In $5d$ Ir oxides with an interplay of spin-orbit coupling and electron correlations, we have tailored a spin-orbital magnetic insulator out of a semimetal ${\mathrm{SrIrO}}_3$ by tuning the structure through superlattices [$({\mathrm{SrIrO}}_3{)}_m$, ${\mathrm{SrTiO}}_3$] ($m=1,2,3,4$, and $\infty$). We observed the systematic decrease of the magnetic ordering temperature and the resistivity as a function of $m$. The transition from the semimetal to the insulator is found to be closely linked to the appearance of magnetism at $m\simeq 3$. Long range magnetic ordering was realized even in the $m=1$ single layer superlattice, implying that the design and realization of novel electronic phases is feasible at the level of a single atomic layer in complex Ir oxides.

We demonstrate a method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy-sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu L2,3 and Nd M4,5 absorption edges of the high-Tc cuprate La1.475Nd0.4Sr0.125CuO4 by measuring the O K$\alpha${} PFY and comparing the total electron yield, total fluorescence yield, and IPFY spectra.

The fundamental electronic structure of the widely used battery material ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ still remains a mystery. Soft x-ray absorption spectroscopy of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ reveals that holes with strong O $2p$ character play an essential role in the electronic conductivity of the ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ mixed valence ${\mathrm{CoO}}_2$ layer. The oxygen holes are bound to the ${\mathrm{Co}}^{4+}$ sites and the Li-ion vacancy, suggesting that the Li-ion flow can be stabilized by oxygen hole back flow. Such an oxygen hole state of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ is unique among the various oxide-based battery materials and is one of the key ingredients to improving their electronic and Li-ion conductivities.

Transition-metal substitution in Fe pnictides leading to superconductivity is usually interpreted in terms of carrier doping to the system. We report on a density functional calculation of the local substitute electron density and demonstrate that substitutions like Co and Ni for Fe do not carrier dope but rather are isovalent to Fe. We find that the extra d electrons for Co and Ni are almost totally located within the muffin-tin sphere of the substituted site. We suggest that Co and Ni act more like random scatterers scrambling momentum space and washing out parts of the Fermi surface.

Resonant soft x-ray scattering with photon energies near the $\text{O}K$ and the $\text{Cu}{L}_3$ edges was used to study the charge ordering in the system ${\text{La}}_{1.8-x}{\text{Eu}}_{0.2}{\text{Sr}}_x{\text{CuO}}_4$ as a function of temperature for $x=0.125$ and 0.15. From the superstructure diffraction intensities, a charge ordering with a doping-dependent wave vector is derived which is in this system well below the transition temperature of the low-temperature tetragonal phase but well above the onset of spin ordering. This indicates that structural changes and magnetic interactions are not in general the driving force for the formation of stripelike phases in two-dimensional-doped cuprates. Analysis of the lineshape of the scattered intensity as a function of photon energy yields evidence for a high hole concentration in the stripes.