It has recently been demonstrated that dynamical magnetic correlations measured by neutron scattering in iron chalcogenides can be described with models of short-range correlations characterized by particular {choices of four-spin plaquettes, where the appropriate choice changes as the} parent material is doped towards superconductivity. Here we apply such models to describe measured maps of magnetic scattering as a function of two-dimensional wave vectors obtained for optimally superconducting crystals of FeTe$_{1-x}$Se$_x$. We show that the characteristic antiferromagnetic wave vector evolves from that of the bicollinear structure found in underdoped chalcogenides (at high temperature) to that associated with the stripe structure of antiferromagnetic iron arsenides (at low temperature); {these can both be described with the same local plaquette, but with different inter-plaquette correlations}. While the magnitude of the low-energy magnetic spectral weight is substantial at all temperatures, it actually weakens somewhat at low temperature, where the charge carriers become more itinerant. The observed change in spin correlations is correlated with the dramatic drop in the electronic scattering rate and the growth of the bulk nematic response on cooling. Finally, we also present powder neutron diffraction results for lattice parameters in FeTe$_{1-x}$Se$_x$ indicating that the tetrahedral bond angle tends to increase towards the ideal value on cooling, in agreement with the increased screening of the crystal field by more itinerant electrons and the correspondingly smaller splitting of the Fe $3d$ orbitals.
Temperature-dependent neutron-scattering experiments have been performed on a polycrystalline sample of ${\mathrm{Ce}}_{0.74}$${\mathrm{Th}}_{0.26}$ which undergoes a first-order $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\alpha}$ valence transition at ${T}_{V}\ensuremath{\sim}150$ K. By a measurement of the temperature dependence of the lattice parameter and use of V\'egard's law, we estimate the temperature behavior of the valence of Ce. The $Q$ dependence of the magnetic scattering is found to follow the form factor of the ${\mathrm{Ce}}^{3+}$ ion surprisingly well. In the inelastic scans, particular attention has been paid to the subtraction of the phonon background via an inelastic study of an identically sized and shaped sample of the nonmagnetic material ${\mathrm{La}}_{0.73}$${\mathrm{Th}}_{0.27}$. The corrected ${\mathrm{Ce}}_{0.74}$${\mathrm{Th}}_{0.26}$ spectra have then been expressed in the form of the imaginary part of the susceptibility ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},\ensuremath{\omega})$. The $\ensuremath{\gamma}$-phase dynamic susceptibility is a broad feature with significant intensity extending beyond 70.0 meV (the limit of our measurement) with a peak near \ensuremath{\sim}20.0 meV. On cooling below ${T}_{V}$ the susceptibility decreases in magnitude and broadens such that the peak is beyond 70.0 meV. These results are compared with the macroscopic magnetic susceptibility which exhibits behavior similar to $\ensuremath{\chi}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}})$ obtained from ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},\ensuremath{\omega})$ by a Kramers-Kronig analysis.
Adiabatic susceptibility measurements have been performed on CeBr3 in the helium range between 3.4 and 22.0 MHz. From the measurements we deduce the specific heat at constant magnetization CM / R≅ (0.0274±0.0011) / T2+(0.009±0.005) / T3 and an antiferromagnetic Curie-Weiss constant θ (sphere) = −0.050±0.045°K. These values are combined with the results of EPR spectra to give estimates of the diagonal terms of the nearest- and next-nearest-neighbors interaction tensors for Ce3+ ions in CeBr3 which are compared with those found for CeCl3. The interaction tensors for the effective spins are found to contain terms which arise from high-degree superexchange of the form J+5(1) J−5(2) acting between the real spins. An analysis of the interactions indicates that CeBr3 should correspond well to a weakly perturbed linear chain system with either antiferromagnetic Ising or ferromagnetic XY interactions within the chain.
Electron correlations often lead to emergent orders in quantum materials. Kagome lattice materials are emerging as an exciting platform for realizing quantum topology in the presence of electron correlations. This proposal stems from the key signatures of electronic structures associated with its lattice geometry: flat band induced by destructive interference of the electronic wavefunctions, topological Dirac crossing, and a pair of van Hove singularities (vHSs). A plethora of correlated electronic phases have been discovered amongst kagome lattice materials, including magnetism, charge density wave (CDW), nematicity, and superconductivity. These materials can be largely organized into two types: those that host magnetism and those that host CDW order. Recently, a CDW order has been discovered in the magnetic kagome FeGe, providing a new platform for understanding the interplay between CDW and magnetism. Here, utilizing angle-resolved photoemission spectroscopy, we observe all three types of electronic signatures of the kagome lattice: flat bands, Dirac crossings, and vHSs. From both the observation of a temperature-dependent shift of the vHSs towards the Fermi level as well as guidance via first-principle calculations, we identify the presence of the vHSs near the Fermi level (EF) to be driven by the development of underlying magnetic exchange splitting. Furthermore, we show spectral evidence for the CDW order as gaps that open on the near-EF vHS bands, as well as evidence of electron-phonon coupling from a kink on the vHS band together with phonon hardening observed by inelastic neutron scattering. Our observation points to the magnetic interaction-driven band modification resulting in the formation of the CDW order, indicating an intertwined connection between the emergent magnetism and vHS charge order in this moderately-correlated kagome metal.
A detailed neutron-scattering investigation of the magnetic properties of the antiferromagnet Fe${\mathrm{Cl}}_{2}$ in zero field has been carried out. In this paper we report inelastic studies of the spin waves and magnetic excitons both at low temperatures and around the phase transition. The spin waves are found to simulate those of a two-dimensional ferromagnet with large anisotropy. The magnon dispersion relations at 5 \ifmmode^\circ\else\textdegree\fi{}K may be accurately described by simple $S=1$ spin-wave theory with an anisotropy field $g{\ensuremath{\mu}}_{B}{H}_{A}=2.0\ifmmode\pm\else\textpm\fi{}0.1$ meV, in-plane isotropic exchange interactions of $2{J}_{1}=0.68\ifmmode\pm\else\textpm\fi{}0.02$ meV, $2{J}_{2}=\ensuremath{-}0.09\ifmmode\pm\else\textpm\fi{}0.02$ meV, and an antiferromagnetic interplanar interaction of $2{J}_{1}^{\ensuremath{'}}=\ensuremath{-}0.03\ifmmode\pm\else\textpm\fi{}0.01$ meV. The temperature dependence of these magnons is quite unusual. Up to 21 \ifmmode^\circ\else\textdegree\fi{}K there is no renormalization at all of the exchange part of the spin-wave energy. However, between 21 \ifmmode^\circ\else\textdegree\fi{}K and ${T}_{N}=23.55$ \ifmmode^\circ\else\textdegree\fi{}K the entire magnon branch collapses precipitously into a continuum of scattering. Magnetic excitons originating in transitions between the spin-orbit-split $^{5}T_{2g}(J=1)$ and $J=2$ states have also been observed. However, these are complicated by coupling to the optical phonons so that only qualitative results are obtained.
In the quest for novel quantum states driven by topology and correlation, kagome lattice materials have garnered significant interest due to their distinctive electronic band structures, featuring flat bands (FBs) arising from the quantum destructive interference of the electronic wave function. The tuning of the FBs to the chemical potential would lead to the possibility of liberating electronic instabilities that lead to emergent electronic orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their participation in emergent electronic orders have been lacking in bulk quantum materials. Here using a combination of Angle-Resolved Photoemission Spectroscopy (ARPES) and Density Functional Theory (DFT), we reveal that the low-energy electronic structure of the recently discovered Cr-based kagome metal superconductor {\Cr} is dominated by a pervasive FB in close proximity to, and below the Fermi level. A comparative analysis with orbital-projected DFT and polarization dependence measurement uncovers that an orbital-selective renormalization mechanism is needed to reconcile the discrepancy with the DFT calculations, which predict the FB to appear 200 meV above the Fermi level. Furthermore, we observe the FB to shift away from the Fermi level by 20 meV in the low-temperature density wave-ordered phase, highlighting the role of the FB in the emergent electronic order. Our results reveal {\Cr} to stand out as a promising platform for further exploration into the effects of FBs near the Fermi level on kagome lattices, and their role in emergent orders in bulk quantum materials.
We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ for $x=0.039$, 0.022, and 0.021 as-grown single crystals. Our investigations probe the behavior near the magnetic tricritical point in the $(x,T)$ plane, ${x}_{\text{tr}}\ensuremath{\approx}0.022$, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples show long-range antiferromagnetic order that may be described near the transition by simple power laws, with $\ensuremath{\beta}=0.306\ifmmode\pm\else\textpm\fi{}0.060$ for $x=0.039$, $\ensuremath{\beta}=0.208\ifmmode\pm\else\textpm\fi{}0.005$ for $x=0.022$, and $\ensuremath{\beta}=0.198\ifmmode\pm\else\textpm\fi{}0.009$ for $x=0.021$. For the $x=0.039$ sample, the data are reasonably well described by the order parameter exponent $\ensuremath{\beta}=0.326$ expected for a three-dimensional Ising model while the $x=0.022$ and $x=0.021$ samples are near the $\ensuremath{\beta}=0.25$ value for a tricritical system in the mean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions.
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