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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.

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.

Spin momentum densities of ferromagnetic ZrFe2 and Zr0.8Sc0.2Fe2 have been measured using magnetic Compton scattering with 182.65 keV circularly polarized synchrotron radiations. Site specific spin moments, which are responsible for the formation of total spin moment, have been deduced from Compton line shapes. At room temperature, the computed spin moment of ZrFe2 is found to be slightly higher than that of Sc doped ZrFe2 which is in consensus with the magnetization data. To compare the experimental data, we have also computed magnetic Compton profiles (MCPs), total and partial spin projected density of states (DOS) and the site specific spin moments using spin-polarized relativistic Korringa-Kohn-Rostoker method. It is observed that the spin moment at Fe site is aligned antiparallel to that of Zr site in both ZrFe2 and Zr0.8Sc0.2Fe2. The MCP results when compared with vibrating sample magnetometer based magnetization data, show a very small contribution of orbital moment in the formation of total magnetic moments in both the compounds. The DOS of ferromagnetic ground state of ZrFe2 and Zr0.8Sc0.2Fe2 are interpreted on the basis of a covalent magnetic model beyond the Stoner rigid band model. It appears that on alloying between a magnetic and a non-magnetic partner (with low valence), a polarization develops on the non-magnetic atom which is anti-parallel to that of the magnetic atom.

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.

Electronic structure, lattice dynamics, and electron-phonon coupling of Li-intercalated bilayer SnS2 are systematically investigated via first-principles density functional theory. The energetically stable configuration for Li-intercalated bilayer SnS2 is /AB/ stacking, which is different from /AA/ stacking for pristine bilayer. There is a charge transfer from Li to bilayer SnS2 and the change of the band structure after Li intercalation can be explained well by a rigid band model, suggesting that the intercalated Li atoms mainly play a role of charge reservoir. Our calculations show that the softening of acoustic phonon near K ¯ high-symmetry point make a large contribution to electron-phonon interaction and the superconducting temperature Tc can achieve 14.0 K. Our study suggest that Li-intercalated SnS2, a potential material of lithium-ion battery, may meanwhile be a quasi two-dimensional superconductor.

The elastic properties and electronic structures of transition metal compounds Cr5B3, Nb5Si3, Ta5Si3, Nb5Ge3, and Ta5Ge3, all having the Cr5B3-type structure, were studied using first-principles calculations. Because the chemical elements involved in the materials are from different groups and periods, the band widths and energy positions are different. However, all of them show very similar features in their electronic structure, in particular a pseudogap in the densities of states. The calculations show that all the materials can be strengthened by adjusting the number of valence electrons to 36 per formula unit, where the electronic states below the pseudogap are occupied and those above empty. The results indicate that the rigid band model can be applied to materials with the transition metal, the main-group element, or both are changed, suggesting a convenient way to design strong materials by changing the number of valence electrons.

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.

The electronic transport coefficients of LiBe for the intermetallic quasi-two-dimensional layered structure are evaluated by means of the semi-classical Boltzmann theory and the rigid band model. It has been found that the charge carrier concentration is about −0.0026 e/uc at low temperature (T = 50 K), then it decreases with increase of temperature to reach −0.0028 e/uc at 250 K. Above this temperature the charge carriers concentration increases rapidly to reach −0.00228 e/uc at 600 K. The negative sign shows that LiBe exhibits n-type conduction at a certain value of chemical potential (μ = E F). The calculated electrical conductivity (σ/τ) at μ = E F shows that at low temperatures (50 K) LiBe exhibits low σ/τ of about 1.318 × 1021 (Ω ms)−1, this value increases rapidly with an increase of temperature to be 1.389 × 1021 (Ω ms)−1 at 300 K. Above 300 K the σ/τ reach to the saturated value of about 1.389 × 1021 (Ω ms)−1. It has been found that LiBe possesses a thermo-power (S) of about −460 μV/K at 50 K, which decreases with increase of temperatures to reach −220 μV/K at around 350 K. Increasing the temperatures above 350 K lead to a decrease S to be −190 μV/K at 600 K. It is clear that at a certain value of chemical potential LiBe exhibits n-type conduction as it has negative S in the whole temperature range; this supports our previous observation about the charge carriers concentration. It is a remarkable finding that LiBe exhibits a large Seebeck coefficient, which is attributed to the non-zero density of states at the Fermi level. The electron thermal conductivity (κ e/τ) increased linearly with the temperature, and we observed that at low temperature LiBe exhibits low κ e/τ of about 0.2 × 1016 W/mKs at 50 K. A rapid increases in κ e/τ occurs as a result of increasing the temperature to reach 2.1 × 1016 W/mKs at 600 K. The obtained values of the power factor (S 2 σ/τ) as function of temperature show that at low temperature (50 K) LiBe possesses a high S 2 σ/τ of about 2.8 × 1010 mW/mK2, and with increasing the temperature a rapid reduction in the S 2 σ/τ occurs till 350 K to reach the value of about 0.5 × 1010 mW/mK2. Above this temperature LiBe exhibits almost a stable S 2 σ/τ of about 0.6 × 1010 mW/mK2 at 600 K. Also, we have calculated the transport properties as a function of chemical potential at the vicinity of E F (±0.15 eV above and below E F) for two different temperatures. In LiBe for the quasi-two-dimensional layered structure, the Li and Be atoms are packed in zig-zag layers along the y-axis. The Li and Be layers are puckered with square nets of Li and triangular nets of Be alternating with each others along the z-axis. Given that the LiBe system exhibits two-dimensional (2-D) doping behavior, it might be justifiable to study its transport properties; however, we would like to explore the unique transport behavior arising from its 2-D electronic structure.

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.

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., Comment: 14 page, 5 figures

The discovery of high critical temperature T c 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 I m 3 ¯ 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 T c in these materials as it makes up 75 ∼ 80 % of the total λ . Our calculations for H3Se and H3Br give a T c ∼ 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 T c . However, according to both of our rigid band model and virtual crystal calculations, we predict a T c ∼ 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 T c because the Hopfield parameters, which are the strongest contributor to high T c , are not large enough.

This chapter presents an experimental and theoretical discussion of the superconducting properties of the transition metals (TM). It discusses the models which have been proposed for the electronic and phonic band structure of the TM. The chapter emphasizes the concept of the rigid band model, which is found experimentally to be appropriate for these metals. It also discusses the electron–electron and electron–phonon interactions and how they can be determined from the experimental properties of these metals in the normal state. The study of the superconducting properties of the TM is divided into two sections. First, concerned with the variation of the pairing interaction among the elements upon alloying, with changing isotopic mass, and with changing atomic volume. It also discusses evidence for two-band superconductors, particularly for La and U. Second, discusses the properties of the superconducting state, and it is shown that generally one finds approximate agreement with the predictions of the BCS theory.

The calculated electronic band structure shows that the g-BC3 phase is a narrow band gap semiconductor constructed from an ABAB,…, stacking sequence. Whereas t-BC3 is a metallic phase constructed from a sandwich-like metal-insulator lattice from an alternately stacking sequence of a metallic CBC block and an insulating CCC block. The electronic transport coefficients of the g-BC3 and t-BC3 phases are obtained with the aid of the semi-classical Boltzmann theory and the rigid band model based on density functional theory within the recently modified Becke–Johnson potential (mBJ). The semiconductor phase (g-BC3) exhibits a linearly dependent charge carrier concentration and electrical conductivity with temperature, whereas for the t-BC3 phase the charge carrier concentration is increased with increasing temperature up to 200 K, then a rapid decrease occurs upon increasing the temperature. The electrical conductivity of t-BC3 shows the highest value at 50 K then a rapid decrease occurs upon increasing the temperature up to 250 K, and it stays roughly constant above this temperature. The Seebeck coefficient of the g-BC3 and t-BC3 phases shows that these materials exhibit p-type conduction and the g-BC3 phase exhibits a higher Seebeck coefficient than the t-BC3 phase. The g-BC3 phase exhibits an almost linearly dependent power factor with temperature. Whereas the power factor of the t-BC3 phase shows a very high value at low temperature then drops to the lower value at 200 K. Above this temperature the power factor of t-BC3 becomes zero along the whole temperature range. It has been found that the g-BC3 phase shows a higher power factor than that of the t-BC3 phase. The electronic thermal conductivity of g-BC3 exponentially increases with increasing temperature. It has a zero value at 50 K and the highest value 4.6 × 1014 (W m−1 K−1 s−1) at 900 K. t-BC3 exhibits linearly dependent electronic thermal conductivity with temperature. At low temperature (50 K) the electronic thermal conductivity of t-BC3 is about 0.15 × 1016 (W m−1 K−1 s−1) while it has a value of about 2.1 × 1016 (W m−1 K−1 s−1) at 900 K. It is clear that the g-BC3 phase exhibits lower electronic thermal conductivity than the t-BC3 phase.

Applying magnetic nanostructures in high density magnetic data storage is hindered by a lack of suitable substrate. Using density functional theory, we explored the potentiality of graphyne as a template for nanomagnetic bits. Due to the unique porous structure of graphyne, Os atom tightly binds to the graphyne at the hollow site with an in-plane MAE of 18 meV. Through a rigid band model, we introduced the strategy to manipulate the MAE by rearrangement of d-orbitals of Os atom. A large MAE of about 48 meV was obtained for F functionalized Os@graphyne. To obtain the out-of-plane easy axis, we have considered the MAE of several transition-metal combinations. We finally identified Os–Os@graphyne as an excellent candidate for room temperature applications due to the high MAE and the structural stability.

The electronic transport coefficients of Nowotny–Juza NaZnX (X = P, As and Sb) compounds were evaluated by using the semi-classical Boltzmann and the rigid band model. The compounds are direct band gap (Γv–Γc) semiconductors with energy band gaps of about 1.8, 1.47 and 0.25 eV, for NaZnX (X = P, As and Sb) respectively. A maximum value of electrical conductivity of about 5.4 × 1020 (Ω ms)−1 and 5.1 × 1020 (Ω ms)−1 is achieved at 0.2 μ (eV) for n-type of NaZnP and NaZnAs, while for NaZnSb a maximum value of about 4.1 × 1020 (Ω ms)−1 is found at −0.15 μ (eV) for p-type and 4.2 × 1020 (Ω ms)−1 at 0.15 μ (eV). The investigated materials exhibit the highest value of Seebeck coefficient at 300 K of about 300, 250, 510 (μV/K) for p-type and −290, −240, −305 (μV/K) for n-type of NaZnX (X = P, As and Sb) respectively. In the chemical potential region between ∓0.6, ∓0.5, ∓0.025 μ (eV) NaZnP, NaZnAs, NaZnSb respectively exhibit a minimum value of electronic thermal conductivity. Therefore in these regions the investigated materials can give maximum efficiency.

By using magnetization and small-angle neutron scattering (SANS) measurements, we have investigated the magnetic behavior of Mn_{1-x}Ir_{x}Si system to explore the effect of increased carrier density and spin-orbit interaction on the magnetic properties of MnSi. We determine estimates of the spin wave stiffness and the Dzyalloshinski-Moria, DM, interaction strength and compare with Mn_{1-x}Co_{x}Si and Mn_{1-x}Fe_{x}Si. Despite the large differences in atomic mass and size of the substituted elements, Mn_{1-x}Co_{x}Si and Mn_{1-x}Ir_{x}Si show nearly identical variations in their magnetic properties with substitution. We find a systematic dependence of the transition temperature, the ordered moment, the helix period and the DM interaction strength with electron count for Mn{1-x}Ir{x}Si, Mn_{1-x}Co_{x}Si, and Mn_{1-x}Fe_{x}Si indicating that the magnetic behavior is primarily dependent upon the additional carrier density rather than on the mass or size of the substituting species. This indicates that the variation in magnetic properties, including the DM interaction strength, are primarily controlled by the electronic structure as Co and Ir are isovalent. Our work suggests that although the rigid band model of electronic structure along with Moira's model of weak itinerant magnetism describe this system surprisingly well, phenomenological models for the DM interaction strength are not adequate to describe this system., Comment: 17 pages, 7 Figures

Using the first-principles full-potential linearized augmented plane-wave method, we investigate the magnetostriction of Fe4N. Its magnetostrictive coefficient is found to be −143 ppm. In order to enhance the magnetostriction, the derivative MnFe3N is taken into our consideration according to the rigid band model. Interestingly, we find that the magnetostrictive coefficient is enhanced to be +373 ppm. To further improve the magnetostriction, we calculate the magnetostrictive coefficient of the derivatives containing 4d and 5d metal due to their strong spin–orbit coupling. The results show that the magnetostrictive coefficient can reach to −564 ppm and +416 ppm for OsFe3N and IrFe3N. Our calculations give a guideline for designing giant magnetostriction material.

In this study, a comprehensive experimental in situ analysis of the evolution of the occupied and unoccupied density of states as a function of the charging state of the Lix≤1CoO2 films has been done by using synchrotron X-ray photoelectron spectroscopy (SXPS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and O K- and Co L3,2-edges XANES. Our experimental data demonstrate the change of the Fermi level position and the Co3d–O2p hybridization under the Li+ removal and provide the evidence for the involvement of the oxygen states in the charge compensation. Thus, the rigid band model fails to describe the observed changes of the electronic structure. The Co site is involved in a Co3+ → Co4+ oxidation at the period of the Li deintercalation (x ∼ 0.5), while the electronic configuration at the oxygen site is stable up to 4.2 V. Further lowering of the Fermi level promoted by Li+ extraction leads to a deviation of the electronic density of states due to structural distor...

Ta3Ru5B2, Zr2.86(5)Ru5.14(5)B2, and Hf2.83(2)Ru5.17(2)B2 have been successfully synthesized as polycrystalline powders as well as single crystals and characterized by powder and single-crystal X-ray diffraction and EDX analysis. Furthermore, their electronic structures and bonding characteristics have been analyzed by DFT calculations. The three phases are isotypic and crystallize with a Ti3Co5B2-type structure (space group P4/mbm). Their electronic densities of states, which all possess a large pseudogap at the Fermi level, closely follow the rigid band model. Bonding analyses indicated that Ru–B and Ru–M (M = Zr, Hf, Ta) heteroatomic interactions are mainly responsible for their structural stability and that the homoatomic M–M interactions behave differently when connecting either two tetragonal or two pentagonal Ru prisms at the same distance.

The temperature dependence of wettability (wetting angle, Θ(T )) for Ag-based melts on graphite and Al2O3 substrates is compared. Typical alloying e ects are found, as the Ag host metal is gradually replaced by various metallic elements. The essence of alloying lies in the change of the electron/atom (e/a) ratio. This ratio is also manifested in the shift of wetting angles on the same substrate. The e ect is also supported by the calculations based on the rigid band model, and is also in qualitative agreement with the Hume-Rothery rules. Nevertheless, the e ects are partially smeared by other (metallurgical) factors, like the interaction between the oxygen-alloying elements and by the graphite substrate oxygen interaction. In contrast, such e ects are not pronounced in the case of Al2O3 substrates. As a consequence, Θ(T ) exhibits an opposite trend in the case of two substrates. Crossovers of the Θ(T ) curves were often found. The positions of crossovers depend on the chemical character and concentration of solute atoms. Segregation and epitaxial texture formation after solidi cation were also observed in certain alloy drops, especially in high concentration range. This phenomenon is not yet explained in every detail. DOI: 10.12693/APhysPolA.124.78

The magnetic susceptibility and the pressure-composition isotherm were measured simultaneously for Pd–Rh–H and Pd–Ag–H systems in order to clarify the effect of Rh or Ag substitution on the hydrogen uptake from viewpoint of the electronic band structure. The magnetic susceptibility of all Pd binary alloys prepared decreased monotonically with increasing hydrogen content. At high hydrogen contents, the magnetic susceptibility became approximately zero for Pd–Rh–H and Pd–Ag–H system, and the hydrogen content at which the magnetic susceptibility gives zero corresponded with the terminal of the plateau region in the isotherm curve. The results indicated that the magnetic susceptibility of hydride phase was almost zero for all Pd binary alloys. On the basis of the band structure of Pd metal, we concluded that atom substitution only affected shift of the energy at Fermi level, and the amount of the hydrogen uptake was dominated by the number of unoccupied d-band in the alloys.

We report systematic angle-resolved photoemission (ARPES) experiments using different photon polarizations and experimental geometries and find that the doping evolution of the normal state of $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$ deviates significantly from the predictions of a rigid band model. The data reveal a nonmonotonic dependence upon doping of key quantities such as band filling, bandwidth of the electron pocket, and quasiparticle coherence. Our analysis suggests that the observed phenomenology and the inapplicability of the rigid band model in Co-doped Ba122 are due to electronic correlations, and not to the either the strength of the impurity potential, or self-energy effects due to impurity scattering. Our findings indicate that the effects of doping in pnictides are much more complicated than currently believed. More generally, they indicate that a deep understanding of the evolution of the electronic properties of the normal state, which requires an understanding of the doping process, remains elusive even for the 122 iron-pnictides, which are viewed as the least correlated of the high-${T}_{C}$ unconventional superconductors.

We present a systematic investigation of the electrical, structural, and antiferromagnetic properties for the series of $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Co}}_{x}{\mathrm{Rh}}_{y})}_{2}{\mathrm{As}}_{2}$ compounds with fixed $x\ensuremath{\approx}0.027$ and $0\ensuremath{\le}y\ensuremath{\le}0.035$. We compare our results for the Co-Rh doped $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Co}}_{x}{\mathrm{Rh}}_{y})}_{2}{\mathrm{As}}_{2}$ compounds with the Co doped $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ compounds. We demonstrate that the electrical, structural, antiferromagnetic, and superconducting properties of the Co-Rh doped compounds are similar to the properties of the Co doped compounds. We find that the overall behaviors of $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Co}}_{x}{\mathrm{Rh}}_{y})}_{2}{\mathrm{As}}_{2}$ and $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ compounds are very similar when the total number of extra electrons per Fe/$\text{TM}$ $(\text{TM}=\text{transition}$ metal) site is considered, which is consistent with the rigid band model. Despite the similarity, we find that the details of the transitions, for example, the temperature difference between the structural and antiferromagnetic transition temperatures and the incommensurability of the antiferromangetic peaks, are different between $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Co}}_{x}{\mathrm{Rh}}_{y})}_{2}{\mathrm{As}}_{2}$ and $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ compounds.

A new Sb-based type-I clathrate family involving group 11 or 12 elements has been studied by density functional theory (DFT) calculations. The hypothetical Cu/Ag−Sb- and Zn/Cd−Sb-based compounds ar...

The effects of La and Sb doping on the electronic structure and optical properties of SrTiO3 are investigated by first-principles calculation of the plane wave ultra-soft pseudo-potential based on density functional theory. The calculated results reveal that corner-shared TiO6 octahedra dominate the main electronic properties of SrTiO3, and its structural stability can be improved by La doping. The La3+ ion fully acts as an electron donor in Sr0.875La0.125TiO3 and the Fermi level shifts into the conduction bands (CBs) after La doping. As for SrSb0.125Ti0.875O3, there is a distortion near the bottom of the CBs for SrSb0.125Ti0.875O3 after Sb doping and an incipient localization of some of the doped electrons trapped in the Ti site, making it impossible to describe the evolution of the density of states (DOS) within the rigid band model. At the same time, the DOSs of the two electron-doped systems shift towards low energies and the optical band gaps are broadened by about 0.4 and 0.6 eV for Sr0.875La0.125TiO3 and SrSb0.125Ti0.875O3, respectively. Moreover, the transmittance of SrSb0.125Ti0.875O3 is as high as 95% in most of the visible region, which is higher than that of Sr0.875La0.125TiO3(85%). The wide band gap, the small transition probability and the weak absorption due to the low partial density of states (PDOS) of impurity in the Fermi level result in the significant optical transparency of SrSb0.125Ti0.875O3.

The metal diboride family has been systematically studied in recent years due to the discovery of superconductivity for magnesium diboride MgB2 at 39 K. TiB2 is the most stable of several titanium–boron compounds, due to its high hardness, extreme melting point and chemical inertness. TiB2 is a candidate for a number of applications; it is used for wear parts and in composites with other materials. In combination with other primarily oxide ceramics, TiB2 is used to constitute composite materials in which the presence of the material serves to increase the strength and fracture toughness of the matrix. In our paper, the electronic structure of AlB2-type transition metal diboride of TiB2, NbB2 and their ternary alloy Ti0.5Nb0.5B2 have been calculated by using the full potential linearized augmented plane wave method with local orbitals (APW+lo). We included the exchange correlation potential by using both the generalized gradient approximation (GGA) and the local density approximation (LDA), respectively, as embodied in the Wien2 K in full relativistic version. The electronic structure is discussed and the rigid band model is shown to provide a fairly good description. The Ti-3d and Nb-4d electron are treated as valence electrons. We explained in some detail the bonding nature of our compounds. The existence of the pseudogap in the total densities is found to be a common feature of these compounds, but we found that the pseudogap at Fermi-levels of TiB2 is the competing effect of Ti-3d resonance and strong hybridization between Ti-3d and B-2p states. The variation of the chemical stabilities of these diborides is analysed. The results are compared with other theoretical and experimental work.

As shown previously, Pb on vicinal Si(5 5 7) refacets the surface into a (2 2 3) facet orientation at a Pb coverage of 1.31 ML. This facet formation is electronically stabilized by Fermi nesting and leads to one-dimensional conductance. Electronic correlation seems to be responsible also for the periodic arrangement of atomic Pb chains which decorate the step edges at concentrations exceeding 1.31 ML, up to a concentration of 1.5 ML. Instead of random step decoration, periodicities up to six (2 2 3)-terrace widths (28 lattice constants, 93 A) have been found. These depend inversely on excess Pb concentration and end at a concentration of 1.52 ML when all steps are decorated with a line density equal to the Si density at steps. These one-dimensional periodicities can be explained assuming that split-off states from surface bands are completely filled by two electrons per Pb atom with corresponding gap opening. This behavior is reminiscent of the formation of charge density waves with tunable wavelengths as a function of excess Pb concentration, and indicates strong electron correlation in this strongly anisotropic 2d system. The alternative, simple band filling within a rigid band model is expected to destabilize the (2 2 3) facet structure upon further adsorption of Pb, which has not been observed.

The compounds Sr3Sn5–xTlx (x = 1.78(4), I; 2.16(5), II), Ca3Tl5 (III), and Sr3Tl5 (IV) have been obtained by direct fusion of the elements in sealed niobium ampoules at high temperature. Their structures were determined by single crystal X-ray diffraction studies and the bonding was investigated by TB–LMTO–ASA calculations. The ternary phases I and II crystallize in the Tm3Ga5 structure type; space group Pnma (No. 62); Z = 4 with a = 13.422(1), b = 10.897(1), c = 6.897(1) A for I and a = 13.482(2), b = 10.938(2), c = 6.850(1) A for II. The binary compounds III and IV crystallize in the Pu3Pd5 structure type; space group Cmcm (No. 63); Z = 4 with a = 10.231(2), b = 8.362(2), c = 10.623(2) A for III and a = 10.592(5), b = 8.667(5), c = 10.973(6) A for IV. Both structure types consist of zigzag chains of distorted square pyramidal {E5} (E = Tl, Sn/Tl) clusters. In the lower symmetric structures I/II the clusters are reoriented with respect to each other and have only two exo–contacts each, whereas in III/IV the clusters are aligned parallel and show a considerably higher degree of polymerization. The DOS for Sr3Sn5–xTlx calculated for x = 0 reveals a pseudo gap for x = 2 within the rigid band model, whereas the electronic band structure for Sr3Tl5 and Ca3Tl5 show metallic properties and strong participation of the cations in the bonding.

The electronic structures of Heusler-type Fe2VAl-based alloys, n-type thermoelectric alloys Fe2VAl1-zXz(X = Si, Ge) and p-type one Fe2(V1-yTiy)Al, have been investigated by photoelectron spectroscopy with use of synchrotron radiation as an excitation photon source. Observed valence-band spectra of Fe2VAl agree quite well with the predicted density of states with a sharp pseudogap across the Fermi energy EF. Small substitution in Fe2VAl1-zXz and Fe2(V1-yTiy)Al reveals a rigid-band-like energy shift of the valence band, although the observed shift is smaller than expected in the rigid band model. Further substitution causes the modification of the electronic structure near EF and the appearance of mid-pseudogap states. However, the dependence of the thermoelectric properties of these alloys on substitution can be qualitatively explained by the observed change in the electronic structure near EF.

Numerous layered structured compounds are interesting materials in which lithium intercalation occurs primarily without destruction of the host lattice. In many cases a rigid band model is a useful first approximation for describing the changes in electronic properties of the host material with intercalation. We observed, nevertheless, that the rigid-band model is not applicable to all of the layered compounds. One may argue that the applicability of the rigid-band model may be taken as a test for the properties most desirable in a good intercalation material. This needs yet to be more extensively documented for their promising applications as insertion electrode in rechargeable lithium batteries. This chapter presents the applicability of the rigid-band model on intercalation compounds with a layered structure namely the transition-metal chalcogenides MX 2 (X = S, Se) and the transition-metal oxides LiMO2 (M = Co, Ni) as well.

The magnetization, magnetic susceptibility, heat capacity and neutron powder diffraction (NPD) data of the series R 3 Ni 13− x Co x B 2 , R = Y and Nd are reviewed, and the magnetic phase diagrams are given for the two R substitutions. The influence of the Y substitution by Nd is to increase the cell constants, and the average Co moment. The Ni substitution by Co modifies the magnetic properties from weak itinerant behavior due to the Ni sublattice to strongly coupled Co moments near to the rigid band model. In the Co rich side of the solid solution Nd 3 Ni 13− x Co x B 2 a spin reorientation transition (SRT) is induced.

The valence-band electronic structures of the Heusler-type alloys Fe 2 VAl 1− z Ge z ( z = 0–0.10) have been investigated with high-resolution soft X-ray photoemission spectroscopy. The overall photoelectron spectral feature of Fe 2 VAl 1− z Ge z with the Ge content z up to 0.10 is not so much changed, except for the main 3d band in the region from the Fermi level E F to the binding energy E B ∼ 1 eV. For z = 0.05, the leading edge of the main band is shifted by ∼0.07 eV, but the shift is much smaller than expected within a rigid band model for a calculated band structure. For z = 0.10, a band at E B ∼ 0.5 eV becomes prominent and the intensity of the Fermi edge is increased. The change in the valence band spectrum near E F suggests the rigid-band-like shift of the 3d bands and the appearance of new 3d states in the pseudogap, which may qualitatively explain the z -dependence of the thermoelectric properties. As z increases, the V 2p spectra show a shift towards the high binding energy and the evolution of a shoulder structure on the low binding energy side of the main line. The shoulder structure is attributed to the charge transfer to the V site induced by the Ge substitution as well as the surface effect. The change in the valence-band electronic structure is discussed in terms of the interaction between the substituted Ge and its surrounding constituent atoms.

Thermodynamic and magnetic properties of the Pd 0.93 Ag 0.07 hydrides were measured. The isotherms of Pd 0.93 Ag 0.07 -H and -D systems in the temperature range 373-523 K showed the plateau regions to be within 0.05 < [Q (Q = H or D)]/[Pd 0.93 Ag 0.07 ] < 0.4. From an investigation of the thermodynamic isotope effect for the formation and decomposition of hydride, the enthalpy changes, ΔH° β→α , for desorption of hydrogen and deuterium were evaluated to be 43.0 kJ/mol H2 and 36.4 kJ/mol D2 , respectively. It was observed that the magnetization of Pd 0.93 Ag 0.07 Hx at ambient temperature increased with increasing magnetic field. Since the magnetization was not saturated up to 7 T, it can be concluded that Pd 0.93 Ag 0.07 H x is paramagnetic. The magnetic susceptibility of Pd 0.93 Ag 0.07 H x progressively decreased with increasing hydrogen content and vanished at [H]/[Pd 0.93 Ag 0.07 ] = 0.4. The disappearance of the susceptibility coincided with the termination of the plateau region of the isotherms. This fact can be explained by the rigid band model, according to which the vacant part of d-band of Pd 0.93 Ag 0.07 can accept electrons from absorbed hydrogen atoms.

A series of crystals of NaxTaS2 with different superconducting transition temperatures (Tc) ranging from 2.0–4.4 K were obtained by flux method. The structural data deduced from X-ray diffraction pattern shows that the sample has the same structure as 2H-TaS2. Compared with the resistivity curve of un-intercalated sample 2H-TaS2 (Tc=0.8 K, TCDW≈70 K), no signal of charge density wave (CDW) was observed in our present samples Na0.1TaS2. It is thus concluded that there is a competition between the superconductivity and the CDW. With the increase of sodium content, the rise of Tc in NaxTaS2 is caused mainly by the suppression to the CDW in 2H-TaS2, and the conventional rigid band model for layered dichalcogenide may be inadequate to explain the changes induced by the slight intercalation of sodium in 2H-TaS2.

Numerous channel- or tunnel-structured compounds are interesting materials in which lithium intercalation occurs primarily without destruction of the host lattice assuming a complete charge transfer. In many cases, the rigid-band model (RBM) is a useful first approximation for describing the changes in electronic properties of the host material with intercalation. The applicability of the rigid-band model is taken as a test for the properties most desirable in a good intercalation material. This review paper presents experimental results, namely optical and electrical measurements, obtained on various intercalation materials such as dichalcogenides, trichalcogenides and oxides to probe the validity of the RBM. The impact of the lithium intercalation is discussed for various structural forms, from well-crystallised to amorphous compounds. We observed, nevertheless, that the rigid-band model is not applicable to all of the layered intercalation materials. This needs yet to be more extensively documented for their promising application as insertion electrode in rechargeable lithium batteries. Compounds such as TiS 2 , TaS 2 , MoS 2 , NiPS 3 , MoO 3 , V 2 O 5 , V 6 O 13 , LiCoO 2 , LiNiO 2 , LiMn 2 O 4 and LiNiVO 4 are tested in this work. Some guidelines are established for improving the performances of these materials in their most eminent applications.

Transition metal (Co and Ni) co-doping effects are investigated on an underdoped Ca0.94La0.06Fe2As2 compound. It is discovered that electron doping from substituting Fe with transition metal (TM = Co, Ni) can trigger high- superconductivity around 35 K, which emerges abruptly before the total suppression of the innate spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Remarkably, the critical temperature for the high- superconductivity remains constant against a wide range of TM doping levels. And the net electron doping density dependence of the superconducting based on the rigid band model can be nicely scaled into a single curve for Co and Ni substitutions, in stark contrast to the case of Ba(Fe1−x TM x )2As2. This carrier density independent superconductivity and the unusual scaling behavior are presumably resulted from the interface superconductivity based on the similarity with the interface superconductivity in a La2−x Sr x CuO4-La2CuO4 bilayer. Evidence of the two-dimensional character of the superfluid by angle-resolved magneto-resistance measurements can further strengthen the interface nature of the high- superconductivity.

The magnetization process of the Kondo insulator YbB12 has been unveiled in ultrahigh magnetic fields of up to 120 T at 4.2 K. We have found a novel metamagnetic transition at Bc2 = 102 T in addition to the previously known transition at Bc1 = 55 T. It has also been observed that the magnetization tends to saturate at around 112 T. Within the rigid band model, the two-energy-gap structure in the density of states (DOS) explains the successive two-step metamagnetism as a result of the Zeeman effect of the DOS. The metamagnetic transition at Bc1 occurs along with an insulator–metal transition and the field-induced phase is expected to be a heavy fermion metallic state. The Kondo effect can weaken at the second transition of Bc2, as theoretically found in the successive two-metamagnetic-transition process in the Kondo semimetal CeNiSn.

The electronic structure of the layered compound 2H-TaSe2 has been studied using angle-resolved photoemission before and after in situ intercalation with Na and Cs. Core level spectra verified that Na and Cs both intercalate easily at room temperature, with only small amounts remaining on the surface. Valence band spectra revealed changes in the electronic band structure which were much more extensive than predicted by the rigid band model, but which were in reasonable agreement with theoretical bands calculated by the LAPW method. Some discrepancies between the experimental and calculated results are probably due to intercalation induced changes in the stacking of host layers. A general similarity with results from transition metal dichalcogenides with 1T structure indicates that the intercalation properties are not critically dependent on the internal structure of the host layers.

The specific heat in the normal and superconducting states of single-crystal intercalation compounds of the host material 2H−TaS2 was measured from 1.7 to 9 K by use of a thermal relaxation method. By studying two extreme cases of charge transfer n, namely the 9 A methylene blue intercalation compound (9 A MB+−TaS2) (n = 0.06) and K0.33TaS2 (n = 0.33), and comparing the results with literature data, it is seen that charge transfer has only minor influence on the electronic specific heat in the normal state. This is in drastic contradiction to the expectation from a rigid band model. The contribution of the phonon-parameter β, however, varies with intercalation. No direct correspondence between Tc and the Debye temperature θD can be seen, but if θD is multiplied by a factor, characterizing the intercalation compound, scaling between Tc and the corrected Debye temperature is found. Furthermore, it is observed that the superconducting transition temperature of the methylene blue intercalation compound is sen...

Employing angle resolved photoemission spectroscopy using synchrotron radiation in combination with band structure calculations we determine experimental and theoretical momentum resolved band structures for the misfit compound (PbS)NbS2 and the reference material NbSe2. The high quality of the crystals was confirmed by X-ray diffraction and transmission electron microscopy. With the extended linear augmented plane wave (ELAPW) k.p method the band structures have been calculated for the misfit compound as well as for NbSe2. The comparison between experiment and theory lays special emphasis on the deviations from the rigid band model due to a more ionic bonding between the layers of the misfit compounds.

Cs was adsorbed at room temperature onto the (0001) cleavage planes of 1T–TaSe2. The deposited Cs forms no metallic overlayer, but intercalates after an initial adsorption stage into the van der Waals gap of the layered crystal. A limit of the relative concentration ratio cCs/cTa of ca. 60% is observed. Due to the electron transfer associated with the intercalation the charge density wave induced p( 13 × 13 )R13.89° superstructure exhibits a phase transition to a c( 2 3 ×4 )rect. structure. As a further consequence of the intercalation a metal to semiconductor transition occurs. The change in the electronic structure could not be interpreted within the rigid band model, but is tentatively explained by an localization effect, caused by the charge density wave phase transition.

The phase stability of Ti(CN) solid solutions was investigated based on the accurate first principles method, the Debye model and the rigid band model beyond a simple ideal mixing model. The Gibbs free energy of formation was calculated using the results from the above models and the standard state energies for the formation of pure TiC and TiN from JANAF tables at varying temperatures. The predicted phases for highest stability at 1700 and 2100 K are in the range of 0.6 and 0.3, whereas those obtained experimentally were near 0.7 and 0.4 nitrogen contents at 1700 and 2100 K, respectively. From the calculation results it can be safely concluded that Ti(CN) solid solutions have higher stability than that predicted by the ideal mixing model.

Ferromagnetic systems described by the double exchange model are investigated. At temperatures close to the Curie temperature, and for a wide range of doping levels, the system is unstable toward phase separation. The chemical potential decreases upon increasing doping, due to the significant dependence of the bandwidth on the number of carriers. The reduction of the electronic bandwidth by spin disorder leads to an enormously enhanced thermopower which peaks near T_c, with a sign opposite that predicted by a rigid band model., 4 pages, 2 encapsulated PostScript figures

YFe 11− x Co x Ti compounds ( x =0, 0.5, 1, 1.5 and 2) with ThMn 12 -type structure have been synthesized by induction melting for x =0, 1 and 2. The carbides YFe 11− x Co x TiC were obtained by solid–solid reaction. X-ray diffraction method was used in order to analyze the influence of Co substitution and carbonation on the cell parameters and interatomic distances of 3d sublattice. The influence of Co substitution and carbonation on the Curie temperature T C and saturation magnetization M S are interpreted in a rigid band model, by the effect of the 3d band filling on the exchange interaction between 3d (Fe and Co) atoms.

The deviations from linearity in the concentration dependence of lattice spacings in AgcPd1-c have been recently related to the topology changes shown by the Fermi Surface, or Electronic Topological Transitions (ETTs), of this alloy. Using an extension of the ETTs theory to finite-temperature and impurity scattering case, and in terms of a simple Rigid Band Model, we study such deviations and interpret them as the results of the filling of the hole pocket centered at theX point of the Brillouin Zone, remarking the role of the finiteness of the quasi-particle lifetime of the relevant states. We also show howab initio calculations based on the Coherent Potential Approximation underestimate these lifetimes.

In order to understand the role of Ni site substitution on the electronic structure and chemical bonding in ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C},$ we have made systematic electronic-structure studies on ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ as a function of Co and Cu substitution using the supercell approach within the local density approximation. The equilibrium volume, bulk modulus ${(B}_{0})$ and its pressure derivative ${(B}_{0}^{\ensuremath{'}}),$ Gr\"uneisen constant $({\ensuremath{\gamma}}_{G}),$ Debye temperature $({\ensuremath{\Theta}}_{D}),$ cohesive energy ${(E}_{c}),$ and heat of formation $(\ensuremath{\Delta}H)$ are calculated for ${\mathrm{YNi}}_{2\ensuremath{-}x}(\mathrm{C}\mathrm{o}/\mathrm{C}\mathrm{u}{)}_{x}{\mathrm{B}}_{2}\mathrm{C}$ $(x=0,0.5,1.0,1.5,2).$ From the total energy, electron-energy band structure, site decomposed density of states, and charge-density contour we have analyzed the structural stability and chemical bonding behavior of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ as a function of Co/Cu substitution. We find that the simple rigid band model successfully explains the electronic structure and structural stability of Co/Cu substitution for Ni. In addition to studying the chemical bonding and electronic structure we present a somewhat speculative analysis of the general trends in the behavior of critical temperature for superconductivity as a function of alloying.

The electronic structure of Sc-doped p-type SrTiO 3 has been studied by photoemission spectroscopy. Comparing the results with those of Nb-doped n-type SrTiO 3 , the Fermi level of p-type is shifted to the low binding energy. As Sc dopant increases, the top of the valence band is shifted to the low binding energy in accord with the rigid-band model. The Ti 3d component in the O 2p band of p-type SrTiO 3 increases compared with that of the n-type one. It was also found that the satellite intensities of Ti core levels drastically increase. The covalency of p-type SrTiO 3 increases by the doping of the Sc ions. The dopant concentration dependence shows that the covalency is strongest when the Sc concentration is 2 mol%. These facts suggest that the covalency is related to the protonic conduction.

The stability of Hume–Rothery9s electron phases is studied using modern bandstructure techniques. For these alloys, empirical evidence has shown a strong correlation between the number of valence electrons per atom and the adopted crystal structure. The generally accepted explanation traces the phenomenon to prominent structure in the electron density of states which arises when the free–electron Fermi sphere encounters the Bragg planes at the surface of the Brillouin zone. By applying a rigid band model on the basis of density functional theory to the archetypal system Cu–Zn, we obtain accurate density–of–states functions which are used to critically evaluate the popular models due to Mott and Jones. We also are able to account for the stability of the γ–phase.

A periodic supercell model based on the large unit cell (LUC) method within the Hartree-Fock molecular orbital (MO) theory is used to explore the stability of hydrogen dimers placed on a number of sites in common semiconductors: Si, GaAs and CdTe. Within the rigid band model approximation the different types of crystals, intrinsic, n-type and p-type, are considered by converting the total number of valence electrons in the supercell. The results show that the most energetically favourable configuration is to form a H2 molecule. The net charges of atoms and the electron populations on a bond are presented, and the values of the charge transfer and energies of defect levels due to the inserted hydrogens are given. The results are compared with the available theoretical and experimental data, and the origin of disagreement of results already reported elsewhere is clarified.

This paper discusses various issues of the multiple-scattering theory of magnetic circular dichroism (MCD) in the x-ray range, especially at the Kedge of 3d transition metals. Using a semirelativistic expansion of the Dirac Green function, we show how MCD can be calculated and related to orbital momentum. Moreover, the rigid band model will be used to relate the high-energy behaviour of MCD to the exchange energy. Finally, we generalise our approach to the case of orbital polarisation, which was found to be relevant to MCD.