Spin Glass and Semiconducting Behavior in 1D BaFe2-{\delta}Se3 Crystals

We investigate the physical properties and electronic structure of BaFe2-{\delta}Se3 crystals, which were grown out of tellurium flux. The crystal structure of the compound, an iron-deficient derivative of the ThCr2Si2-type, is built upon edge-shared FeSe4 tetrahedra fused into double chains. The semiconducting BaFe2-{\delta}Se3 with {\delta} \approx 0.2 ({\rho}295K = 0.18 {\Omega}\cdotcm and Eg = 0.30 eV) does not order magnetically, however there is evidence for short-range magnetic correlations of spin glass type (Tf \approx 50 K) in magnetization, heat capacity and neutron diffraction results. A one-third substitution of selenium with sulfur leads to a slightly higher electrical conductivity ({\rho}295K = 0.11 {\Omega}\cdotcm and Eg = 0.22 eV) and a lower spin glass freezing temperature (Tf \approx 15 K), corroborating with higher electrical conductivity reported for BaFe2S3. According to the electronic structure calculations, BaFe2Se3 can be considered as a one-dimensional ladder structure with a weak interchain coupling.Comment: 17 pages, 9 figure

Pressure induced renormalization of energy scales in the unconventional superconductor FeTe0.6Se0.4

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. We have extracted the neutron spin resonance energy and the superconducting transition temperature as a function of applied pressure. Both increase with pressure up to a maximum at ~1.3 GPa. This analogous qualitative behavior is evidence for a correlation between these two fundamental parameters of unconventional superconductivity. However, Tc and the resonance energy do not scale linearly and thus a simple relationship between these energies does not exist even in a single sample. The renormalization of the resonance energy relative to the transition temperature is here attributed to an increased hybridization. The present results appear to be consistent with a pressure-induced weakening of the coupling strength associated with the fundamental pairing mechanism.Comment: 5 pages, 4 figure

Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe

Local symmetry breaking in complex materials is emerging as an important contributor to materials properties but is inherently difficult to study. Here we follow up an earlier structural observation of such a local symmetry broken phase in the technologically important compound PbTe with a study of the lattice dynamics using inelastic neutron scattering (INS). We show that the lattice dynamics are responsive to the local symmetry broken phase, giving key insights in the behavior of PbTe, but also revealing INS as a powerful tool for studying local structure. The new result is the observation of the unexpected appearance on warming of a new zone center phonon branch in PbTe. In a harmonic solid the number of phonon branches is strictly determined by the contents and symmetry of the unit cell. The appearance of the new mode indicates a crossover to a dynamic lower symmetry structure with increasing temperature. No structural transition is seen crystallographically but the appearance of the new mode in inelastic neutron scattering coincides with the observation of local Pb off-centering dipoles observed in the local structure. The observation resembles relaxor ferroelectricity but since there are no inhomogeneous dopants in pure PbTe this anomalous behavior is an intrinsic response of the system. We call such an appearance of dipoles out of a non-dipolar ground-state "emphanisis" meaning the appearance out of nothing. It cannot be explained within the framework of conventional phase transition theories such as soft-mode theory and challenges our basic understanding of the physics of materials

MCViNE -- An object oriented Monte Carlo neutron ray tracing simulation package

MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo (MC) neutron ray-tracing program that provides researchers with tools for performing computer modeling and simulations that mirror real neutron scattering experiments. By adopting modern software engineering practices such as using composite and visitor design patterns for representing and accessing neutron scatterers, and using recursive algorithms for multiple scattering, MCViNE is flexible enough to handle sophisticated neutron scattering problems including, for example, neutron detection by complex detector systems, and single and multiple scattering events in a variety of samples and sample environments. In addition, MCViNE can take advantage of simulation components in linear-chain-based MC ray tracing packages widely used in instrument design and optimization, as well as NumPy-based components that make prototypes useful and easy to develop. These developments have enabled us to carry out detailed simulations of neutron scattering experiments with non-trivial samples in time-of-flight inelastic instruments at the Spallation Neutron Source. Examples of such simulations for powder and single-crystal samples with various scattering kernels, including kernels for phonon and magnon scattering, are presented. With simulations that closely reproduce experimental results, scattering mechanisms can be turned on and off to determine how they contribute to the measured scattering intensities, improving our understanding of the underlying physics.Comment: 34 pages, 14 figure

Spin-dynamics of the low-dimensional magnet (CH3)2NH2CuCl3

Dimethylammonium copper (II) chloride (also known as DMACuCl3 or MCCL) is a low dimensional S=1/2 quantum spin system proposed to be an alternating ferro-antiferromagnetic chain with similar magnitude ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. Subsequently, it was shown that the existing bulk measurements could be adequately modeled by considering DMACuCl3 as independent AFM and FM dimer spin pairs. We present here new inelastic neutron scattering measurements of the spin-excitations in single crystals of DMACuCl3. These results show significant quasi-one-dimensional coupling, however the magnetic excitations do not propagate along the expected direction. We observe a band of excitations with a gap of 0.95 meV and a bandwidth of 0.82 meV.Comment: 3 pages, 2 figures included in text, submitted to proceedings of International Conference on Neutron Scattering, December 200

The valence-fluctuating ground state of plutonium

A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise because of the competition of localized and itinerant electronic degrees of freedom. Although the respective limits of well-localized or entirely itinerant ground states are well understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and nonbonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the long-standing controversy between experiment and theory on plutonium’s magnetism but also suggest an improved understanding of the effects of such electronic dichotomy in complex materials.JRC.E.6-Actinide researc

Tracing cosmic evolution with clusters of galaxies

The most successful cosmological models to date envision structure formation as a hierarchical process in which gravity is constantly drawing lumps of matter together to form increasingly larger structures. Clusters of galaxies currently sit atop this hierarchy as the largest objects that have had time to collapse under the influence of their own gravity. Thus, their appearance on the cosmic scene is also relatively recent. Two features of clusters make them uniquely useful tracers of cosmic evolution. First, clusters are the biggest things whose masses we can reliably measure because they are the largest objects to have undergone gravitational relaxation and entered into virial equilibrium. Mass measurements of nearby clusters can therefore be used to determine the amount of structure in the universe on scales of 10^14 to 10^15 solar masses, and comparisons of the present-day cluster mass distribution with the mass distribution at earlier times can be used to measure the rate of structure formation, placing important constraints on cosmological models. Second, clusters are essentially ``closed boxes'' that retain all their gaseous matter, despite the enormous energy input associated with supernovae and active galactic nuclei, because the gravitational potential wells of clusters are so deep. The baryonic component of clusters therefore contains a wealth of information about the processes associated with galaxy formation, including the efficiency with which baryons are converted into stars and the effects of the resulting feedback processes on galaxy formation. This article reviews our theoretical understanding of both the dark-matter component and the baryonic component of clusters. (Abridged)Comment: 54 pages, 15 figures, Rev. Mod. Phys. (in press

Phonon Lifetime Investigation of Anharmonicity and Thermal Conductivity of UO₂ by Neutron Scattering and Theory

Inelastic neutron scattering measurements of individual phonon lifetimes and dispersion at 295 and 1200 K have been used to probe anharmonicity and thermal conductivity in UO2. They show that longitudinal optic phonon modes carry the largest amount of heat, in contrast to past simulations and that the total conductivity demonstrates a quantitative correspondence between microscopic and macroscopic phonon physics. We have further performed first-principles simulations for UO2 showing semiquantitative agreement with phonon lifetimes at 295 K, but larger anharmonicity than measured at 1200 K

Effect of Molybdenum 4d Hole Substitution in BaFe2As2

We investigate the thermodynamic and transport properties of molybdenum-doped BaFe2As2 (122) crystals, the first report of hole doping using a 4d element. The chemical substitution of Mo in place of Fe is possible up to ~ 7%. For Ba(Fe1-xMox)2As2, the suppression rate of the magnetic transition temperature with x is the same as in 3d Cr-doped 122 and is independent of the unit cell changes. This illustrates that temperature-composition phase diagram for hole-doped 122 can be simply parameterized by x, similar to the electron-doped 122 systems found in literature. Compared to 122 with a coupled antiferromagnetic order (TN) and orthorhombic structural transition (To) at ~ 132 K, 1.3% Mo-doped 122 (x = 0.013) gives TN = To = 125(1) K according to neutron diffraction results and features in specific heat, magnetic susceptibility and electrical resistivity. The cell volume expands by ~ 1% with maximum Mo-doping and TN is reduced to ~ 90 K. There is a new T* feature that is identified for lightly Cr- or Mo-doped (< 3%) 122 crystals, which is x dependent. This low-temperature transition may be a trace of superconductivity or it may have another electronic or magnetic origin.Comment: 10 pages, 6 figure