The hidden order phase in URu$_2$Si$_2$ is highly sensitive to electronic doping. A special interest in silicon-to-phosphorus substitution is due to the fact that it may allow one, in part, to isolate the effects of tuning the chemical potential from
the complexity of the correlated $f$ and $d$ electronic states. We investigate the new antiferromagnetic phase that is induced in URu$_2$Si$_{2-x}$P$_x$ at $xgtrsim0.27$. Time-of-flight neutron diffraction of a single crystal ($x=0.28$) reveals $c$-axis collinear $mathbf{q}_mathrm{m}=(frac12,frac12,frac12)$ magnetic structure with localized magnetic moments ($approx2.1,mu_mathrm{B}$). This points to an unexpected analogy between the (Si,P) and (Ru,Rh) substitution series. Through further comparisons with other tuning studies of URu$_2$Si$_2$, we are able to delineate the mechanisms by which silicon-to-phosphorus substitution affects the system. In particular, both the localization of itinerant 5$f$ electrons as well as the choice of $mathbf{q}_m$ appears to be consequences of the increase in chemical potential. Further, enhanced exchange interactions are induced by chemical pressure and lead to magnetic order, in which an increase in inter-layer spacing may play a special role.
We study the magnetic structure of the stuffed (Tb-rich) pyrochlore iridate Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$, using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were per
formed at the Ir $L_3$ and Tb $M_5$ edges, which provide selective sensitivity to Ir $5d$ and Tb $4f$ magnetic moments, respectively. At the Ir $L_3$ edge, we found the onset of long-range ${bf k}={bf 0}$ magnetic order below $T_{N}^text{Ir}sim$ 71 K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of $mu_{5d}^{text{Ir}} approx 0.34(3),mu_B$ at 5 K. At the Tb $M_5$ edge, long-range ${bf k}={bf 0}$ magnetic order appeared below $sim40$ K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the $M_5$ edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb$_{2}$Ir$_{2}$O$_{7}$, for which $T_{N}^{text{Ir}} =130$ K and $mu_{5d}^{text{Ir}}=0.56,mu_B$, indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.
We report an Fe $L$-edge resonant inelastic x-ray scattering (RIXS) study of the unusual superconductor $beta$-FeSe. The high energy resolution of this RIXS experiment ($approx,$55$,$meV FWHM) made it possible to resolve low-energy excitations of the
Fe $3d$ manifold. These include a broad peak which shows dispersive trends between 100-200$,$meV along the $(pi,0)$ and $(pi,pi)$ directions of the one-Fe square reciprocal lattice, and which can be attributed to paramagnon excitations. The multi-band valence state of FeSe is among the most metallic in which such excitations have been discerned by soft x-ray RIXS.
We use resonant elastic x-ray scattering to determine the evolution of magnetic order in EuCd$_2$As$_2$ below $T_textrm{N}=9.5$,K, as a function of temperature and applied magnetic field. We find an A-type antiferromagneticstructure with in-plane mag
netic moments, and observe dramatic magnetoresistive effects associated with field-induced changes in the magnetic structure and domain populations. Our textit{ab initio} electronic structure calculations indicate that the Dirac dispersion found in the nonmagnetic Dirac semimetal Cd$_3$As$_2$ is also present in EuCd$_2$As$_2$, but is gapped for $T < T_textrm{N}$ due to the breaking of $C_3$ symmetry by the magnetic structure.
We have performed inelastic neutron scattering measurements on a powder sample of the superconductor lithium iron selenide hydroxide Li$_{1-x}$Fe$_{x}$ODFe$_{1-y}$Se ($x simeq 0.16, y simeq 0.02$, $T_{rm c} = 41$,K). The spectrum shows an enhanced in
tensity below $T_{rm c}$ over an energy range $0.64times2Delta < E < 2Delta$, where $Delta$ is the superconducting gap, with maxima at the wave vectors $Q_1 simeq 1.46$,AA$^{-1}$ and $Q_2 simeq 1.97$,AA$^{-1}$. The behavior of this feature is consistent with the spin resonance mode found in other unconventional superconductors, and strongly resembles the spin resonance observed in the spectrum of the molecular-intercalated iron selenide, Li$_{0.6}$(ND$_{2}$)$_{0.2}$(ND$_{3}$)$_{0.8}$Fe$_{2}$Se$_{2}$. The signal can be described with a characteristic two-dimensional wave vector $(pi, 0.67pi)$ in the Brillouin zone of the iron square lattice, consistent with the nesting vector between electron Fermi sheets.
We have performed powder inelastic neutron scattering measurements on the unconventional superconductor $beta$-FeSe ($T_{rm c} simeq 8,mathrm{K}$). The spectra reveal highly dispersive paramagnetic fluctuations emerging from the square-lattice wave v
ector $(pi,0)$ extending beyond 80 meV in energy. Measurements as a function of temperature at an energy of $sim 13,mathrm{meV}$ did not show any variation from $T_{rm c}$ to $104,mathrm{K}$. The results show that FeSe is close to an instability towards $(pi,0)$ antiferromagnetism characteristic of the parent phases of the high-$T_{rm c}$ iron arsenide superconductors, and that the iron paramagnetic moment is neither affected by the orthorhombic-to-tetragonal structural transition at $T_{rm s} simeq 90,mathrm{K}$ nor does it undergo a change in spin state over the temperature range studied.
