Jürgen Röhler

LIII-absorption on valence fluctuating materials

Abstract A detailed description of a deconvolution technique for valence determination from mixed-valent L III -spectra is given. It is shown that from L III -spectra valence numbers can be read separately from hybridization and non-linear volume effects. Such separation cannot be obtained from valence measurements by volume or Mossbauer isomer shift.

The valence of Eu under high pressure

Abstract The valence of elementary Eu was investigated through LIII X-ray absorption spectroscopy up to 340 kbar at 300 K. Eu transforms into a mixed valent state already at p

Anomalies in the CuO structure of EuBa2Cu3O7−δ at 110 K and 250 K

Abstract The temperature dependence (77 − 300 K) of the Cu O bonds in EuBa 2 Cu 3 O 7−δ has been investigated through the extended x-ray absorption fine structures (EXAFS) of Cu. The absorption spectra were recorded from a compact sample in the fluorescence mode simultaneously with the electrical resistance. Increasing the temperature the effective Cu O Debye-Waller factors Δσ 2 (T) exhibit an anomalous kink at 110 K and a strong singularity at 253 K. The anomalies are interpreted through a characteristic temperature dependence of the oxygen defect structure.

The valence of cerium in CeCu2Si2 under high pressure

Abstract We have measured the fractional valence ( υ = 3 + ν ) of Ce in CeCu 2 Si 2 (nonsuperconducting at ambient pressure) at pressures up to 200 kbar (300 K) through L III X-ray absorption. υ increases with increasing pressure from 3.06 (0 kbar) to 3.38 (150 kbar) and then saturates. At 38 kbar we have observed a first order valence transition by 0.04 from υ = 3.10 to 3.14. A superconducting reference sample at ambient pressure exhibits υ = 3.11.

Evidence for charge redistribution below Tc in YBa2Cu3O7-δ

Abstract We have measured the temperature dependence of the K absorption near-edge fine structure of Cu in polycrystalline YBa 2 Cu 3 O 7−δ between 47 and 300 K. Temperature dependent differences in the fine structure are resolved within 0.2%. The most pronounced temperature dependence is observed at ≅ 11 eV, and is attributed to intensity variations of the out-of-plane transitions Cu(2) 1s→4p z π and Cu(1) 1s→4p x π . At T c their average intensity exhibits a disctince minimum. Below T c the intensity of the transition Cu(1) 1s→4p x π is found to increase by ≅ + 1%, indicating an increase of electron density at the Cu(1) site. We attribution the experimental findings to a redistribution of charge between chains and planes below the onset of superconductivity. The associated structural distortion is interpreted to increase the symmetry of the Cu(1)O 2 (1)O 2 (4) rhombs in the doping block of the crystal. No evidence is found for a bimodal axial lattice instability across the superconducting transition.

Valence instability in magnetically ordered Eu(Pd1−xAux)2Si2

Abstract 151 Eu Mossbauer effect (300, 77, 4.2 K), X-ray diffraction (300 K), L III -X-ray absorption (300 K) and magnetic susceptibility measurements have been performed on the series Eu(Pd 1−x Au x ) 2 Si 2 (0⩽ x ⩽ 1). The variation of both the isomer shift and the magnetic hyperfine field with volume is found to be unusually large. Although mixed valence behaviour is deduced from L III -X-ray absorption, the magnetic susceptibility suggests divalent 4f 7 states.

Local structure study of the underdoped-overdoped transition in YBa2Cu3Ox (x=6.806–6.984) by yttrium EXAFS

Abstract We have measured the x-ray absorption-fine-structure (EXAFS) at the Y- K edge of YBa 2 Cu 3 O x for x =6.806, 6.826, 6.886, 6.947, 6.968, 6.984 at T =20–300 K. Optimum doping is found to be a notable point in the x - T phase diagram, also concerning the c -axis related displacements of the planar O2, 3 and Cu2 atoms. The O2,3-Cu2 spacing along c is found to be largest near x opt . In the underdoped regime doping increases the O2,3-Cu2 spacing by displacing the Cu2 layer from the O2,3 layer. In the overdoped regime doping reduces the O2,3-Cu2 spacing by displacing the O2,3 layer towards the Cu2 layer. Possible implications of these c -axis related displacements for the electronic structure of doped CuO 2 layers are discussed.

LIII X-ray absorption in lanthanides at high pressures☆

Abstract We discuss the feasibility of “high-pressure tuning” in elemental lanthanides through X-ray absorption. The potential of the K, L and M core-level excitations at high pressures is addressed. Some of the experimental problems associated with high-pressure X-ray absorption are out-lined. We discuss in detail the volume and valence effects on the LIII absorption lines of the elements. In particular we studied the effects of valence transitions and of the semiconductor-metal transition in ytterbium, and the saturation of valence in cerium and europium at low volumes.

Chapter 71 X-ray absorption and emission spectra

Publisher Summary The chapter discusses X-ray absorption and emission spectra. This chapter is dedicated to a review of modern X-ray absorption spectroscopy of lanthanides, in particular of mixed valent materials, using synchrotron radiation. X-ray emission in lanthanide solid stare research is discussed. The energies of X-ray emission lines and absorption edges in the past have been extensively used for the determination of the atomic energy levels. Since the energies of the X-ray emission lines provide accurate data for the determination of the atomic levels only on a relative scale, in most cases the energies of the well-defined Lm edges are chosen as the fundamental reference. The spectra from X-ray absorption turn out to be less complicated than those from X-ray photoemission (XPS). X-ray absorption spectra exhibit a more transparent signature of the atom in the ground state of the solid. Deep core level X-ray absorption in the lanthanides is understood in the so-called one electron picture (as far as the transition is concerned). Satellite structures due to many-electron and final-state effects are found to be insignificant. The chapter discusses experimental aspects of X-ray absorption spectroscopy.

The underdoped-overdoped transition in YBa2Cu3Ox

Abstract Oxygen doping in metallic YBa 2 Cu 3 O x induces quadrupolar “α-ortho”, and breathing “β-ortho” deformations of the CuO 2 planes. β-ortho deformations favour hybridizations of the Cu3 d x 2 − y 2 − O 2 p x , y , with the pdπ bands relaxing the confin ement of the carriers in the overdoped regime, x ≥ 6.95.