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Register a new userEffect of Anisotropic Hybridization in YbAlB$_4$ Probed by Linear Dichroism in Core-Level Hard X-ray Photoemission Spectroscopy
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We have probed the crystalline electric-field ground states of pure $|J = 7/2, J_z = pm 5/2rangle$ as well as the anisotropic $c$-$f$ hybridization in both valence fluctuating systems $alpha$- and $beta$-YbAlB$_4$ by linear polarization dependence of angle-resolved core level photoemission spectroscopy. Interestingly, the small but distinct difference between abyb was found in the polar angle dependence of linear dichroism, indicating the difference in the anisotropy of $c$-$f$ hybridization which may be essential to a heavy Fermi liquid state in $alpha$-YbAlB$_4$ and a quantum critical state in $beta$-YbAlB$_4$.
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Electronic properties of V$_2$OPO$_4$ have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and subsequent theoretical calculations. The V 1$s$ and 2$p$ HAXPES spectra are consistent with the charge ordering of V$^{2+}$ and V$^{3+}$. The binding energy difference between the V$^{2+}$ and V$^{3+}$ components is unexpectedly large indicating large bonding-antibonding splitting between them in the final states of core level photoemission. The V 1$s$ HAXPES spectrum exhibits a charge transfer satellite which can be analyzed by configuration interaction calculations on a V$_2$O$_9$ cluster. The V 3$d$ spectral weight near the Fermi level is assigned to the 3$d$ $t_{2g}$ orbitals of the V$^{2+}$ site. The broad V 3$d$ spectral distribution is consistent with the strong hybridization between V$^{2+}$ and V$^{3+}$ in the ground state. The core level and valence band HAXPES results indicate substantial charge transfer from the V$^{2+}$ site to the V$^{3+}$ site.7 figure
We have successfully observed linear dichroism in angle-resolved Yb3+ 3d5/2 core-level photoemission spectra for YbB12 in cubic symmetry. Its anisotropic 4f charge distribution due to the crystal-field splitting is responsible for the linear dichroism, which has been verified by spectral simulations using ionic calculations with the full multiplet theory for a single-site Yb3+ ion in cubic symmetry. The observed linear dichroism as well as the polarization-dependent spectra in two different photoelectron directions for YbB12 are quantitatively reproduced by theoretical analysis for the Gamma_8 ground state, indicating the Gamma_8 ground-state symmetry for the Yb3+ ions mixed with the Yb2+ state.
We revisit the formulations and simulations of angular distributions in polarization-dependent core-level photoemission spectra of strongly correlated electron systems, in order to explain the recently discovered linear dichroism (LD) in the core-level photoemission of 4f-based rare-earth compounds. Owing to the selection rules for the optical process of core-level excitations, the LD originating from the anisotropic outer localized charge distributions determined by the ground-state orbital symmetry can be observed. Our simulations show that core d-level excitations are essential for the LD in localized ions having a cubic symmetry, which is absent in the p-orbital excitations.
We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type CuIr$_2$S$_4$ using ultra-violet photoemission spectroscopy (UPS). The Ir$^{3+}$/Ir$^{4+}$ charge-ordered gap appears below the metal-insulator transition temperature. However, in the insulating phase, in-gap spectral features with $softgap$ are observed in UPS just below the Fermi level ($E_F$), corresponding to the variable range hopping transport observed in resistivity. The spectral weight at $E_F$ is not increased by light illumination, indicating that the Ir$^{4+}$-Ir$^{4+}$ dimer is very robust although the long-range octamer order would be destructed by the photo-excitation. Present results suggest that the Ir$^{4+}$-Ir$^{4+}$ bipolaronic hopping and disorder effects are responsible for the conductivity of CuIr$_2$S$_4$.
We report an angle-resolved photoemission (ARPES) study of $beta$-YbAlB$_4$, which is known to harbor unconventional quantum criticality (QC) without any tuning. We directly observe a quasiparticle peak (QP), emerging from hybridization, characterized by a binding energy and an onset of coherence both at about 4 meV. This value conforms with a previously observed reduced Kondo scale at about 40 K. Consistency with an earlier study of carriers in $beta$-YbAlB$_4$ via the Hall effect strongly suggests that this QP is responsible for the QC in $beta$-YbAlB$_4$. A comparison with the sister polymorph $alpha$-YbAlB$_4$, which is not quantum critical at ambient pressure, further supports this result. Indeed, within the limitation of our instrumental resolution, our ARPES measurements do not show tangible sign of hybridization in this locally isomorphic system, while the conduction band we observe is essentially the same as in $beta$-YbAlB$_4$. We therefore claim that we identified by ARPES the carriers responsible for the QC in $beta$-YbAlB$_4$. The observed dispersion and the underlying hybridization of this QP are discussed in the context of existing theoretical models.
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