Partial Density Of Unoccupied States Research Articles

Electronic and atomic structure of Ba8Ga16Ge30−xSix type I clathrates: Ge and Ga XAFS study

X-ray absorption fine structure spectroscopy at the Ga and Ge K-edges was used to study changes in the Ga and Ge electronic structure and local coordination geometry as a function of composition in Ba8Ga16Ge30−xSix type I clathrates (x = 0, 7.5, 9.1, 10.7, 13.4 and 30). Based on XANES data, the partial density of unoccupied states with p character is modified for both Ga and Ge upon Si substitution. Among the specimens which contain both Ge and Si, we found that the specimen with the highest measured power factor (i.e., x = 7.5) has the lowest density of unoccupied states for both Ga and Ge. Our experimental results are qualitatively consistent with computational results based on density functional theory, indicating that a series of pertinent electronic states are modified by Si p states. This suggests that an increase in the electron density near the Fermi level for an optimal Si substitution leads to an increase in the Seebeck coefficient and consequently in the power factor, according to the Cutler–Mott relation. Based on quantitative analysis of EXAFS spectra, we found that Ga has more Si neighbors than Ge, indicating that Si resides preferentially next to Ga. Both the Ge–Ga/Ge and Ga–Ge/Ga coordination distances remain relatively unchanged (∼2.51 Å) regardless of the degree of Si substitution. Furthermore, the Ge–Si and Ga–Si coordination distances remain relatively unchanged at ∼2.41 and ∼2.45 Å, respectively, regardless of the degree of Si substitution. For the Ba8Ga16Si30 specimen, on average, Ga is coordinated with 0.9 Ga and 3.1 Si at roughly the same distance of ∼2.50 Å. The number of Ga–Ga bonds is consistent with the fact that Ga is distributed on the framework sites in a way which reduces the number of Ga–Ga bonds relative to that based on a random distribution. An understanding of the underlying physics of the structure–property relationship provides for potential additional routes for tuning the electronic properties of clathrates for thermoelectric applications.

Localized magnetic moments in the Heusler alloy Rh2MnGe

X-ray magnetic circular dichroism (XMCD) of core-level absorption (x-ray absorption spectroscopy, XAS) spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Rh2MnGe at the Rh M3,2 and Mn L3,2 edges. The ratio of Rh and Mn spin moments amounts to 0.05 which is smaller than the ratio of 0.1 determined by a local density approximation electronic band structure calculation. We have found that the orbital moments of the Rh 4d and Mn 3d states are very small. The observed Rh 2p XAS spectrum can be understood on the basis of the Rh 3d partial density of unoccupied states as is typical for metals. The observed features of the Mn 2p XAS and XMCD spectra are dominated by final state multiplets as is typical for oxides. The comparison of experimental and ab initio calculated XAS/XMCD spectra reveals a strong narrowing of the Mn 3d bands, indicating strongly localized Mn moments. The magnetic moments are considerably more localized for Rh2MnGe in comparison with the isoelectronic compound Co2MnGe. In spite of the strong localization of the Mn moment, the temperature dependences of sublattice magnetization are equal for the Mn and Rh sublattices in contrast to the prediction by a Heisenberg model. This might be attributed to the remaining itinerant character of the Rh moment.

Neat Alq 3 thin film and metal/Alq 3 interfaces studied by NEXAFS spectroscopy

Thin film of Alq 3 and its interfaces with various metals (K, Li, Ca, Al)/Alq 3 and the Al/LiF/Alq 3 interface were investigated using NEXAFS spectroscopy. The assignments of structures in NEXAFS spectra at the C, N and O K-edges were performed using theoretical calculations taking account of the core-excitonic effect. The NEXAFS spectra at the three edges are well reproduced by the simulated results. The spectra at the N and O K-edges of Alq 3 are similar to the partial density of unoccupied states (PDOUS) of the ground state, but the spectrum at the C K-edge is different from the PDOUS of ground state due to the core-excitonic effect. The present study is the first trial of the detailed analysis of the C K-edge spectrum of Alq 3. The spectral changes at the metal/Alq 3 and Al/LiF/Alq 3 interface were found to depend on the metal. By the comparison with the theoretical simulations based on the 1:1 metal-Alq 3 complex, we found that the electron transfer and orbital overlap occur at the interfaces of K/Alq 3 and Li/Alq 3, while the observed changes at the Ca/Alq 3 and Al/Alq 3 interfaces could not be understood by the simulated results. At the LiF/Alq 3 interface, we observed the spectral changes which may result from some chemical interaction. Some of the spectral changes at Al/LiF/Alq 3 interface differ from those of Al/Alq 3 interface, indicating chemical interaction specific for the co-existence of Al, LiF, and Alq 3.

First-principles Calculation of L<SUB>3</SUB> X-ray Absorption Near Edge Structures (XANES) and Electron Energy Loss Near Edge Structures (ELNES) of GaN and InN Polymorphs

First principles calculations of L3 XANES/ELNES of GaN and InN with both wurtzite and zinc-blende structures have been made using OLCAO (orthogonalized linear combinations of atomic orbitals) method. Supercells with more than 100 atoms were employed. A core-hole was rigorously included in the calculation, and the photo absorption cross section (PACS) between the initial and final states was computed. Quantitative reproduction of experimental spectrum that is available in literature can be found when the PACS was computed. Although spectral shapes of two phases look similar, characteristic differences are predicted to appear at the first peak of the L3 XANES/ELNES. The first peak is notably broader in the zinc-blende phases. The origin of the broadness is analyzed using partial density of unoccupied states (PDOS) and Mulliken charge. We then conclude that the broadness can be related to greater covalency of the zinc-blende phase as compared to the wurtzite phase.

Electron dynamics within Ru-2,2′-bipyridine complexes—an N1s core level excitation study

The dynamics of electron transfer within an excited ruthenium complex as well as between a ruthenium complex and a nanostructured TiO2 film is addressed on the core hole few femtosecond lifetime scale. The ruthenium complexes studied are Ru(bpy)32+·2Cl− (where bpy is 2,2′-bipyridine) as well as Ru(bpy)2(dcbpyH2)2+·2PF6− (dcbpy is 4,4′-dicarboxy-2,2′-bypyridine) anchored to a nanostructured TiO2 surface, where the latter system constitutes the photoactive part of a dye-sensitized solar cell. The N1s core level of the ruthenium complexes was excited by using synchrotron radiation, and the experimental techniques used were X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). The occupied molecular orbital structure and the N1s partial density of unoccupied states are mapped and compared to calculated orbital structures.

Bonding in silicates; investigation of the Si L (sub 2,3) edge by parallel electron energy-loss spectroscopy

The Si L2,3 core-loss edge can be used to probe the crystal chemistry around Si, providing information on the s- and d-like partial density of unoccupied states of the Si-O bonds. We present Si L 2,3 edges from 59 silicates, glasses, and amorphous materials acquired by parallel electron energy-loss spectroscopy (PEELS) with a transmission electron microscope (TEM) at an energy resolution of 0.7 eV. The Si L 2,3 edge spectrum of α-quartz is interpreted using the results of a recent pseudopotential band-structure calculation. A combination of Si s- and d-like partial density of states derived from this calculation resembles the Si L2,3 energy-loss near-edge structure (ELNES) of α-quartz. The Si L2,3 ELNES of the silicates are interpreted using the results of the band-structure calculation of α-quartz. The Si L2,3 edges of Q 4 , Q 3 , Q 2 , some Q 1 silicates, and amorphous materials have ELNES similar to that of α-quartz, and the Q 0 and some Q 1 silicates have ELNES different from that of α-quartz. A “coordination fingerprint” is defined for Q 4 , Q 3 , and Q 2 Si L2,3 ELNES because of their similarity to the α-quartz spectrum. The similarities between the L2,3 core-loss edge shapes of the third-row XO 4 n‐ (X = Al, Si, S, and P) series attests to a common molecular‐orbital picture of their bonding. For Q 0 and some Q 1 spectra a “structure fingerprint” is defined because the Si L2,3-edge shapes are indicative of the number, distribution, and nature of the non-nearestneighbor atoms. Spectra of olivine glasses and metamict zircon more closely resemble the α-quartz spectrum than their crystalline analogs. In contrast to previous studies, we show that distortion of the SiO 4 tetrahedron is of secondary importance as an ELNES-modifying parameter. Polyhedral distortions become less important with increase in polymerization. There is a positive linear correlation between the energies of the Si L 2,3-edge onsets and polymerization, Si 2p and 2s binding energies, and the

Location of interstitial Cr in mullite by incoherent channeling patterns from characteristic X-ray emission

The Si L2,3 core-loss edge can be used to probe the crystal chemistry around Si, providing information on the s- and d-like partial density of unoccupied states of the Si-O bonds. We present Si L2,3 edges from 59 silicates, glasses, and amorphous materials acquired by parallel electron energy-loss spectroscopy (PEELS) with a transmission electron microscope (TEM) at an energy resolution of 0.7 eV. The Si L2,3 edge Spectmm of α-quartz is interpreted using the results of a recent pseudopotential band-structure cal­culation. A combination of Si s- and d-like partial density of states derived from this calculation resembles the Si L2,3 energy-loss near-edge structure (ELNES) of α-quartz. The Si L2,3 ELNES of the silicates are interpreted using the results of the band-structure calculation of α-quartz. The Si L2,3 edges of Q4, Q3, Q2, some Q1 silicates, and amorphous materials have ELNES similar to that of α-quartz, and the Q0 and some Q1 silicates have ELNES different from that of α-quartz. A “coordination fingerprint” is defined for Q4, Q3. and Q2 Si L2,3 ELNES because of their similarity to the α-quartz spectrum. The similarities between the L2,3 core-loss edge shapes of the third-row XOn-4 (X = Al, Si, S, and P) series attests to a common molecular-orbital picture of their bonding. For Q0 and some Q1 spectra a “structure fingerprint” is defined because the Si L2,3-edge shapes are indicative of the number, distribution, and nature of the non-nearest- neighbor atoms. Spectra of olivine glasses and metamict zircon more closely resemble the α-quartz spec­trum than their crystalline analogs. In contrast to previous studies, we show that distortion of the SiO4 tetrahedron is of secondary importance as an ELNES-modifying parameter. Polyhedral distortions be­come less important with increase in polymerization. There is a positive linear correlation between the energies of the Si L2,3-edge onsets and polymerization. Si 2p and 2s binding energies, and the 29Si NMR isotropic chemical shifts. The shift to higher energies of the edge onsets with polymerization corresponds to an increase in effective charge on the Si atom with higher Qn. For silicates with isolated SiO4 tetrahedra. increases in L2,3-energy onsets correlate with increases in polarizing power of the next-nearest-neighbor cations. The Si L2,3-edge shapes are affected by the types and coordinations of the next-nearest-neighbor cations. For example, andradite, ilvaite, fayalite, and γ-Fe2SiO4 have FeO6 bonded to SiO4 and exhibit similar ELNES. Topaz, diunortierite, staiuolite, and kyanite have similar Si L2,3 ELNES. with AlO6 bonded to the SiO4. Their edge shapes are distinct from those of silicates with SiO4 bonded to AlO4, as in the feldspars. A comparison of the Al and Si L2,3 and Al, Si, 0, and F K core-loss edges of topaz illustrates the influence of neighbor effects and mixing of unoccupied states. This mixing illustrates the limitations of ab initio methods that model core-loss edges that neglect non-nearest-neighbor interactions.

Sulphur K-edge x-ray absorption fine-structure studies of EuS and GdS

High-resolution sulphur K-edge x-ray absorption near-edge structure spectra have been recorded for europium and gadolinium monosulphides. A comparison of the experimental spectra with the element-projected partial density of unoccupied states calculated using a spin-polarized linear muffin-tin orbital method revealed a rather good agreement. Our analysis suggests that there is a strong hybridization of the 4f and 5d orbitals of the rare earths with the p orbitals of the sulphur which also contribute to the conduction band. The strong d-p and f-p interactions result in a large sulphur p-band polarization which has been confirmed experimentally by the detection of a strong x-ray magnetic circular dichroism signal at the S K edge in EuS.

Crystal-structure effects in the Ce L3-edge x-ray-absorption spectrum of CeO2: Multiple-scattering resonances and many-body final states.

Here we report the interpretation of all spectral features of Ce ${\mathit{L}}_{3}$ x-ray-absorption near-edge structure (XANES) of ${\mathrm{CeO}}_{2}$ over a 40 eV range. The local partial unoccupied density of states in the initial state and in the fully relaxed final state have been calculated by the full multiple-scattering approach. The wave function of the excited electron in the final states of the Ce ${\mathit{L}}_{3}$-edge XANES spectrum is found to be determined by the multiple-scattering processes in a large size cluster formed by at least 45 atoms. We predict, in good agreement with the experimental data, the crystal-field splitting of 5d states in the final state ${\mathrm{\ensuremath{\Delta}}}_{\mathit{f}}$=4.0\ifmmode\pm\else\textpm\fi{}0.2 eV and its variation from the initial to the final state. The many-body final states for the 2p\ensuremath{\rightarrow}5,\ensuremath{\varepsilon}d transition, arising from the configuration interaction due to mixing of Ce 4f and O 2p valence orbitals, have been calculated taking into account the crystal-field splitting of the 5,\ensuremath{\varepsilon}d states and they give a full explanation for the low-energy shoulder on the Ce ${\mathit{L}}_{3}$ x-ray-absorption spectrum white line that was the object of a long-standing discussion. The origin of the pre-edge peak at about 10 eV below the white line maximum is explained as due to transitions at the bottom of the conduction band.

Cell-by-cell mapping of carrier concentrations in high-temperature superconductors

The correlation between atomic and electronic structure in the vicinity of grain boundaries in YBa2Cu3O7-δ is investigated on the scale of the coherence length, using a combination of Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope. The detector arrangement of the microscope enables both techniques to be performed simultaneously, allowing the exact crystallographic location of the probe to be determined from the image. As the pre-edge feature of the oxygen K-absorption edge is very sensitive to the partial density of unoccupied states, it can be calibrated to give an accurate measure of local hole concentrations (±5%). Spatial resolution is limited only by the 2.2 A probe size, thus allowing the electronic properties of the material to be inferred on a scale less than the coherence length. Results are discussed from the study of YBa2Cu3O7-δ films prepared by laser ablation on yttria-stabilized-zirconia (YSZ) substrates, where a symmetric (near Σ5) grain boundary shows no significant hole depletion in the boundary region, while an asymmetric (near Σ17) boundary shows depletion extending over a region far greater than the atomic disorder shown in the image. Application of this technique to the study of interfaces in other superconducting systems is discussed.

Electronic structure studies on the n-type doped superconductors R2-xMxCuO4- delta (R=Pr,Nd,Sm; M=Ce,Th) and Nd2CuO4-xFx by electron-energy-loss spectroscopy.

The electronic structure of the high-${\mathit{T}}_{\mathit{c}}$ superconductors ${\mathit{R}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (R=Pr, Nd, Sm; M=Ce, Th) and ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{3.8}$${\mathrm{F}}_{0.2}$ has been studied by high-energy electron-energy-loss spectroscopy in transmission. Core-level spectroscopy, particularly on O 1s and Cu 2p levels yields information on the partial unoccupied density of states near the Fermi level, while that on R and Ce 3d and 4d levels may serve as a valence monitor for these constituents. A pre-edge structure in the O 1s spectrum is polarized in the ${\mathrm{CuO}}_{2}$ plane and assigned to transitions into a two-dimensional conduction band consisting of Cu 3d hybridized to O 2p states. In the Cu 3${\mathit{d}}^{9}$ configuration we found a slight admixture (\ensuremath{\sim}10%) of Cu 3${\mathit{d}}_{3\mathit{z}}^{2}$-${\mathit{r}}^{2}$ to the Cu 3${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ hole states similar to the p-type doped superconductors. Upon doping we recognized a reduction (\ensuremath{\sim}14%) of the excitonic Cu 2${\mathit{p}}_{3/2}$ line, indicating that the introduced electrons go onto the Cu sites, thus producing charge carriers having predominantly Cu 3d character.

Electron energy-loss studies on high-temperature superconductors

The electronic structure of the high-temperature superconductors La2−xSrxCuO4, Bi2Sr2Ca1−x YxCu2O8, and Nd2−xCexCuO4−δ have been studied by high-energy electron energy-loss spectroscopy in transmission. Core-level spectroscopy, in particular on the O 1s and Cu 2p levels, yields information on the partial unoccupied density of states, i.e., on the character of hole states close to the Fermi level.

Valency and covalency in high-Tc-superconductors from x-ray absorption

The local electronic structure of the high-Tc superconductors YBa2Cu3O7−δ and Nd2−xCexCuO4−δ is probed by soft x-ray absorption spectroscopy at the CuLII,III, OK, CeMIV,V, and NdMIV,V thresholds. The near-edge structures provide information on the partial unoccupied density of states above EF as well as on valency and covalency of the constituent elements.

Partial density of unoccupied states and L 2,3-x-ray absorption spectrum of bulk silicon and of the Si(1 1 1) 2 × 1 surface

The L 2,3 surface absorption spectrum of the clean Si(1 1 1) 2 × 1 surface has been measured and compared with the bulk spectrum. The results are interpreted by comparison with tight binding calculations of the s density of states. The 2 p core hole final state effect is shown to shift all absorption features in the bulk up to 3 eV above threshold by ≈ 0.3 eV. A surface feature 2.3 eV above the conduction band bottom has been identified.