional theory, the linear-response formalism, and the plane-wave pseudopotential method. An analysis of the electronic structure with the help of electronic density of states shows that the density of states at the Fermi level (N (E_F)) is found to be governed by the Mn 3d electrons with some contributions from the 2p states of O atoms. It is important to note that the contribution of Mn 3d states to N(_EF)$N\left(E_F\right)$ is as much as 85%. From our phonon calculations, we have obtained that the main contribution to phonon density of states (below 250 cm⁻¹) comes from the coupled motion of Mn and O atoms while phonon modes between 250 cm⁻¹ and 375 cm⁻¹ are characterized by the vibrations of all the three types of atoms. The contribution from Li increases rapidly at higher frequency (above 375 cm⁻¹) due to the light mass of this atom. Finally, the specific heat and the Debye temperature at 300 K are calculated to be 249.29 J/mol K and 820.80 K respectively.     

Spintronic properties of the Ti2CoB Heusler compound by density functional theory

The electronic structure and magnetic properties of the Ti₂CoB Heusler compound with a high-ordered CuHg₂Ti structure were investigated using the self-consistent full potential linearized augmented plane wave (FPLAPW) method within the density functional theory (DFT). Spin-polarized calculations show that the Ti₂CoB compound is half-metallic ferromagnetic with a magnetic moment of 2 μ_B at the equilibrium lattice constant, a=5.74 Å. The Ti₂CoB Heusler compound is ferromagnetic below the equilibrium lattice constant and ferrimagnetic above the equilibrium lattice constant. A large peak in majority-spin DOS and an energy gap in minority-spin DOS are observed at the Fermi level, yielding a spin polarization of 100%. A spin polarization higher than 90% is achieved for a wide range of lattice constants between 5.6 and 6.0 Å.     

Potential parent compound of superconductor: Sr2CuM2As2O2 (M = Mn, Fe)

The electronic structure of Sr₂CuMn₂As₂O₂ and Sr₂CuFe₂As₂O₂ are studied by the first-principle calculations. These compounds have a body-centered-tetragonal crystal structure that consists of the CuO₂ layers similar to those in the high-Tc cuprate superconductor, and intermetallic MAs (M = Mn, or Fe) layers similar to the FeAs layers in high-Tc pnictides. Such special structure makes them as interesting candidates for new type of superconductor since they have two types of superconducting layers. However, our calculations indicate that the states in the range from −2.0 eV to +2.0 eV are dominated by Mn-3d or Fe-3d states, while the states of Cu-3d are far away from the Fermi level (in the range from −3.0 eV to −1.0 eV). Such results are significantly different with the Cu-based superconductor, like La₂CuO₄, where the states around Fermi level are dominated by Cu-3d states. Besides, we find that the mean-field magnetic ground state is the checkerboard antiferromagnetic in Cu sublattice and the stripe antiferromagnetic in Fe (or Mn) sublattice.     

Heat capacity measurements of itinerant electron magnetism in Y3Ni13−xCOxB2 system

The heat capacity of the Y₃Ni_13−xCo_xB₂ series has been measured from 300 mK to RT. The magnetic ordering phase transitions have been characterized as second-order type and the T_c's determined. The electronic contribution to the low-temperature heat capacity for x=0 yields an electronic constant γ=54 mJ mol K², which is higher than those of YNi₅ and YNi₄B, proving experimentally that its density of states at the Fermi surface is larger than in those other compounds. The substitution of Ni by Co increases γ linearly. Electronic band calculations could explain these features.