Soft and hard x-ray orbital-resolved photoemission study of a strongly correlated Cd-Ce quasicrystal approximant

TL;DR

The paper investigates orbital-resolved electronic states in Cd6Ce, a Cd-based quasicrystal approximant, using soft and hard X-ray photoemission to disentangle 4f and valence-band contributions. Ce 4f electrons are found to be largely localized with weak hybridization at the Fermi level, while Cd 5p states dominate the valence band near E_F; noncrossing-approximation (NCA) calculations within a SIAM framework reveal a strong, energy-dependent c-f coupling that peaks around ~1.1 eV, indicating dominant 4f–Cd 5p hybridization away from E_F. This atypical hybridization suppresses Kondo screening and favors RKKY-type magnetic interactions, potentially explaining the low-temperature magnetism and multi-step transitions observed in Cd-based ACs. The results position Cd-based approximants as a new platform for exploring exotic magnetic phenomena beyond conventional EF-hybridization pictures in 4f systems.

Abstract

We have investigated the orbital-dependent electronic states of Cd6Ce, a prototype of strongly correlated rare-earth-based Tsai-type quasicrystals and approximants (ACs) by soft and hard x-ray photoemission spectroscopy. Our results reveal that the 4f orbitals are predominantly hybridized with the valence-band electrons far from the Fermi level EF, in sharp contrast to the hybridization with conduction electrons at EF seen for the intermetallic Ce-based compounds. This anomalous hybridization effect is likely responsible for the unresolved magnetic ground state in Cd6Ce. These findings suggest that Cd-based ACs, some of which show the multi-step magnetic transitions, provide a new platform for investigating exotic magnetic properties that cannot be understood within the conventional framework of hybridization at EF.

Figures (5)

• Figure 1: (a) Ce $M_5$-edge XAS spectrum of AC $\rm{Cd_{6}}$Ce. The labels A-H indicate the selected photon energies for the Ce 3$d$-4$f$ RPES of AC $\rm{Cd_{6}}$Ce. (b) Ce 3$d$-4$f$ RPES spectra of AC $\rm{Cd_{6}}$Ce at the photon energies A-H indicated in (a). The dashed lines indicate the peak binding energy of the $4f^0$ and $4f^1$ final states. The arrows show the hump structure of the $4f^1$ final states in the spectra at E-H.
• Figure 2: (a) On- and off-resonance (on-res., photon energy E and off-res., photon energy A indicated in Fig. \ref{['Fig1']}(a)) valence-band photoemission spectra of AC $\rm{Cd_{6}}$Ce. (b) Enlarged view of the on-res. spectrum of AC $\rm{Cd_{6}}$Ce with the energy resolution $\sim$ 130 meV. The solid lines indicate the $4f^1$$_{5/2}$ and $4f^1$$_{7/2}$ final states. The arrow shows the hump structure at around 0.7 eV.
• Figure 3: (a) Linearly polarized valence-band HAXPES spectra of AC $\rm{Cd_{6}}$Ce normalized by the photon flux. (b) Enlarged view of linearly polarized valence-band HAXPES spectra of AC $\rm{Cd_{6}}$Ce (bottom) and the photoelectron intensity ratio $I_s$/$I_p$ (top), which is calculated from the HAXPES spectra in (a). Both HAXPES spectra are normalized to the peak around 1 eV to enable a detailed comparison of the linear-polarization dependence of the spectral shape. The bold bar indicates the peak structure in both polarizations. An asterisk denotes the $I_s$/$I_p$ value of Cd 5$p$ orbital obtained with In 5$p$ calculation parameters, whereas unmarked values are calculated with the parameters for each orbital.
• Figure 4: (a) Optimized $\rho$$V^2$$(E)$ in the NCA calculation compared with the off-res. spectrum at $h\nu = 875$ eV and valence-band HAXPES spectrum in the $s$-polarization configuration for AC $\rm{Cd_{6}}$Ce. The arrow shows the shoulder structure around 0.3 eV in the off-res. spectrum. (b) Comparison of the on-res. spectrum and that obtained by the NCA calculation. (c) Enlarged view of the on-res. spectrum of AC $\rm{Cd_{6}}$Ce and the simulated spectrum from the NCA calculation.
• Figure 5: Ce 3$d$-4$f$ RPES spectra of AC $\rm{Cd_{6}}$Ce at the photon energies D-H indicated in Figure \ref{['Fig1']}(a) as a function of the photoelectron kinetic energy. The arrows show the hump structure of the $4f^1$ final states in the spectra at D-H.