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We report the direct evidence of valence fluctuation in the thermoelectric material Yb3Si5 by using synchrotron X-ray photoemission spectroscopy. The Yb 3d core level photoemission spectrum of a Yb3Si5 single crystal shows clear separation of the Yb2+ peak and Yb3+ multiplet peaks. From the spectral weight of each component, the mean valence of Yb ions is estimated to be +2.67 at 20 K. Furthermore, a Kondo resonance peak just below the Fermi energy is also observed by high-resolution soft X-ray photoemission spectroscopy, suggesting the valence fluctuating state in Yb3Si5.

We have probed the crystalline electric-field ground states of pure $|J=7/2,{J}_{z}=\ifmmode\pm\else\textpm\fi{}5/2⟩$ as well as the anisotropic $c\text{\ensuremath{-}}f$ hybridization in both valence fluctuating systems $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-${\mathrm{YbAlB}}_{4}$ by linear polarization dependence of angle-resolved core level photoemission spectroscopy. Interestingly, the small but distinct difference between $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-${\mathrm{YbAlB}}_{4}$ was found in the polar angle dependence of linear dichroism, indicating the difference in the anisotropy of $c\text{\ensuremath{-}}f$ hybridization, which may be a key to understanding a heavy Fermi liquid state in $\ensuremath{\alpha}$-${\mathrm{YbAlB}}_{4}$ and a quantum critical state in $\ensuremath{\beta}$-${\mathrm{YbAlB}}_{4}$.

In this paper, we show the linearly polarized hard X-ray core-level photoemission spectroscopy of valence fluctuating system β-YbAlB4. Interestingly, we observed subtle but distinct linear dichroisms in Yb2+ 3d5/2 core-level spectra at several angles of β-YbAlB4. Basically linear dichroism in Yb2+ 3d5/2 core-level spectrum is not expected in the ionic picture because Yb2+ 4f orbital is fully occupied. These contradiction indicates the breakdown of ionic picture in β-YbAlB4.

We report the core-level photoemission study of cubic PrBe13, PrIr2Zn20 and PrB6 using linearly polarized hard X-ray. We have successfully observed linear dichroism (LD) in Pr3+ 3d core-level photoemission spectra reflecting the 4f ground-state symmetry for all the compounds. The observed LDs and polarization-dependent spectra are reproduced by ionic-model calculations including the full multiplet theory and the local crystalline electric field (CEF) splitting. We have confirmed that LDs in Pr3+ 3d core-level photoemission spectra are able to determine unambiguously the Pr3+ 4f ground-state symmetry.

We have probed the crystalline electric-field ground states of pure |J=7/2,J_{z}=±5/2⟩ as well as the anisotropic c-f hybridization in both valence fluctuating systems α- and β-YbAlB_{4} by linear polarization dependence of angle-resolved core level photoemission spectroscopy. Interestingly, the small but distinct difference between α- and β-YbAlB_{4} was found in the polar angle dependence of linear dichroism, indicating the difference in the anisotropy of c-f hybridization, which may be a key to understanding a heavy Fermi liquid state in α-YbAlB_{4} and a quantum critical state in β-YbAlB_{4}.

We present a detailed study on the $4f$ ground-state symmetry of the pressure-induced superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Ge}}_{2}$ probed by soft x-ray absorption and hard x-ray photoemission spectroscopy. The revised Ce $4f$ ground states are determined as $|{\mathrm{\ensuremath{\Gamma}}}_{7}\ensuremath{\rangle}=\sqrt{0.45}|{J}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{5}{2}\ensuremath{\rangle}\ensuremath{-}\sqrt{0.55}|\ensuremath{\mp}\frac{3}{2}\ensuremath{\rangle}$ with $\mathrm{\ensuremath{\Sigma}}$-type in-plane rotational symmetry. This gives an in-plane magnetic moment consistent with the antiferromagnetic moment as reported in neutron measurements. Since the in-plane symmetry is the same as that for the superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, we propose that the charge distribution along the $c$ axis plays an essential role in driving the system into a superconducting phase.

We report experimentally observed linear dichroism in angle-resolved core-level photoemission spectra of PrIr2Zn20 and PrB6 in cubic symmetry. The different anisotropic 4f charge distributions between the compounds due to the crystalline-electric-field splitting are responsible for the difference in the linear dichroism, which has been verified by spectral simulations with the full multiplet theory for a single-site Pr3+ ion in cubic symmetry. The observed linear dichroism and polarization-dependent spectra in two different photoelectron directions for PrIr2Zn20 are reproduced by theoretical analysis for the Gamma_3 ground state, whereas those of the Pr 3d and 4d core levels indicate the Gamma_5 ground state for PrB6., 6 pages, 4 figures