Ab Initio Electronic Band Structure Research Articles

Energy-efficient sintering-free Chemically synthesized carbon nanofibers for high-performance supercapacitors

This work introduces a cost-effective method to synthesize carbon nanofibers (CNFs) at a lower temperature (∼100 °C), addressing the synthesis cost and precursor selection challenges associated with traditional methods requiring temperatures of ⁓3000 °C. Acid-etched banana fibers served as sustainable precursors, while KClO3 acted as a liquid carbonization medium. The resulting CNFs (referred MCCNFs), exhibited hollow structures with intertwined tubular morphology (OD ∼20–40 nm and ID∼ 5–10 nm). They possessed a hexagonal porous structure (P63/mmc) with a high specific surface area (152.09 m2g-1), significant pore volume (0.459 ccg−1), and impressive tensile strength exceeding 2.71 TPa. Smaller intensity ratio of D and G bands (0.396) confirmed their purity and structural integrity. MCCNFs displayed semi-metallic behaviour with superior electrical conductivity (0.0386 S-m−1), validated by ab initio electronic band structure calculations. UV–vis absorption spectra revealed a linear band structure. Notably, MCCNFs displayed a record-breaking capacitance of 1412.5 Fg-1 at 5 Ag-1, making them promising for supercapacitor electrode applications. Furthermore, these CNFs were used to fabricate a flexible all-solid-state supercapacitor, successfully powering 6 LEDs for 10 s. Hence, this work set a pave for low-cost, low-temperature synthesis of CNFs having superior physical and chemical properties and immense promise for advancing energy storage applications.

Thorium dicarbide under high pressure and high temperature: Ab initio investigation

A systematic study on the structural stability of thorium dicarbide (ThC2) under hydrostatic compression has been carried out by exploiting the evolutionary structure search algorithm as implemented in the universal structure predictor: evolutionary Xtallography (USPEX) code in conjunction with the ab initio electronic band structure calculation method. At ambient conditions, ThC2 exists in a monoclinic crystallographic phase with space group (SG) C2/c. Our calculations under generalized gradient approximation (GGA) predict the high-pressure structural sequence of monoclinic-I (SG C2/c) → monoclinic-II (SG C2/m) → orthorhombic-I (SG Pmma) → orthorhombic-II (SG Immm) → hexagonal (SG P6/mmm) for this material with transition pressures of ∼3.3, 58.3, 191.6, and 255 GPa, respectively. Out of this theoretically predicted high-pressure structural phase transition sequence, only the first transition, i.e., monoclinic-I → monoclinic-II, could be compared with the available high-pressure experimental study by Guo et al. [Sci. Rep. 7, 45872 (2017)]. The theoretically determined phase transition qualitatively agrees with the experimental results [Y. Guo et al. Sci. Rep. 7, 45872 (2017)]. Interestingly, our predicted intermediate orthorhombic-I (SG Pmma) phase has an enthalpy lower than that of the previously predicted orthorhombic Cmmm phase by Guo et al. [Sci. Rep. 7, 45872 (2017)]. The high-pressure structural sequence so predicted through static lattice calculations has been further substantiated by confirming the elastic and lattice dynamic stability of each structure in the pressure regime of its structural stability. Additionally, the superconducting transition temperature for all these structures has been determined and it is found that the monoclinic-II (C2/m) phase has the highest transition temperature of 17 K at 5 GPa. Furthermore, the thermo-physical properties along with the temperature-induced phase transitions in ThC2 have also been investigated through both the lattice dynamic simulations (within quasi-harmonic approximation) and ab initio molecular dynamics simulations.

Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi2B2C

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds.

Anisotropy of electronic correlations: On the applicability of local theories to layered materials

Besides the chemical constituents, it is the lattice geometry that controls the most important material properties. In many interesting compounds, the arrangement of elements leads to pronounced anisotropies, which reflect into a varying degree of quasi two-dimensionality of their low-energy excitations. Here, we start by classifying important families of correlated materials according to a simple measure for the tetragonal anisotropy of their ab initio electronic (band) structure. Second, we investigate the impact of a progressively large anisotropy in driving the non-locality of many-body effects. To this end, we tune the Hubbard model from isotropic cubic in three dimensions to the two-dimensional limit and analyze it using the dynamical vertex approximation. For sufficiently isotropic hoppings, we find the self-energy to be well separable into a static non-local and a dynamical local contribution. While the latter could potentially be obtained from dynamical mean-field approaches, we find the former to be non-negligible in all cases. Further, by increasing the model-anisotropy, we quantify the degree of quasi two-dimensionality which causes this "space-time separation" to break down. Our systematic analysis improves the general understanding of electronic correlations in anisotropic materials, heterostructures and ultra-thin films, and provides useful guidance for future realistic studies.

High pressure theoretical and experimental analysis of the bandgap of BaMoO4, PbMoO4, and CdMoO4

We have investigated the origin of the bandgap of BaMoO4, PbMoO4, and CdMoO4 crystals on the basis of optical absorption spectroscopy experiments and ab initio electronic band structure, density of states, and electronic localization function calculations under high pressure. Our study provides an accurate determination of the bandgaps Eg and their pressure derivatives dEg/dP for BaMoO4 (4.43 eV, −4.4 meV/GPa), PbMoO4 (3.45 eV, −53.8 meV/GPa), and CdMoO4 (3.71 eV, −3.3 meV/GPa). The absorption edges were fitted with the Urbach exponential model which we demonstrate to be the most appropriate for thick crystals with direct bandgaps. So far, the narrowing of the bandgap of distinct PbMoO4 had been qualitatively explained considering only the presence of the Pb 6s levels at the top of its valence band. Its fast pressure dependent redshift and the occurrence of its direct bandgap away from Γ in contrast to the other scheelites had remained unsolved. Here, we show that in contrast to what had been proposed and different from the other scheelites, in PbMoO4, the bandgap takes place between the Pb 6s levels at the top of the valence band and the antibonding O 2p levels at the bottom of the conduction band. For this reason, the direct bandgap is pushed away from the zone center in order to allow s–p mixing. Its pressure dependence is one order of magnitude faster than that in the other scheelites due to two effects: its delocalized character and the higher compressibility of dodecahedral units, PbO8, compared to tetrahedral units, MoO4.

Pressure-induced metallization in layered ReSe2

The evolution of the crystal structure and electrical transport properties of distorted layered transition metal dichalcogenide ReSe2 was studied under high pressure up to ~90 GPa by Raman spectroscopy and electrical resistivity measurements accompanied by ab initio electronic band structure calculations. Raman spectroscopy studies indicate an isostructural phase transition due to layer sliding at ~7 GPa, to the distorted 1T-phase which remains stable up to the highest pressures employed in these experiments. From a direct band gap semiconductor at ambient pressure, ReSe2 undergoes pressure-induced metallization at pressures ~35 GPa, in agreement with the ab initio calculations. Resistivity measurements performed with different loading conditions reveal the possible emergence of superconductivity, which is most likely not an intrinsic property of ReSe2, but is rather conditioned by internal stresses upon compression.

Accuracy and Transferability of Ab Initio Electronic Band Structure Calculations for Doped BiFeO3

BiFeO3 is a multiferroic material and, therefore, highly interesting with respect to future oxide electronics. In order to realize such devices, pn junctions need to be fabricated, which are currently impeded by the lack of successful p-type doping in this material. In order to guide the numerous research efforts in this field, we recently finished a comprehensive computational study, investigating the influence of many dopants onto the electronic structure of BiFeO3. In order to allow for this large scale ab initio study, the computational setup had to be accurate and efficient. Here we discuss the details of this assessment, showing that standard density-functional theory (DFT) yields good structural properties. The obtained electronic structure, however, suffers from well-known shortcomings. By comparing the conventional DFT results for alkali and alkaline-earth metal doping with more accurate hybrid-DFT calculations, we show that, in this case, the problems of standard DFT go beyond a simple systematic error. Conventional DFT shows bad transferability and the more reliable hybrid-DFT has to be chosen for a qualitatively correct prediction of doping induced changes in the electronic structure of BiFeO3.

High pressure behaviour of uranium dicarbide (UC2): Ab-initio study

The structural stability of uranium dicarbide has been examined under hydrostatic compression employing evolutionary structure search algorithm implemented in the universal structure predictor: evolutionary Xtallography (USPEX) code in conjunction with ab-initio electronic band structure calculation method. The ab-initio total energy calculations involved for this purpose have been carried out within both generalized gradient approximations (GGA) and GGA + U approximations. Our calculations under GGA approximation predict the high pressure structural sequence of tetragonal → monoclinic → orthorhombic for this material with transition pressures of ∼8 GPa and 42 GPa, respectively. The same transition sequence is predicted by calculations within GGA + U also with transition pressures placed at ∼24 GPa and ∼50 GPa, respectively. Further, on the basis of comparison of zero pressure equilibrium volume and equation of state with available experimental data, we find that GGA + U approximation with U = 2.5 eV describes this material better than the simple GGA approximation. The theoretically predicted high pressure structural phase transitions are in disagreement with the only high experimental study by Dancausse et al. [J. Alloys. Compd. 191, 309 (1993)] on this compound which reports a tetragonal to hexagonal phase transition at a pressure of ∼17.6 GPa. Interestingly, during lowest enthalpy structure search using USPEX, we do not see any hexagonal phase to be closer to the predicted monoclinic phase even within 0.2 eV/f. unit. More experiments with varying carbon contents in UC2 sample are required to resolve this discrepancy. The existence of these high pressure phases predicted by static lattice calculations has been further substantiated by analyzing the elastic and lattice dynamic stability of these structures in the pressure regimes of their structural stability. Additionally, various thermo-physical quantities such as equilibrium volume, bulk modulus, Debye temperature, thermal expansion coefficient, Gruneisen parameter, and heat capacity at ambient conditions have been determined from these calculations and compared with the available experimental data.

Structural, Vibrational, and Electronic Study of α-As2Te3 under Compression

We report a study of the structural, vibrational, and electronic properties of layered monoclinic arsenic telluride (α-As2Te3) at high pressures. Powder X-ray diffraction and Raman scattering measurements up to 17 GPa have been complemented with ab initio total-energy, lattice dynamics, and electronic band structure calculations. Our measurements, which include previously unreported Raman scattering measurements for crystalline α-As2Te3, show that this compound undergoes a reversible phase transition above 14 GPa at room temperature. The monoclinic crystalline structure of α-As2Te3 and its behavior under compression are analyzed by means of the compressibility tensor. Major structural and vibrational changes are observed in the range between 2 and 4 GPa and can be ascribed to the strengthening of interlayer bonds. No evidence of any isostructural phase transition has been observed in α-As2Te3. A comparison with other group 15 sesquichalcogenides allows understanding the structure of α-As2Te3 and its behavior under compression based on the activity of the cation lone electron pair in these compounds. Finally, our electronic band structure calculations show that α-As2Te3 is a semiconductor at 1 atm, which undergoes a trivial semiconducting–metal transition above 4 GPa. The absence of a pressure-induced electronic topological transition in α-As2Te3 is discussed.

β−Bi2O3under compression: Optical and elastic properties and electron density topology analysis

We report a joint experimental and theoretical study of the optical properties of tetragonal bismuth oxide ($\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$) at high pressure by means of optical absorption measurements combined with ab initio electronic band structure calculations. Our results are consistent with previous results that show the presence of a second-order isostructural phase transition in $\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ (from $\ensuremath{\beta}$ to ${\ensuremath{\beta}}^{\ensuremath{'}}$) around 2 GPa and a phase transition above 15 GPa combined with a pressure-induced amorphization above 17--20 GPa. In order to further understand the pressure-induced phase transitions and amorphization occurring in $\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$, we theoretically studied the mechanical and dynamical stability of the tetragonal structures of $\ensuremath{\beta}$- and ${\ensuremath{\beta}}^{\ensuremath{'}}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ at high pressure through calculations of their elastic constants, elastic stiffness coefficients, and phonon dispersion curves. The pressure dependence of the elastic stiffness coefficients and phonon dispersion curves confirms that the isostructural phase transition near 2 GPa is of ferroelastic nature. Furthermore, a topological study of the electron density shows that the ferroelastic transition is not caused by a change in number of critical points (cusp catastrophe), but by the equalization of the electron densities of both independent O atoms in the unit cell due to a local rise in symmetry. Finally, from theoretical simulations, ${\ensuremath{\beta}}^{\ensuremath{'}}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ is found to be mechanically and dynamically stable at least up to 26.7 GPa under hydrostatic conditions; thus, the pressure-induced amorphization reported above 17--20 GPa in powder ${\ensuremath{\beta}}^{\ensuremath{'}}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ using methanol-ethanol-water as pressure-transmitting medium could be related to the frustration of a reconstructive phase transition at room temperature and the presence of mechanical or dynamical instabilities under nonhydrostatic conditions.

Large enhancement of superconducting transition temperature of SrBi3 induced by Na substitution for Sr

The Matthias rule, which is an empirical correlation between the superconducting transition temperature (Tc) and the average number of valence electrons per atom (n) in alloys and intermetallic compounds, has been used in the past as a guiding principle to search for new superconductors with higher Tc. The intermetallic compound SrBi3 (AuCu3 structure) exhibits a Tc of 5.6 K. An ab-initio electronic band structure calculation for SrBi3 predicted that Tc increases on decreasing the Fermi energy, i.e., on decreasing n, because of a steep increase in the density of states. In this study, we demonstrated that high-pressure (~ 3 GPa) and low-temperature ( < 350 °C) synthesis conditions enables the substitution of Na for about 40 at.% of Sr. With a consequent decrease in n, the Tc of (Sr,Na)Bi3 increases to 9.0 K. A new high-Tc peak is observed in the oscillatory dependence of Tc on n in compounds with the AuCu3 structure. We have shown that the oscillatory dependence of Tc is in good agreement with the band structure calculation. Our experiments reaffirm the importance of controlling the number of electrons in intermetallic compounds.

Ab initio electronic band structure study of the valence bands of II-VI C(2 × 2) reconstructed surfaces

The structural and electronic properties of CdTe(001), CdSe(001), and ZnSe(001) C(2 x 2) reconstructed surfaces have been investigated through the use of first-principles calculations. To simulate the surface, we employed the slab model. Using the experimentally determined lattice parameters as inputs, we relaxed the internal atomic positions of the outer atomic layers. We demonstrate that our model appropriately reproduces both the surface structural parameters and the known electronic properties found for these semiconductor compounds in bulk. Finally, we discuss our results of the projected bulk bands and the surface and resonance states found for these surfaces.

Theoretical and experimental studies of electronic band structure for GaSb1−xBix in the dilute Bi regime

Photoreflectance (PR) spectroscopy was applied to study the band gap in GaSb1−xBix alloys with Bi < 5%. Obtained results have been interpreted in the context of ab initio electronic band structure calculations in which the supercell (SC) based calculations are joined with the alchemical mixing (AM) approximation applied to a single atom in the cell. This approach, which we call SC-AM, allows on the one hand to study alloys with a very small Bi content, and on the other hand to avoid limitations characteristic of a pure AM approximation. It has been shown that the pure AM does not reproduce the GaSb1−xBix band gap determined from PR while the agreement between experimental data and the ab initio calculations of the band gap obtained within the SC-AM approach is excellent. These calculations show that the incorporation of Bi atoms into the GaSb host modifies both the conduction and the valence band. The shift rates found in this work are respectively −26.0 meV per % Bi for the conduction band and 9.6 meV per % Bi for the valence band that consequently leads to a reduction in the band gap by 35.6 meV per % Bi. The shifts found for the conduction and valence band give a ∼27% (73%) valence (conduction) band offset between GaSb1−xBix and GaSb. The rate of the Bi-related shift for the split-off band is −7.0 meV per % Bi and the respective increase in the spin–orbit split-off is 16.6 meV per % Bi.

Ab initio electronic band structure study of III–VI layered semiconductors

We present a total energy study of the electronic properties of the rhombohedral γ-InSe, hexagonal e-GaSe, and monoclinic GaTe layered compounds. The calculations have been done using the full potential linear augmented plane wave method, including spin-orbit interaction. The calculated valence bands of the three compounds compare well with angle resolved photoemission measurements and a discussion of the small discrepancies found has been given. The present calculations are also compared with recent and previous band structure calculations available in the literature for the three compounds. Finally, in order to improve the calculated band gap value we have used the recently proposed modified Becke-Johnson correction for the exchange-correlation potential.

Effect of submonolayer MgO coverages on the electron-spin motion in Fe(001): Experiment and theory

It is shown that the spin polarization direction of the reflected electrons on Fe(001) strongly changes with minute amounts of MgO. Our ab initio electronic band structure and spin-dependent electron reflection calculations reveal that the MgO-induced out-of-plane relaxation of the Fe surface layer is responsible for this behavior. Our study points toward the subtle feature that the major change of the spin-dependent electron reflection properties of the Fe(001) surface is already caused by the very first MgO coverage.

Straightforward Route to the Adamantane Clusters [Sn4Q10]4− (Q = S, Se, Te) and Use in the Assembly of Open-Framework Chalcogenides (Me4N)2M[Sn4Se10] (M = MnII, FeII, CoII, ZnII) Including the First Telluride Member (Me4N)2Mn[Ge4Te10

The reaction of K(2)Sn(2)Q(5) (Q = S, Se, Te) with stoichiometric amounts of alkyl-ammonium bromides R(4)NBr (R = methyl or ethyl) in ethylenediamine (en) afforded the corresponding salts (R(4)N)(4)[Sn(4)Q(10)] (Q = S, Se, Te) in high yield. Although the compound K(2)Sn(2)Te(5) is not known, this reaction is also applicable to solids with a nominal composition "K(2)Sn(2)Te(5)" which in the presence of R(4)NBr in en are quantitatively converted to the salts (R(4)N)(4)[Sn(4)Te(10)] on a multigram scale. These salts contain the molecular adamantane clusters [Sn(4)Q(10)](4-) and can serve as soluble precursors in simple metathesis reactions with transition metal salts to synthesize the large family of open-framework compounds (Me(4)N)(2)M[Sn(4)Se(10)] (M = Mn(2+), Fe(2+), Co(2+), Zn(2+)). Full structural characterization of these materials as well as their magnetic and optical properties is reported. Depending on the transition metal in (Me(4)N)(2)M[Sn(4)Se(10)], the energy band gaps of these compounds lie in the range of 1.27-2.23 eV. (Me(4)N)(2)Mn[Ge(4)Te(10)] is the first telluride analogue to be reported in this family. This material is a narrow band gap semiconductor with an optical absorption energy of 0.69 eV. Ab initio electronic band structure calculations validate the semiconductor nature of these chalcogenides and indicate a nearly direct band gap.

Star-Shaped Molecule of MnII4O6 Core with an St = 10 High-Spin State. A Theoretical and Experimental Study with XPS, XMCD, and Other Magnetic Methods

We report a comprehensive study of the electronic and magnetic properties of a star-shaped molecule comprising a MnII4O6 core. One feature of this compound is weak magnetic coupling constants compared to other similar polyoxo compounds. This leads to complicated low-lying magnetic states in which the ground state is not well separated from the upper-lying states, yielding a high-spin molecule with a giant magnetic moment of up to 20 microB/formula unit. We apply X-ray diffraction and magnetometry as well as other X-ray spectroscopic techniques, namely, X-ray photoelectron spectroscopy, X-ray magnetic circular dichroism, and X-ray emission spectroscopy. We compare our experimental results with ab initio electronic band structure calculations as well as the localized electronic structure around the Mn2+ ions with charge-transfer multiplet calculations.

Structural and optical properties of CaO

CaO is a wide band gap material yet unexplored for optoelectronics, but which was recently proposed as a candidate for spintronics applications. In the present work we report the results of an ab initio electronic band structure calculation of cubic CaO using both the local-density and the generalized gradient approximations. We performed the structural CaO crystal optimization, and calculated its optical properties, which are compared with the available experimental data and with other theoretical results for the cubic CaO structure.

High pressure phase transition in metallic LaB 6: Raman and X-ray diffraction studies

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB 6 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.

Transport properties of Mn-doped Ru 2Si 3

Transport properties of Mn-doped ruthenium silicide Ru 2Si 3 were studied both experimentally and theoretically. The precipitation-free Ru 2Si 3 single crystals were grown by the zone melting technique with radiation heating. The temperature dependence of the electrical resistivity and Hall coefficients of the crystals were measured. The electrical resistivity of 1% Mn-doped Ru 2Si 3 was lower than that of undoped crystals. The carrier concentration in the doped samples is about 10 18 cm −3 at room temperature. Mn-doped Ru 2Si 3 has a twice higher carrier mobility compared to the undoped one. Theoretical calculation of the charge carrier mobility is based on the effective masses which are estimated from the ab initio electronic band structure and classical scattering mechanisms.