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We have studied in-gap states in epitaxial CoFe2O4(111), which potentially acts as a perfect spin filter, grown on a Al2O3(111)/Si(111) structure by using ellipsometry, Fe L2,3-edge x-ray absorption spectroscopy (XAS), and Fe L2,3-edge resonant inelastic x-ray scattering (RIXS), and revealed the relation between the in-gap states and chemical defects due to the Fe2+ cations at the octahedral sites (Fe2+ (Oh) cations). The ellipsometry measurements showed the indirect band gap of 1.24 eV for the CoFe2O4 layer and the Fe L2,3-edge XAS confirmed the characteristic photon energy for the preferential excitation of the Fe2+ (Oh) cations. In the Fe L3-edge RIXS spectra, a band-gap excitation and an excitation whose energy is smaller than the band-gap energy (Eg = 1.24 eV) of CoF2O4, which we refer to as "below-band-gap excitation (BBGE)" hereafter, were observed. The intensity of the BBGE was strengthened at the preferential excitation energy of the Fe2+ (Oh) cations. In addition, the intensity of the BBGE was significantly increased when the thickness of the CoFe2O4 layer was decreased from 11 to 1.4 nm, which coincides with the increase in the site occupancy of the Fe2+ (Oh) cations with decreasing the thickness. These results indicate that the BBGE comes from the in-gap states of the Fe2+ (Oh) cations whose density increases near the heterointerface on the bottom Al2O3 layer. We have demonstrated that RIXS measurements and analyses in combination with ellipsometry and XAS are effective to provide an insight into in-gap states in thin-film oxide heterostructures.

An X-ray free-electron laser oscillator (XFELO) is a new type of hard X-ray source that would produce fully coherent pulses with meV bandwidth and stable intensity. The XFELO complements existing sources based on self-amplified spontaneous emission (SASE) from high-gain X-ray free-electron lasers (XFEL) that produce ultra-short pulses with broad-band chaotic spectra. This report is based on discussions of scientific opportunities enabled by an XFELO during a workshop held at SLAC on June 29 - July 1, 2016, Comment: 21 pages, 12 figures

We have probed the crystalline electric-field ground states of pure $|J = 7/2, J_z = \pm 5/2\rangle$ 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$_4$.

Orbital angular momentum (OAM) of photons is carried upon the wave front of an optical vortex and is important in physics research due to its fundamental degree of freedom. As for the interaction with materials, the optical OAM was shown to be transferred to the valence electron based on modified selection rules where a single ion is carefully aligned to the center of a vortex [Schmiegelow et al.(2016)]. We here demonstrate an elaborate way of extracting the distributed OAMs in the two-dimensional x-ray wave field at the exit-face of the specimen by a vorticity-sensitive microscopy, which detects vorticity in the reciprocal-space wave field downstream of the optical vortices. This method enables us to measure the distributed topological charges and OAMs by a single image with the wide field of view over 300 $\mu$m. Among various wavelength, the research of x-ray OAM is especially interesting since it could be closely related to the predicted OAM-induced x-ray dichroism [Veenendaal et al.(2007)]., Comment: 4 pages, 3 figures

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., Comment: 6 pages, 4 figures

We consider the optimal investment problem when the traded asset may default, causing a jump in its price. For an investor with constant absolute risk aversion, we compute indifference prices for defaultable bonds, as well as a price for dynamic protection against default. For the latter problem, our work complements Sircar & Zariphopoulou (2007), where it is implicitly assumed the investor is protected against default. We consider a factor model where the asset's instantaneous return, variance, correlation and default intensity are driven by a time-homogenous diffusion X taking values in an arbitrary region E. We identify the certainty equivalent with a semi-linear degenerate elliptic partial differential equation with quadratic growth in both function and gradient. Under a minimal integrability assumption on the market price of risk, we show the certainty equivalent is a classical solution. In particular, our results cover when X is a one-dimensional affine diffusion and when returns, variances and default intensities are also affine. Numerical examples highlight the relationship between the factor process and both the indifference price and default insurance. Lastly, we show the insurance protection price is not the default intensity under the dual optimal measure., Comment: 2 figures

We report 3D coherent diffractive imaging of Au/Pd core-shell nanoparticles with 6 nm resolution on 5-6 femtosecond timescales. We measured single-shot diffraction patterns of core-shell nanoparticles using very intense and short x-ray free electron laser pulses. By taking advantage of the curvature of the Ewald sphere and the symmetry of the nanoparticle, we reconstructed the 3D electron density of 34 core-shell structures from single-shot diffraction patterns. We determined the size of the Au core and the thickness of the Pd shell to be 65.0 +/- 1.0 nm and 4.0 +/- 0.5 nm, respectively, and identified the 3D elemental distribution inside the nanoparticles with an accuracy better than 2%. We anticipate this method can be used for quantitative 3D imaging of symmetrical nanostructures and virus particles., Comment: 14 pages, 4 figures

We performed an X-ray diffraction experiment while palladium bulk absorbed and desorbed hydrogen to investigate the behavior of the crystalline lattice during the phase transition between the $\alpha$ phase and $\beta$ phase. Fast growth of $\beta$ phase was observed around x=0.1 and x=0.45 of PdH$_x$. In addition, slight compression of the lattice at high hydrogen concentration and increase in the lattice constant and the line width of the $\alpha$ phase after a cycle of absorption and desorption of hydrogen was observed. These behavior correlated with the change in the sample length, which may infer that the change in shape was related to the phase transition., Comment: 7 pages, 7 figures

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., Comment: 5 pages, 3 figures, to be published in J. Phys. Soc. Jpn. (Letters)