Previous Band Structure Calculations Research Articles

Fermi Surface Modeling of Light‐Rare‐Earth Hexaborides using Positron Annihilation Spectroscopy

2D angular correlation of the positron annihilation radiation (2D‐ACAR) spectra are measured for along high‐symmetry directions and compared with first‐principles calculations based on density functional theory (DFT). This allows the modeling of the Fermi surface in terms of ellipsoid electron pockets centered at X‐points elongated along the Σ axis (Γ direction). The obtained structure is in agreement with quantum oscillation measurements and previous band structure calculations. For the isostructural topologically nontrivial , the similar ellipsoids are connected through necks that have significantly smaller radii in the case of . A theoretical analysis of the 2D‐ACAR spectra is also conducted for including the on‐site repulsion U‐correction to the local density approximation (LDA+U) of the DFT. The similarities of the 2D‐ACAR spectra and the Fermi surface projections of these two compounds allow to infer that both and are topologically trivial correlated metals.

Extremely Large Magnetoresistance in the Hourglass Dirac Loop Chain Metal β-ReO2

The transport and thermodynamic properties of $\beta$-ReO$_{2}$ crystallizing in a nonsymmorphic structure were studied using high-quality single crystals. An extremely large magnetoresistance (XMR) reaching 22,000 $\%$ in a transverse magnetic field of 10 T at 2 K was observed. However, distinguished from other topological semimetals with low carrier densities that show XMR, $\beta$-ReO$_{2}$ has a high electron carrier density of 1 $\times$ $10^{22}$ cm$^{-3}$ as determined by Hall measurements and large Fermi surfaces in the electronic structure. In addition, a small Fermi surface with a small effective mass was evidenced by de Haas-van Alphen oscillation measurements. The previous band structure calculations [S. S. Wang, et al., Nat. Commun. 8, 1844 (2017)] showed that two kinds of loops made of Dirac points of hourglass-shaped dispersions exist and are connected to each other by a point to form a string of alternating loops, called the Dirac loop chain (DLC), which are protected by the multiple glide symmetries. Our first-principles calculations revealed the complex Fermi surfaces with the smallest one corresponding to the observed small Fermi surface, which is just located near the DLC. The XMR of $\beta$-ReO$_{2}$ is attributed to the small Fermi surface and thus is likely caused by the DLC.

Pairing symmetry of an intermediate valence superconductor CeIr3 investigated using μSR measurements

We have investigated the bulk and microscopic properties of the rhombohedral intermediate valence superconductor ${\mathrm{CeIr}}_{3}$ by employing magnetization, heat capacity, and muon spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements. The magnetic susceptibility indicates bulk superconductivity below ${T}_{\mathrm{C}}=3.1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Heat capacity data also reveal a bulk superconducting transition at 3.1 K with a second weak anomaly near 1.6 K. Zero-field $\ensuremath{\mu}\mathrm{SR}$ data show no strong evidence of broken time-reversal symmetry but support the presence of spin fluctuations below ${T}_{\mathrm{C}}$. Transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements suggest a fully gapped, isotropic, $s$-wave superconductivity with $2\mathrm{\ensuremath{\Delta}}(0)/{k}_{\mathrm{B}}{T}_{\mathrm{C}}=3.76(3)$, very close to 3.53, the Bardeen-Cooper-Schrieffer gap value for weak-coupling superconductors. From the temperature variation of the magnetic penetration depth, we have also determined the London penetration depth ${\ensuremath{\lambda}}_{\mathrm{L}}(0)=435(2)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, the carrier effective-mass enhancement ${m}^{*}=1.69(1){m}_{\mathrm{e}}$, and the superconducting carrier density ${n}_{\mathrm{s}}=2.5(1)\ifmmode\times\else\texttimes\fi{}{10}^{27}$ carriers ${\mathrm{m}}^{\ensuremath{-}3}$. The fact that ${\mathrm{LaIr}}_{3}$, with no $4f$ electrons, and ${\mathrm{CeIr}}_{3}$ with $4{f}^{n}$ electrons where $n\ensuremath{\le}1$ (Ce ion in a valence fluctuating state), both exhibit the same $s$-wave gap symmetry indicates that the $\mathrm{Ir}\text{\ensuremath{-}}d$ band governs the physics of these two compounds near the Fermi level, which is in agreement with previous band structure calculations.

Re-evaluation of experimental measurements for the validation of electronic band structure calculations for LiFePO4 and FePO4.

Experimental measurements used to validate previous electronic band structure calculations for olivine LiFePO4 and its delithiated phase, FePO4, have been re-investigated in this study. Experimental band gaps of LiFePO4 and FePO4 have been determined to be 6.34 eV and 3.2 eV by electron energy loss spectroscopy (EELS) and UV-Vis-NIR diffusion reflectance spectroscopy, respectively. X-ray photoemission (XPS) and Raman spectroscopy show that the surfaces of very carefully synthesized LiFePO4 display Li-depletion, which affects optical reflectance determinations. Based on these experimental measurements, functionals for density functional theory (DFT) calculations of the electronic properties have been revisited. Overall, electronic structures of LiFePO4 and FePO4 calculated using sX-LDA show the best self-consistent match to combined experimentally determined parameters. Furthermore, the open-circuit voltages of the LiFePO4 half-cell have been interpreted in terms of both Fermi levels and Gibbs free energies, which provides additional support for the electronic band structures determined by this research.

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.

Structure, electronic properties and electron energy loss spectra of transition metal nitride films

We present a thorough and critical study of the electronic properties of the mononitrides of the group IV–V–VI metals (TiN, ZrN, HfN, NbN, TaN, MoN, and WN) grown by Pulsed Laser Deposition (PLD). The microstructure and density of the films have been studied by X-Ray Diffraction (XRD) and Reflectivity (XRR), while their optical properties were investigated by spectral reflectivity at vertical incidence and in-situ reflection electron energy loss spectroscopy (R-EELS). We report the R-EELS spectra for all the binary TMN and we identify their features (metal-d plasmon and N-p+ metal-d loss) based on previous ab-initio band structure calculations. The spectral positions of p+d loss peak are rationally grouped according to the electron configuration (i.e. of the respective quantum numbers) of the constituent metal. The assigned and reported R-EELS spectra can be used as a reference database for the colloquial in-situ surface analysis performed in most laboratories.

Relevance of Fe atomic volumes for the magnetic properties of Fe-rich metallic glasses

The atomic volume Va-Fe that can be assigned to Fe atoms in Fe–metalloid (Fe–MD) and Fe–early transition metal (Fe–TE) glasses was deduced in a previous paper (I. Bakonyi, Acta Materialia 53 (2005) 2509) from an analysis of available density data for such amorphous alloys. In the present paper, based on a similarity of the amorphous and face-centered cubic (fcc) structures, the distinctly different magnetic behaviors of these two families of amorphous alloys are discussed in terms of the relative position of Va-Fe and the critical volume Vfcc⁎-Fe≈11.7Å3/atom separating the so-called low-spin (LS) and high-spin (HS) state of fcc-Fe. For Fe–MD systems, Va-Fe is found to be definitely larger than Vfcc*-Fe whereas for Fe-TE systems Va-Fe is fairly close to Vfcc*-Fe. Since in topologically disordered alloys a distribution of atomic volumes is inherently present, in Fe–MD glasses the Fe atoms can be assumed to exhibit exclusively the HS state whereas in Fe–TE amorphous alloys a comparable fraction of Fe atoms can be either in the LS or the HS state. According to previous theoretical band structure calculations, an antiferromagnetic state can also be stable just around Vfcc*-Fe. The simultaneous presence of Fe atoms with such a rich variety of magnetic states due to the specific position of the average of the atomic volume distribution can well explain the complex magnetic behavior observed in Fe-rich Fe–TE metallic glasses such as, e.g., in amorphous Fe–Zr alloys around 90at% Fe.

Electronic Structure and E1 Excitons of CuInS2 Energy-Related Crystals Studied by Temperature-Dependent Thermoreflectance Spectroscopy

Single crystals of were grown by a chemical vapor transport method using as a transport agent. The electronic structure of was experimentally characterized by temperature-dependent thermoreflectance (TR) measurements in a wide energy range of 1.25–6 eV between 20 and 320 K. A pair of excitonic doublets of and near 3.4 eV were observed in the low temperature TR spectrum at 20 K. The temperature– energy shift of the and features identified the origin of the excitonic doublets. The excitons were confirmed to be interband transitions from the nonbonding Cu 3d state in the valence band to the S 3p antibonding state at the conduction band bottom. There were also a lot of derivative-like spectral features observed near the bandedge as well as high lying interband transitions of . The origins of the critical-point transitions of were evaluated. X-ray photoelectron spectroscopic (XPS) measurements were implemented to identify the band blocks below the top of the valence band. Based on the experimental analyses of TR and XPS, together with previous band-structure calculations of , an experimental band structure that describes a fundamental band-edge structure of was then constructed.

Fourier-transform scanning tunneling microscopy investigation of the energy versus wave vector dispersion of electrons at the Au(111) surface

We have investigated the contributions of surface-state electrons and bulk state electrons to the spatially oscillating local density of states (LDOS) of the Au(111) surface by scanning tunneling microscopy and spectroscopy at low temperatures. Based on the Fourier-transform images of LDOS maps obtained at various energies, we were able to determine the dispersion of the surface-state electrons in a broad range of energies, up to 3 eV. The energy versus wave vector dispersion relation was found to clearly deviate from the parabolic free-electron-like behavior at higher energies above 1 eV. Moreover, relying on two-dimensional Fourier-transform analysis, we develop an original approach to determine the dispersion behavior of bulk state electrons that are scattered at (sub)surface defects in the top atomic layers of the Au(111) surface. This ``additional set'' of electrons is found to exhibit a dispersion for the occupied states that is consistent with the calculated band structure of bulk Au and previous photoemission experiments. On the other hand, an anomalous bulk state dispersion behavior occurs in the empty state regime, where the wavelength of the spatial oscillations of the LDOS is observed to remain constant for the investigated energy range. This behavior is inconsistent with previous band-structure calculations and requires further theoretical investigation that takes into account the influence of inelastic electron scattering.

Structure and optical characterization of sulfur incorporated As2Se3 crystals

In this study, structural and optical properties of crystalline sulfur incorporated As2Se3 have been characterized using x-ray diffraction and thermoreflectance (TR) measurements at 300K. The crystalline layer compounds of As2(Se1−xSx)3 (0⩽x⩽0.4) [c-As2(Se1−xSx)3] were grown by vertical Bridgman method. The lattice parameters were determined by x-ray diffraction measurements. The band structure of c-As2(Se1−xSx)3 was characterized experimentally by TR measurements in the energy range of 1.25–6eV. TR spectra of the c-As2(Se1−xSx)3 (0⩽x⩽0.4) exhibit a multitude of derivativelike features in the vicinity of band edge as well as the higher lying interband transition regions. Interband transition features of the c-As2(Se1−xSx)3 (0⩽x⩽0.4) are analyzed and the proper transition origins are assigned. Based on the experimental analyses and on the previous band-structure calculations of As2X3 (X=S,Se), a probable experimental band scheme near the fundamental edge of As2(Se1−xSx)3 is then constructed.

Visible luminescence and structural property of GaSe 1− xS x (0≤ x≤1) series layered crystals

Luminescence and structural properties of GaSe 1− x S x (0≤ x≤1) series optical materials were characterized using photoluminescence (PL) and piezoreflectance (PzR) measurements at 300 K. From the comparison of PL and PzR spectra, the whole series GaSe 1− x S x (0≤ x≤1) layers were confirmed to possess three different kinds of stacking phase when increasing the sulfur composition from x=0 to 1. Observing the peak positions of the PL spectra the light emissions of whole series GaSe 1− x S x are shown to cover from red to blue visible region. The PL and PzR spectra of whole series GaSe 1− x S x are analyzed. The structural variations in the layers are discussed. Based on the experimental results together with referring to previous band-structure calculations, a probable band scheme which indicating the transition assignments in the fundamental edge of GaSe 1− x S x is proposed.

Thermoreflectance study of the electronic structure ofGe(Se1−xSx)2

The band structure of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ series solids was probed experimentally by thermoreflectance (TR) measurements in the energy range of $2--6.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The TR measurements were done with the layered samples put in air and operated at room temperature. The TR spectra of the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ series layers exhibit derivative-like spectral features in the vicinity of band edge as well as higher-lying interband transitions. The energies and broadening parameters of the interband transition features of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ are analyzed by detailed line-shape fits to the TR spectra. Compositional dependences of the transition energies of the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ are analyzed. The presence of higher-lying interband transitions as well as the analysis of line-width broadening of the direct band gap indicates that the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ series should not be amorphous but belong to the polycrystalline solids with medium-range order. Based on the experimental results of TR measurements together with referring to previous band-structure calculations of crystalline and glassy $\mathrm{Ge}{X}_{2}$ $(X=\mathrm{S},\mathrm{Se})$, a probable experimental band structure near the fundamental edge of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ is proposed.

Electronic structure ofKD2xH2(1−x)PO4studied by soft x-ray absorption and emission spectroscopies

The surface and bulk electronic structure of tetragonal (at 300 K) and orthorhombic (at 77 K) $\mathrm{K}{\mathrm{D}}_{2x}{\mathrm{H}}_{2(1\ensuremath{-}x)}\mathrm{P}{\mathrm{O}}_{4}$ single crystals (so-called KDP and DKDP), with a deuteration degree $x$ of 0.0, 0.3, and 0.6, is studied by soft x-ray absorption near-edge structure (XANES) and x-ray emission (XES) spectroscopies. High-resolution $\mathrm{O}\phantom{\rule{0.2em}{0ex}}K$-edge, $\mathrm{P}\phantom{\rule{0.2em}{0ex}}{L}_{2,3}$-edge, and $\mathrm{K}\phantom{\rule{0.2em}{0ex}}{L}_{2,3}$-edge XANES and XES spectra reveal that the element-specific partial density of states in the conduction and valence bands is essentially independent of deuteration $x$. We give assignment of XANES and XES peaks based on previous molecular orbital and band-structure calculations. Projected densities of states in the conduction band also appear to be essentially identical for tetragonal (at 300 K) and orthorhombic (at 77 K) phases, consistent with previous band structure calculations. However, a decrease in sample temperature from 300 to 77 K results in measurable changes to the valence band (probed by XES) but not to the conduction band (probed by XANES). The lower limit on the room-temperature band gap of KDP and DKDP is estimated as $\ensuremath{\sim}7.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Results also show that high-intensity x-ray irradiation results in decomposition of these hydrogen-bonded materials into water and $\mathrm{K}\mathrm{P}{\mathrm{O}}_{3}$ cyclophosphates and polyphosphates.

Spin‐dependent momentum density distribution and Fermi surface of Ho via 2D‐ACAR measurements

AbstractThe first direct measurements of the spin‐dependent positron–electron momentum density and Fermi surface of Ho are presented. The measurements were performed using two‐dimensional angular correlation of annihilation radiation (ACAR) experiments with reversal magnetic field directions parallel and anti‐parallel to the polarization direction of the positron. The analysis confirmed that two hybrid bands influence the Fermi surface of Ho. They are 5d–6s conduction hybrid bands and partial hybridization of 4f–5d bands. In fact, the measured Fermi surface revealed the behavior of the magnetic electrons. Further, the reciprocal lattice points revealed the electronic spin density distribution behavior. The general layout of the Fermi surface of Ho showed a multiply connected surface as an open hole running along the ΓA axis with minority spin distribution and two electron surfaces centered on K and H points, respectively. Furthermore, this Fermi surface showed anti‐ferromagnetic character. The measured Fermi surface of Ho showed agreement with the results of a previous band structure calculation method. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vacuum-ultraviolet spectroscopy of poly(methylphenylsilylene).

The first vacuum-ultraviolet spectrum of a polysilylene (chain-type polysilane) with aromatic substituents is presented. Assignments of the absorption bands of the model compound poly(methylphenylsilylene) are based on previous experimental data and theoretical electronic band structure calculations for poly(alkylsilylenes) and on ultraviolet spectra of phenyl-containing monomers and polymers. Although aryl orbitals mix with the sigma-conjugated orbitals located along the catenated silicon backbone, some transitions are largely localized on the phenyl groups. These assignments elucidate the nature of the bonding in polysilylenes and should be useful in understanding photodegradation mechanisms and in the design of related new optical materials.

Low-energy spin-wave excitations in the bilayered magnetic manganiteLa2−2xSr1+2xMn2O7(0.30<~x<~0.50)

We have studied the low-temperature behavior of the magnetization and the specific heat of the bilayered perovskite system $% La_{2-2x}Sr_{1+2x}Mn_{2}O_{7}$, for $0.30\leq x\leq 0.50$. Our analysis reveals that below 30 K the temperature dependence of the magnetization of the ferromagnetic samples, $x=0.30$, 0.32, and 0.36, in a field of 1T can be interpreted in terms of the thermal excitations of a two-dimensional gas of ferromagnetic magnons. The specific heat in zero field for these samples as for the $x=0.50$ antiferromagnetic one, is linear with temperature between the range of 1.8K $\leq $ T $\leq $ 10K. This behavior can be also explained by the magnon gas model. By comparing specific heat measurements in zero field with those taken in a field of 9T we are able to extract the lattice and electronic contributions and determine the in-plane exchange interactions. That are found to be in reasonable agreement with the values inferred from the analysis of the magnetization data and also with the values reported by inelastic neutron scattering studies. In addition, we found that the electronic density of states obtained for the $x=0.50$ sample is in agreement with previous band structure calculations.

Ferromagnetic superconductivity driven by changing Fermi surface topology.

We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase dia-gram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetization. The twin-peak DOS may be derived from a tight-binding, quasi-one-dimensional band structure, inspired by previous band-structure calculations.

Evolution of the electronic structure of metastable Fe1−xCux alloys produced by mechanical alloying

We studied the evolution of the electronic structure of metastable Fe1−xCux alloys using X-ray photoelectron spectroscopy (XPS). The absence of chemical shifts in the core level spectra indicates that the charge transfer between Fe and Cu is negligible. On the other hand, the valence band spectra of the alloys deviate from a linear combination of the corresponding Fe and Cu bands. These differences in the valence band of the alloys are attributed to bonding interactions between the Fe 3d and Cu 3d states. The most relevant effect of these bonding interactions is to shift the Cu 3d derived band to higher binding energies. The energy position and the overall shape of the Cu 3d band approaches that of the pure metal as the Cu concentration is increased. The valence band spectra are in good agreement with previous band structure calculations based on the LMTO–CPA method.

Electronic Structure of Doped La-Mn-O Perovskites

The electronic structure of the doped perovskite manganese oxides La1-x Srx MnO3 and La1xBax MnO3 has been investigated by X-ray photoelectron and X-ray emission spectroscopy. By comparing the O Kα and Mn Lα X-ray emission spectra with the valence band X-ray photoelectron spectra we could localize the Mn 3d and O 2p states into the valence band. Our data are compared with previous band structure calculations. The value of the Mn 3s splitting does not change with Sr-doping in the concentration region from x = 0.0 till x = 0.3. This shows that the doping holes have mainly oxygen p character. The large value of the splitting indicates that the 3d electrons are in the high-spin state.

Temperature and pressure dependence of the optical absorption in hexagonal MnTe

The absorption edge of hexagonal (NiAs structure) antiferromagnetic MnTe has been measured by means of light transmission experiments carried out at different temperatures in the range 16--420 K $(P=1\mathrm{bar})$ and hydrostatic pressures up to 9 GPa $(T=295\mathrm{K}).$ An indirect band gap has been found, in agreement with previous band-structure calculations, with an energy of ${E}_{\mathrm{ig}}=1.272\ifmmode\pm\else\textpm\fi{}0.013\mathrm{eV}$ at room temperature and pressure. The temperature dependence of the absorption edge is linear above the N\'eel temperature ${T}_{N}=310\mathrm{K},$ with a temperature coefficient $dE/dT=\ensuremath{-}(3.5\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\mathrm{e}\mathrm{V}/\mathrm{K}.$ Below ${T}_{N}$ an additional blueshift is found, with a maximum value of 0.1 eV at low temperatures. The temperature dependence of this anomalous shift is proportional to the square of the magnetization, a result which is consistent with previous second-order perturbation calculations. Regarding the measurements under pressure, a negative pressure coefficient with a value of $dE/dP=\ensuremath{-}(59\ifmmode\pm\else\textpm\fi{}2\mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{G}\mathrm{P}\mathrm{a})$ has been found. The N\'eel temperature is known to increase with pressure in hexagonal MnTe, due to an increment of the exchange interaction, or equivalently, of the sublattice magnetization. Consequently a positive pressure shift could be expected at room temperature, derived from both the antiferromagnetic splitting of Mn $3d$ orbital and second-order electron and hole interaction with fixed Mn spins. The negative pressure coefficient has thus been interpreted as a sum of that positive contribution and a larger negative one derived from an enhanced p-d repulsion which would lead to an upwards shift of the valence-band maximum.