Calculation of the Knight shift in palladium

The direct and the exchange core polarization (ECP) contributions of the conduction electrons to the Knight shift of palladium are evaluated. To obtain the wave functions for the conduction electrons and the partial densities of states at the Fermi surface a KKR energy band calculation was performed. The contributions of the core electrons to the Knight shift were determined by using the moment perturbation method (MP). Electron-electron interactions are taken into account by individual enhancement factors for thed ands electrons. The agreement between the theoretical results and the available experimental data is quite satisfactory.     

Theory of knight shift due to indirect nuclear hyperfine interactions

We derive a theory of Knight shift (K) in solids including the effects of periodic potential, spin-orbit interaction, magnetic hyperfine interactions and indirect nuclear hyperfine interaction. We use a temperature dependent Green's function technique to evaluate the thermodynamic potential which is then used to obtain a general expresion for the Knight shift. Our formula for K is expressed as a sum of contributions due to conduction electrons and localized electrons of either d- or f-type: K_cond and K_loc. While K_cond is the same as our previous expression for K derived in the absence of localized magnetic moments, K_loc is a new contribution and is due to the hybridization of conduction and localized electron magnetic moments. We also briefly discuss the many-body effects on the different contributions to K. Finally, the importance of the present theory in possible applications to metals, alloys and compounds containing transition and rare-earth elements, and magnetic semiconductors is discussed.     

Knight shift and paramagnetic susceptibility study in U3P4

The Knight shift and paramagnetic susceptibility have been measured on one sample of U₃P₄ in a rather wide temperature region. Observed deviations from the Curie-Weiss law are ascribed to the conduction electrons. The s-f coupling constantΓ=?2·4 eV has been obtained from the experimental data in the approximation of homogeneously polarized conduction electrons.     

Nuclear spin relaxation in disordered conductors

The relaxation timesT ₁ andT ₂ for nuclear spins interacting with the conduction electrons of disordered metals are calculated. An explicite expression for Warren's relaxation enhancement in terms of electron spectra is obtained, showing in particular that the proportionality ofT ₁ and the conductivity is a universal feature of high resistivity conductors. A formula for an inhomogeneous line width contribution due to disorder induced static Knight shift fluctuations is found. Two dimensional electron spin diffusion is shown to imply various logarithmic line width corrections.     

Magnetic properties of CuRh alloys

By means of rapid quenching techniques single phased samples of Cu_xRh_1?x(0?x?1) were obtained. For these alloys the Knight shift of ⁶³Cu and ¹⁰³Rh has been determined employing pulsed NMR at low temperatures, furthermore the magnetic susceptibility was measured for temperatures between 4.2 and 300 K. While the Knight shift of ¹⁰³Rh is dominated by s-electron contributions in spite of a high density of d-states at the Fermi level, for the susceptibility, however, the d-electron contributions prevail. In addition the susceptibility shows a pronounced maximum at about 10 at.% Cu. Using the extrapolated Knight shift of copper (x→0) we estimate a net copper hyperfine field of — 15 T in close agreement with the corresponding values for CuPd and CuPt.     

NMR and magnetic susceptibility of CeCu6

NMR and magnetic susceptibility of CeCu₆ intermetallic compound were investigated. The most important contributions to the magnetic susceptibility are the Curie-Weiss term, expressing the paramagnetism of the localized f-electrons, and a temperature independent term for which all the contributions were estimated. The phenomenological exchange constantJ _sf between 4f — electron spins and conduction — electron spins is derived to be ?0·012 eV. The Knight shift and Curie temperature are discussed in terms of the Ruderman-Kittel-Kasuya-Yosida theory and in the Rocher's virtual bound states model. The s — f exchange integralΓ and the Fermi wavevectork _F are derived to be — 0·8 eV and 1·32 Å^?1, respectively.     

Pseudopotential in metals and alloys

In this communication, the pseudopotential investigation of, the various properties of non-transition metals and alloys, is discussed. Various one parametric model pseudopotentials, derived from well known spherical functions s_l(x), are employed in the calculations. Many recurrence relations of the s_l(x) function have been described. The effects of exchange and correlation on conduction electrons are also considered separately by using different dielectric screenings in various properties. The ion-ion interaction, force constants, phonon spectrum, temperature coefficient of Knight shift and electronic transport coefficients of certain metals and alloys are evaluated. The results are compared with available experimental values. Generally good agreement is achieved. The screening charge density of certain metals in low and high density region are also determined.     

Knight shift in superconducting vanadium

The Knight shift in metallic vanadium in the normal and superconducting states has been measured. In contrast to the previously obtained results, this shift appears to change after the transition to the superconducting state. The behavior of the Knight shift in the superconducting state in vanadium samples doped with iron impurities has been found to be different from that in the “pure” samples. As a possible explanation of the effect, the broadening of the peak of the density of states near the Fermi level due to the scattering of conduction electrons on the iron impurities and the earlier predicted impurity polarization shift of the NMR line are discussed.     

NMR and magnetic susceptibility of the intermetallic pseudo-binary compounds Ce1−xRxAl3 with R=Gd,Tb and Er

The NMR and magnetic susceptibility of the intermetallic pseudo-binary compounds Ce_1?xGd_xAl₃ (x=0·01; 0·03; 0·05; 0·07), Ce_0·95Tb_0·05Al₃ and Ce_0·975Er_0·025Al₃ were investigated. The susceptibility is given by the sum of a temperature independent term and a Curie-Weiss one. The last one results from the contribution of the localized free-ion magnetic moments of all the rare-earth ions which are also responsible for the strong temperature dependent Knight shifts of the NMR lines of ²⁷Al nuclei via the exchange polarization of the conduction electrons. The ²⁷Al NMR spectra exhibit besides the line due to Al sites surrounded in the first coordination sphere as in CeAl₃ a second peak with a lower Knight shift due to Al nuclei positioned in the vicinity of a Gd, Tb or Er ion.     

Magnetic susceptibility and nuclear magnetic resonance of vanadium sulfides

Measurements of the magnetic susceptibility in the temperature range 4–1000°K are reported for the vanadium sulfides VS, V₃S₄-V₂S₃ and V₅S₈. At higher temperatures the susceptibility of all compounds approaches a constant, positive value, indicating a Pauli-paramagnetic contribution of itinerant electrons. At lower temperatures an increase of the susceptibility is observed. This effect is particularly pronounced in V₅S₈, and is taken as evidence for the presence of localized magnetic moments.Nuclear magnetic resonance spectra of V₅S₈ show two lines, one with a temperature-independent Knight shift, and one with a positive Knight shift which increases strongly at lower temperatures. The two lines are attributed to vanadium atoms with itinerant d electrons, and to vanadium atoms with localized d electrons, respectively. This interpretation is consistent with the magnetic susceptibility data. The difference in properties between two types of vanadium atoms in V₅S₈ can be understood by considering the crystal structure.     

Magnetic susceptibilities and knight shifts of the intermetallic compounds NdCu4 and NdCu5

The magnetic susceptibility and Knight shift of the intermetallic compounds NdCu₄ and NdCu₅ were investigated over the temperature range 80–850 K. The most important contributions to the magnetic susceptibility are the Curie-Weiss term, expressing the paramagnetism of the localized 4?-electrons, and a temperature independent term, both of which have been determined. The phenomenological quantity J_s? between the 4?-electron and conduction electron spins was found to be ?2.46.10^?3eV for NdCu₄ and 1.35.10^?3 eV for NdCu₅. A reversal in the sign of the s-? coupling for CeCu₅ was noted.     

Electronic structure,spin polarization and high critical fields in Chevrel compounds

Results are presented of an extensive theoretical study of the origin of high field superconductivity and/or magnetism in a number of Chevrel phase ternary compounds, MMo₆X₈ (with M=Sn, Eu, Gd and X=S and/or Se) based on self-consistent linear muffin-tin orbital (LMTO) energy band calculations using the local density approach (Hedin et al. exchange correlation) for the paramagnetic structures and local spin density formalism (Gunnarsson and Lundqvist) for the ferromagnetic structures. All electrons and all 15 atoms/cell are included with the core electrons (including the 4f's) recalculated in each iteration in a fully relativistic representation and the conduction electrons treated semirelativistically (all relativistic terms except spin-orbit). Superconductivity is found to be due to the high Mo d-band density of states (DOS) at E_F resulting from the unusual large charge transfer of Mo electrons to the chalcogen sites. There is also a large charge transfer from the metal site to the cluster (≈2 electrons in Sn and Eu) giving essentially no occupied conduction bands, for example, at the Eu site and a divalent ion isomer shift in very good agreement with the experiments of Dunlap et al. The conduction-electron DOS at the Eu site is found to be reduced by an order of magnitude from its metallic state value - in close agreement with their spin - lattice relaxation rate measurements. This low conduction-electron DOS yields very weak coupling of the 4f electrons to the conduction electrons and only a very weak Ruderman-Kittel-Kasuya-Yosida magnetic interaction showing why all the Chevrel rare-earth compounds - except Ce and Eu - are superconducting despite their having large local magnetic moments. The unusually high upper critical fields, H_c2, in these materials is found to be due to the unusully flat energy bands near F_F. The ferromagnetic (spin polarized) results for the Eu- and Gd-compounds show a net small but positive magnetic moment on the metal site and a small but negative induced spin magnetic moment on the Mo site in the Eu compound. Fermi-contact contributions to the hyperfine field are calculated and found to be in good agreement with the Eu Mössbauer results and the negative NMR Knights shift results of Fradin et al. These results demonstrate theoretically for the first time the validity of the Fischer et al. and Fradin et al. conclusion that the Jaccarino-Peter mechanism is responsible for the large increase in the H_c2 when large concentrations of Eu magnetic impurities are substituted in SnMo₆S₈. Finally, calculated Stoner factors for the paramegnetic phase and spin magnetization densities for the ferromagnetic phase are used to discuss qualitatively the origin of the different behavior observed for GdMo₆S₈ and EuMo₆S₈.     

The band structure of lithium with a screened non-local crystal potential

Wave functions and energy eigenvalues for Li have been calculated with the MAPW-method, using a one-particle potential constructed from the Coulomb potentials of the nuclei and the electrons and a non-local screened exchange potential. Using different interpolation schemes starting from the results along the principal axes, the Fermi energy, the Fermi surface, its extremal cross sections, different masses and the contribution of the conduction electrons to the Knight shift are determined.     

Knight shifts and magnetic susceptibilities of the intermetallic compounds CeCu4 and CeCu5

The magnetic susceptibility and Knight shift of the compounds CeCu₄ and CeCu₅ have been measured over the temperature ranges 80–800 and 140–400 K, respectively. The most important contributions to the magnetic susceptibility are the Curie-Weiss term, expressing the paramagnetism of the localized ?-electrons, and a temperature independent term, which have both been determined. The phenomenological exchange integral F_s? between the 4?-electron spins and conduction electron spins was found to be ?10.43× 10^?3 eV for CeCu₄ and 3.9 × 10^?3 eV for CeCu₅. A reversal in the sign of the s?? coupling for CeCu₅ is noted.     

Cluster growth of Cu on graphite: XPS,Auger and electron energy loss studies

Auger, XPS and EELS techniques have been used to investigate the core levels, the d-valence band and the electronic transitions of different UHV deposited Cu clusters on graphite. The decreasing of the Cu particle size produces core levels and valence band shifts towards higher binding energies. Lower extra-atomic screening of the conduction electrons near the excited atom and shift of the d-band towards the isolated atom levels are claimed to explain these effects. The EELS results suggest that, for smallest clusters, no structural change but only a lattice parameter contraction of the f.c.c. cage occur.     

μ+ Knight shift in Be

A first principles calculation of the Knight shift of a ⁺ in beryllium was carried out using a cellular multiple scattering technique (Keller 1972, 1977). The main conclusion is that the ⁺ induces the segregation of one state (of each spin) from the conduction band to lower energies, a molecular orbital of the ⁺ and its nearest neighbours. This state is occupied (with two antiparallel spin electrons) but it has less than 35% of ⁺ character. The screening is then completed with the conduction electrons properly. The observed Knight shift is then the sum of a diamagnetic shielding and a typical enhanced spin susceptibility term of opposite sign and smaller value.     

Studies of impure cadmium oxide semiconductors using NMR,EPR and conductivity measurements

Cadmium oxide semiconductors were prepared by heating CdO which also contained hydroxide and carbonate species formed on storage. Reaction of the impurities on heating, and high temperature annealing, produced an unusually wide range of electrical conductivities, EPR spectra and NMR spectra. Changes in EPR spectra were correlated with the ¹¹³Cd shift, which was shown to be proportional to the square root of the relaxation rate, and also proportional to the line width. This allowed the Knight shift contribution to the resonance position to be separated from the paramagnetic shielding (due to covalency). The latter was found to be 234 ± 6 ppm relative to Cd(H₂O)²⁺₆. It could then be shown that the Korringa relationship (for degenerate electrons) was satisfied, with T₁TK² = 3.9 ± 0.2 × 10⁻⁶ sK. The variations of the asymmetric line shape with Knight shift were examined in the light of various models for impurity semiconductors.     

NMR of 69Ga, 71Ga,and 51V in V3−xFex Ga alloys

We have studied at room temperature the intensities and the Knight shifts of the nuclear magnetic resonances of ⁶⁹Ga, ⁷¹Ga, and ⁵¹V in V_3?xFe_xGa for Fe concentrations ranging from x = 0.04 to x = 0.6. The results interpreted in terms of a redistribution of the conduction electrons and a paramagnetism due to the Fe impurities.     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.     

Diamagnetism and muon Knight shift in semiconducting Bi1−x Sb x single crystals

The Knight shift of positive muons (⁺) implanted in semiconducting single crystals of Bi_1–x Sb _x (x=0.085, 0.14, 0.19) has been measured as a function of temperature and sample orientation. The Knight shift (KS) is generally negative and is found to scale with the negative total macroscopic susceptibility; the scaling is independent ofx but dependent on the orientation. One concludes that only the valence and conduction bands near theL-symmetry point in the Brillouin zone contribute to the ⁺ Knight shift, most likely by the contact hyperfine interaction. Furthermore the valence bands and the conduction bands seem to be associated with the same hyperfine coupling constants. These conclusions are quite unexpected and call for a new theoretical approach.