Polarization Of Valence Electrons Research Articles

Magnetic hyperfine interactions on Cd sites of the rare-earth cadmium compoundsRCd(R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er)

This paper reports the investigation of the magnetic hyperfine field ${B}_{hf}$ in a series of rare-earth ($R$) cadmium intermetallic compounds $R\mathrm{Cd}$ and ${\mathrm{GdCd}}_{2}$ measured by perturbed angular correlation (PAC) spectroscopy using $^{111}\mathrm{In}/^{111}\mathrm{Cd}$ as probe nuclei at Cd sites as well as first-principles calculations of ${B}_{hf}$ at Cd sites in the studied compounds. Vapor--solid state reaction of $R$ metals with Cd vapor and the $^{111}\mathrm{In}$ radioisotope was found to be an appropriate route of doping rare-earth cadmium compounds with the PAC probe $^{111}\mathrm{In}/^{111}\mathrm{Cd}$. The observation that the hyperfine parameters depend on details of the sample preparation provides information on the phase preference of diffusing $^{111}\mathrm{In}$ in the rare-earth cadmium phase system. The $^{111}\mathrm{Cd}$ hyperfine field has been determined in the compounds $R\mathrm{Cd}$ for the $R$ constituents Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er, in several cases as a function of temperature. For most $R$ constituents, the temperature dependence ${B}_{hf}(T)$ of $^{111}\mathrm{Cd}$:$R\mathrm{Cd}$ is consistent with ferromagnetic order of the compound. DyCd, however, presents a remarkable anomaly: a finite magnetic hyperfine field is observed only in the temperature interval 35 K $\ensuremath{\le} T \ensuremath{\le}$ 80 K which indicates a transition from ferromagnetic order to a spin arrangement where all $4f$-induced contributions to the magnetic hyperfine field at the Cd site cancel. First-principles calculation results for DyCd show that the ($\ensuremath{\pi}, \ensuremath{\pi}$, 0) antiferromagnetic configuration is energetically more favorable than the ferromagnetic. The approach used in the calculations to simulate the $R\mathrm{Cd}$ system successfully reproduces the experimental values of ${B}_{hf}$ at Cd sites and shows that the main contribution to ${B}_{hf}$ comes from the valence electron polarization. The de Gennes plot of the hyperfine field ${B}_{hf}$ of $^{111}\mathrm{Cd}$:$R\mathrm{Cd}$ vs the $4f$-spin projection $(g\ensuremath{-}1)J$ reflects a decrease of the strength of indirect $4f\ensuremath{-}4f$ exchange across the $R$ series. Possible mechanisms are discussed and the experimental results indicate that the indirect coupling is provided by the intra-atomic $4f\ensuremath{-}5d$ exchange and interatomic $5d\ensuremath{-}5d$ interaction between the spin-polarized $5d$ electrons of neighboring $R$ atoms. The ratio of the hyperfine fields of GdCd and ${\mathrm{GdCd}}_{2}$ scales with the number of nearest Gd neighbors. In the paramagnetic phases of the $R\mathrm{Cd}$ compounds, the PAC spectra indicate the presence of a broad distribution of weak quadrupole interactions suggesting a perturbation of the cubic CsCl symmetry of the Cd site, most probably due to chemical disorder of the $R$ and Cd sublattices. A substantial interchange of $R$ and Cd atoms is also reflected in the temperature dependence of the linewidth of the magnetic hyperfine interaction in the magnetically ordered phase of $R\mathrm{Cd}$ and ${\mathrm{GdCd}}_{2}$. Its critical increase towards the order temperature is evidence for a distribution of the order temperature with a width of about 10 K.

Enhanced quantum size effect in Li and Na clusters via rare gas doping

AbstractAlbeit its chemical inertness, rare gas doping can substantially enhance the quantum size effect of nanocrystals, yet little attention has been paid on this fascination and the mechanism behind remains unclear. Here, we show that the rare gas dopant breaks bonds of its neighboring atoms, which effects the same to atomic under‐coordination on the bond strain, energy quantum entrapment, and valence electron polarization of Li and Na clusters. Consistency between density functional theory calculation and the bond‐order‐length‐strength correlation prediction revealed that the bond strain by 16.86% and 21.12% before and after He doping for Na30 clusters. Observations suggest an effective yet simple means to modulate the physical properties by doping the inert gases.

Coordination-Resolved Spectrometrics of Local Bonding and Electronic Dynamics of Au Atomic Clusters, Solid Skins, and Oxidized Foils.

By using combination of bond-order-length-strength (BOLS) correlation, the tight-binding (TB) approach, and zone-selective photoelectron spectroscopy (ZPS), we were able to resolve local bond relaxation and the associated 4f7/2 core-level shift of Au atomic clusters, Au(100, 110, 111) skins, and Au foils exposed to ozone for different lengths of time. In addition to quantitative information, such as local bond length, bond energy, binding-energy density, and atomic cohesive energy, the results confirm our predictions that bond-order deficiency shortens and stiffens the bond between undercoordinated atoms, which results in local densification and quantum entrapment of bonding electrons. The entrapment perturbs the Hamiltonian, and hence, shifts the core-level energy accordingly. ZPS also confirms that oxidation enhances the effect of atomic undercoordination on the positive 4f7/2 energy shift, with the associated valence electron polarization contributing to the catalytic ability of undercoordinated Au atoms.

Ab initio study of 59Co NMR spectra in Co2FeAl1−xSix Heusler alloys

Ab initio electronic structure calculation of a series of Co2FeAl1−xSix Heusler alloys has been performed, using the Korringa–Kohn–Rostoker-coherent potential approximation method to explain experimental 59Co NMR spectra. Two prominent features are explained semi-quantitatively—a global shift of the 59Co resonance line due to alloying with Al and Si atoms in Co2FeAl1−xSix, and the effect of local disorder in creating distinct satellite lines of 59Co NMR in Co2FeAl. The importance is stressed of the positive contribution to the 59Co hyperfine field from valence electron polarization, which emerges from the half-metallic band structure inherent in Co-based Heusler alloys.

Local bond-electron-energy relaxation of Mo atomic clusters and solid skins

Mo(100, 110) skins and atomic clusters share the common nature of atomic undercoordination induced local bond strain and the associated quantum entrapment and valence electron polarization.

59Co NMR experiment as a probe of electron doping in Co2FeAl1−xSixHeusler alloys

A systematic ${}^{59}$Co NMR study has been carried out at 4.2 K in a series of quaternary Co${}_{2}$FeAl${}_{1\ensuremath{-}x}$Si${}_{x}$ polycrystalline bulk Heusler alloys ($x$ = 0, 0.3, 0.5, 0.7, 1). It was shown that the effect of Si substitution consists in a significant modification of ${}^{59}$Co hyperfine field and that this modification is mainly due to the contribution from $s$ valence electron polarization, suggesting a shift of the Fermi energy level inside the half-metallic gap. This observation supports the theoretical predictions that the Fermi-level position in Co${}_{2}$Fe$Z$ Heusler alloys can be effectively tuned by varying the composition of the $Z$ sublattice.

Edge versus interior in the chemical bonding of graphene materials

A notable phenomenon is described involving the polarization of valence electrons that operates in the edge regions of graphene materials, including compact polyaromatic hydrocarbon (PAH) molecules, because of the mismatch between interior and perimeter chemical bonds. The topology of hexagonal-shaped zigzag-edged systems, with alternating perimeter CC bond lengths, enhances the effect, making recognition easier, compared to graphene ribbons where it is weak. The phenomenon is delineated in ab initio density-functional theory calculations of the electronic structure of a 4 nm diameter hexagonal zigzag-edged molecule ${\text{C}}_{486}{\text{H}}_{54}$, chosen because it is a representative of larger ones. Using CC bond length as a surrogate measure, we map the polarization of valence electrons from the perimeter to the center. In a pattern that mimics the bonding, the polarization of the two outermost sets of edge bonds (transverse along rows and radial connecting nested rows of carbon atoms) is impressed on the graphene interior bonds with amplitude that decreases rapidly from the perimeter to the center. All bonds display a decaying oscillation with radial bonds longer than transverse bonds at the same location. This difference leads to a characteristic quinoidal pattern of CC bonds linking the zigzag hexagonal edges joined at each apex. The phenomenon and its analysis are applicable to geometries periodic or not, substituent modified edges and interior.

Plasmon-phonon-polaritons in p-doped Bi-Sb alloys

The phonon spectra of Bi crystals and Bi-Sb alloys indicate the presence of phonon-polaritons caused by the polarization of valence electrons, which reduces the rigidity of the transverse mode of optical lattice vibrations and increases the rigidity of the longitudinal mode of vibrations. Doping Bi-Sb alloys with Sn (p-type impurity) decreases the difference in the energy of plasma oscillations of free charge carriers and the energy of longitudinal optical phonons. In this case, the behavior of the dielectric function is described in terms of the model corresponding to the excitation of plasmon-phonon-polaritons.

Ionicity Dependent Transverse Effective Charge of ANB8-NCompounds

We have calculated the transverse effective charge of A N B 8- N compounds by taking account of the electronic polarization of valence electrons induced by the ionic displacements. It is shown that the transverse effective charge can be expressed as a function of the Phillips ionicity f i from the relation between the most important pseudopotential and band gaps on the Jones-zone faces for zincblende and rock-salt structures. We have found a critical ionicity F c that the ionicity dependent transverse effective charge Z T ( f i ) of both structures coincide with together. From the comparison with simple ionic model, the F c is assigned as identical to the Phillips critical ionicity. We have found that when taking account of the effective valence number based on the modified f -sum rule the Z T ( f i ) well agree with the experimental results for tetrahedrally coordinated compounds. For rock-salt compounds, the two set model of most strongest pseudopotential is qualitatively discussed.

Local magnetic moments and hyperfine magnetic fields in disordered metal-metalloid alloys

The local magnetic moments and hyperfine magnetic fields (HFF's) in the ordered alloys ${\mathrm{Fe}}_{15}\mathrm{Sn}$ and ${\mathrm{Fe}}_{15}\mathrm{Si}$ are calculated with the first-principles full-potential linear augmented plane wave method. The results are compared with the experimental data on Fe--M (M=Si, Sn) disordered alloys at small metalloid concentration. The relaxation of the lattice around the impurity and its influence on the quantities under consideration are studied. The mechanism of the local magnetic moment formation is described. It is proved that the main distinction between these alloys is connected with the different lattice parameters. Three contributions to the HFF are discussed: the contributions of the core and valence electron polarization to the Fermi-contact part, and the contribution from the orbital magnetic moment.

Nuclear magnetic resonance study of expanded lattice permanent magnet nitrides and carbides at high pressure

The nuclear magnetic resonance spin-echo spectra of 89Y and 147Sm for Y2Fe17, Y2Fe17C0.4, Y2Fe17C1.5, Y2Fe17N2.5, Y2Fe17CN1.5, Y2Fe17H1.6, and Sm2Fe17 have been measured at pressures up to 10 kbar at 4.2 K. A reduction of the magnitude of the valence electron polarization contribution to the effective field Be at Y and Sm sites with pressure is observed. The effect is strongly related to the nearest neighbor environment of the site. A significant reduction of ∂B/∂p is observed for the sites with nitrogen neighbors. Carbon increases (decreases) ∂B/∂p in the rhombohedral (hexagonal) structure of the compound. A decrease of the lattice electric field gradient at the Sm site in Sm2Fe17 with pressure is observed. A corresponding decrease of the crystal electric field coefficient A02 at the Sm site with pressure of 0.7% per kbar is derived, indicating an anisotropic compressibility of the material.

Charge distribution and bonding nature of silicon compounds. An ab initio theoretical study

The charge distribution and bonding nature of MX bonds (M is C, Si; X is C, H, F, Cl, Li, O) have been investigated with various ab initio levels of theories including HF/6-31G ∗ and MP2/6-311++G ∗∗. We have evaluated the excess charge value for each atom type from the analyses based on Mulliken population, natural population, and electrostatic potential derived populations. We have attempted to obtain a reasonable charge value for each atom type in organosilanes. Various schemes for partitioning charges provide different values and correlate poorly with other schemes. According to the limited test for simple C and Si compounds, the charge values obtained from electrostatic potential derived methods appear to reproduce both ab initio and experimental dipole moments reasonably well. We have also examined the electron densities (valence and total) and electrostatic potential maps of the plane including an MX bond. An electron density analysis of bonds reveals that the electron densities of Si bonds are almost half those of C bonds in the bond critical point. Low electron densities of Si bonds may be ascribed to both 1. (1) the longer Si bonds 2. (2) the intrinsic polarization of valence electrons of the Si atom.

Acceleration of convoy electrons produced by MeV He ions for glancing angle scattering from a clean surface

Dependences of the most probable energy of convoy electrons ontheergy of MeV H and He ions are studied for glancing angle scattering of the ions from a clean (001) surface of SnTe. The convoy electrons emitted at He scattering are faster than the projectile He ions, while at H scattering they are almost isotachic to the projectile H ions. The acceleration of convoy electrons at He scattering is explained as the result of polarization of valence electrons induced by the projectile He ion.

The Positron as a Probe for Studying Bulk and Defect Properties in Semiconductors

AbstractPositron lifetime measurements are used to study various doped and undoped III–V compound semiconductors like GaAs, GaP, InAs and InP. In some as‐grown crystals native vacancies (V, V) or their complexes with dopants (TeAsV, VPZnInV) are detected with maximum concentrations of a few 1018 cm−3. In undoped GaAs neutral As vacancies V are identified, the concentration of which varies locally. The vacancies have their origin in deviations from the stoichiometrie composition of the compound. They disappear at 500°C. At this temperature defects created by irradiation with fast neutrons anneal also out. The relation between the bulk positron lifetime and the density and polarization of valence electrons is analyzed. Further, the potential of the positron annihilation method in identifying and characterizing vacancy‐type defects in semiconductors is discussed.

Paramagnetic Resonance ofS-State Ions in Metals of High Paramagnetic Susceptibility

The paramagnetic resonance of ${\mathrm{Gd}}^{3+}$ in metals shows $g$ shifts with respect to the free ionic $g$ value which are due to the valence-electron polarization in metallic hosts, and the effective exchange interaction of ${\mathrm{Cd}}^{3+}$ with these valence electrons. These shifts have been studied in metals and intermetallic compounds with high paramagnetic susceptibility such as Pd, ${\mathrm{Ni}}_{5}$Y, and ${\mathrm{Pd}}_{3}$U and in many alloys involving these metals and compounds. The effective exchange interaction is found to be generally much smaller than expected from the atomic spectra. It is negative for valence bands of $d$ character and positive in valence bands of $5f$ and $s$ character, and is therefore not the result of simple atomic exchange only. The shape of the Gd resonance lines gives information on the spatial variations in the valence-electron polarization of the host metals. Thus, it was found that Pd alloyed with La or H segregates into two phases. The valence-electron polarization can be altered by admixture of other magnetic ions, and it was therefore possible to measure the exchange interaction for many rare earths and Fe, Co, Ni in Pd, and some rare earths in ${\mathrm{Ni}}_{5}$Y. The Gd line shape in these experiments allowed a study of the nonlocal character of the valence-electron susceptibility, and it appears that in Pd and in ${\mathrm{Ni}}_{5}$Y this susceptibility has a larger range than predicted by the free-electron calculation of Ruderman-Kittel-Yosida.