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Exotic magnetic phases often have frustration as a key ingredient. Chemically and structurally simple materials are particularly prized as testbeds for concepts found in more complex ones. Using a combination of neutron diffraction, inelastic neutron scattering, and laboratory-based characterization methods, the authors find long-range antiferromagnetic order in Nd${}_{2}$O${}_{3}$ only below 0.55 K, despite indications that the magnetic energy scales are significantly higher. The ground state possesses alternating stripes of local moments in the plane of the triangular lattice and is characterized by strong XY anisotropy that originates from the local crystal field. These results suggest that Nd${}_{2}$O${}_{3}$ may be a model system for studying frustration originating from competing interactions between magnetic moments subject to strong spin-orbit coupling on a centrosymmetric lattice.

We report the crystal field levels of several newly discovered rare-earth kagome compounds: ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$, ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$, and ${\mathrm{Pr}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$. We determine the crystal electric field (CEF) Hamiltonian by fitting to neutron scattering data using a point charge Hamiltonian as an intermediate fitting step. The fitted Hamiltonians accurately reproduce bulk susceptibility measurements, and the results indicate easy-axis ground-state doublets for ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$ and ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$ and a singlet ground state for ${\mathrm{Pr}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$. These results provide the groundwork for future investigations of these compounds and a template for CEF analysis of other low-symmetry materials.

We present inelastic neutron scattering results of phonons in ${({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5})}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te}$ powders, with $x=0$ and 0.3. The $x=0$ sample is a topological crystalline insulator, and the $x=0.3$ sample is a superconductor with a bulk superconducting transition temperature ${T}_{c}$ of 4.7 K. In both samples, we observe unexpected van Hove singularities in the phonon density of states at energies of 1--2.5 meV, suggestive of local modes. On cooling the superconducting sample through ${T}_{c}$, there is an enhancement of these features for energies below twice the superconducting-gap energy. We further note that the superconductivity in ${({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5})}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te}$ occurs in samples with normal-state resistivities of order 10 $\mathrm{m}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$, indicative of bad-metal behavior. Calculations based on density functional theory suggest that the superconductivity is easily explainable in terms of electron-phonon coupling; however, they completely miss the low-frequency modes and do not explain the large resistivity. While the bulk superconducting state of ${({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5})}_{0.7}{\mathrm{In}}_{0.3}\mathrm{Te}$ appears to be driven by phonons, a proper understanding will require ideas beyond simple BCS theory.

Sr$_2$FeOsO$_6$ is an insulating double perovskite compound which undergoes antiferromagnetic transitions at 140 K ($T_{N1}$) and 67 K ($T_{N2}$). To study the underlying electronic and magnetic interactions giving rise to this behavior we have performed inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) experiments on polycrystalline samples of Sr$_2$FeOsO$_6$. The INS data reveal that the spectrum of spin excitations remains ungapped below T$_{N1}$, however below T$_{N2}$ a gap of 6.8 meV develops. The RIXS data reveals splitting of the T$_{2g}$ multiplet consistent with that seen in other 5d$^3$ osmium based double perovskites. Together these results suggest that spin-orbit coupling is important for ground state selection in 3d-5d$^3$ double perovskite materials., 5 figures, supplementary material available on request

The formation of polar nanoregions through solid-solution additions is known to enhance significantly the functional properties of ferroelectric materials. Despite considerable progress in characterizing the microscopic behavior of polar nanoregions (PNR), understanding their real-space atomic structure and dynamics of their formation remains a considerable challenge. Here, using the method of dynamic pair distribution function, we provide direct insights into the role of solid-solution additions towards the stabilization of polar nanoregions in the Pb-free ferroelectric of Ba(Zr,Ti)O3. It is shown that for an optimum level of substitution of Ti by larger Zr ions, the dynamics of atomic displacements for ferroelectric polarization are slowed sufficiently below THz frequencies, which leads to increased local correlation among dipoles within PNRs. The dynamic pair distribution function technique demonstrates a unique capability to obtain insights into locally correlated atomic dynamics in disordered materials, including new Pb-free ferroelectrics, which is necessary to understand and control their functional properties. (Less)

The electronic ground state of ${\mathrm{Ca}}_{3}{\mathrm{LiOsO}}_{6}$ was recently considered within an intermediate coupling regime that revealed $J=3/2$ spin-orbit entangled magnetic moments. Through inelastic neutron scattering and density functional theory we investigate the magnetic interactions and probe how the magnetism is influenced by the change in hierarchy of interactions as we move from ${\mathrm{Ca}}_{3}{\mathrm{LiOsO}}_{6}$ ($5{d}^{3}$) to ${\mathrm{Ca}}_{3}{\mathrm{LiRuO}}_{6}$ ($4{d}^{3}$). An alteration of the spin gap and ordered local moment is observed, however the magnetic structure, N\'eel temperature, and exchange interactions are unaltered. To explain this behavior it is necessary to include both spin-orbit coupling and hybridization, indicating the importance of an intermediate coupling approach when describing $5d$ oxides.

We use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped ${\mathrm{NaFe}}_{0.9785}{\mathrm{Co}}_{0.0215}\mathrm{As}$, with coexisting superconductivity $({T}_{\mathrm{c}}\ensuremath{\approx}19$ K) and weak antiferromagnetic order $({T}_{\mathrm{N}}\ensuremath{\approx}30$ K, ordered moment $\ensuremath{\approx}0.02\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe})$. A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly upon approaching ${T}_{\mathrm{c}}$. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped ${\mathrm{NaFe}}_{0.935}{\mathrm{Co}}_{0.045}\mathrm{As}$ but different from optimal electron-, hole-, and isovalently doped ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in ${\mathrm{NaFe}}_{0.9785}{\mathrm{Co}}_{0.0215}\mathrm{As}$ is instead present at energies below the resonance, which becomes partially gapped below ${T}_{\mathrm{c}}$, similar to the situation in optimally doped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.9}$. Our results indicate that anisotropic spin fluctuations in ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{As}$ appear in the form of a resonance in the underdoped regime, become partially gapped below ${T}_{\mathrm{c}}$ near optimal doping, and disappear in overdoped compounds.

The dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe$_4$As$_4$ was investigated by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations. Powder INS measurements show that the spin resonance at $Q_\text{res}=1.17(1)$~$\text{\AA}^{-1}$, corresponding to the $(\pi,\pi)$ nesting wave vector in tetragonal notation, evolves below $T_\text{c}$. The characteristic energy of the spin resonance $E_\text{res}=12.5$~meV is smaller than twice the size of the superconducting gap ($2\Delta$). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe$_4$As$_4$ is the $s_\pm$ symmetry., Comment: http://journals.jps.jp/doi/10.7566/JPSJ.86.093703

The behavior of matter near a quantum critical point is one of the most exciting and challenging areas of physics research. Emergent phenomena such as high-temperature superconductivity are linked to the proximity to a quantum critical point. Although significant progress has been made in understanding quantum critical behavior in some low dimensional magnetic insulators, the situation in metallic systems is much less clear. Here, we demonstrate that NiCoCrx single crystal alloys are remarkable model systems for investigating quantum critical point physics in a metallic environment. For NiCoCrx alloys with x ≈ 0.8, the critical exponents associated with a ferromagnetic quantum critical point are experimentally determined from low temperature magnetization and heat capacity measurements. All of the five exponents (γ T ≈ 1/2, β T ≈ 1, δ ≈ 3/2, νzm ≈ 2, $${\bar {\rm \alpha}}$$ T ≈ 0) are in remarkable agreement with predictions of Belitz–Kirkpatrick–Vojta theory in the asymptotic limit of high disorder. Using these critical exponents, excellent scaling of the magnetization data is demonstrated with no adjustable parameters. We also find a divergence of the magnetic Gruneisen parameter, consistent with a ferromagnetic quantum critical point. This work therefore demonstrates that entropy stabilized concentrated solid solutions represent a unique platform to study quantum critical behavior in a highly tunable class of materials. A family of metallic solid solutions—alloys characterized by extreme chemical disorder—are found to exhibit quantum critical behaviour. Few metallic systems are known to show this type of phase transition, and research on the topic has mainly focused on low dimensional magnetic insulators. Our current understanding of this phenomenon is consequently very limited. Now, Brian C. Sales and colleagues at USA’s Oak Ridge National Laboratory, and collaborators from National Institute of Standards and Technology and University of Maryland, report quantum criticality as a function of temperature and magnetic field in NiCoCrx single crystals, which are metallic ferromagnetic solid solutions. Over two hundred ternary solid solutions have been predicted to exist, and they may therefore represent the perfect platform for studying these quantum phase transitions.

In common with many strongly correlated electron systems, intermediate valence compounds are believed to display a crossover from a high-temperature regime of incoherently fluctuating local moments to a low-temperature regime of coherent hybridized bands. We show that inelastic neutron scattering measurements of the dynamic magnetic susceptibility of CePd

Inelastic neutron scattering measurements have been performed to investigate the spin waves of the quasi-one-dimensional antiferromagnetic ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$, where a superconducting transition was observed under pressure [H. Takahashi et al., Nat. Mater. 14, 1008 (2015); T. Yamauchi et al., Phys. Rev. Lett. 115, 246402 (2015)]. By fitting the spherically averaged experimental data collected on a powder sample to a Heisenberg Hamiltonian, we find that the one-dimensional antiferromagnetic ladder exhibits a strong nearest-neighbor ferromagnetic exchange interaction ($S{J}_{R}=\ensuremath{-}71\ifmmode\pm\else\textpm\fi{}4$ meV) along the rung direction, an antiferromagnetic $S{J}_{L}=49\ifmmode\pm\else\textpm\fi{}3$ meV along the leg direction, and a ferromagnetic $S{J}_{2}=\ensuremath{-}15\ifmmode\pm\else\textpm\fi{}2$ meV along the diagonal direction. Our data demonstrate that the antiferromagnetic spin excitations are a common characteristic for the iron-based superconductors, while specific relative values for the exchange interactions do not appear to be unique for the parent states of the superconducting materials.

While it is mostly well-known for its excellent thermoelectric properties at high temperatures, Yb${}_{14}$MnSb${}_{11}$ has also come to be characterized as a ferromagnetic semiconductor. Such systems support ferromagnetic correlations similar to metallic compounds, although with semiconducting electronic configurations. As such, they are promising materials for use in spintronic devices. The authors here use neutron scattering techniques to examine the distribution of the magnetization density and the energy scale of the magnetic exchange interactions in the Yb${}_{14}$MnSb${}_{11}$ compound. They find that there is a large ferromagnetic moment localized to the Mn site in the crystal structure with a `Kondo cloud' of negative moments distributed over the other atomic sites. The results provide long sought answers to questions regarding the valence, magnetization distribution, and energy scales of the magnetic interactions in this compound. The ferromagnetic spin-wave spectrum supports this premise and is well characterized by inclusion of nearest and next-nearest neighbor exchange interactions.

The perovskite SrIrO3 is an exotic narrow-band metal owing to a confluence of the strengths of the spin-orbit coupling (SOC) and the electron-electron correlations. It has been proposed that topological and magnetic insulating phases can be achieved by tuning the SOC, Hubbard interactions, and/or lattice symmetry. Here, we report that the substitution of nonmagnetic, isovalent Sn4+ for Ir4+ in the SrIr1-xSnxO3 perovskites synthesized under high pressure leads to a metal-insulator transition to an antiferromagnetic (AF) phase at TN > 225 K. The continuous change of the cell volume as detected by x-ray diffraction and the lamda-shape transition of the specific heat on cooling through TN demonstrate that the metal-insulator transition is of second-order. Neutron powder diffraction results indicate that the Sn substitution enlarges an octahedral-site distortion that reduces the SOC relative to the spin-spin exchange interaction and results in the type-G AF spin ordering below TN. Measurement of high-temperature magnetic susceptibility shows the evolution of magnetic coupling in the paramagnetic phase typical of weak itinerant-electron magnetism in the Sn-substituted samples. A reduced structural symmetry in the magnetically ordered phase leads to an electron gap opening at the Brillouin zone boundary below TN in the same way as proposed by Slater., 15 pages, 4 figures

The dispersive magnetic excitations in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ have previously been well described within an isospin-1/2 Heisenberg model on a square lattice that revealed parallels with ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. Here we investigate the inelastic spectra of ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{0.8}{\mathrm{Ru}}_{0.2}{\mathrm{O}}_{4}$ with resonant inelastic x-ray scattering (RIXS) at the Ir ${L}_{3}$ edge. The results are well described using linear spin-wave theory within a similar Heisenberg model applicable to ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$; however, the disorder induced by the substitution of $20%\phantom{\rule{4pt}{0ex}}{\mathrm{Ir}}^{4+}$ ions for ${\mathrm{Ru}}^{4+}$ removes longer range exchange interactions. A large spin gap (40 meV) is measured indicating strong anisotropy from spin-orbit coupling that is manifest due to the altered magnetic structure in ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{0.8}{\mathrm{Ru}}_{0.2}{\mathrm{O}}_{4}$ with $c$-axis aligned moments compared to the basal plane moments in the parent. Collectively the results indicate the robustness of a Heisenberg model description even when the magnetic structure is altered and the ${J}_{\mathrm{eff}}=1/2$ moments are diluted.

The A-site spinel material, CoAl2O4, is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which is predicted to contain unique incommensurate or `spin-spiral liquid' ground states. Our previous single-crystal neutron scattering study instead classified it as a `kinetically-inhibited' antiferromagnet, where the long ranged correlations of a collinear Neel ground state are blocked by the freezing of domain wall motion below a first-order phase transition at T* = 6.5 K. The current paper expands on our original results in several important ways. New elastic and inelastic neutron measurements are presented that show our initial conclusions are affected by neither the sample measured nor the instrument resolution, while measurements to temperatures as low as T = 250 mK limit the possible role being played by low-lying thermal excitations. Polarized diffuse neutron measurements confirm reports of short-range antiferromagnetic correlations and diffuse streaks of scattering, but major diffuse features are explained as signatures of overlapping critical correlations between neighboring Brillouin zones. Finally, and critically, this paper presents detailed elastic and inelastic measurements of magnetic correlations in a single-crystal of MnAl2O4, which acts as an unfrustrated analogue to CoAl2O4. The unfrustrated material is shown to have a classical continuous phase transition to Neel order at T_N = 39 K, with collective spinwave excitations and Lorentzian-like critical correlations which diverge at the transition. Direct comparison between the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse correlations, but rather in the nature of the frozen state below T*. The higher level of cation inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater next-nearest-neighbor exchange.

Intermediate valence compounds containing rare earth or actinide ions are archetypal systems for the investigation of strong electron correlations. Their effective electron masses of 10 to 50 times the free electron mass result from a hybridization of the highly localized $f$-electrons with the more itinerant $d$-electrons, which is strong enough that their properties are dominated by on-site electron correlations. To a remarkable degree, this can be modeled by the Anderson Impurity Model, even though the $f$-electrons are situated on a periodic lattice. However, in recent years, there has been increasing evidence that the dynamic magnetic susceptibility of intermediate valence compounds is not purely local, but shows variations across the Brillouin zone that have been ascribed to $f$-band coherence. So far, this has been based on simplified qualitative models. In this article, we present a quantitative comparison of inelastic neutron scattering from a single crystal of CePd$_3$, measured in four-dimensional (Q,$\omega$)-space, with ab initio calculations, which are in excellent agreement on an absolute scale. Our results establish that the Q-dependence of the scattering is caused by particle-hole excitations within $f$-$d$ hybridized bands that grow in coherence with decreasing temperature., Comment: 8 page article with 6 figures, 4 page supplementary information with 8 figures

A special type of anisotropic spin exchange called the ``Kitaev interaction'' provides rare exactly-soluble models of gapless quantum spin liquids in 2d and 3d. The main goals of this work were to identify other lattice geometries that support the elusive Kitaev interactions and understand the magnetic properties of such materials. By performing inelastic neutron scattering on La${}_{2}$MgIrO${}_{6}$ and La${}_{2}$ZnIrO${}_{6}$, the authors find that these magnets, which possess long-range antiferromagnetic order, yield clear signatures of gapped and nearly-dispersionless magnon excitations. They also show that spin-wave models with a dominant Kitaev interaction can naturally account for the magnetic ordering pattern as well as the inelastic neutron scattering data.

Neutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this work, we study the doping evolution of the resonances in NaFe$_{1-x}$Co$_x$As covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum, but retains approximately the same peak position even when $T_c$ drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe$_{1-x}$Co$_x$As is neither simply proportional to $T_c$ nor the superconducting gap, but is controlled by the multi-orbital character of the system and doped impurity scattering effect., Comment: accepted by PRB

We report a combined study of the spin resonances and superconducting gaps for underdoped (${T}_{c}=19$ K), optimally doped (${T}_{c}=25$ K), and overdoped (${T}_{c}=19$ K) $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals with inelastic neutron scattering and angle resolved photoemission spectroscopy. We find a quasi-two-dimensional spin resonance whose energy scales with the superconducting gap in all three compounds. In addition, anisotropic low energy spin excitation enhancements in the superconducting state have been deduced and characterized for the under and optimally doped compounds. Our data suggest that the quasi-two-dimensional spin resonance is a spin exciton that corresponds to the spin singlet-triplet excitations of the itinerant electrons. However, the intensity enhancements of the anisotropic spin excitations are dominated by the out-of-plane spin excitations of the ordered moments due to the suppression of damping in the superconducting state. Hence we offer an interpretation of the double energy scales differing from previous interpretations based on anisotropic superconducting energy gaps and systematically explain the doping-dependent trend across the phase diagram.

Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of compounds, there exists no information for structurally simple HgBa2CuO4+δ. Here we report neutron-scattering results for HgBa2CuO4+δ (superconducting transition temperature Tc≈71 K, pseudogap temperature T*≈305 K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of the cuprates: the X-shaped ‘hourglass' response and the resonance mode in the superconducting state. Instead, the response is Y-shaped, gapped and significantly enhanced below T*, and hence a prominent signature of the pseudogap state., In the cuprates, antiferromagnetic correlations might be the cause of the pseudogap phenomenon. Here the authors use neutron scattering on the tetragonal cuprate HgBa2CuO4+δ revealing commensurate antiferromagnetic excitations as a signature of the pseudogap state.

Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit N\'eel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (Ts = 90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here, we report inelastic neutron-scattering experiments that reveal both stripe and N\'eel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the N\'eel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ~ 60% larger than that in the iron pnictide BaFe2As2. Our results suggest that FeSe is a novel S = 1 nematic quantum-disordered paramagnet interpolating between the N\'eel and stripe magnetic instabilities., Comment: Supplementary information included; accepted by Nature Communications

Low-energy spin excitations have been studied on polycrystalline LaFeAsO 1− x F x samples by inelastic neutron scattering. The Q -integrated dynamical spin susceptibility χ ″( ω ) of the superconducting samples is found to be comparable to that of the magnetically ordered parent sample. On the other hand, χ ″( ω ) almost vanishes at x = 0.158, where the superconducting transition temperature T c is suppressed to 7 K. In addition, χ ″( ω ) in optimally doped LaFeAsO 0.918 F 0.082 with T c = 29 K exhibits a spin resonance mode. The peak energy, E res , when scaled by k B T c is similar to the value of about 4.7 reported in other high- T c iron-based superconductors. This result suggests that there is intimate relationship between the dynamical spin susceptibility and high- T c superconductivity in iron-based superconductors, and is consistent with a nesting condition between Fermi surfaces at the Γ and M points.

Phonons are exquisitely sensitive to finite-length scale effects in complex materials because they are intimately connected to charge, polarizability, and structure, and a quantitative analysis of their behavior can reveal microscopic aspects of chemical bonding. To investigate these effects in a model correlated oxide, we measured the infrared vibrational properties of 8-nm particles of MnO, compared the results with the analogous bulk material, and quantified the phonon confinement with a calculation of the Born effective charge. Our analysis reveals that the Born effective charge decreases by {approx}20%, compared to the bulk material. Moreover, this change impacts both ionicity and polarizability. Specifically, we find that MnO nanoparticles are {approx}12% less ionic than the corresponding bulk. This discovery is important for understanding finite-length scale effects in this simple binary oxide and the more complicated functional oxides that emanate from this parent compound.

The nature of the superconducting state in the recently discovered Fe-based superconductors1-3 is the subject of intense scrutiny. Neutron scattering investigations have already elucidated a strong correlation between magnetism and superconductivity in the form of a spin resonance in the magnetic excitation spectrum4-7. A central unanswered question concerns the nature of the normal state spin fluctuations which may be responsible for the pairing mechanism. Here we show inelastic neutron scattering measurements of Fe1.04Te0.73Se0.27, not superconducting in bulk, and FeTe0.51Se0.49, a bulk superconductor. These measurements demonstrate that the spin fluctuation spectrum is dominated by two-dimensional incommensurate excitations near the (1/2,1/2) (square lattice (pi,0)) wavevector, the wavevector of interest in other Fe-based superconductors, that extend to energies at least as high as 300 meV. Most importantly, the spin excitations in Fe1+yTe1-xSex exhibit four-fold symmetry about the (1,0) (square lattice (pi,pi)) wavevector and are described by the identical wavevector as the normal state spin excitations in the high-TC cuprates8-12 demonstrating a commonality between the magnetism in these classes of materials which perhaps extends to a common origin for superconductivity., Comment: 19 pages, 5 figures

We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting $\mathrm{Ba}(\mathrm{Fe}{}_{0.957}\mathrm{Cu}{}_{0.043}){}_{2}\mathrm{As}{}_{2}$, a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and superconducting $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{As}}_{2}$ compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of $\mathrm{Ba}{({\mathrm{Fe}}_{0.957}{\mathrm{Cu}}_{0.043})}_{2}{\mathrm{As}}_{2}$, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spin-spin correlation length $\ensuremath{\xi}(T)$ increases rapidly as the temperature is lowered and find $\ensuremath{\omega}/T$ scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.

Enhanced coupling of material properties offers new fundamental insights and routes to multifunctional devices. In this context 5d oxides provide new paradigms of cooperative interactions that drive novel emergent behaviour. This is exemplified in osmates that host metal–insulator transitions where magnetic order appears intimately entwined. Here we consider such a material, the 5d perovskite NaOsO3, and observe a coupling between spin and phonon manifested in a frequency shift of 40 cm−1, the largest measured in any material. The anomalous modes are shown to involve solely Os–O interactions and magnetism is revealed as the driving microscopic mechanism for the phonon renormalization. The magnitude of the coupling in NaOsO3 is primarily due to a property common to all 5d materials: the large spatial extent of the ion. This allows magnetism to couple to phonons on an unprecedented scale and in general offers multiple new routes to enhanced coupled phenomena in 5d materials., Transition metal oxides with 5d ions present novel emergent behaviour based on the enhanced coupling of material properties compared to those with 3d ions. Here, the authors demonstrate a large spin-phonon coupling in NaOsO3 which results from a large Os–O electronic orbital overlap.

We investigate the magnetic properties of the series ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{4}$ with neutron, resonant x-ray, and magnetization measurements. The results indicate an evolution and coexistence of magnetic structures via a spin-flop transition from $ab$-plane to $c$-axis collinear order as the $5d {\mathrm{Ir}}^{4+}$ ions are replaced with an increasing concentration of $4d {\mathrm{Ru}}^{4+}$ ions. The magnetic structures within the ordered regime of the phase diagram ($xl0.3$) are reported. Despite the changes in magnetic structure no alteration of the ${J}_{\mathrm{eff}}=1/2$ ground state is observed. The behavior of ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{4}$ is consistent with electronic phase separation and diverges from a standard scenario of hole doping. The role of lattice alterations with doping on the magnetic and insulating behavior is considered. The results presented here provide insight into the magnetic insulating states in strong spin-orbit coupled materials and the role perturbations play in altering the behavior.

We report elastic and inelastic neutron-scattering measurements of the high-${T}_{\mathrm{C}}$ ferromagnet ${\mathrm{Mn}}_{1+\ensuremath{\delta}}\mathrm{Sb}$. Measurements were performed on a large, ${T}_{\mathrm{C}}=434$ K, single crystal with an interstitial Mn content of $\ensuremath{\delta}\ensuremath{\approx}0.13$. The neutron-diffraction results reveal that the interstitial Mn has a magnetic moment, and that it is aligned antiparallel to the main Mn moment. We perform density functional theory calculations including the interstitial Mn and find the interstitial to be magnetic, in agreement with the diffraction data. The inelastic neutron-scattering measurements reveal two features in the magnetic dynamics: (i) a spin-wave-like dispersion emanating from ferromagnetic Bragg positions $(\mathrm{H}\phantom{\rule{4pt}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}2n)$, and (ii) a broad, nondispersive signal centered at forbidden Bragg positions $(\mathrm{H}\phantom{\rule{4pt}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}2n+1)$. The inelastic spectrum cannot be modeled by simple linear spin-wave-theory calculations and appears to be significantly altered by the presence of the interstitial Mn ions. The results show that the influence of the interstitial Mn on the magnetic state in this system is more important than previously understood.

In this work, we study crystalline electric field effects in the heavy fermion superconductor CeIrIn5. We observe two regions of broad magnetic response in the inelastic neutron scattering (INS) spectra at 10 K. The first corresponds to the transition between the Γ7 groundstate doublet and the first excited state doublet at 4 meV interwoven with a broad quasielastic contribution. The second region corresponds to the transition between the ground state and the second excited state doublet at 28 meV. The large Lorentzian half-widths of the peaks (∼10 meV) calls into question calculations for the specific heat and magnetic susceptibility that assume sharp crystal field (CF) levels. Consequently, we have calculated the INS spectra and magnetic susceptibility using the Anderson impurity model within the non-crossing approximation including the effects of CF level splitting.

We report on the temperature variation of the interatomic distances in isostructural U 2 Ni 2 In and U 2 Pd 2 Sn, both of which order antiferromagnetically at low temperatures. Both compounds exhibit complex non-collinear arrangements of the magnetic moments confined to the tetragonal basal plane, which is perpendicular to the shortest interuranium distance along the c -axis at low temperatures. The different temperature dependencies of the shortest interatomic links between uranium and the transition metal (Ni or Pd) provide evidence for the dual nature of 5f–d hybridization in these two compounds. We argue that magnetic ordering in U 2 Pd 2 Sn arises due to increased 5f–d hybridization (promoting stronger exchange) while the reduced hybridization in U 2 Ni 2 In allows for the formation of stable U magnetic moments.

Superconductivity is observed in the filled skutterudite compound \PrOsSb{} below a critical temperature temperature $T_\mathrm{c} = 1.85$ K and appears to develop out of a nonmagnetic heavy Fermi liquid with an effective mass $m^{*} \approx 50 m_\mathrm{e}$, where $m_\mathrm{e}$ is the free electron mass. Features associated with a cubic crystalline electric field are present in magnetic susceptibility, specific heat, electrical resistivity, and inelastic neutron scattering measurements, yielding a Pr$^{3+}$ energy level scheme consisting of a $\Gamma_{3}$ nonmagnetic doublet ground state, a low lying $\Gamma_{5}$ triplet excitied state at $\sim 10$ K, and much higher temperature $\Gamma_{4}$ triplet and $\Gamma_{1}$ singlet excited states. Measurements also indicate that the superconducting state is unconventional and consists of two distinct superconducting phases. At high fields and low temperatures, an ordered phase of magnetic or quadrupolar origin is observed, suggesting that the superconductivity may occur in the vicinity of a magnetic or quadrupolar quantum critical point., Comment: 11 pages, 4 figures, presented at the 3rd international symposium on Advance Science Research (ASR 2002), JAERI Tokai, Ibaraki, Japan

The interplay of magnetism and superconductivity has been a recurring theme in heavy fermion compounds. Recently, alloys of CeRhIn 5 (antiferromagnet at 3.8 K) and CeIrIn 5 (superconductor at 0.4 K) have been grown, i.e. CeRh 1-x Ir x In 5. For x = 0.5, specific heat and magnetic susceptibility measurements indicate at ~3.6 K an antiferromagnetic transition occurs. Decreasing temperature still further, a superconducting transition is observed at 0.8 K. Thus this alloy series offers the possibility of exploring the coexistence of superconductivity and magnetism in a Ce-based heavy fermion system. We have initiated a magnetotransport study from 1.4 to 300 K with applied fields to 18 T, to probe the behavior of these systems at temperatures above the possible coexistence region. In this paper we will focus on comparing the high field magnetotransport properties of CeRh 0.9 Ir 0.1 In 5 to the end compounds CeRhIn 5 and CeIrIn 5.

Relaxor ferroelectric behavior occurs in many disordered ferroelectric materials but is not well understood at the atomic level. Recent experiments and theoretical arguments indicate that Anderson localization of phonons instigates relaxor behavior by driving the formation of polar nanoregions (PNRs). Here, we use inelastic neutron scattering to observe phonon localization in relaxor ferroelectric PZN-5%PT (0.95[Pb(Zn1/3 Nb2/3)O3]–0.05PbTiO3) and detect additional features of the localization process. In the lead, up to phonon localization on cooling, the local resonant modes that drive phonon localization increase in number. The increase in resonant scattering centers is attributed to a known increase in the number of locally off centered Pb atoms on cooling. The transition to phonon localization occurs when these random scattering centers increase to a concentration where the Ioffe-Regel criterion is satisfied for localizing the phonon. We also model the effects of damped mode coupling on the observed p...

Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni$_{0.935}$Pd$_{0.065}$)$_2$Ge$_2$ in order to study the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for $T \leq$ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP ($\rho(T) \sim T^{3/2}$ and $\gamma(T) \sim \gamma_0 - b T^{1/2}$). However, for 2 $\leq T \leq$ 10 K, the susceptibility and specific heat vary as $log T$ and the resistivity varies linearly with temperature. Furthermore, despite the fact that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. We suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder., Comment: 6 pages, 8 figures, To bu published on J. Phys.: Condens. Matter

Neutron spectroscopy is used to investigate the magnetic fluctuations in Fe_{1+x}Te - a parent compound of chalcogenide superconductors. Incommensurate "stripe-like" excitations soften with increased interstitial iron concentration. The energy crossover from incommensurate to stripy fluctuations defines an apparent hour-glass dispersion. Application of sum rules of neutron scattering find that the integrated intensity is inconsistent with an S=1 Fe^{2+} ground state and significantly less than S=2 predicted from weak crystal field arguments pointing towards the Fe^{2+} being in a superposition of orbital states. The results suggest that a highly anisotropic order competes with superconductivity in chalcogenide systems., (7 pages and 8 pages supplementary information, to be published in Physical Review B)

A method to determine spatially resolved changes in magnetic exchange energy from magnetic neutron scattering data reveals that a significant reduction in the magnetic interaction energy exceeding the superconducting condensation energy by more than an order of magnitude occurs in Fe${}_{1+\ensuremath{\delta}}$Se${}_{x}$Te${}_{1-x}$ upon entering the superconducting state.

By means of neutron inelastic scattering, magnetic excitations and phonons were measured for a single crystal of slightly overdoped superconductor Ca10Pt4As8(Fe1-xPtxAs)10 (x ~ 0.2) with the transition temperature Tc of ~33 K. Below Tc, magnetic excitation spectra \{chi}"(Q, {\omega}) measured at Q = QM [magnetic {\Gamma} points] are gapped, and in the relatively higher {\omega} region, the \{chi}"(QM, {\omega})-increase was observed with decreasing T, where the maximum of the increase was found at ~18 meV at 3 K (<< Tc). These characteristics are favorable to orbital-fluctuation-mediated superconductivity with the so-called S++symmetry of the order parameter. The energy dependence of {\delta}Q-temperature(T) curve of the magnetic excitations seems to have anomalous behavior in rather wide T region above Tc, {\delta}Q being the width of the Q scan profile. In the phonon measurements, we observed softening of the in-plane TA mode, which corresponds to the elastic constant C66. This softening seems to start at rather high temperature T, as T is lowered. Additionally, anomalous increase in spectral weights of the TO-phonons at around QM in the region 35 < {\omega} < 40 meV was found even above Tc, as T is lowered from ambient T . Because the spectral weights in this {\omega} region mainly correspond to the in-plane motions of Fe atoms and because orbital fluctuations are expected to be strong at around QM, the result may present clues to investigate a possible coupling between the fluctuations of the orbitals and lattice system.

The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in $\mathrm{FeV_2O_4}$, a recent example of a ferrimagnetic spinel vanadate which is available in single crystal form. We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, $\mathrm{MnV_2O_4}$. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semi- classical spin wave calculations, using a minimal model that implies a sizeable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of $\mathrm{VO_6}$ octahedra, previously observed via neutron powder diffraction measurements. We further report on the spin gap, which is an order-of-magnitude larger than that observed in $\mathrm{MnV_2O_4}$. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in $\mathrm{FeV_2O_4}$ and $\mathrm{MnV_2O_4}$., Comment: 8 pages, 6 figures, includes supplementary figure; accepted for publication in Physical Review B

We report neutron scattering measurements on low-energy ($\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}5$ meV) magnetic excitations from a series of ${\mathrm{Fe}}_{1+y\ensuremath{-}z}$(Ni/Cu)${}_{z}$${\mathrm{Te}}_{1\ensuremath{-}x}$${\mathrm{Se}}_{x}$ samples which belong to the ``11'' Fe-chalcogenide family. Our results suggest a strong correlation between the magnetic excitations near (0.5,0.5,0) and the superconducting properties of the system. The low-energy magnetic excitations are found to gradually move away from (0.5,0.5,0) to incommensurate positions when superconductivity is suppressed, either by heating or chemical doping, confirming previous observations.

We report inelastic neutron scattering experiments on a single crystal of the intermediate valence compound CePd3. At 300 K the magnetic scattering is quasielastic, with half-width G = 23 meV, and is independent of momentum transfer Q. At low temperature, the Q-averaged magnetic spectrum is inelastic, exhibiting a broad peak centered near E-max = 55 meV. These results, together with the temperature dependence of the susceptibility, 4f occupation number, and specific heat, can be fit by the Kondo/Anderson impurity model. The low temperature scattering near Emax, however, shows significant variations with Q, reflecting the coherence of the 4f lattice. The intensity is maximal at (1/2, 1/2, 0), intermediate at (1/2, 0, 0) and (0, 0, 0), and weak at (1/2, 1/2, 1/2). We discuss this Q-dependence in terms of current ideas about coherence in heavy fermion systems.

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$ coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from $x=0.04$ to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials.

Comprehensive Research Organization for Sci. and Soc., Tokai 319-1106, Japan, b Dept. of Phys., Nagoya Univ., Nagoya 464-8602, Japan, Department of Physics, School of Science and Technology, Meiji University, Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan, Materials and Life Science Division, J-PARC Center, JAEA, Tokai, 319-1195, Japan, Lab. Leon Brillouin, CEA-CNRS, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France, Quantum Condensed Matter Division, ORNL, Oak Ridge, TN 37831, USA CCSE, JAEA, 5-1-5 Kashiwanoha, Kashiwa, 277-8587, Japan

We present a single crystal neutron scattering study of BaMn2Bi2, a recently synthesized material with the same ThCr2Si2-type structure found in several Fe-based unconventional superconducting materials. We show long range magnetic order, in the form of a G-type antiferromagnetic structure, to exist up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering we observe a spin-gap of 16meV, with spin-waves extending up to 55 meV. We find these magnetic excitations to be well fit to a J1-J2-Jc Heisenberg model and present values for the exchange interactions. The spin wave spectrum appears to be unchanged by the 100 K structural phase transition.

The ${J}_{\mathrm{eff}}=\frac{1}{2}$ state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, $U$. $5d$ transition-metal oxides host parameters in this regime and strong evidence for this state in ${\text{Sr}}_{2}$${\text{IrO}}_{4}$, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical ${J}_{\mathrm{eff}}=\frac{1}{2}$ state, impacting the development of a robust model of this novel insulating and magnetic state. We present experimental and theoretical results that not only show ${\text{Ca}}_{4}$${\text{IrO}}_{6}$ hosts the state, but furthermore uniquely resides in the limit required for a canonical unmixed ${J}_{\mathrm{eff}}=\frac{1}{2}$ state.

The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase-space for three-phonon scattering processes, rather than just the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optical ferroelectric mode.

ac susceptibility measurements of the charge-density-wave (CDW) compound alpha-(BEDT-TTF)(2)-KHg(SCN)(4) at magnetic fields, mu0H >23 T, above its Pauli paramagnetic limit, reveal unambiguously that the magnetic hysteresis observed previously within this CDW phase is diamagnetic and can only be explained by induced currents. It is argued that the ensemble of experimental techniques amounts to a strong case for dissipationless conductivity within this phase.

We have measured c -axis magnetotransport properties of the tetragonal YbNi 2 B 2 C compound down to 2 K and up to 18 T. Transverse and longitudinal magnetoresistance have been measured with current applied along the c -axis of the tetragonal structure. A well-defined maximum in the magnetoresistance is observed at temperatures below 10 K at approximately 5 T. At higher temperatures the magnetoresistance is always negative and weakens as the temperature is increased.

Pr3In is a single ground state compound which exhibits antiferromagnetic order below 11.4 K due to the exchange induced admixture of crystalline electric field levels. Additional information regarding the complex magnetic behavior of this compound can be gained through application of magnetic fields. We report specific heat and magnetocaloric effect measurements to 15 T and magnetization measurements to 44 T on single crystal samples of Pr3In. A new magnetic phase is revealed above 1.9 T and below 11.4 K.

We have performed resistivity and inelastic neutron scattering measurements on three samples of Fe${}_{0.98\ensuremath{-}z}$Cu${}_{z}$Te${}_{0.5}$Se${}_{0.5}$ with $z=0$, 0.02, and 0.1. It is found that with increasing Cu doping the sample's resistivity deviates progressively from that of a metal. However, in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations, the low-energy ($\ensuremath{\le}12$ meV) magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations. In either case, the localization of the conduction states induced by the Cu doping should play a critical role. Our results are not applicable to models that treat 3$d$ transition metal dopants simply as effective electron donors.