Contribution Of Conduction Electrons Research Articles

Electronic Structure of Third-Stage Lithium Intercalated Graphite

Self-consistent, first-principles electronic-structure calculations were carried out for Li${\mathrm{C}}_{18}$, a model high- (third-) stage graphite intercalation compound. The results illustrate the charge distribution and screening mechanisms in this class of compounds and account for many of their unusual electronic properties. The total charge transferred is found to be well concentrated on the bounding carbon layers, while the conduction-electron contribution is much more delocalized.

Measurements of the heat capacity of praseodymium at liquid helium4 temperatures

Abstract The heat capacity of a single crystal of praseodymium has been measured between 1 and 6K in magnetic fields from 0–4T applied in the crystallographic b direction. The conduction electron contribution is far larger than that expected from band structure caclulations, and furthermore is field-dependent. This may be due to a new electron mass enhancement of magnetic origin.

Proton NMR in a hydrogen molybdenum bronze

Solid state nuclear magnetic resonance (NMR) has been used to probe the nature of hydrogen in the hydrogen bronze H1.64MoO3. The line shape of the proton NMR at room temperature is found to be associated only with shielding anisotropy. Below 230 K, dipolar interactions become important in broadening the line. The response of the protons under a magic angle spinning experiment indicates that there is predominantly only one type of hydrogen in this sample. The shielding anisotropy indicates that at room temperature this species is not a hydroxyl proton. The longitudinal relaxation time of protons in this material exhibits a minimum between 190–320 K indicating that dipolar interactions modulated by motion are responsible for T1 relaxation. Comparison of longitudinal relaxation times of partially deuterated and nondeuterated samples indicates that there exists a contribution to T1 relaxation not dependent upon interproton distance. There does not appear to be a conduction electron contribution to the proton T1.

Hydrogen locations, diffusion and the electronic density of states in yttrium dihydrides: A nuclear magnetic resonance investigation

We report results of wide-line and pulsed proton magnetic resonance measurements on yttrium dihydrides, YH x (1.63 ⩽ x ⩽ 1.98), covering the temperature range 140–760 K and yielding information on hydrogen locations, diffusion and the density of states in the dihydride phase. Our principal conclusions are as follows. (1) At x = 1.98, wide-line second moment measurements indicate that 15% of the octahedral (O) sites in the f.c.c. CaF 2 structure are occupied by hydrogen. (2) Spin-lattice relaxation time measurements in the rotating frame ( T 1 ρ versus T) yield hydrogen jump frequencies ν j showing three distinct regions in which the activation energy E act for hydrogen diffusion increases with T. (3) For x = 1.98 ν j is consistently higher than for x = 1.92, corroborating the partial occupation of O sites for x values below 2.0. (4) At the lower limiting composition ( x ≈ 1.8) E act becomes anomalously large (approximately 1.1 eV atom −1), compared with the value 0.5 eV atom −1 at x = 1.92, and the diffusion rate is about 10 3 times smaller. (5) At low temperatures the spin-lattice relaxation rate T −1 1 is dominated by the conduction electron contribution T −1 1 e . The quantity (T 1eT) −1 2 which is proportional to the density of states at the Fermi level, is 0.061 ± 0.002 s −1 2 K −1 2 for the dihydride compositions studied, indicating a constant density of states in the dihydride phase.

Self-consistent calculation of hyperfine fields at impurity sites in ferromagnetic host

The spin-density functional formalism is used to study the hyperfine-field systematics of interstitial and substitutional impurities in a ferromagnetic host. Both the core- and conduction-electron contributions to the spin density at the impurity site are treated fully self-consistently. The unperturbed conduction electrons of the host metal are approximated by plane waves, the initial polarization of which is generated by smoothing the host-ion magnetic moments into a homogeneous magnetic field outside the impurity cell. A good qualitative agreement with experimental results is obtained for the hyperfine-field systematics of both light impurities (H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}Ne) and heavy impurities (Cd\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}Cs) in Ni. The inability of the mere jellium model to calculate the hyperfine field of ${\ensuremath{\mu}}^{+}$ in magnetic materials is demonstrated. It is shown that the hyperfine field of ${\ensuremath{\mu}}^{+}$ does not depend entirely on the local polarization of the host conduction electrons at the interstitial regions. The effect of the muon zero-point motion on the hyperfine field is discussed.

Bonding and phase transitions in transition metal dichalcogenide layer compounds

The electronic structure of the transition metal dichalcogenide layer compounds is studied using hybridised orbitals appropriate to the transition metal atom coordination. Non-bonding orbitals on the transition metal atoms give the conduction electron bands which are responsible for the charge-density-wave phase transitions in the VB compounds, and the band structure of 1T- and 2H-TaS2 is found using tight binding. The phase transitions are studied by a tight-binding calculation of the conduction electron contribution to the dynamical matrix, Phi el. In 1T-TaS2 there is a Kohn anomaly in Phi el close to the displacement wavevector of the incommensurate phase, sufficiently large to make the ideal structure unstable. The ideal 2H structure is also unstable, though there is only a weak Kohn anomaly. As Phi el is fairly temperature independent, the stability of the high-temperature phase must be due to phonon entropy, and this is studied using a mean-field theory of anharmonic effects. The low-temperature commensurate phases are stabilised by chemical bonding effects.

NMR-analysis of indirect magnetic interactions in intermetallic rare-earth compounds

NMR Spectroscopy has given several pieces of information with regard to the indirect magnetic interactions in intermetallic compounds of gadolinium or europium. These comprise information on the contribution of non-s conduction electrons, on the predominance of short-range interactions, on the anisotropy of the transferred hyperfine interaction and on the range of d electron spin polarization.

Bonding and charge density wave phase transitions

Abstract A simple tight-binding method, based on hybridised directed orbitals, is used to calculate the electronic structure of the transition metal dichalcogenide layer compounds. The method is also used to find the conduction electron contribution to the dynamical matrix in 1T- and 2H-TaS 2 to study the charge density wave transitions. It is found that the ideal structures are unstable, and it is suggested that the ideal-incommensurate phase transitions are mainly due to phonon anharmonicity. Clustering effects are important in stabilising the low temperature commensurate structures.

Hyperfine fields at nonmagnetic atoms in metallic ferromagnets

The various models used to describe the hyperfine field at nonmagnetic impurities in metallic ferromagnets are discussed. These models fall into two main categories. In one the impurity is considered to introduce a strongly perturbing charge disturbance and solute-host effects are nonseparable. These models use scattering-theory concepts and formalism and the host and impurities are represented by parameters which are difficult to relate to, and often in conflict with other known information about these systems. In the other approach the perturbation of the $s$-conduction-electron polarization caused by the solute is considered to be small enough so that the hyperfine field at the impurity can be separated into a host $s$-conduction-electron polarization contribution and a contribution associated with the impurity atom. This model is formulated in terms of atomic and band properties and thus can give information about the impurity and host wave functions. This separable volume misfit model is modified from earlier presentations to take into account the changes in the $d$-conduction-electron polarization in the vicinity of impurity atoms with $d$ valence electrons. The impurity contribution, also known as the transferred hyperfine field or volume overlap contribution, has been calculated for the $4sp$ and $5sp$ solute atoms in Fe. The main differences from previous calculations of this type are that in order to better represent the solid state of the host we have used a more diffuse $3d$ orbital for the Fe atoms which closely resembles the Fe band orbital. We have also included contributions from the 2nn (2nd nearest neighbor) and 3nn shells. With these improvements we get excellent agreement between the experimental solute-atom contribution obtained from the volume misfit model and the calculated overlap hyperfine fields.

Temperature dependence of the electric field gradient at Ta impurities in the heavy-rare-earth metals Gd, Dy, Ho, and Er

The time-differential perturbed-angular-correlation (TDPAC) technique has been used to study the temperature dependences of the electric field gradient (EFG) at /sup 181/Ta impurities in the heavy-rare-earth (RE) metals Gd, Dy, Ho, and Er. At room temperature the ratio ..cap alpha.. equivalent vertical-bar V/sub z/z/ (1-..gamma../sub infinity/) V/sup 1at//sub z/z vertical-bar of the measured EFG V/sub z/z and the calculated ionic EFG (1-..gamma../sub infinity/) V/sup 1at//sub z/z decreases linearly with increasing rare-earth atomic number. A linear but much stronger decrease of ..cap alpha.. has previously been reported for the impurity /sup 111/Cd. A simple model is proposed which explains the linear decrease of ..cap alpha.. and the different slopes for /sup 181/Ta and /sup 111/Cd in terms of the lanthanide contraction. This model assumes the conduction-electron contribution to the EFG to be mainly determined by the number of electrons in the Wigner-Seitz cell of the impurity. In all rare-earth hosts the EFG decreases with increasing temperature. This decrease, which is slightly stronger for Gd than for Er, is better described by a linear function of temperature than by a T/sup 3/2/ behavior, observed in many other impurity-host systems. The temperature dependence of the EFG is much stronger than expected from themore » lattice expansion. The difference between the temperature dependence of the measured EFG and of the calculated lattice EFG decreases across the rare-earth series. This can be attributed to a decrease of the amplitudes of the lattice vibrations between Gd and Er.« less

Temperature dependence of the static magnetic susceptibility of brominated (SN) x

The static magnetic susceptibility of brominated (SN) x differs markedly from that of the parent material. The conduction electron contribution is increased by a factor of three, and a positive linear temperature dependence is observed in samples with 0.4 Br per SN unit. The magnitudes of both the susceptibility enhancement and the temperature coefficient depend monotonically on bromine content, for concentrations in the range 20–40 atomic %. No simple explanation for this behaviour can be found on the basis of minor perturbations of the properties of the parent material, suggesting that brominated (SN) x has a band-structure radically altered from that of pristine (SN) x.

Hyperfine field distribution in amorphous (FexNi1-x)80P14B6

The magnetic hyperfine field distributions (P(H)) of amorphous (FexNi1-x)80P14B6 have been measured by 57Fe Mossbauer spectroscopy. The saturation P(H) and their concentration dependences have been determined using a Fourier series method. The reliability and shortcomings of this method are illustrated. For all samples of (FexNi1-x)80P14B6, the P(H) are well defined, single-maximum and asymmetric functions of H with a higher population at the low-field side. The P(H) shifts toward smaller H values for samples with decreasing Fe content, but the extrapolated P(H) for dilute Fe in amorphous Ni80P14B6 remains finite. The shape of P(H) is found to be insensitive to the Fe content and practically temperature-independent. For amorphous Fe80P14B6, the ratio of the average hyperfine field and the average magnetic moment is 150 kOe/ mu B, which is essentially the same as those observed in a large number of crystalline Fe metalloid compounds. This suggests that the Fe hyperfine field is likely to be proportional to its moment, due to a negligible conduction electron contribution. An appreciable amount of Fe atoms in samples with x>or approximately=0.3 has been found to have hyperfine fields larger than 340 kOe, the saturation value of alpha Fe.

Optical properties and valence mixing in CeN

The reflectivity spectrum of CeN shows a strong temperature dependence in the photon energy range up to 2.5 eV. The conduction electron contribution in terms of the optical constants has been analysed by means of the Drude theory.

Nuclear-Acoustic-Resonance Determination of the Electronic Contribution to the Electric-Field-Gradient Tensor in Molybdenum

Nuclear acoustic-resonance absorption and dispersion of both isotopes Mo/sup 95/ and Mo/sup 97/ have been observed for the first time in molybdenum. From quantitative evaluation of the nuclear-acoustic-resonance results in the conduction-electron contribution to the tensor S connecting the electric field gradient to elastic strains have been determined at high magnetic fields. Comparison of the electronic contributions to the diagonal and off-diagonal elements of the electric-field gradient in Mo, Ta, and Nb show a disparity in the dependence on the density of states.

Nuclear quadrupole interaction in tin metal

Precise M\"ossbauer measurements of the quadrupole splitting in tin metal over the range 4-450 K are reported. The analysis of the transmission spectra takes into account self-absorption effects in the emission profile and in the absorption, as well as line-broadening due to source granularity. At 296 K, $\frac{{e}^{2}\mathrm{Qq}}{h}=\ensuremath{-}9.73(40)$ MHz, in good agreement with a time-differential-perturbed-angular-correlation measurement. The splitting decreases by 25% over the temperature range studied. The experimental electric-field gradient is compared with calculations of the lattice and conduction-electron contributions. Results from augmented-plane-wave calculations of the electronic contribution in which the effects of lattice vibrations were explicitly considered are briefly reported, and the axial ratio dependences of the two contributions are discussed.

Field induced magnetic form factor of γ-Ce

In the present experiment polarized neutron scattering techniques have been used to assess the spatial distribution and temperature dependence of the magnetization induced in a single crystal of γ-Ce by a magnetic field of 60 kG. The measured form factor is found to be in agreement with the free ion 4f magnetic form factor of Ce3+. Comparison of the localized susceptibility, obtained in the present experiment, with bulk susceptibility measurements indicate that there is a rather large positive conduction electron contribution to the static susceptibility of γ-Ce. The localized moment is found to follow, between 300 and 115 K, a Curie-Weiss law with a paramagnetic Curie temperature of (−44±2.5) K. The measurements indicate that at approximately 100 K some ions of the γ-Ce crystal undergo a change in their electronic configuration.

Temperature dependence of electric field gradient and Knight shift in Mg

The hyperfine properties of Mg such as Knight shift and electric field gradients (EFG) have been calculated from first principles as a function of temperature. The conduction-electron wave functions were obtained in the orthogonalized-plane-wave model by taking into account both the thermal expansion of the lattice and the effect of thermal vibrations of the ions on the crystal - field potential. The decrease in the ${p}_{x}\ensuremath{-}{p}_{y}$-like and ${p}_{z}$-like electrons with temperature leads to a decrease in the conduction-electron contribution to EFG. The lattice contribution is found to be rather insensitive to temperature and the conduction-electron part is largely responsible for the observed ${T}^{\frac{3}{2}}$ temperature dependence of the EFG. The decrease in the $p$ content of the wave function with temperature is accompanied by a simultaneous increase in the $d$ content and, to a small amount, of the $s$ content of the wave function. This small rise in the $s$-like electrons makes the Knight shift less dependent upon temperature than the EFG---an observation consistent with experimental data.

Theory of nuclear quadrupole interaction in hexagonal close packed metals—cadmium and zinc

The conduction-electron contribution to the electric field gradient has been calculated for zinc and cadmium. From a study of the conduction and core electron distributions relative to each other, it is concluded that different components of the field gradient due to the conduction electrons, such as from the local and plane-wave components of the conduction-electron densities, should be shielded differently. A value of about ..gamma../sub infinity/is estimated for the field gradient due to the plane-wave component of the density. Using the calculated field gradients in the two metals, the quadrupole moments of /sup 67/Zn and /sup 111/Cd are obtained as 0.50 and 0.76 b, the latter being in good agreement with the value derived earlier from ionic crystal measurements. The effect of the larger antishielding factor on the earlier calculated field gradients in beryllium and magnesium is discussed and it is concluded that the earlier good agreement between theory and experiment for these metals is not significantly affected. Finally, the bearing of the results of the present work on the empirical correlation obtained recently between the conduction-electron and lattice contributions is discussed.

Optical Properties of Cu and Ag in the Intermediate Region between Pure Drude and Interband Absorption

AbstractThe complex dielectric constant ϵ of thick Cu and Ag films on fused quartz substrates is determined at 300 and 1.5 K by a modified v. Fragstein‐Kampermann method in the spectral region between 0.41 and 0.71 m̈n. The relaxation frequency of the conduction electrons is determined and the contributions of electron‐phonon, electron‐surface, electron‐lattice, and electron‐electron scattering are separated. No further absorption mechanisms have to he taken into consideration to explain the experimental results. By decomposing ϵ into interband and conduction electron contributions, the temperature dependence of the interband transition edge of Cu is evaluated. The edge is shifted from 2.18 eV (1.5 K) to 2.15 eV (300 K). There are indications of indirect transitions, but a quantitative determination is not possible.

Fluctuation spectra in weakly coupled Van Vleck paramagnets: Some theoretical and experimental aspects

The ESR relaxation of impurities in rare-earth Van Vleck paramagnets with weak coupling is investigated both theoretically and experimentally. It is shown that the impurity relaxation is appreciably dependent on the fluctuation spectra of the host rare-earth ions. The line shapes of the fluctuation spectra are calculated using the method of moments for cases of interest. The role of the host exchange interaction and that of the rare-earth crystalline-field splitting in the relaxation process are demonstrated. The theory is applied to interpret the ESR linewidth behavior of Gd in the metallic Van Vleck paramagnets Pr${\mathrm{In}}_{3}$ (reported for the first time) as well as the linewidth behavior of Gd in the Van Vleck pnictides $\mathrm{Pr}X$ ($X=\mathrm{P},\mathrm{As},\mathrm{Sb},\mathrm{Bi}$) (reported previously and in the present work). The host exchange interaction in the pnictides was extracted by fitting the theory to the experimental ESR thermal broadening. These exchange parameters are compared with those estimated by means of other techniques. The rapid relaxation phenomenon in Pr${\mathrm{In}}_{3}$:Gd is partially attributed to a relatively low-lying ${\ensuremath{\Gamma}}_{5}$ crystalline-field-splitting level in this system. The conduction-electron contribution to the relaxation rate is shown to be significant in Pr${\mathrm{In}}_{3}$:Gd and La${\mathrm{In}}_{3}$Gd. The exchange interaction between the Gd and conduction electrons was extracted in both systems.