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Compounds based on the Fe2P structure have continued to attract interest because of the interplay between itinerant and localized magnetism in a noncentrosymmetric crystal structure, and because of the recent developments of these materials for magnetocaloric applications. Here we report the growth and characterization of millimeter-sized single crystals of FeMnP0.8Si0.2 with the Fe2P structure. Single-crystal x-ray diffraction, magnetization, resistivity, and Hall and heat capacity data are reported. The crystals exhibit itinerant antiferromagnetic order below 158 K with no hint of ferromagnetic behavior in the magnetization curves and with the spins ordered primarily in the ab plane. The room-temperature resistivity is close to the Ioffe-Regel limit for a metal. Single-crystal x-ray diffraction indicates a strong preference for Mn to occupy the larger pyramidal 3g site. The cation site preference in the as-grown crystals and the antiferromagnetism are not changed after high-temperature anneals and a rapid quench to room temperature. [ABSTRACT FROM AUTHOR]

PbTe is a well-known thermoelectric material. Recent x-ray total scattering studies suggest that Pb moves off center along 100 in PbTe, by ∼0.2 Å at 300 K, producing a split Pb-Te pair distribution. We present an extended x-ray absorption fine structure spectroscopy (EXAFS) study of PbTe (and Ti doped PbTe) to determine if Pb or Te is off center. EXAFS provides sensitive r- or k-space phase information which can differentiate between a split peak for the Pb-Te distribution (indicative of off-center Pb) and a thermally broadened peak. We find no evidence for a split peak for Pb-Te or Te-Pb. At 300 K, the vibration amplitude for Pb-Te (or Te-Pb) is large; this thermally induced disorder is indicative of weak bonds, and the large disorder is consistent with the low thermal conductivity at 300 K. We also find evidence of an anharmonic potential for the nearest Pb-Te bonds, consistent with the overall anharmonicity found for the phonon modes. This effect is modeled by a "skew" factor (C3) which significantly improves the fit of the Pb-Te and Te-Pb peaks for the high temperature EXAFS data; C3 becomes significant above approximately 150-200 K. The consequences of these results will be discussed. [ABSTRACT FROM AUTHOR]

When PbTe, a good thermoelectric material, is doped with a few percent Tl, the figure of merit ZT=TS2/(ρκ) [S is the Seebeck coefficient, ρ the electrical resistivity, and κ the thermal conductivity] is dramatically improved. The maximum value of ZT occurs for approximately 2% Tl, but the factors limiting ZT are as yet poorly understood. From a detailed local structure study of PbTe:Tl using the extended x-ray absorption fine structure (EXAFS) technique, we find that Tl substitutes primarily as Tl(+1) on the Pb site, with no evidence for any solubility issue, any significant fraction of Tl(+3), or any Tl interstitials. However it is not a simple substitution as there is evidence for increasing Te vacancies on neighboring sites with increasing Tl concentration, x. In addition there is also increasing disorder with x—Tl-Te bond length disorder as well as the vacancy defects—that will scatter the hole carriers and begin to increase the electrical resistivity in spite of an increase in hole concentration. This increased disorder is likely an important factor limiting ZT. [ABSTRACT FROM AUTHOR]

We study the high-temperature magnetic order in a quasi-two-dimensional honeycomb compound SrRu2O6 by measuring magnetization and neutron powder diffraction with both polarized and unpolarized neutrons. SrRu2O6 crystallizes into the hexagonal lead antimonate (PbSb2O6, space group P31¯m) structure with layers of edge-sharing RuO6 octahedra separated by Sr2+ ions. SrRu2CO6 is found to order at TN = 565 K with Ru moments coupled antiferromagnetically both in plane and out of plane. The magnetic moment is 1.30(2) μB/Ru at room temperature and is along the crystallographic c axis in the G-type magnetic structure. We perform density functional calculations with constrained random-phase approximation (RPA) to obtain the electronic structure and effective intra- and interorbital interaction parameters. The projected density of states shows strong hybridization between Ru 4d and O 2p. By downfolding to the target t2g bands we extract the effective magnetic Hamiltonian and perform Monte Carlo simulations to determine the transition temperature as a function of interand intraplane couplings. We find a weak interplane coupling, 3% of the strong intraplane coupling, permits three-dimensional magnetic order at the observed TN. [ABSTRACT FROM AUTHOR]

A high-pressure study of the lattice dynamics in the filled skutterudite Eu0.84Fe4Sb12 was carried out by means of x-ray powder diffraction and nuclear inelastic scattering. The anharmonicity of particular phonon modes was characterized by mode and element specific Griineisen parameters. The large anharmonicity of the rattling optical mode that is hybridized with the acoustical phonons at ambient pressure is reduced at high pressure as the phonon modes decouple. This result suggests that anharmonic coupling between acoustic and optical phonon modes plays a central role in the reduced thermal conductivity. [ABSTRACT FROM AUTHOR]

MnFePSi compounds are promising materials for magnetic refrigeration as they exhibit a giant magnetocaloric effect. From first principles calculations and experiments on bulk materials, it has been proposed that this is due to the Mn and Fe atoms preferentially occupying two different sites within the atomic lattice. A recently developed technique was used to deconvolve the obscuring effects of both multiple elastic scattering and thermal diffuse scattering of the probe in an atomic resolution electron energy-loss spectroscopy investigation of a MnFePSi compound. This reveals, unambiguously, that the Mn atoms preferentially occupy the 3g site in a hexagonal crystal structure, confirming the theoretical predictions. After deconvolution, the data exhibit a difference in the Fe L2.3 ratio between the 3f and 3g sites consistent with differences in magnetic moments calculated from first principles, which are also not observed in the raw data. [ABSTRACT FROM AUTHOR]

The vibrational behavior of heavy substitutional impurities (M=Ir, Os) in Fe1-xMxSi(x=0, 0.02, 0.04, 0.1) was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and first-principles simulations. Our INS measurements on single crystals mapped the four-dimensional dynamical structure factor, S(Q, E), for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers, leads to softened interatomic force constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon S(Q, E) from INS through a Green's-function model incorporating the phonon self-energy based on first-principles density functional theory simulations, and we study the disorder-induced lifetimes on large supercells. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

La0.4Na0.6Fe2As2 single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single-crystal x-ray and neutron diffraction. La0.4Na0.6Fe2As2 single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at TS = 125 K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the a direction with an ordered moment of 0.7(1) μB at T=5 K. The low-temperature stripe antiferromagnetic structure is the same as that in other AFe2 As2 (A = Ca, Sr, Ba) compounds. La0.5-xNa0.5+x:Fe2As2 provides a material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying the La/Na ratio. [ABSTRACT FROM AUTHOR]

Neutron scattering is used to probe magnetic interactions as superconductivity develops in optimally doped Fe1+δSexTe1-x. Applying the first moment sum rule to comprehensive neutron scattering data, we extract the change in magnetic exchange energy Δ[JR-R' (SR · SR')] in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements ∣ΔR∣ < ξ, where ξ = 1.3(1) nm is the superconducting coherence length. Dominated by a large reduction in the second nearest neighbor exchange energy [-1.2(2) meV/Fe], the overall reduction in magnetic interaction energy is Δ(Hmag) = -0.31(9) meV/Fe. Comparison to the superconducting condensation energy ΔESC = -0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe1+δSexTe1-x. [ABSTRACT FROM AUTHOR]

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe1-xCox)2As2 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. [ABSTRACT FROM AUTHOR]

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, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode. [ABSTRACT FROM AUTHOR]

The Jeff = ½ 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 Sr2IrO4, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical Jeff = ½ 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 Ca4IrO6 hosts the state, but furthermore uniquely resides in the limit required for a canonical unmixed Jeff = ½ state. [ABSTRACT FROM AUTHOR]

Measurements of magnetization, specific heat, electrical resistivity, Hall effect, and magnetoresistance on single crystalline samples of LaFeAsO grown in a NaAs flux are reported. While this material is known to be a semimetal, the temperature dependence of the electrical resistivity data presented herein is reminiscent of semiconducting behavior and exhibits distinct features associated with a structural transition and spin density wave (SDW) order. Low-temperature x-ray diffraction measurements have confirmed that the structural transition in these samples occurs near 140 K, compared to a transition temperature of 156 K observed in polycrystalline samples. Magnetoresistance and Hall coefficient measurements were performed in magnetic fields up to 9 T applied perpendicular to the basal plane using a van der Pauw configuration. The charge carrier density and mobility indicate that electrons are the majority charge carriers and exhibit features indicative of the structural transition and SDW formation. Isotherms of magnetoresistivity measured as a function of magnetic field can be scaled onto a single curve. [ABSTRACT FROM AUTHOR]

M-doped IrTe2 (M=Mn, Fe, Co, Ni) compounds were synthesized by solid-state reaction. Single crystal x-ray diffraction experiments indicate that part of the doped M ions (M=Fe, Co, and Ni) substitute for Ir, and the rest intercalate into the octahedral interstitial sites located in between IrTe2 layers. Due to the lattice mismatch between MnTe2 and IrTe2, Mn has limited solubility in IrTe2 lattice. The trigonal structure is stable in the whole temperature range 1.80 K= T = 300 K for all doped compositions. No long-range magnetic order or superconductivity was observed in any doped compositions above 1.80 K. A spin glass behavior below 10 K was observed in Fe-doped IrTe2 from the temperature dependence of magnetization, electrical resistivity, and specific heat. The low temperature specific heat data suggest the electron density of states is enhanced in Fe- and Co-doped compositions but reduced in Ni-doped IrTe2. With the 3d transition metal doping the trigonal a-lattice parameter increases but the c-lattice parameter decreases. Detailed analysis of the single crystal x-ray diffraction data shows that interlayer Te-Te distance increases despite a reduced c lattice. The importance of the Te-Te, Te-Ir, and Ir-Ir bonding is discussed. [ABSTRACT FROM AUTHOR]

The Fe1+y,Te1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials, we present systematic inelastic neutron scattering measurements of the low-energy spin excitations for x = 0.27, 0.36, 0.40, and 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2,1/2,0) wave vector with doping. Concentrations (x = 0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2,1/2,0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2,1/2,0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2,1/2,0) wave vector and the role that interstitial Fe may play. [ABSTRACT FROM AUTHOR]

The spin and orbital ordering in Dy1-xTbxVO3 (x = 0 and 0.2) was studied by measuring x-ray powder diffraction, magnetization, specific heat, and neutron single-crystal diffraction. The results show that G-OO/C-AF and C-OO/G-AF phases coexist in Dy0.8Tb0.20VO3 in the temperature range 2-60 K, and the volume fraction of each phase is temperature and field dependent. The ordering of Dy moments at T" = 12 K induces a transition from G-OO/C-AF to a C-OO/G-AF phase. Magnetic fields suppress the long-range order of Dy moments and thus the C-OO/G-AF phase below T". The polarized moments induced at the Dy sublattice by external magnetic fields couple to the V 3d moments, and this coupling favors the G-OO/C-AF state. Also discussed is the effect of the Dy-V magnetic interaction and local structure distortion on the spin and orbital ordering in Dy1-xTbxVO3. [ABSTRACT FROM AUTHOR]

The effects of temperature and pressure on phonons in B20 compounds FeSi1-xAlx were measured using inelastic neutron scattering and nuclear-resonant inelastic x-ray scattering. The effect of hole doping through Al substitution is compared to results of alloying with Co (electron doping) in Fe1-xCOxSi. While the temperature dependence of phonons in FeSi is highly anomalous, doping with either type of carriers leads to a recovery of the normal quasiharmonic behavior. Density functional theory (DFT) computations of the electronic band structure and phonons were performed. The anomaly in the temperature dependence of the phonons in undoped FeSi was related to the narrow band gap, and its sensitivity to the effect of thermal disordering by phonons. On the other hand, the pressure dependence of phonons at room temperature in undoped FeSi follows the quasihannonic behavior and is well reproduced by the DFT calculations. [ABSTRACT FROM AUTHOR]

Millimeter sized single crystals of Mo3Sb7 are grown using the self-flux technique and a thorough characterization of their structural, magnetic, thermal, and transport properties is reported. The structure parameters for the high-temperature cubic phase and the low-temperature tetragonal phase were determined with neutron single crystal diffraction. Both x-ray powder diffraction and neutron single crystal diffraction at room temperature confirmed that Mo3Sb7 crystallizes in Ir3Ge7-type cubic structure with space group Im ̅3 m. The cubic-tetragonal structure transition at 53 K is verified by the peak splitting of (4 0 0) reflection observed by x-ray single crystal diffraction and the dramatic intensity change of the (12 0 0) peak observed by neutron single crystal diffraction. The structural transition is accompanied by a sharp drop in magnetic susceptibility, electrical resistivity, and thermopower while cooling. A weak lambda anomaly was also observed around 53 K in the temperature dependence of specific heat, and the entropy change across the transition is estimated to be 1.80 J/mol Mo K. The temperature dependence of magnetic susceptibility was measured up to 750 K, and it follows a Curie-Weiss behavior above room temperature. Analysis of the low-temperature magnetic susceptibility suggests a spin gap of 110 K around 53 K. A typical phonon thermal conductivity was observed in the low temperature tetragonal phase. A glassy phonon thermal conductivity above 53 K suggests a structural instability in a wide temperature range. Superconductivity was observed at 2.35 K in the as-grown crystals, and the dimensionless specific heat jump ΔC(T)/γnTc was determined to be 1.49, which is slightly larger than the BCS value of 1.43 for the weak-coupling limit. [ABSTRACT FROM AUTHOR]

The chiral helimagnet Cr1/3NbS2 has been investigated by magnetic, transport, and thermal properties measurements on single crystals and by first-principles electronic structure calculations. From the measured field and temperature dependence of the magnetization for fields applied perpendicular to the c axis, the magnetic phase diagram has been constructed in the vicinity of the phase transition. A transition from a paramagnetic to a magnetically ordered phase occurs near 120 K. The transition is found to strongly affect the electrical transport. The resistivity decreases sharply upon cooling near 120 K, and the spin reorientation from the helimagnetic ground state to the commensurate ferromagnetic state is evident in the magnetoresistance. At high fields, a large magnetoresistance (55% at 140 kOe) is observed near the magnetic transition temperature. Heat capacity and electronic structure calculations show that the density of states at the Fermi level is low in the magnetically ordered state. [ABSTRACT FROM AUTHOR]

We have performed systematic studies of the lattice dynamics in LFeAsO (L = La, Ce, Pr, Nd, Sm) in the parent and in the --10% F-doped compounds as a function of pressure and temperature. We have found that the modifications in the partial Fe density of phonon states are mainly governed by the Fe-As bond length. The change of this bond length explains the change of the Fe density of phonon states above 25 meV. We further observe anomalies in the behavior of the phonon mode near 16 meV. In the parent phase, this mode softens anomalously upon cooling through the structural phase transition. Upon F doping, this mode hardens indicating a strong electron-phonon coupling. This suggests that the corresponding phonons play an important role in the competition between superconductivity and magnetism in these materials. [ABSTRACT FROM AUTHOR]

We have carried out a pressure study of the unconventional superconductor FeTe0.6Se0.4 up to 1.5 GPa by neutron scattering, resistivity, and magnetic susceptibility measurements. The neutron spin resonance energy and the superconducting transition temperature have been extracted as a function of applied pressure in samples obtained from the same crystal. Both increase with pressure up to a maximum at ≈1.3 GPa, directly demonstrating a correlation between these two fundamental parameters of unconventional superconductivity. A comparison between the quantitative evolution of Tc and the resonance energy as a function of applied pressure is also discussed. These measurements serve to demonstrate the feasibility of using pressure dependent inelastic neutron scattering to explore the relationship between the resonance energy and Tc in unconventional superconductors. [ABSTRACT FROM AUTHOR]

The layered 5d transition-metal oxide Sr2IrO4 has been shown to host a novel Jeff=½Mott spin-orbit insulating state with antiferromagnetic ordering, leading to comparisons with the layered cuprates. Here we study the effect of substituting Mn for Ir in single crystals of Sr2Ir0.9Mn0.1O4 through an investigation involving bulk measurements and resonant x-ray and neutron scattering. We observe a new long-range magnetic structure emerge upon doping through a reordering of the spins from the basal plane to the c axis with a reduced ordering temperature compared to Sr2IrO4. The strpng enhancement of the magnetic x-ray scattering intensity at the L3 edge relative to the L2 edge indicates that the Jeff=½ state is robust and capable of hosting a variety of ground states. [ABSTRACT FROM AUTHOR]

High-resolution scanning tunneling microscopy measurements on the (001) surface of BaFe²As², a parent compound of Fe-based high-T( superconductors, reveal a complex surface with coexisting structural phases. Large areas display a squarelike (√2 × √2)R45° structure with surface-pinned antiphase domains. The domain walls exhibit C2η symmetry, in contrast with the bulk C2„ geometric structure. We argue that the strong spin-lattice coupling at the surface results in the coexistence of structure and spin antiphase domain boundaries with C² symmetry. [ABSTRACT FROM AUTHOR]

We use core-valence-valence Auger spectra to probe the Coulomb repulsion between holes in the valence band of Fe pnictide superconductors. By comparing the two-hole final-state spectra to density functional theory calculations of the single-particle density of states, we extract a measure of the electron correlations that exist in these systems. Our results show that the Coulomb repulsion is highly screened and can definitively be considered as weak. We also find that there are differences between the 1111 and 122 families and even a small variation as a function of the doping x in Ba(Fe1-xCox)2As2. We discuss how the values of the hole-hole Coulomb repulsion obtained from our study relate to the onsite Coulomb parameter U used in model and first-principles calculations based on dynamical mean field theory and establish an upper bound for its effective value. Our results impose stringent constraints on model-based phase diagrams that vary with the quantity U or U/W by restricting the latter to a rather small range of values. [ABSTRACT FROM AUTHOR]

The noncentrosymmetric ferromagnet Cr11Ge19 has been investigated by electrical transport, AC and DC magnetization, heat capacity, x-ray diffraction, resonant ultrasound spectroscopy, and first principles electronic structure calculations. Complex itinerant ferromagnetism in this material is indicated by nonlinearity in conventional Arrott plots, unusual behavior of AC susceptibility, and a weak heat capacity anomaly near the Curie temperature (88 K). The inclusion of spin wave excitations was found to be important in modeling the low temperature heat capacity. The temperature dependence of the elastic moduli and lattice constants, including negative thermal expansion along the c axis at low temperatures, indicates strong magneto-elastic coupling in this system. Calculations show strong evidence for itinerant ferromagnetism and suggest a noncollinear ground state may be expected. [ABSTRACT FROM AUTHOR]

We present inelastic neutron scattering measurements and first principles calculations examining the intermetallic marcasite CrSb2. The observed spin-wave dispersion implies that the magnetic interactions are strongly one-dimensional with antiferromagnetic chains parallel to the crystalline c axis. Such low-dimensional excitations are unexpected in a semiconducting intermetallic system. Moreover, we observe a clear anisotropic thermal conductivity indicating that the magnetic anisotropy enhances thermoelectric properties along particular crystallographic directions. [ABSTRACT FROM AUTHOR]

Lattice dynamics and magnetic excitations are investigated to elucidate the origin of magneto-dielectric effects in the S = 1/2 two-dimensional quantum spin compound K2V308. We find evidence for lattice instabilities at 110 K and 60 K as optical phonon anomalies and a soft mode at 26 cm-1 in A, symmetry. Two-magnon excitations in B1 symmetry show an unconventional double-peak structure and temperature dependence. This suggests the existence of a split mode near the zone boundary caused by a mixing of spin and lattice modes. [ABSTRACT FROM AUTHOR]

The lattice dynamics of the filled skutterudite YbFe4Sb12 was studied by resonant ultrasound spectroscopy and an anomalous softening in the temperature dependence of the elastic constants at ~50 K was observed. This anomaly can not be explained by the dynamics of the filler, in contrast to other filled skutterudites. We have further investigated the origin of this anomaly using macroscopic and microscopic measurements. A rearrangement of the spectral weight of the Yb phonon states was observed in the temperature dependence of the density of phonon states, obtained by inelastic neutron scattering. We suggest that the anomaly is due to a change of the Yb valence state and that the anomaly and the phonon spectral weight rearrangement have the same origin. [ABSTRACT FROM AUTHOR]

We report the temperature dependence of the resistivity and thermoelectric power (TEP) under hydrostatic pressure of the itinerant antiferromagnet BaFe2As2 and the electron-doped superconductor Ba(Fe0.9Co0.1)2As2. We observe a hole-like contribution to the thermopower below the structural/magnetic (S-M) transition in the parent compound that is suppressed in magnitude and temperature with pressure. Pressure increases the contribution of electrons to transport in both the doped and undoped compound. In the 10% Co-doped sample we used a two-band model for thermopower to estimate the carrier concentrations and determine the effect of pressure on the band structure. [ABSTRACT FROM AUTHOR]

Atomically resolved structural and electronic properties of FeTe1-xSex (x = 0 and 0.45) have been studied with scanning tunneling microscopy/spectroscopy (STM/STS). In contrast to the extreme flatness of the Te-terminated FeTe surface, nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe0.55Se0.45. A statistical counting of the two kinds of atoms has the same ratio as that in the bulk. Remarkably, there is no electronic phase separation seen in the tunneling spectroscopy. This indicates that the optimally doped superconductor is chemically inhomogeneous but electronically homogeneous, in contrast to many correlated electron materials. [ABSTRACT FROM AUTHOR]

We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of Tc compared to LaFeAsO1-xFx is not due to rare-earth magnetism. [ABSTRACT FROM AUTHOR]

The CeFeAsO and CeFePO iron pnictide compounds were studied via electrical transport measurements under high pressure. In CeFeAsO polycrystals, the magnetic phases involving the Fe and Ce ions coexist for hydrostatically applied pressures up to 15 GPa, and with no signs of pressure-induced superconductivity up to 50 GPa for the less hydrostatic pressure techniques. For the CeFePO single crystals, pressure further stabilizes the Kondo screening of the Ce 4f-electron magnetic moments. [ABSTRACT FROM AUTHOR]

We systematically investigate the magnetic properties and local structure of Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this 5d4 compound. The experimental investigation involved comparison of the magnetic properties and atomic imaging of (1) a slow-cooled crystal, (2) a crystal quenched from 900°C after growth, and (3) a crystal grown using a faster cooling rate during growth than the slow-cooled one. Atomic-scale imaging by scanning transmission electron microscopy (STEM) shows that quenching from 900°C introduces Ir-rich antiphase boundaries in the crystals, and a faster cooling rate during crystal growth leads to clusters of Y and Ir antisite disorder. Compared to the slow-cooled crystals, Ba2YIrO6 crystals with clusters of antisite defects have a larger effective moment and a larger saturation moment, while quenched crystals with Ir-rich antiphase boundary show a slightly suppressed moment. Our DFT and model magnetic Hamiltonian calculations suggest magnetic condensation is unlikely, as the energy to be gained from superexchange is small compared to the spin-orbit gap. However, once Y is replaced by Ir in the antisite disordered region, the picture of local nonmagnetic singlets breaks down and magnetism can be induced. This is because of (a) enhanced interactions due to increased orbital overlap and (b) increased number of orbitals mediating the interactions. Our work highlights the importance of lattice defects in understanding the experimentally observed magnetism in Ba2YIrO6 and other J=0 systems. [ABSTRACT FROM AUTHOR]

The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (Tc). However, the absence of hole pockets near the Fermi level of the iron-selenide (FeSe) derived high-Tc superconductors raises a fundamental question of whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-Tc phase induced by pressure in bulk FeSe from magnetotransport measurements and first-principles calculations. With increasing pressure, the low-Tc superconducting phase transforms into the high-Tc phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in contrast to other FeSe-derived high-Tc systems. Moreover, the Hall coefficient is enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-Tc phase. These results in FeSe highlight similarities with high-Tc phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity. [ABSTRACT FROM AUTHOR]

Yb14MnSb11 is a rare example of a Kondo lattice compound with ferromagnetic dominated RKKY interactions. As a ferromagnetic semiconductor with Tc≈53 K, it is also a potential compound for exploration of spintronic devices. Here we describe measurements which answer remaining questions regarding the energy scales of the exchange interactions, the valence, and the magnetization density distribution in this system. We find that the system consists of RKKY exchange coupled Mn2+ sites with nearest and next nearest exchange interactions dominating the magnetic spectrum with no significant magnetization density localized on other atomic sites. The extended spread of a negative magnetization around each of the Mn ions supports a Kondo screening cloud scenario for Yb14MnSb11. [ABSTRACT FROM AUTHOR]

Prior to the superconducting transition at Tc × 2.3K, Mo3Sb7 undergoes a symmetry-lowering, cubic-to-tetragonal structural transition at Ts = 53K. We have monitored the pressure dependence of these two transitions by measuring the resistivity of Mo3Sb7 single crystals under various hydrostatic pressures up to 15 GPa. The application of external pressure enhances Tc but suppresses Ts until Pc 10GPa, above which a pressure-induced first-order structural transition takes place and is manifested by the phase coexistence in the pressure range 8 = P = 12GPa. The cubic phase above 12 GPa is also found to be superconducting with a higher Tc 6K that decreases slightly with further increasing pressure. The variations with pressure of Tc and Ts satisfy the Bilbro-McMillan equation, i.e. TcnTs1-n = constant, thus suggesting the competition of superconductivity with the structural transition that has been proposed to be accompanied with a spin-gap formation at Ts. Our first-principles calculations suggest the importance of magnetism that competes with the superconductivity in Mo3Sb7. [ABSTRACT FROM AUTHOR]

We report a neutron diffraction study of the multiferroic mechanism in (ND4)2FeCl5 · D2O, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at TN = 7.3 K, followed by a second transition to an IC cycloidal spin state at TFE = 6.8 K, the latter of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling, as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The second-order harmonic appears at TFE, thereby providing unambiguous evidence that the onset of the electric polarization is accompanied by a lattice modulation due to spin-lattice interaction. The neutron results, in conjunction with the negative thermal expansion and large magnetostriction observed in Ref. [19], indicate that spin-lattice coupling plays a critical role in the ferroelectric mechanism of (ND4)2FeCl5 · D2O. [ABSTRACT FROM AUTHOR]

The singlet ground-state magnetism in pyrochlore osmates Y2Os2O7 and Ho2Os2O7 is studied by dc and ac susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected nonmagnetic singlet in the strong spin-orbit coupling (SOC) limit for Os4+ (5d4), Y2Os2O7 exhibits a spin-glass ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the nonmagnetic state in the strong SOC limit and a magnetic state in the strong superexchange limit. Ho2Os2O7 has the same structural distortion as in Y2Os2O7; however, the Os sublattice in Ho2Os2O7 shows long-range magnetic ordering below 36 K. The sharp difference of the magnetic ground state between Y2Os2O7 and Ho2Os2O7 signals that the singlet ground-state magnetism in R2Os2O7 is fragile and can be disturbed by the weak 4f-5d interactions. [ABSTRACT FROM AUTHOR]

We investigate the magnetic properties of the series Sr2Ir1-xRuxO4 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 Ir4+ ions are replaced with an increasing concentration of 4d Ru4+ ions. The magnetic structures within the ordered regime of the phase diagram (x<0.3) are reported. Despite the changes in magnetic structure no alteration of the Jeff=1/2 ground state is observed. The behavior of Sr2Ir1-xRuxO4 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. [ABSTRACT FROM AUTHOR]

The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (T&#95;{c}). However, the absence of hole pockets near the Fermi level of the iron-selenide (FeSe) derived high-T&#95;{c} superconductors raises a fundamental question of whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-T&#95;{c} phase induced by pressure in bulk FeSe from magnetotransport measurements and first-principles calculations. With increasing pressure, the low-T&#95;{c} superconducting phase transforms into the high-T&#95;{c} phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in contrast to other FeSe-derived high-T&#95;{c} systems. Moreover, the Hall coefficient is enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-T&#95;{c} phase. These results in FeSe highlight similarities with high-T&#95;{c} phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity.

It is shown that attempts to accurately deduce the magnetic penetration depth λ of overdoped BaFe(1.82)Co(0.18)As₂ single crystals by transverse-field muon spin rotation (TF μSR) are thwarted by field-induced magnetic order and strong vortex-lattice disorder. We explain how substantial deviations from the magnetic field distribution of a nearly perfect vortex lattice by one or both of these factors is also significant for other iron-arsenic superconductors, and this introduces considerable uncertainty in the values of λ obtained by TF μSR.

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.

We report neutron scattering measurements on single crystals of BaFe1.92Co0.08As2. The magnetic Bragg peak intensity is reduced by 6% upon cooling through TC. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional interactions. Below TC additional intensity appears at an energy of approximately 4.5(0.5) meV, similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below TC for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance, demonstrating the direct competition between static magnetic order and superconductivity.

BaFe2As2 exhibits properties that are characteristic of the parent compounds of the newly discovered iron (Fe)-based high-T(C) superconductors. By combining real-space imaging of scanning tunneling microscopy and spectroscopy (STM+STS) with momentum-space quantitative low-energy electron diffraction (LEED), we have identified the surface plane of cleaved BaFe2As2 crystals as the As terminated Fe-As layer-the plane where superconductivity occurs. LEED and STM+STS data on the BaFe2As2(001) surface indicate an ordered arsenic (As) terminated metallic surface without reconstruction or lattice distortion. It is surprising that STM images the different Fe-As orbitals associated with the orthorhombic structure, but not the As atoms in the surface plane.

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wave vectors (1/2 1/2 L) in tetragonal notation. On cooling below T&#95;{C}, a clear resonance peak is observed at this wave vector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) k&#95;{B}T&#95;{C}. This is in good agreement with the canonical value of 5 k&#95;{B}T&#95;{C} observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c axis, indicating that the magnetic fluctuations are two dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.