We have successfully observed linear dichroism in angle-resolved Yb 3+ 3d 5/2 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 Yb 3+ 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 Γ8 ground state, indicating the Γ8 ground-state symmetry for the Yb 3+ ions mixed with the Yb 2+ state.Rare-earth-based strongly correlated electron systems show various interesting phenomena such as competition between magnetism and unconventional superconductivity, charge and/or multipole ordering, and the formation of a narrow (∼meV) gap at low temperatures. Among them, YbB 12 is known as a Kondo semiconductor [1][2][3][4][5], which has been recently recognized as a candidate for topological insulators [6], as intensively discussed for another Kondo semiconductor, SmB 6 [7-9]. The mean valence of YbB 12 has been estimated as ∼2.9+ by bulksensitive 3d core-level hard X-ray photoemission (HAX-PES) spectroscopy [10]. To discuss the mechanisms of the gap opening at low temperatures [5,[11][12][13][14][15][16] Generally, it is difficult to experimentally determine the 4f ground-state symmetry. Inelastic neutron scattering is useful, but other excitations such as phonon excitations often hamper the observation of magnetic 4f −4f excitations. Linear dichroism (LD) in 3d−to−4f soft Xray absorption spectroscopy (XAS) for single crystals is powerful owing to the dipole selection rules, as reported for Ce compounds [19][20][21][22]. However, it is not applicable to compounds in cubic symmetry, in which there is no anisotropic axis relative to the electric field of the incident light. On the other hand, the selection rules work also in the photoemission process while the excited electron energy is much higher than that in the absorption. Furthermore, there is another controllable measurement parameter in photoemission, called the "photoelectron detection direction" relative to the single-crystalline axis, in addition to the polarization direction of the excitation light. Indeed, by using LD in 3d core-level HAXPES spectra, the Yb 3+ 4f ground state has been determined for tetragonal YbCu 2 Si 2 and YbRh 2 Si 2 [23]. LD in the core-level HAXPES for cubic Yb compounds is also expected to be observed, as discussed below.In the case of Yb 3+ ions in tetragonal symmetry, the eightfold degenerate J = 7/2 state splits into four dou-

We show that the strongly correlated 4f -orbital symmetry of the ground state is revealed by linear dichroism in core-level photoemission spectra as we have discovered for YbRh2Si2 and YbCu2Si2. Theoretical analysis tells us that the linear dichroism reflects the anisotropic charge distributions resulting from crystalline electric field. We have successfully determined the ground-state 4f symmetry for both compounds from the polarization-dependent angle-resolved core-level spectra at a low temperature well below the first excitation energy. The excited-state symmetry is also probed by temperature dependence of the linear dichroism where the high measuring temperatures are of the order of the crystal-field-splitting energies.

An angle-resolved linearly polarized hard X-ray photoemission spectroscopy (HAXPES) system has been developed to study the ground-state symmetry of strongly correlated materials. The linear polarization of the incoming X-ray beam is switched by a transmission-type phase retarder composed of two diamond (100) crystals. The best value of the degree of linear polarization was found to be À0.96, containing a vertical polarization component of 98%. A newly developed low-temperature two-axis manipulator enables easy polar and azimuthal rotations to select the detection direction of photoelectrons. The lowest temperature achieved was 9 K, offering the chance to access the ground state even for strongly correlated electron systems in cubic symmetry. A co-axial sample monitoring system with long-working-distance microscope enables the same region on the sample surface to be measured before and after rotation. Combining this sample monitoring system with a micro-focused X-ray beam by means of an ellipsoidal Kirkpatrick-Baez mirror (25 mm  25 mm FWHM), polarized valence-band HAXPES has been performed on NiO for voltage application as resistive random access memory to demonstrate the micropositioning technique and polarization switching.

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