Motivated by the search for design principles of rare-earth-free strong magnets, we present a study of electronic structure and magnetic properties of the ferromagnetic metal ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ within the local-density approximation (LDA) of the density-functional theory, and its combination with dynamical mean-field theory (DMFT). To compare these calculations, we measure magnetic and thermodynamic properties as well as x-ray magnetic circular dichroism and the photoemission spectrum of single-crystal ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. We find that the experimentally determined Sommerfeld coefficient is enhanced by an order of magnitude with respect to the LDA value. This enhancement can be partially explained by LDA+DMFT. In addition, the inclusion of dynamical electronic correlation effects provides the experimentally observed magnetic moments, and the spectral density is in better agreement with photoemission data. These results establish the importance of electronic correlations in this ferromagnet.

The diffusion coefficient of ${\mathrm{Li}}^{+}$ ions ($D_{\mathrm{Li}}$) in the battery material ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ has been investigated by muon-spin relaxation (${\mu }^{+}\mathrm{SR}$). Based on experiments in zero and weak longitudinal fields at temperatures up to 400 K, we determined the fluctuation rate ($\nu$) of the fields on the muons due to their interaction with the nuclear moments. Combined with susceptibility data and electrostatic potential calculations, clear ${\mathrm{Li}}^{+}$ ion diffusion was detected above $\sim 150\mathrm{K}$. The $D_{\mathrm{Li}}$ estimated from $\nu$ was in very good agreement with predictions from first-principles calculations, and we present the ${\mu }^{+}\mathrm{SR}$ technique as an optimal probe to detect $D_{\mathrm{Li}}$ for materials containing magnetic ions.

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on ${\mathrm{SrTiO}}_3$ consists of nondegenerate $t_{2g}$ states with different dimensional characters. While the $d_{xy}$ bands have quasi-2D dispersions with weak $k_z$ dependence, the lifted $d_{xz}/d_{yz}$ bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the $d_{xz}/d_{yz}$ Fermi surface components are essentially the same for different sample preparations. Irradiating ${\mathrm{SrTiO}}_3$ in ultrahigh vacuum is one method observed so far to induce the “universal” surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O $1s$ core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-$E_F$ electrons in the surface region.

We report an angle-resolved photoemission study of the charge stripe ordered ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (Nd-LSCO) system. A comparative and quantitative line-shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side ($x=0.20$), a normal-state antinodal spectral gap opens upon cooling below 80 K. In this process, spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different line shape is observed in the antinodal region of charge ordered Nd-LSCO $x=1/8$. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral-weight redistribution and gapping originating from charge stripe ordering.

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of ${\mathrm{Ba}}_3{\mathrm{ZnIr}}_2{\mathrm{O}}_9$ is a realization of a novel spin-orbital liquid state. Our results reveal that ${\mathrm{Ba}}_3{\mathrm{ZnIr}}_2{\mathrm{O}}_9$ with ${\mathrm{Ir}}^{5+}$ ($5d^4$) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive $J=0$ state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK.

A review to delineate the chemistry and physics for honeycomb layered oxides.