Except in the very early stage of the development of X-ray photoemission spectroscopy (XPS) by Kai Siegbahn and his coworkers, the excitation sources for XPS studies have predominantly been the Al Kα and Mg Kα emission lines. The advent of synchrotron radiation sources opened up the possibility of tuning the excitation photon energy with much higher throughputs for photoemission spectroscopy, however the excitation energy range was limited to the vacuum ultra violet and soft X-ray regions. Over the past 5–6 years, bulk-sensitive hard X-ray photoemission spectroscopy using high-brilliance high-flux X-rays from third generation synchrotron radiation facilities has been developed. This article reviews the history of HXPES covering the period from Kai Siegbahn and his coworkers’ pioneering works to the present, and describes the fundamental aspects, instrumentation, applications to solid state physics, applied physics, materials science, and industrial applications of HXPES. Finally, several challenging new developments which have been conducted at SPring-8 by collaborations among several groups are introduced.

Hard X-ray photoemission spectroscopy

Magnetic properties were investigated on a single-crystalline DyB 2 C 2 compound with the tetragonal structure. Spontaneous magnetizations appear along the a- and [1 1 0]-directions below T C =15.3 K. A very small shoulder is observed only along the c -axis at T Q =24.7 K, although large λ-type anomalous specific heats are observed at T C and T Q . There exists a large magnetic anisotropy in the tetragonal basal plane which is observed in magnetization processes below T C . Neutron powder diffraction experiments reveal that magnetic reflections are observed only below T C . In the magnetically ordered phase, the Dy moments are arranged to be perpendicular to neighbors along the c -axis. The results are well interpreted by postulating that the phase between T C and T Q is an antiferroquadrupolar ordered one. DyB 2 C 2 is a novel compound with a high antiferroquadrupolar ordering temperature of 24.7 K.

Antiferroquadrupolar ordering and magnetic properties of the tetragonal DyB2C2 compound

A survey of experimental findings for Ce-Cu- and Yb-Cu-based compounds is given. Most of these compounds exhibit unusual physical properties as a consequence of various competing mechanisms such as a magnetic interaction of the Ruderman-Kittel-Kasuya-Yoshida type, Kondo interaction and crystal-field splitting. The ground-state properties are then dominated by whichever process surpasses the others. A successful description of the observed behaviour is possible in terms of the Coqblin-Schrieffer model, which is based on the Anderson Hamiltonian which is more general than the Kondo Hamiltonian.

Anomalous properties of Ce-Cu- and Yb-Cu-based compounds

Rare earth–transition metal–magnesium compounds—An overview

Publisher Summary This chapter reviews the synthesis techniques and the crystal chemistry of rare earth-transition metal-indides. It focuses on the phase relations and physical properties, and discusses the structure-property relationships. The crystal chemistry and chemical bonding of the R – T in compounds strongly depend on the composition. The rare earth metal-rich compounds exhibit typical intermetallic structures with relatively large coordination numbers, as found in close packed arrangements. The ternary rare earth metal-transition metal-indium systems contain a vast number of individual intermetallic compounds. The crystallographic data (lattice parameters) of all known indides is discussed in the chapter. The compounds are grouped with respect to the d element component. The chemical and various physical properties of rare earth-transition metal-indides are discussed in this chapter. Most of these indides are light gray in polycrystalline form. Single crystals have metallic luster. Although the rare earth metals are relatively electropositive elements similar to the alkaline earth metals, there is a significant difference in the stability of the ternary compounds.

Rare earth–transition metal–indides

Yb 3d and valence-band photoemission spectra of the first-order valence-transition compound YbInCu4 have been measured with hard x ray at an excitation energy of 5.95 keV. Abrupt changes are clearly observed in both spectra around the transition temperature T(V)=42 K, in comparison with ultraviolet and soft x-ray photoemission (VUV-PES and SX-PES) spectra. From the Yb 3d spectra, the Yb valence has been estimated to be approximately 2.90 from 220 down to 50 K and approximately 2.74 at 30-10 K. We propose that Yb 3d hard x-ray photoemission spectroscopy is a very powerful method to estimate the valence of Yb with high accuracy. On the other hand, the Yb2+ 4f-derived peaks in the valence-band spectra exhibit a remarkable enhancement below T(V). The shape of the valence-band spectra is different from those of the VUV-PES and SX-PES spectra above T(V), reflecting the In 5s and 5p contributions.

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Valence Transition of YbInCu4 Observed in Hard X-Ray Photoemission Spectra

Crystal phases and Yb valence transition in Ybx>In1−x>Cu2

The crystal structure of YbInCu 4 has been refined by the Rietveld analysis of TOF neutron powder diffraction data. This compound has a cubic AuBe 5 (C15b, F\bar43m)-type structure with the lattice constant a =7.1575(6) A, and Yb ions are completely ordered at the 4c site. X-ray powder diffraction data have also been taken as a function of temperature. The lattice constant shows an abrupt change of 0.15% at the temperature of Yb valence phase transition (about 40 K) without any change in the crystal symmetry.

Neutron and X-Ray Diffraction Studies of a Valence Fluctuating Compound YbInCu4

ESR and magnetic susceptibility measurements were made in insulating Y2BaCuO5. A uniaxial type of the ESR spectrum was observed at room temperature and was found to be due to Cu2+ ions by an intensity measurement. The g values parallel and perpendicular to the symmetry axis are 2.24±0.02 and 2.06±0.02, respectively. Susceptibility measurements showed that the Curie constant is (0.22±0.1) emuK/mol and the paramagnetic Curie temperature is (-8±1) K. The small value of the Curie constant suggests that the sample contains some oxygen deficiency.

https://iopscience.iop.org/article/10.1143/JJAP.26.L766/pdf

ESR and magnetic susceptibility in Y2BaCuO5

Valence-band electronic structure of ${\mathrm{YbInCu}}_{4}$ has been investigated by means of high-resolution photoemission spectroscopy with an excitation energy of $h\ensuremath{\nu}=800\mathrm{eV}$ With decreasing temperature from 50 to 40 K, the intensity of the ${\mathrm{Yb}}^{2+}$ $4f$ peak increases remarkably, while that of the ${\mathrm{Yb}}^{3+}$ $4f$ multiplet structures decreases due to the valence transition at 42 K The Yb valence directly derived from the photoemission spectra is $z\ensuremath{\sim}281$ at 100 K, decreases continuously to 50 K and changes sharply to $z\ensuremath{\sim}268$ at 40 K The larger valence compared with the other photoemission experiments with smaller probing depth suggests that there is a subsurface region in ${\mathrm{YbInCu}}_{4}$ We have also found that the ${\mathrm{Yb}}^{3+}$ $4f$ multiplet structures shift toward the deeper binding-energy side by $\ensuremath{\sim}50\mathrm{meV}$ with decreasing temperature from 50 to 40 K This energy shift is assumed to reflect that the energy to add the bare Yb $4f$ hole increases for the low-temperature region in accordance with the valence transition

Kenichi Kojima

Papers

Valence Transition of YbInCu4 Observed in Hard X-Ray Photoemission Spectra

Crystal phases and Yb valence transition in Ybx>In1−x>Cu2

Neutron and X-Ray Diffraction Studies of a Valence Fluctuating Compound YbInCu4

ESR and magnetic susceptibility in Y2BaCuO5

Soft-x-ray high-resolution photoemission study on the valence transitions in YbInCu 4