Rigid Band Model Research Articles

Probing the mechanism of guest-framework bonding interactions through a first-principles study on the structural and electronic properties of type-II clathrate A x Si136 (A = Na, K, Rb; 0 ≤ x ≤ 24) under pressure.

The role of noncovalent bonding, including multiatomic interactions (van der Waals-like forces) and ionic characteristics, in the intermetallic clathrate A x Si136 (A = Na, K, Rb; 0 < x ≤ 24) is qualitatively discussed. Using the local density approximation (LDA) to density functional theory (DFT), we investigated the effect of different guest filling and pressure parameters on the structural and electronic properties of these materials. In the context of the rigid-band model, we first noted that the competition between van der Waals-like multiatomic interactions and ionicity due to the extent of charge transfer responsible for guest-framework complexes accounts for the nonmonotonic structural response upon guest filling in A x Si136 (0 ≤ x ≤ 8), which is in good agreement with previous experimental findings as well as theoretical predictions. In comparison with computational work initiated under zero temperature and pressure conditions, the DFT calculations at high pressure (P = 3 GPa) show no apparent variation with respect to the electronic structure. Regarding the A16Si136 compound, the encapsulated sodium atoms residing in the 20-atom cage cavity act as centers of somewhat localized electrons compared with the alkaline metal sites inside Si28 cage voids. Moreover, the substitution of heavier guest atoms (e.g., Rb) for all the Na atoms in Na8Si136 yields less significant charge transfer between the guest and framework constituents. The net effect of quickly increasing multiatomic interactions and slowly decreasing ionic bonding between the encapsulated atom and Si28 cage may prevent the entire lattice configuration from contracting in a more rapid way when guest species are tuned from Na to Rb in A x Si136 (A = Na, Rb; 0 < x ≤ 8) with increased composition x. In other words, the coulombic attraction due to ionic bonding slightly outweighs the repulsive interaction between the Rb atom and Si28 cage. In addition, the determined formation energy per conventional unit cell in K8Si136, Rb8Si136 and Na12Si136 attains a minimum value, demonstrating the stabilizing effect of guests incorporated into "oversized" cage cavities.

Unusual Transport and Impact of Nonparabolic Electronic Band Structure on the Thermoelectric Performance in n‐Type In4Se3‐Based Thermoelectric Materials

Unusual thermoelectric (TE) transport in n‐type In4Se3 has been reported in the context of first‐principles electronic structure calculation and Boltzmann transport theory. The density of states (DOS) shows certain peaks due to a nontopological Mexican hat and parabolic capping around the edges of Kane bands. The effect of such hard shifts of bands shows anomalously high Seebeck coefficient (S = 550 μV K−1 at ≈1017 cm−3) at room temperature due to very slow decreasing S and rapidly increasing DOS effective mass with carrier concentration compared to that obtained from Kane and parabolic band model. The best TE performance is predicated in Kane transport region (≈1018 cm−3) over the wide range of temperature, 300–700 K. The calculated values of relaxation time (≈10−14 s at 660 K), electrical conductivity, power factor and figure of merit in iodine‐doped sample and those obtained from the rigid band model of pure sample show excellent agreement with the reported experimental results.

Specific Features of the Electronic Structure of Oxygen-Deficient Perovskites SrFe1 – xMoxO3 – y

By using the VASP package, within the framework of the DFT approach, the properties of the ground state of the SrFe1 – xMoxO3 – y oxide with the perovskite structure are calculated for the various values of molybdenum content and oxygen nonstoichiometry. It is shown that the doping procedure as well as the procedure of varying the oxygen content give rise to changes in the charge stage of oxygen ions in the system, which is accompanied by a shift of the Fermi level with respect to the invariant band structure (rigid band model) and the transition to the semimetal conduction type.

The rendering from the periodic system of elements on the stability, elastic, and electronic properties of M2AC phases

MAX phases are nanolaminated ternary materials that combine metallic and ceramic properties. Currently, the A-site elements replacement in traditional ones by later transition-metals opens a door to explore new types of MAX phases. In this work, we performed a systematic first-principle study to explore trends in stability, electronic structure and elastic properties of 288 compositions of M2AX phases (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; A=Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb, Te, Tl, Pb, Bi, Mn, Fe, Co, Ni, Cu, Zn, Tc, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, Au, Hg; X=C). Such a dataset, combined with the rigid-band model been applied to most transition metal carbides, shows us the fundamental trends in bonding mechanisms and elastic properties of MAX phases endowed with the periodic arrangements of M/A-site elements. In particular, the M-A d-d interactions of MAX phases uniquely contribute to the elastic constant C33.

The carrier mobility and superconducting properties of monolayer oxygen-terminated functionalized MXene Ti2CO2.

In this study, the carrier mobility of monolayer Ti2CO2 was evaluated by employing the Boltzmann transport equation and superconducting transition temperature (Tc) of Ti2CO2 was determined by utilizing the Migdal and Eliashberg formalism in the first-principles framework. In contrast to previous studies, the results reveal that optical phonons in monolayer Ti2CO2 have dominant roles in scattering processes, which significantly reduce the mobility of carriers. Alongside the rigid band model, the jellium model is implemented to investigate the screening effects on electron-phonon interactions. Based on the jellium model and full-band electron-phonon calculations, the predicted maximum electron mobility at room temperature is 38 cm2 V-1 s-1 in which 80% of the total scattering rate originates from the intra-valley transitions within the M-valleys, indicating the crucial role of the long wavelength phonon wavevectors in scattering processes. On the other hand, for the p-type material, a maximum room temperature mobility of about 285 cm2 V-1 s-1 is calculated, which can be explained by a relatively small effective mass and tiny scattering phase space. Moreover, a maximum Tc of 39 (10) K is obtained for the n-type monolayer Ti2CO2 based on the rigid (jellium) model. Outcomes indicate that the important peaks of α2F(ω) are mainly caused by the optical phonons. The remarkable couplings between the electron states and phonons are related to the non-zero slope of (near the Brillouin zone center) the longitudinal optical branch denoted by Eu caused by the displacements of oxygen and carbon atoms at intermediate and high energy ranges of phonon dispersion, respectively.

Large magnetostriction of heavy-metal-element doped Fe-based alloys

Using density functional theory calculation and rigid band model, we investigate the electronic structure and magnetostrictive properties of transition heavy-metal doped Fe-based (Fe–Al, Fe–Si, Fe–B, and Fe–Be) alloys. It is found that a small amount of addition of 4d/5d heavy-metal atoms greatly enhances the coefficient of tetragonal magnetostriction of Fe-based alloys, reaching up to about 1000 ppm in Fe87.5Al6.25Pt6.25 and Fe75Al18.75Rh6.25 alloys. The underlying mechanism is mainly ascribed to combined factors of band narrowing induced by non-bonded states in pure Fe layer, strong spin–orbit coupling effect by heavy metals, and improved mechanical properties, through analysis of the electronic density of states near Fermi level and k-mesh resolved magnetocrystalline anisotropy energy in momentum space. These results provide useful guidance for optimizing the magnetostrictive performance of Fe-based alloys for practical application.

Atomic site preference, electronic structures, and magnetic properties of γ-brass type pseudo-binary Mn2Zn11–Ni2Zn11 at high Mn-contents

Samples loaded as MnxNi2Zn11−x (x = 0.1–5) were synthesized using conventional high-temperature solid-state techniques and characterized using X-ray diffraction, neutron powder diffraction, and energy dispersive X-ray spectroscopy. A Hume-Rothery-type mechanism was applied, following the rigid band model, to rationalize the electronic stability of these phases. This shows that at low Mn substitution for Zn, there is a structural preference for Mn atoms to be farthest apart within the same cluster, but that with increasing Mn concentration, there is a favorability in maximizing shortest Mn–Mn contacts between adjacent clusters of the γ-brass-type structure. The properties involving temperature-dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetization, susceptibility, M(H) hysteresis curves, thermoremanent magnetization, and memory effect were studied for Mn1.5Ni2Zn9.5. A spin glass state has been observed below the transition temperature of ∼58.5 K, which originates predominately from the disordered substitution of Mn for Zn atoms in the inner tetrahedral and octahedral shells of the structure.

Enhanced low-energy magnetic excitations evidencing the Cu-induced localization in the Fe-based superconductor Fe0.98Te0.5Se0.5

We have performed inelastic neutron scattering measurements on optimally doped ${\mathrm{Fe}}_{0.98}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ and 10% Cu-doped ${\mathrm{Fe}}_{0.88}{\mathrm{Cu}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ to investigate the substitution effects on the spin excitations in the whole energy range up to 300 meV. It is found that substitution of Cu for Fe enhances the low-energy spin excitations ($\ensuremath{\le}100\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$), especially around the (0.5, 0.5) point, and leaves the high-energy magnetic excitations intact. In contrast to the expectation that Cu with spin 1/2 will dilute the magnetic moments contributed by Fe with a larger spin, we find that the 10% Cu doping enlarges the effective fluctuating moment from 2.85 to 3.13 ${\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$, although there is no long- or short-range magnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhanced magnetic excitations in the 10% Cu doped sample which is in the insulating state indicates that the magnetic excitations must have some contributions from the local moments, reflecting the dual nature of the magnetism in iron-based superconductors. We attribute the substitution effects to the localization of the itinerant electrons induced by Cu dopants. These results also indicate that the Cu doping does not act as electron donor as in a rigid-band shift model, but more as scattering centers that localize the system.

Studying Electronic and Thermoelectric Properties of Ga-doped ZnO using Rigid Band Model

Studying Electronic and Thermoelectric Properties of Ga-doped ZnO using Rigid Band Model

First-principles disordered local-moment study on temperature dependence of spin polarization in Co2Fe(Ga0.5Ge0.5) Heusler alloy

Magnetoresistance (MR) devices fabricated with half-metallic Co-based Heusler alloys (Co2YZ) have a large MR ratio but are subject to substantial temperature degradation. Reduction of the spin polarization in the bulk electrode at finite temperatures is one possible reason for the reduction in the MR ratio. In this study, we investigated the temperature dependence of the spin polarization of Co2Fe(Ga0.5Ge0.5) (CFGG) using density-functional theory and the disordered local-moment method. We found that the reduction in the spin polarization at finite temperatures is smaller in CFGG compared to the well known Co2MnSi (CMS) half-metal, which is attributed to the higher Curie temperature of CFGG than CMS. On the other hand, it was also found that modulation of the Fermi level position in the half-metallic gap of CFGG within the rigid band model by electron and hole doping can improve the temperature dependence of the spin polarization. However, off-stoichiometric Co-rich and Fe-rich CFGG corresponding to electron and hole doping show a reduction in spin polarization compared to that of stoichiometric CFGG at 0 K. Therefore, we propose that modulation of the Fermi level position through varying the Y and Z site compositions of Co2YZ will be necessary to improve the temperature dependence of the spin polarization.

Reduction in Work Functions of Transition-Metal Carbides and Oxycarbides upon Oxidation.

Herein, the work functions of group 4 and group 5 transition-metal (Ti, Zr, Hf, V, Nb, and Ta) carbides and transition-metal oxycarbides (TMCOs) were investigated by first-principles calculations for their potential application as electron emitters. The work functions of both groups decreased as the substitution of carbon atoms with oxygen proceeded, and the reduction in group 4 was more than that of group 5. In particular, ZrC1–xOx and HfC1–xOx (x ≥ 0.25) exhibited work functions of less than 3 eV, which were comparable with those of LaB6- and ZrO-coated tungsten. The reduction in the work functions could be explained by the rigid-band model of the electronic density of states. The increase in valence electrons increased the Fermi energy, while it demonstrated a less significant influence on the vacuum potential, resulting in a reduction in the work functions. The phonon dispersion curves indicated that the NaCl-type group 5 TMCOs were less stable than the group 4 TMCOs. This agrees with the experimental findings that TaC1–xOx was not synthesized and NbC1–xOx was synthesized only for smaller values of x (i.e., x < 0.28). From the viewpoints of the work functions and structural stabilities, group 4 (Ti, Zr, and Hf) TMCOs exhibit better potential for application as electron emitters than group 5 (V, Nb, and Ta) TMCOs.

First-Principles Disordered Local-Moment Study on Temperature Dependence of Spin Polarization in Co &lt;sub&gt;2&lt;/sub&gt;Fe(Ga &lt;sub&gt;0.5&lt;/sub&gt;Ge &lt;sub&gt;0.5&lt;/sub&gt;) Heusler Alloy

Magnetoresistance (MR) devices fabricated with half-metallic Co-based Heusler alloys (Co2YZ) have a large MR ratio but are subject to substantial temperature degradation. Reduction of the spin polarization in the bulk electrode at finite temperatures is one possible reason for the reduction in the MR ratio. In this study, we investigated the temperature dependence of the spin polarization of Co2Fe(Ga0.5Ge0.5) (CFGG) using density-functional theory and the disordered local-moment method. We found that the reduction in the spin polarization at finite temperatures is smaller in CFGG compared to the well known Co2MnSi (CMS) half-metal, which is attributed to the higher Curie temperature of CFGG than CMS. On the other hand, it was also found that modulation of the Fermi level position in the half-metallic gap of CFGG within the rigid band model by electron and hole doping can improve the temperature dependence of the spin polarization. However, off-stoichiometric Co-rich and Fe-rich CFGG corresponding to electron and hole doping show a reduction in spin polarization compared to that of stoichiometric CFGG at 0 K. Therefore, we propose that modulation of the Fermi level position through varying the Y and Z site compositions of Co2YZ will be necessary to improve the temperature dependence of the spin polarization.

Soft X-ray spectroscopic study on the electronic structure of WO3 thin films fabricated under various annealing temperature and gas flow conditions

The electrical properties of WO3 thin films vary significantly depending on the growth conditions. In this work, the influence of O2 gas on the band gap of WO3 thin films during growth was investigated via electronic structure characterization using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). A substantial decrease in the electrical conductivity of the WO3 films was observed with an increase in the O2 partial pressure during growth. Spectral differences in the peak energy and intensity were apparent for WO3 films grown under only Ar and those grown in Ar:O2. It is difficult to explain the acquired spectrum of WO3 with oxygen defects through the rigid-band model in terms of the simple addition of electrons to the conduction band of WO3. Our results show that an oxygen deficiency in WO3 moves the conduction band to the Fermi edge.

A Sequential Electron Doping for Quadruple Perovskite Oxides ACu3Co4O12 (A = Ca, Y, Ce).

A novel quadruple perovskite oxide CeCu3Co4O12 has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A-site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the ACu3Co4O12 series (A = Ca, Y, Ce) from Ca2+Cu3+3Co3.25+4O12 to Y3+Cu3+3Co3+4O12 to Ce4+Cu2.67+3Co3+4O12, where the electrons are doped in the order from B-site (Co3.25+ → Co3+) to A'-site (Cu3+ → Cu2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO3-type perovskite and quadruple perovskites CaCu3B4O12 (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na1-xLaxMn3Ti4O12 and A'- and B-site electron doping for AMn3V4O12 (A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.

Large magnetic anisotropy of single transition metal adatoms on WS2

We performed systematic density functional theory calculations to study the structural stability and magnetic properties of single transition metal adatoms on a WS2 monolayer. It was found that Re adatom takes the hollow site on WS2 and has a large perpendicular magnetic anisotropy energy (MAE) up to ~30 meV/atom. Electronic structure and rigid band model analyses revealed that this large MAE originates from the spin-orbital coupling interaction between the dxz and dyz orbitals in the spin down and through cross spin channels. This work provides a practical single-atom magnetic system for potential applications and gives useful insights for further developments.

Two-dimensional conducting layer on the SrTiO3 surface induced by hydrogenation

We found that a surface state induced by hydrogenation on the surface of SrTiO$_{3}$(001) (STO) did not obey the rigid band model, which was confirmed by in situ electrical resistivity measurements in ultrahigh vacuum. With exposure of atomic hydrogen on the STO, a new surface state (H-induced donor state, HDS) appears within the bulk band gap (an in-gap state), which donates electrons thermally activated to the bulk conduction band, resulting in downward bending of the bulk bands beneath the surface. The doped electrons flow through the space-charge layer in two-dimensional manner parallel to the surface. The observed semiconducting behavior in the temperature dependence of electronic transport is explained by the thermal activation of carriers. The HDS and the bulk conduction band are non-rigid in energy position; they come closer with increasing the hydrogen adsorption. Eventually the HDS saturates its position around 88 meV below the bottom of the bulk conduction band. The sheet conductivity, accordingly, also saturates at 1.95$\pm$ 0.02 $\mu$ S/sq. with increasing hydrogen adsorption, corresponding to completion of the hydrogenation of the surface.

A non-rigid shift of band dispersions induced by Cu intercalation in 2H-TaSe2

The intercalation of metal is a promising method for the modulating electronic properties in transition metal dichalcogenides (TMDs). However, there still lacks enough knowledge about how the intercalated atoms directly impact the two-dimensional structural layers and modulate the band structures therein. Taking advantage of X-ray absorption fine structure and angle-resolved photoemission spectroscopy, we studied how Cu intercalation influences the host TaSe2 layers in Cu0.03TaSe2 crystals. The intercalated Cu atoms form bonds with Se of the host layers, and there is charge transfer from Cu to Se. By examining the changes of band dispersions, we show that the variation of electronic structures is beyond a simple rigid band model with merely charge doping effect. This work reveals that the unusual change of band dispersions is associated with the formation of bonds between the intercalated metal elements and anion ions in the host layers, and provides a reference for the comprehensive understanding of the electronic structures in intercalated materials.

Pressure-induced high-temperature superconductivity in hypothetical H3X (X=As, Se, Br, Sb, Te and I) in the H3S structure with [formula omitted] symmetry

The discovery of high critical temperature Tc superconductivity in highly compressed H3S has opened up the question of searching for strong electron–phonon coupling in the hydrides outside the transition metal series. The specific objective of this work is to explore the possibility of discovering a material that exceeds the superconducting transition temperature of H3S. Our study includes the materials H3X (X=As, Se, Br, Sb, Te, and I), is limited to the Im3¯m crystal structure. The procedure we adopt involves performing linearized augmented plane wave (LAPW) calculations for many different volumes to compute the electronic densities of states and their pressure variation. This is combined with Quantum-ESPRESSO (QE) calculations from which we obtain the phonon frequencies and the electron–phonon coupling constant λ, and followed by applying the multiple scattering-based theory of Gaspari and Gyorffy (GG) to obtain the Hopfield parameters and the McMillan–Allen–Dynes theory. It should be stressed that the GG approach decouples the electronic contribution to λ from the corresponding phonon contribution, and provides additional insights for the understanding of superconductivity in these materials. Based on our analysis, the hydrogen is the main contributor to the Tc in these materials as it makes up 75∼80 % of the total λ. Our calculations for H3Se and H3Br give a Tc∼100 K. For the other materials in our study we find that H3As is unstable and H3Sb, H3Te and H3I have small values of the McMillan–Hopfield parameters which makes it unlikely to give high Tc. However, according to both of our rigid band model and virtual crystal calculations, we predict a Tc∼150 K for H3Br with a small amount of hydrogen doping. Our basic conclusion is that the materials studied here could not reach very high Tc because the Hopfield parameters, which are the strongest contributor to high Tc, are not large enough.

µ-PES Studies on TiNCl and Quasi-two-dimensional Superconductor Na-intercalated TiNCl

Electron-doped TiNCl is a superconductor, for which exotic mechanisms of the superconductivity have been discussed. However, difficulty in preparing large single-phase samples has prevented the direct observation of its electronic structure and how that changes with electron doping. In this study, micro-photoemission spectroscopy ({\mu}-PES) was used to reveal the electronic structures of TiNCl and Na-intercalated TiNCl (Na-TiNCl). Comparison of Ti 2p core-level spectra shows the enhancement of a spectral feature in Na-TiNCl that suggests an increase in its metallic character. This indicates the introduction of electron carriers to the TiNCl layers. Although the overall valence band electronic structure of parent TiNCl could be reproduced by first-principle calculations, that of Na-TiNCl showed a marked deviation from the rigid band model near the Fermi level (EF). The spectral shape observed near EF of Na-TiNCl was found to be similar to a result in early transition metal oxides that has been attributed to the effect of strong electron correlation. The present study also demonstrated the usefulness of {\mu}-PES to obtain reliable electronic structure data for samples that are difficult to make at large sizes.

Unified description of the electronic structure of M2AC nanolamellar carbides

The nanolamellar ${M}_{2}A\mathrm{C}$ or carbon-based ``211 MAX'' phases, where $M$ is an early transition metal, $A$ belongs to groups 13--15, and C is carbon, can be described by rigid band models. The same band model applies to all possible $A$ elements belonging to a given group of the periodic table. Changing $M$ for a given $A$ is then equivalent to shifting the Fermi energy ${E}_{F}$ through a band structure common to all phases in the group. This is shown by comparing predictions of density functional theory (DFT) to angle-resolved photoemission spectroscopy (ARPES) measurements. In particular, the Fermi surface of a given Al-based MAX phase can be obtained with an acceptable degree of accuracy by simply selecting the appropriate ARPES isoenergy surface of another Al-based phase. In ${\mathrm{V}}_{2}\mathrm{AlC}$, and in addition to conventional metal energy bands, both DFT and ARPES show the existence of a gapped nodal line located around 0.2 eV below ${E}_{F}$ or complex crossing points at ${E}_{F}$ with Dirac-like features in some directions. Application of the rigid band model suggests that these topological features as well as others, also predicted by DFT, can be positioned at or close to ${E}_{F}$ by an appropriate choice of $M$ and $A$, or by using an appropriate combination of various $M$ and $A$ elements.