Hybridization Of Conduction Band Research Articles

Nature of electron correlation and hybridization in NixCu1−xMnSb Heusler alloys

The electronic structure of Heusler alloys having mixed magnetic phases, comprising of vicinal anti-ferromagnetic and ferromagnetic orders, is of great significance. We present the results of an electronic structure study on NixCu1−xMnSb Heusler alloys, using Mn-2p core-level photoemission spectroscopy. Room temperature data in the paramagnetic phase reveal a non-monotonic variation of both electron correlation strength and conduction-band hybridization such that the former enhances while the latter weakens for compositions showing a mixed phase relative to compositions at the phase boundaries to the ordered phases. The results suggest a possible electronic driving force for settling mixed-magnetic phases.

The Yb2Al1−xMgxSi2 series from aspin fluctuation (x = 0) to a magnetically ordered ground state(x = 1)

The structural and magnetic properties ofYb2Al1−xMgxSi2 (x = 0, 0.5 and 1), crystallizing in the tetragonalMo2FeB2-type structure, arereported in this work. Yb2AlSi2 exhibits a Pauli paramagnetic ground state arising due to spin/valence fluctuations inducedby a significant Yb 4f conduction band hybridization. High-field magnetization(up to 120 kOe) indicates a nearly temperature-independent susceptibility of8.6 × 10−3 emu/Yb mol below 10 K. On the other hand, Yb ions inYb2MgSi2 order antiferromagnetically at a relatively high temperatureTN of 9.5 K. The intermediate composition alloyYb2Al0.5Mg0.5Si2 is a Kondo lattice,antiferromagnet with TN = 5.5 K. The coefficient of the linear term of the electronic heat capacity,γ, ofYb2AlSi2 is foundto be 305 mJ mol−1 K−2, indicating a significant electronic mass enhancement due to strong electroniccorrelations. Below 12 K, an additional contribution to the heat capacity of the formT3lnT is observed. The170Yb Mössbauer spectra inthe ordered state of Yb2MgSi2 indicate a strong coupling of the 4f quadrupolar moment with the magnetic momentthrough a magneto-elastic coupling.

Tuning the4fstate occupancy of Ce in highly correlatedCeSi∕Femultilayers: An x-ray absorption spectroscopy study

Spectra of x-ray absorption were measured at the ${L}_{2,3}(2p)$ and ${M}_{4,5}(3d)$ edges of Ce in multilayers $[{\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}∕\mathrm{Fe}]xn$ ($x$ between 0.1 and 0.65), with single-phase amorphous ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ sublayers. The study uncovers the highly correlated nature of this layered system; an $\ensuremath{\alpha}$-phase-like electronic configuration of Ce is observed, which indicates considerable hybridization between the $4f$ and conduction-band states. This is at variance with single alloy films ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ which show a $\ensuremath{\gamma}$-phase-like Ce configuration already at $x=0.1$. X-ray magnetic circular dichroism measured at the ${L}_{2,3}$ edges of Ce in the multilayers reveals magnetic order on the $5d$ electrons, induced by the interaction with Fe at the interfaces. With increasing the Si content, the strength of the $4f$-conduction-band hybridization is reduced, which is reflected in a growing occupation of the $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ states. Variations of the line shape and intensity of the ${L}_{2,3}$-edge dichroism spectra are very complex. The spectra not only are related to the magnetic $5d$ polarization in the ground state but are largely controlled by the exchange interaction between the photoexcited $5d$ electron and the $4f$ electron, which generates a spin-dependent enhancement of the radial parts of the $2p$-to-$5d$ matrix element. The strength of the $4f\text{\ensuremath{-}}5d$ exchange interaction can be controlled by varying the composition of the ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ sublayers. At high Si concentration and low temperature, it induces a change in sign of the dichroic signal. We present a detailed discussion within a simple phenomenological model.

Indium-cadmium-oxide films having exceptional electrical conductivity and optical transparency: clues for optimizing transparent conductors.

Materials with high electrical conductivity and optical transparency are needed for future flat panel display, solar energy, and other opto-electronic technologies. In(x)Cd(1-x)O films having a simple cubic microstructure have been grown on amorphous glass substrates by a straightforward chemical vapor deposition process. The x = 0.05 film conductivity of 17,000 S/cm, carrier mobility of 70 cm2/Vs, and visible region optical transparency window considerably exceed the corresponding parameters for commercial indium-tin oxide. Ab initio electronic structure calculations reveal small conduction electron effective masses, a dramatic shift of the CdO band gap with doping, and a conduction band hybridization gap caused by extensive Cd 5s + In 5s mixing.

Theory of resonant spectroscopies in nickel, cerium compounds and other systems

Abstract In this paper, we discuss theoretically information about electronic structures in strongly correlated systems obtained from resonant X-ray spectroscopies. From a many-body point of view on the basis of an impurity Anderson model or a cluster model including the intra-atomic multipole interaction between electrons, we calculate spectra by exactly diagonalizing our Hamiltonian. First, for ferromagnetic Ni, through an analysis of magnetic circular dichroism in 3 d resonant photoemission spectroscopy (RPES) and resonant inverse photoemission spectroscopy (RIPES), it is shown that, in addition to 3 d 9 and 3 d 10 , the 3 d 8 configuration with weight of about 20% is contained as electron configuration of a Ni atom in Ni metal. The 3 d 8 configuration is furthermore shown to be mainly composed of 3 F and its relation to the origin of ferromagnetism is discussed. Second, for mixed valent Ce compounds, including CeNi 2 , it is shown, through an analysis of RIPES, tht the 4 f -conduction band hybridization must be reduced by the presence of a core hole. Third, for some 3 d transition metal (TM) oxides, 3 d RPES at TM ion 2 p threshold is shown to be a powerful method for estimating the magnitude of 3 d –3 d interaction strength and the charge transfer energy.

Magnetoluminescence oscillations of a doped (Al,Ga)As/GaAs single heterojunction

We have studied the photoluminescence spectra of a modulation doped (Al,Ga)As/GaAs single heterojunction in magnetic fields to 50 T at 4 K. When an external excitation is introduced to the system, photocreated electrons push the Fermi energy close to the second subband and lead to the formation of a hybridized eigenstate of the second subband (E1) exciton and the first subband (E0) two-dimensional electron state. The conduction-band hybridization enhances nonlinear behavior of magnetoexciton transitions in the optical process. At low fields, the E1 magnetoexciton transition is dominant, because the E1 hole wave-function overlap is larger than that of E0 hole. Beyond the {nu}=2 quantum Hall state, where electron screening becomes negligible, electron-hole attraction is dominant, and holes tend to move to the interface. As a consequence, the photoluminescence oscillator strength switches from the magneto-exciton transition to the lowest-Landau-level subband transition above 13 T. {copyright} {ital 1999} {ital The American Physical Society}

Screening of the effect of delocalization on disordered transition metals by including the d states in the conduction band hybridization

The critical resistivity required for the temperature coefficient of electrical resistivity to change sign, for weakly disordered transition metals, may be larger than the Mooij universal boundary. A theoretical modification of the conduction electron wavefunction that involves including the d-state hybridization for the transition metals has a screening effect on the electron-phonon interaction and enhances the effect of localization on the inelastic scattering,.

Suppression of superconductivity in PrBa2Cu3O7: 4f and conduction-band hybridization effect.

In order to understand the suppression of superconductivity in Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7} (0.6{le}{ital x}{le}1.0) and also the electronic properties of the high-{ital T}{sub {ital c}} superconductors {ital R}Ba{sub 2}Cu{sub 3}O{sub 7} ({ital R} denoting the rare earths except Ce, Pr, Pm, and Tb), we have performed self-consistent, spin-polarized electronic structure and all-electron total-energy calculations for three representative materials: {ital R}Ba{sub 2}Cu{sub 3}O{sub 7} ({ital R}=Y, Pr, and Gd). The salient results are that the band structures of the {ital R}Ba{sub 2}Cu{sub 3}O{sub 7} compounds are nearly the same as that of YBa{sub 2}Cu{sub 3}O{sub 7} when {ital R} ions assume trivalency, and extra electrons donated by the hypothetical tetravalent Pr ions would mainly fill the conduction bands in a rigid-band manner, thereby substantially reducing the important hole'' account. However, the tetravalent Pr compound is shown to be energetically less favorable than the trivalent Pr compound; finally, large spin splittings (up to 75 meV) are found in the conduction bands around the Fermi level in PrBa{sub 2}Cu{sub 3}O{sub 7} when the 4{ital f} and conduction-band hybridization effects are taken into account. In contrast, such spin splittings are ten to five times smaller in GdBa{sub 2}Cu{sub 3}O{submore » 7}. This result corroborates that the magnetic pair breaking is responsible for the suppression of superconductivity in the Pr compound.« less

Magnetic behavior of CeFe2: Effects of Ru, Rh, and Pd substitutions.

Results are presented for the effects of substitution of Ru, Rh, and Pd for Fe in ${\mathrm{CeFe}}_{2}$. For Rh and Pd the Curie temperature is lowered for up to 10% substitution without change in magnetic character. Ru substitutions produce (for 4% to 8%) the low-temperature loss of ferromagnetism observed for around 10% Co. At larger Ru substitutions no ferromagnetic regime is found but resistivity, susceptibility, and magnetization results suggest the presence of magnetic freezing which has some antiferromagnetic and some spin-glass features. These results are discussed in terms of the effects that might be produced by 4f--conduction-band hybridization, its modification by alloying and theories of the types of phase diagram that can be produced in itinerant-electron systems with competing interactions.

Photoemission study of valence fluctuation in YbCu2.

Mixed-valence ${\mathrm{YbCu}}_{2}$ has been studied by x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). UPS results reveal a bulk divalent (4${f}^{14}$\ensuremath{\rightarrow}4${f}^{13}$) peak \ensuremath{\sim}0.3 eV below the Fermi level, and a valence of 2.2 is deduced from XPS. These values are compared with the ground-state magnetic susceptibility, the specific heat, and the magnetic relaxation rate of 4f electrons by using the results of the degenerate impurity Anderson model. The 4f--5d--conduction-band hybridization is suggested to be more important than the 4f--sp--conduction-band hybridization.

Magnetic and nonmagnetic charge states in YbPd.

In YbPd, $^{170}\mathrm{Yb}$ M\ossbauer measurements show that two different Yb charge states, one magnetic and one nonmagnetic, are present in essentially equal proportions. For the magnetic fraction the saturated spontaneous magnetic moment is $1.15{\ensuremath{\mu}}_{\mathrm{B}}$, which is lower than that in any of the ${\mathrm{Yb}}^{3+}$ cubic crystal-field levels, and there is no evidence of any anisotropy in the $4f$-shell charge distribution. These results suggest the presence of $4f$-conduction-band hybridization. Both above and below the ordering temperature of the magnetic fraction, the remaining Yb possess a negligibly small magnetic susceptibility and an isomer-shift value characteristic of a charge state close to ${\mathrm{Yb}}^{2+}$.

Core-level spectra of Th compounds.

We report theoretical and experimental core-level x-ray-photoemission spectra of ${\mathrm{ThCo}}_{2}$ and ${\mathrm{ThPt}}_{3}$. Estimates are obtained for the core-hole--5f Coulomb interaction (${U}_{\mathrm{fc}}$\ensuremath{\sim}5 eV) and the strength of the 5f--conduction-band hybridization (${\ensuremath{\Delta}}_{\mathrm{av}\mathrm{\ensuremath{\sim}}0.1}$ eV). It is found that the shape of the occupied conduction-band density of states influences the shape of the core-level spectrum qualitatively.

Hydrogen-induced change in the 4f localization in CeRu2 studied with x-ray photoemission spectroscopy.

Our x-ray photoemission spectroscopy study of ${\mathrm{CeRu}}_{2}$${\mathrm{H}}_{\mathrm{\ensuremath{\approxeq}}4}$ shows the disappearance of a Ce 3d satellite peak which is generally at- tributed to an ${f}^{0}$ final state and a reduction of the ${f}^{2}$-final-state-related intensity. An interpretation in terms of an Anderson model leads to an increase in the f count ${n}_{f}$ from \ensuremath{\approxeq}0.8 to nearly unity and to a change in the 4f\char21{}conduction-band hybridization energy \ensuremath{\Delta} from 120 meV to 60\ifmmode\pm\else\textpm\fi{}20 meV. The observed binding-energy shifts of 0.7 eV (Ce 3d, 5p) and 0.4 eV (Ru 3d) are partly due to the lattice expansion as the 4f bonding contribution disappears. The valence-band spectrum shows weak hydrogen-induced emission at 5\char21{}8 eV. The observation of peaks at \ensuremath{\approxeq}1 eV and \ensuremath{\approxeq}2.6 eV in ${\mathrm{CeRu}}_{2}$ and ${\mathrm{CeRu}}_{2}$${\mathrm{H}}_{\mathrm{\ensuremath{\approxeq}}4}$, respectively, fits well into the ${f}^{1}$-${f}^{0}$ screening scheme.

Valence fluctuation and electron spectroscopies in CeN: 4f-ligand versus 4f–conduction-band hybridization

Valence mixing in CeN is attributed to two different hybridization processes, namely 4f-ligand and 4f--conduction-band hybridization. It is shown that the Ce 4f level mixes strongly with ligand N 2p orbitals and becomes nonmagnetic. This nonmagnetic state and the 4${f}^{1}$ local-moment state are nearly degenerate and hybridize weakly via 4f--5d-conduction-band hybridization, giving rise to the valence-fluctuation properties. Core-level, valence-band photoemission, and bremsstrahlung isochromat spectra are calculated with the use of a cluster model based on this picture, and good agreement with experiment is obtained.

THE EFFECT OF Nb ON PHONONS OF SUPERCONDUCTORS C-15 V2Zr AND V2(HfZr)

Inelastic neutron scattering spectra from superconducting material C-15 V2Zr, V2Zr0.95Nb0.05 and V2Hf0.5Zr0.5,V2Hf0.5Zr0.3Nb0.2 were measured by neutron time-of-flight method. It was found that the additional content of Nb influenced the superconducting transition temperature Tc and phonon frequencies of V2Zr1-xNbx in a way different from that of V2Hf0.5Zr0.5-xNbx. The Tc increases and the phonon frequencies "soften" with increasing x for the former, while the Tc slightly decreases and the phonon frequencies "harden" for the latter. The result that the phonon frequencies of V2Zr0.95Nb0.05 are softer than that of V2Zr is somewhat in contradiction with the previous results, which show that the magnetic susceptibilities decrease and the geometric mean phonon frequencies increase. The result that the phonon frequencies of V2Hf0.5Zr0.3Nb0.2 are harder than that of V2Hf0.5Zr0.5 can be considered as the result of hybridization of conduction band, which is studied for close-packed compounds by means of LMTO (linear combination of muffin-tin orbitals) method.

The cerium metallic radius in AB 2 (C15) laves compounds (A ≡ Ce)

The anomalous contraction that cerium undergoes when it is the A component in AB 2 Laves phase compounds is used to estimate its metallic radius when it is joined to a given B component. In each case, isostructural compounds of trivalent (rare earths) and tetravalent (zirconium, hafnium and thorium) A elements with the same B element are taken as the reference system. The trivalent metallic radius ( r 3 = 1.846 A ̊ ) of cerium proposed by Gschneidner and Smoluchowski was confirmed, but a tetravalent metallic radius ( r 4 = 1.688 A ̊ ) of cerium which exceeds their value by 1% was obtained. The presence of A-B contacts is related to the intermediate valency behaviour, and the valency calculated for each compound is compared with other physical properties. A distinction between localized conduction band hybridization and interconfigurational fluctuation is proposed.

SUPERCONDUCTING T<sub>c</sub> AND HETEROGENEOUS HYBRIDIZATION IN CLOSE-PACKED COMPOUNDS

The hybridization of conduction band is studied for close-packed compounds by means of LMTO method. We emphasize that the heterogeneous coordination would be a necessary condition for intensifying hybridization. In view of the hitherto basic understanding on electron-phonon coupling, a scheme of relation can be outlined as follows: high Tc => high η => significant hybridization=> significant heterogeneous coordination Various experimental results on A15, Bl and other close-packed compouds are in fair agreement with this basic principle.