3d Holes Research Articles

Superconductivity and magnetism in transition-element-substituted YBa2Cu3O7 compounds

Superconducting properties of YBa2Cu3O7 have been investigated by a systematic substitution of 3d transition elements for 10% of the Cu ions. Early transition elements Ti, V, and Cr affect the superconducting temperature of YBa2Cu3O7 only slightly. Local magnetic moment formation is observed in Mn, Fe, Co, and Ni substituted compounds. Tc strongly correlates with the size of the paramagnetic moments, possibly due to scattering from the magnetic sites. Zn displays a phenomenal ability to suppress Tc because of the elimination of the 3d holes. The Zn compositional dependence of Tc indicates that the superconductivity completely disappears at 13–14 at. % of Zn. It is suggested that Zn preferentially substitutes the Cu(2) sites.

Significance of plane versus chain sites in high-temperature oxide superconductors

One of the outstanding questions concerning the high-temperature superconductor YBa2Cu3O7 is the relative importance of the CuO2 planes (Cu(2) site) and the CuO chains (Cu(1) site). Many theories1,2 have stressed the importance of the 3d holes at the Cu sites, which provide an antiferromagnetic correlated background necessary for the coupling of the superconducting electrons. We have substituted Zn(3d104s2) and Ga(3d104s24p1) in the Cu(2) and Cu(1) sites, respectively, while maintaining the oxygen stoichiometry at 7. In the valence states of 2 + and 3 + respectively, the 3d bands of both elements are completely full, without the complication of a magnetic moment. Here we discuss the structural changes that accompany Zn and Ga substitution, as deduced from X-ray and neutron diffraction, and correlate these changes with the superconducting properties. We show that the integrity of the planes is much more important than that of the chains in sustaining high-Tc superconductivity.

Mott‐Insulation, High Frequencies, and the High Transition Temperatures of the New Ceramic Superconductors

AbstractA recent proposal of Fröhlich concerning the origin of the high transition temperatures, Tc, which characterize the newly discovered ceramic superconductors, is examined for the case of materials based on La2CuO4. Whilst one of the conditions specified by Fröhlich — namely the existence of a narrow energy band based on incomplete inner shells of certain metal atoms is indeed realized in terms of a quasi two‐dimensional lower Hubbard band of Cu d‐orbitals — it is noted that certain ionic (Cu–O) vibrational breathing modes exist whose frequency is already high enough (≈ 10 × 1013 Hz) to account, via BCS theory, for the high Tc's, without having to invoke the higher frequency excitonic‐like modes envisaged by Fröhlich. Evidence is adduced that the carriers responsible for the superconductivity are essentially Cu 3d holes introduced into the lower Hubbard band by divalent dopants, and/or deviations from La stoichiometry; their density, n, is thus rather low, which could well account for the low critical currents, Jc, found experimentally. It is noted that n is most probably less than the degree of doping, x, on account of the possible contributions of oxygen vacancies to the maintenance of charge neutrality; this, in turn, imposes a lower limit on x if n is to be > 0.

High-temperature superconductivity in the presence of O 2p-Cu 3d holes: A spectroscopic study.

We report core-level spectroscopic measurements of several cuprate perovskites comparing those which exhibit superconductivity above 90 K to others which do not. Both x-ray photoemission and x-ray absorption results show that in the former Cu is more oxidized, as in CuO (nominally ${\mathrm{Cu}}^{2+}$), than in the latter. Trivalent Cu is clearly excluded. The O $1s$ core level is at higher binding energy in the superconductors, implying the presence of O $2p$ holes. The rare-earth valence in Ce- and Pr-based perovskites is more than 3+, apparently related to their failure to superconduct.

Photoemission satellites and electronic structure of Fe2O3.

The electronic structure of \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ with the high-spin ${d}^{5}$ ground-state configuration has been studied by ultraviolet photoemission spectroscopy with use of synchrotron radiation as well as by x-ray photoemission and Auger-electron spectroscopy. The results are interpreted in terms of the configuration-interaction theory based on a ${\mathrm{FeO}}_{6}$-cluster model. The main lines of the valence-band photoemission spectra are identified with Fe?d sup 5 ndash---ligand-hole final states produced by ligand-to-3d charge-transfer screening of 3d holes (3${d}^{4}$ states), whereas the satellite at higher binding energies is assigned to unscreened (or poorly screened) 3${d}^{4}$ final states. The Fe 3d versus O 2p partial density of states and symmetry characteristics of 3d-derived peaks are found to be quite different from assignments based on ligand-field theory or band theory. These results indicate that ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ cannot be considered as a Mott-Hubbard insulator in its original sense but is classified as a charge-transfer-type insulator according to a theory of Zaanen, Sawatzky, and Allen. A possibility is suggested that the lowest unoccupied state is not Fe?d ndash---like but is the bottom of the Fe 4s band. The large exchange energy of the high-spin 3${d}^{5}$ configuration is shown to greatly stabilize the localized 3d states relative to the itinerant state.

Spin-polarized Auger electron spectroscopy on Gd.

The spin polarization of all prominent Auger lines of Gd is investigated experimentally. High spin polarization signals are found due to the fully polarized 4f shell. In the low energetic Auger lines initiating from a 4d hole normal and resonant excitations are identified. In particular, the resonant core hole only transition ${N}_{45}$${N}_{67}$${N}_{67}$ splits in multiplets where the spin polarization gives evidence of spin-flip processes. A reduced polarization of Auger lines involving 3d holes indicates the shortcoming of the LS coupling scheme. We point out that the Gd Auger spin polarization is a key for monitoring this breakdown of spin conservation.

Spin polarized inverse photoemission studies of surface magnetism and electronic structure

Abstract Spin polarized inverse photoelectron spectroscopy (SPIPES) is shown to be a powerful new technique to study surface and near-surface electronic structure and magnetism. The process, the information obtained, and the apparatus required in a spin polarized inverse photoemission measurement are compared to the complementary spin polarized photoemission measurement. In SPIPES spectra of a clean Ni(110) surface, transitions to the minority spin 3d holes that give rise to ferromagnetism in Ni can be directly observed, as can sp states which are spin slit as an indirect consequence (via s-d hybridization) of the exchange splitting of the d bands. The adsorption of oxygen or carbon monoxide on Ni(110) causes a dramatic decrease in the number of minority 3d holes, that is, in the magnetic moment of the Ni atoms. The data allow us to severely limit the possible models of how chemisorption induces changes in surface magnetism. Other SPIPES studies, such as the temperature dependent behavior of empty bands in ferromagnetic Fe, and future directions and applications of SPIPES will be reviewed.

Chemisorption-induced changes in surface magnetism and electronic structure: Oxygen on Ni(110).

The effect of oxygen chemisorption on the Ni minority-spin 3d holes---and thus the Ni magnetic moment---is measured by spin-polarized inverse photoemission. A dramatic reduction of the minority-spin 3d holes is observed, indicating a strong involvement of these states in the chemisorptive bond. This reduction can be explained by a Ni 3d--O 2p interaction which redistributes the density of states; no indication of a reduced exchange splitting is found. Majority-spin sp states are shown to be unchanged at coverages below the onset of nucleation and oxide formation.

Photoemission satellites “on” and “off” resonance in transition metals and their oxides

Resonant photoemission has attracted considerable interest since 1977, when an enhancement of the valence band satellite in Ni at photon energies close to the 3p core level binding energy was first observed. Interpretations for the presence of this satellite, and similar satellites found in other transition metals, have included autoionization, M 2,3VV Auger processes, interband transitions, shake-up excitations and various combinations of these. We explain the energies of these photoemission satellites using a simple model, corrected for the final state effect of hole-hole Coulomb interaction and relaxation by using U eff determined empirically from our Auger results. “Off” resonance the satellites observed are interpreted in terms of a shake-up process and at resonance as merely the M 2,3VV Auger feature occurring at the energy of the shake-up satellite. We show that the recent conclusions of Chandesris et al. [Phys. Rev. B27 (1983) 2630], which state that the correlation energy between two 3d holes is constant for Cr through to Ni, are incorrect and that the second satellite, observed at resonance in Ni is merely the manifestation of a peak observed in the M 2,3VV Auger spectra. Further we conclude that the 3 eV increase in the satellite to main photoemission peak separation in going from Ni to NiO, is unlikely to be due to an increase in U eff as tentatively assigned by Thuler et al. [Phys. Rev. B27 (1983) 2082]. We suggest that for “off” resonance it is a shake-up process involving empty 4sp states at the bottom of the conduction band, whilst at resonance it is due simply to changes in the energy of the M 2,3VV Auger transition.

Hole localization in ionized and bound excitation states of monovalent copper halides

Abstract The results of self-consistent field calculations on the ground states, ionized stated and some excited states of the clusters [CuCl4]3−, [CuBr4]3−, [Cu4Cl]3+ and [Cu2Cl]+ are presented and discussed. The Madelung field of the rest of the crystal has been incorporated in the calculations. High and low point-group symmetries are considered, showing the importance of relaxing symmetry constraints. The calculated ionization energies show good overall agreement with experiment; however, the calculated crystal field splitting of the 3d levels is much smaller than the observed one. The calculated 3d–4s-like excitation energies are very sensitive to both the external field and the basis sets used, and more studies are required before firm conclusions can be drawn. The results indicate that at the level of the single-configuration SCF approximation, open-shell orbitals describing 3d holes are strongly localized on copper. Their ‘covalency’ is small, so that a seemingly necessary contribution to an understanding of the spectral properties is missing at this level of approximation, which includes one-electron band theory.

Solid state phase transitions in linear transition metal polymers: Variations of the unit cell dimension in bisglyoximato nickel (II)

The band structure of bisglyoximato nickel(II) has been studied as a function of the unit cell dimension ( c) in a staggered and an eclipsed chain conformation by means of crystal orbital (CO) calculations based on the tight-binding approximation. The crystal Hamiltonian is a semiempirical Hartree-Fock (HF) self-consistent field (SCF) operator developed in the framework of the INDO formalism. c has been modified in the interval between 3.0 and 4.2 Å, numbers that are typical for metallomacrocycles of the 3d series. The dispersion curves for metal 3d and ligand bands, the density of states distributions, charge reorganizations and the nature of intercell interactions are studied as a function of the separation between the bisglyoximato moieties. The width of the ligand bands is continuously reduced with increasing c values. This simple ε( k) pattern is not found for the “Ni 3d bands” of the low-dimensional material. The weak NiNi overlap in comparison to metal-ligand interactions of the intracell and intercell type leads to a complicated relation between the energetic width of the Ni 3d states and the unit cell dimension c. The character and the shape of the filled Ni 3d bands are analyzed in detail and are compared with the results of band structure calculations on one-dimensional materials with 5d centers (e.g. Krogmann's salt) that show broad dispersions for the metal bands. Physical phenomena in materials with injected (3d) holes, that must be expected as a result of the narrow metal bands, are shortly discussed. The interaction energies between atoms in neighbouring unit cells are decomposed into Coulomb, resonance and exchange contributions. The electrostatic potentials exceed the covalent resonance interactions as well as the exchange coupling. The interaction energy between the Ni centers is highly repulsive. General rules are formulated that give some insight into modification of the electronic structure of organometallic polymers in the 3d series as a function of the lattice spacing.

Liquid ESCA measurements and ECP calculations on the 3d spectrum of I 3−

ESCA measurements and effective core potential (ECP) calculations have been performed on the 3d hole states of the I 3 − ion. The ESCA spectra, obtained from liquid solutions show substantial shake-up structure as well as a non-stoichiometric intensity ratio between the 3d lines for the two types of iodine in the I 3 − ion. A mechanism is proposed involving a preferential excitation of shake-up transitions in the ionization of the non-central iodine atoms in the I 3 − ion. The ECP calculations confirm the proposed explanation and predict line energies and intensities in good agreement with the experimental I 3d spectrum.

Symmetry breaking and ionization from the 3d and 4s shells in copper clusters

SCF calculations indicate that the 3d hole in Cu + 2 is localized. while the 4s hole prefers to be delocalized. Extrapolation to copper clusters indicates that the d band is overlapped by the 5 band for clusters as small as Cu 5. Overlap of the s and d bands does not seem to be an appropriate criterion for convergence of properties of clusters to bulk properties.

XPS core level and valence band spectra of LaH 3

XPS analysis of LaH 3 shows that the conduction band of the La metal disappears and that a new hydrogen induced band is formed 5.8 eV below E F. The La 3d core levels of LaH 3 are split into two peaks with about 1 3 of the intensity being in the low binding energy peak. We propose to describe the observed 3d structure by the screening of the 3d hole by means of a charge transfer from the new hydrogen band to the empty 4f level, the latter being pulled down below the hydrogen band. This would be a shake down process as in La metal.

On the Hartree-Fock and Xα descriptions of small copper cluster electronic structures

A proper description of photoemission requires localized 3d holes. This resolves the reported discrepancy between Hartree-Fock and Xα densities of states for metal clusters (2-9 atoms) by LCAO Xα calculations are reported. Although quantitatively different from Hartree-Fock values, they show similar behaviour as a function of cluster size.

Investigation of the electronic characteristics of Ni3 (Fe, Ti) and Ni3 (Mn, Ti) alloys

The anomalous Hall (Rs) and the Nerst-Ettingshausen (QS) constants are measured for hardened Ni3 (Fe, Ti) and Ni3 (Mn, Ti) alloys. Scattering by phonons is detected, con tributing primarily to RS as compared to scattering by magnetic inhomogeneities. Based on the positive signs of Rg and QS, it is concluded that 3d holes wi th antiparallel spin make the main contribution to the conductivity. A model is proposed for the electronic structure of the alloys studied.

The Auger and autoionisation spectra of clean and oxidised samarium and erbium

The decay of 3d and 4d holes in clean and oxidised Er and Sm is studied by electron spectroscopy at high energy resolution. Comparison of the X-ray excited and electron excited Auger spectrum of Er gives evidence that electron excited 4d10 4fn to 4d9 4fn+1 transitions decay partially by direct recombination. Oxidation leads to Auger peaks shifted by several eV relative to the clean metals, but direct recombination peaks are shifted less than Auger peaks.

Investigation of electronic configurations of Ni3(Fe, Ti) and Ni3(Mn, Ti) alloys

The ordinary Hall constant Ro of quenched Ni3(Fe, Ti) and Ni3(Mn, Ti) alloys was measured. The number of 4s and 3d electrons per atom was calculated on the basis of a four-band model. At low Ti concentrations in Ni3(Fe, Ti) the contribution of 3d holes to Ro is negligible, but at high concentrations it is significant. The contribution of 3d holes to Ro for Ni3(Mn, Ti) is considerable, and for Ni3Mn and Ni3(Mn +9.8% Ti) it is dominant. It is concluded that the changes in band energy of the bond between Ni and Ti in the considered systems are of a different nature.

The electronic structure of small nickel atom clusters

The ground state electronic structure of small nickel atom clusters (Nin, n=1–6) has been calculated using the ab initio effective core potential self-consistent field (SCF) method in a Gaussian expansion basis. The electronic configuration of the nickel atoms in the clusters is found to be very close to 3d94s1. The ground state electronic configurations for Nin generally have n unpaired 3d electrons in molecular orbitals (MO’s) spanning the same irreducible representations as the 4s atomic orbitals while the n 4s electrons fill their MO’s in accord with a simple three-dimensional Hückel model with overlap. Exceptions to this description are found in the cases of linear systems where the 3d holes prefer δ over σ symmetry and in octahedral Ni6 where a different preferred set of 3d holes is obtained. The SCF ground state wave functions correspond roughly to a model in which the 3d electrons can be viewed as weakly interacting localized 3d9 units. The clusters are bound together primarily by the 4s electrons with the 4p orbital contribution increasing in importance with cluster size and dimensionality. The binding energy per nickel atom generally increases as the size of the cluster increases, although at six atoms this quantity has not yet converged with cluster size. The density of states diagram for the occupied one electron energy levels in Ni6 is found to be very different from the corresponding types of diagrams obtained in the muffin tin (MT)–Xα method for small nickel atom clusters. This difference is examined in detail, with consideration given to the effects of relaxation energy and to the different orbital level filling criteria used in the two methods.

THE EFFECT OF HYDROGEN ON THE MAGNETIC STRUCTURE OF INVAR ALLOYS

The effect of absorbed hydrogen on the magnetic structure of f.c.c. invar alloys, Fee5Ni35, Fe6GNi31Mn3 and Fe69Ni2eC3, was studied by means of Fe Mossbauer effect. As the electrolytic hydrogenation preceeded, the characteristic absorption spectrum with low internal field components, which is believed to be an origin of the invar effect of these alloys, was drastically changed and hydride phases appeared in large fractions. The spectral analysis showed that the hydrides with the same f.c. c. structure as the matrix phase lost the weak field components and only strong ferromagnetic components remained. This effect of hydrogen on the magnetic structure of invar alloys was interpreted in terms of 3d hole filling with electrons from solute hydrogen in the Stoner's band model. A reverse effect found by a previous study on a non-invar Fe Ni alloy that the internal field of the hydride became about 15% smaller than that of the matrix phase was explained by the same model.