Angle-resolved photoemission and LEED from rare-earth metals

Angle-resolved photoemission and LEED studies of single-crystal surfaces of the rare-earth metals are still relatively few in number. The reasons for this are outlined and comparisons are made between the procedures used to obtain clean and well-ordered surfaces suitable for study. Valence-band photoemission spectra are compared with surface electronic structure and photocurrent calculations in order to identify surface and bulk features. Core-level spectroscopy indicates that the 5p levels of the lanthanides show intensity variations with emission angle that are not explicable in terms of a simple core-level shift. Although the 4p levels of Y have some band character, detailed comparison of the 4p photoemission lineshape with those produced by photocurrent calculations enables the surface core-level shift to be determined. Quantitative LEED studies of close-packed (0001) surfaces show essentially a bulk-terminated structure for Sc, a structural reconstruction of the divalent surface of Sm and a magnetic reconstruction of ferromagnetic Gd. The more open (11 0) and (10 0) surfaces of all the rare-earth metals studied undergo extensive reconstructions into a close-packed structure almost identical to that of a (0001) surface. Angle-resolved photoemission and LEED have yet to distinguish between these structures, indicating a reconstruction depth of at least five atomic layers.     

Effective temperature scattering matrix and kinematic mechanism of cooper instability in antiferromagnetic rare-earth intermetallides

An effective temperature-dependent scattering matrix for a localized subsystem is obtained in the periodic Anderson model by summing itinerant variables in the diagrammatic series of perturbation theory. The effective interaction that occurs in this case can lead to an antiferromagnetic phase, a superconducting phase, and the phase of coexisting superconductivity and antiferromagnetism observed in heavy-fermion intermetallic compounds.     

Resonant photoemission from SmS(100)

A strong, sharp resonance enhancement of 4f photoemission has been observed on SmS(100) surfaces for photon energies in the region of the 4d-4f transitions at about 126 eV. The discrete final state reached via the excitation hν+4d¹⁰4f⁶→4d⁹4f⁷ autoionizes primarely via a super Coster-Kronig transition of the type 4d⁹4f⁷→4d¹⁰4f⁵ + unbound electron. Other decay channels, e.g, Sm 5p emission, as well as a surface induced binding energy shift in the Sm³⁺ final state are identified and discussed.     

Resonant photoemission at the L3 absorption edge of Mn and Ti and the electronic structure of 1T-Mn0.2TiSe2

Resonant valence band x-ray photoelectron spectra (ResPES) excited near the 2p(3/2) core level energies, 2p x-ray photoelectron spectra (XPS) and L(3,2) x-ray absorption spectra (XAS) of Ti and Mn in single crystals of 1T-Mn(0.2)TiSe(2) were studied for the first time. The ionic-covalent character of the bonds formed by the Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with the results of atomic multiplet calculations of Ti and Mn L(3,2) XAS, it is found that the Ti atoms are in the ionic state of 4 + and the Mn atoms are in the state of 2 +. In ResPES of Mn(0.2)TiSe(2) excited near the Ti 2p(3/2) and Mn 2p(3/2) absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to theoretical calculations of E(k) the Ti 3d states are localized in the vicinity of the Γ point and the Mn 3d states are localized along the direction K-Γ-M in the Brillouin zone of the crystal.     

Electron transport in CrO2 and Mn x Cr1−x O2

The resistivity-temperature curve of chromium dioxide, crystallizing in the rutile structure, is rather flat with a smooth maximum in the vicinity of the Curie point and, for higher temperature, an exponential decrease corresponding to a gap of 0.1 to 0.3 eV depending on samples and authors. The similarity of the Seebeck coefficient-temperature curves in chromium oxide and manganese telluride is pointed out. The oxide powder was prepared by thermal decomposition of the chromic anhydride, and had a CrO₂ content higher than 95%. An infrared transmission spectrum was obtained by mixing 2% of oxide to 98% of potassium bromide and by pressing pellets under vacuum. The reliability of this technique is discussed.Mixed oxides Mn _x Cr_1–x O₂ were prepared and their electrical and optical properties investigated forx varying from 0 to 0.5. No discontinuity could be observed in the resistivity-temperature curves from chromium oxide to chromium-manganese oxide. Both electrical and optical properties point to an energy gap of 0.3 eV, as for the rutile phase of manganese dioxide. The influence of these data on the interpretation of the electron transport phenomena in chromium dioxide is discussed and an alternative to Goodenough's energy levels scheme suggested. The disappearance of the resistivity maximum in mixed oxides forx>0.2 is explained by the magnetic scattering of broad band carriers at any temperature by the disorder of chromium and manganese spins.     

Resonant photoemission spectroscopy study of electronic structures of LiMn2O4

The valence-band resonant photoemission spectra (RPES) of LiMn₂O₄ have been measured throughout the Mn3p absorption edge. Based on the RPES data, the contribution of Mn3d states to the valence band of LiMn₂O₄ has been described and, consequently, the detailed hybridization between O2p and Mn3d states in the valence-band was determined.     

Resonant photoemission in highly localized versus weakly localized solids

We report on comparative study of angle-resolved photoelectron spectra of Ni metal and CeGd at theM _III andN _IV,V soft-x-ray absorption threshold, respectively. OnM _III resonance, the valence-band photoemission (PE) intensity of Ni follows a cos²-behavior, while the intensity of the 6-eV satellite is independent of the angle of emission relative to the E-vector of the incoming beam. On the other hand, the 4f Ce PE signal behaves cos² onN _IV,V resonance. This clearly shows that the resonant enhancement of the 6-eV PE satellite of Ni is mainly caused by an incoherent Auger decay and cannot be described as a resonant PE process.     

Mossbauer study of ferrite systems Co x Mn1−x Fe2O4 and Ni x Mn1−x Fe2O4

Mössbauer spectra of Co _x Mn_1?x Fe₂O₄ and Ni _x Mn_1?x Fe₂O₄ ferrites withx values ranging from 0·1 to 0·8 in steps of 0·1 have been recorded at room temperature. All spectra exhibit well-defined Zeeman hyperfine patterns. It has been observed that hyperfine field at Fe³⁺ nucleus increases more rapidly by nickel substitution than by cobalt substitution. This has been explained in terms of exchange interactions and cation distribution in the spinels. Hyperfine fields, isomer shifts and quadrupole splittings have been determined.     

Anomalous properties and coexistence of antiferromagnetism and superconductivity near a quantum critical point in rare-earth intermetallides

Mechanisms of the appearance of anomalous properties experimentally observed at the transition through the quantum critical point in rare-earth intermetallides have been studied. Quantum phase transitions are induced by the external pressure and are manifested as the destruction of the long-range antiferromagnetic order at zero temperature. The suppression of the long-range order is accompanied by an increase in the area of the Fermi surface, and the effective electron mass is strongly renormalized near the quantum critical point. It has been shown that such a renormalization is due to the reconstruction of the quasiparticle band, which is responsible for the formation of heavy fermions. It has been established that these features hold when the coexistence phase of antiferromagnetism and superconductivity is implemented near the quantum critical point.     

Circular dichroism in 4f photoemission from magnetically ordered rare-earth materials

4f Photoemission (PE) spectra from magnetically ordered rare-earth materials using circularly polarized X-rays exhibit strong Magnetic Circular Dichroism (MCD), i.e., the intensities of the individual multiplet lines depend on the relative orientation of sample magnetization and photon spin. On the example of the wellresolved Tb 4f PE multiplet, it is shown that in relevant cases 4f PE lines are essentially only observed for one magnetization direction, either parallel or antiparallel to the photon spin. These large MCD effects in 4f PE open new perspectives in the analysis of surface and thin-film magnetism and provide a sensor for the degree of circular polarization of soft X-rays over a wide photon-energy range. To demonstrate the potential of MCD in 4f PE as a magnetometer, we studied Gd(0001) and Tb(0001), where the magnetization of the topmost atomic (0001) layer can be easily separated from the bulk magnetization via the surface core-level shift. In multicomponent magnetic thin films containing different rare-earth elements 4f PE allows to monitor magnetization in an element-specilic way, e.g., in case of the hetero-magnetic interface 1 ML En/Gd(0001).     

Resonant photoemission from 4f systems,especially Ce

Resonant photoemission has provided important new means to study the properties of the 4f electron states in the light lanthanides. These investigations have revealed unexpectedly complex 4f spectral shapes. This has led to new insight into the problem of the 4f systems. Some recent results for Ce are reviewed and it is shown how these disprove the previously accepted promotional model for Ce. The results are briefly discussed in terms off-hybridization and possible band formation. Some expected future experimental developments are also discussed.     

Resonant photoemission study of Ti interaction with GaN surface

Ti/GaN interface formation on GaN(0 0 0 1)-(1 × 1) surface has been investigated by means of resonant photoelectron spectroscopy (for photon energies near to Ti 3p → 3d excitation). The sets of photoelectron energy distribution curves were recorded for in situ prepared clean GaN surface and as a function of Ti coverage followed by post-deposition annealing. Manifestations of chemical reactions at the Ti/GaN interface were revealed in the valence band spectra as well as in the Ga 3d core level peak—the discerned contribution of Ti 3d states to the valence band turned out to be similar to that reported in the literature for titanium nitride. The interaction between Ti and N was further enhanced by post-deposition annealing. The study was complemented with SIMS and AFM measurements.     

Resonant photoemission at the 3s threshold of Ni

A resonant enhancement of valence band photoemission features in Ni near the 3s threshold is presented. The emission behavior with photon energy of the Ni-3d band is characteristic of a Fano-type resonance. In addition to the main 3d-band emission and 6eV binding energy satellite, a weak satellite is observed at 7.2eV below the Fermi level with photon energies in the vicinity of the 3s threshold.     

Resonant photoemission at the 3p-threshold of Ge and GeO2

Resonant photoemission final states are reported for the elemental semiconductor Ge and its oxide GeO₂ at the Ge3p photothresholds. The line positions and the multiplet structure of the resonating peaks identifies them as an atomic-like 3d⁸ configuration. Differences in the intensity profiles and the exact location of the resonance reveal the importance of a change in initial state electron configuration for this basically atomic effect. The effective Coulomb interaction between the two 3d-holes is $\text{U}{}_{\mathrm{eff}}\text{(}{}^1\text{G) = 12.8, 13.2 ± .2}\text{eV}$ for Ge and GeO₂ respectively. A review of all elemental results then allows the restriction of this resonance to elements between Fe and Rb. The line positions of the resonant final states of the remaining elements As, Se, Br, Kr can then approximately be predicted.     

Resonant photoemission spectroscopy of Cu(InGa)Se2 materials for solar cells

The electron structure of CuIn_{1 ? x} Ga _x Se₂ single crystals is determined via resonant photoemis-sion and the main regularities of its transformation upon varying concentration x from 0 to 1 are established. The dependence of the shape of valence band spectra on the photon energy is studied. Integral photoemission intensities are shown to be determined by atomic photoionization cross sections. Processes of the direct and two-step creation of photoelectrons accompanying photoemission and the participation of internal states in the spectra of electrons from valence bands are studied. Two-hole final states in photoemission are obtained upon threshold excitation of the Cu 2p level. The strong interaction of holes leads to the multiplet splitting of these states. Partial densities of the components’ states are determined using the energy dependence of atomic photoionization cross sections.     

Phase transitions in the ferromagnetic alloys Ni2+x Mn1−x Ga

The phase diagram of the cubic ferromagnetic shape-memory alloy Ni_2+x Mn_1−x Ga is constructed theoretically for the case when the Curie temperature is close to the structural transition temperature. This diagram agrees well with the experimental data obtained from resistance and magnetic susceptibility measurements. It is shown that the transition from the paramagnetic cubic phase to the ferromagnetic tetragonal phase can be second-order or first-order. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 212–216 (10 February 1998)     

Resonant photoemission study ofK-derived valence-band states in K X C60

The electronic structure of K-doped C₆₀ was investigated by photoemission (PE) and X-ray absorption near-edge structure (XANES) studies at the C-1s and K-2p thresholds. In addition, information on the local K-derived partial density of states in superconducting K₃C₆₀ was obtained by resonant PE at the K-2p _1/2 threshold. The experimental observations support a complete charge transfer from K to C₆₀ and we clearly observe a finite density of states atE _F . From resonant PE, occupied states with K-p, d character could be identified in the binding-energy region from 1.5 to 8 eV below, but not directly at the Fermi level. This partial-density-of-states structure agrees well with the results of our band-structure calculations based on the local-density approximation.