Search Results (23 total)

Neutron diffraction measurements have been performed on a powder sample of BaMn₂As₂ over the temperature T range from 10 to 675 K. These measurements demonstrate that this compound exhibits collinear antiferromagnetic ordering below the Néel temperature T_N=625(1) K. The ordered moment μ=3.88(4)μ_B/Mn at T=10 K is oriented along the c axis and the magnetic structure is G type, with all nearest-neighbor Mn moments antiferromagnetically aligned. The value of the ordered moment indicates that the oxidation state of Mn is Mn²⁺ with a high spin S=5/2. The T dependence of μ suggests that the magnetic transition is second order in nature. In contrast to the closely related AFe₂As₂ (A=Ca, Sr, Ba, and Eu) compounds, no structural distortion is observed in the magnetically ordered state of BaMn₂As₂. Our results indicate that while next-nearest-neighbor interactions are important in the AFe₂As₂ materials, nearest-neighbor interactions are dominant in BaMn₂As₂.

The S=2 anisotropic triangular lattice α-NaMnO₂ is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T≤45  K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Δ≃7.5  meV) has been analyzed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k_B≃73  K), along the monoclinic b axis and a sizable easy-axis magnetic anisotropy (|D|/k_B≃3  K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. α-NaMnO₂ therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.

The new α-Fe(Te,Se) superconductors share the common iron building block and ferminology with the LaFeAsO and BaFe₂As₂ families of superconductors. In contrast with the predicted commensurate spin-density-wave order at the nesting wave vector (π, 0), a completely different magnetic order with a composition tunable propagation vector (δπ, δπ) was determined for the parent compound Fe_1+yTe in this powder and single-crystal neutron diffraction study. The new antiferromagnetic order survives as a short-range one even in the highest T_C sample. An alternative to the prevailing nesting Fermi surface mechanism is required to understand the latest family of ferrous superconductors.

We report the observation of superconductivity in La₃Ni₄P₄O₂ at 2.2 K. The layer stacking in this compound results in an asymmetric distribution of charge reservoir layers around the Ni₂P₂ planes. The estimated Wilson ratio, R_W≈5, indicates the presence of strongly enhanced normal-state susceptibility, but many of the basic superconducting characteristics are conventional. The estimated electronic contribution to the specific heat, γ≈6.2 mJ mol-Ni⁻¹ K⁻², is about 2/3 of that found in layered nickel borocarbide superconductors.

Using single crystal inelastic neutron scattering with and without the application of an external magnetic field and powder neutron diffraction, we have characterized magnetic interactions in Ba₃Cr₂O₈. Even without a field, we found that there exist three singlet-to-triplet excitation modes in the (h, h, l) scattering plane. Our complete analysis shows that the three modes are due to spatially anisotropic interdimer interactions that are induced by lattice distortions of the tetrahedron of oxygens surrounding the Jahn-Teller active Cr⁵⁺(3d¹). The strong intradimer coupling of J₀=2.38(2)  meV and weak interdimer interactions (|J_inter|≤0.52(2)  meV) makes Ba₃Cr₂O₈ a good model system for weakly coupled s=1/2 quantum spin dimers.

The recent discovery of superconductivity in (Ba,K)Fe₂As₂, which crystallizes in the ThCr₂Si₂ (122) structure as compared with the LnFeAsO (Ln is lanthanide) systems that possess the ZrCuSiAs (1111) structure, demonstrates the exciting potential of the FeAs-based materials for high-T_C superconductivity. Here we report neutron diffraction studies that show a tetragonal-to-orthorhombic distortion associated with the onset of q=(101) antiferromagnetic order in BaFe₂As₂, with a saturation moment 0.87(3)μ_B per Fe that is orientated along the longer a axis of the ab planes. The simultaneous first-order structural and magnetic transition is in contrast with the separated transitions previously reported in the 1111-type materials. The orientational relation between magnetic alignment and lattice distortion supports a multiorbital nature for the magnetic order.

The transition temperature T_C≈26  K of the recently discovered superconductor LaFeAsO_1-xF_x is extremely sensitive to the lanthanide ion, reaching 55 K for the Sm containing oxypnictides. Therefore, it is important to determine how the moment on the lanthanide affects the overall magnetism in these systems. Here we report a neutron diffraction study of the Nd oxypnictides. Long-ranged antiferromagnetic order is apparent in NdFeAsO below 1.96 K. Rietveld refinement shows that both Fe and Nd magnetic ordering are required to describe the observed data with the staggered moment 1.55(4)μ_B/Nd and 0.9(1)μ_B/Fe at 0.3 K. The other structural properties such as the tetragonal-orthorhombic distortion are found to be very similar to those in LaFeAsO. Neither the magnetic ordering nor the structural distortion occur in the superconducting sample NdFeAsO_0.80F_0.20 at any temperatures down to 1.5 K.

Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe₂As₂ undergoes a pressure-induced transition to a nonmagnetic volume “collapsed” tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.

Approximants consisting of 1–165 Si atoms with complete fluorine (F), hydroxyl (OH), amino (NH₂), methyl (CH₃), and hydrogen (H) termination were investigated by density-functional-Hartree-Fock (DF-HF) ground-state (GS) calculations within the B3LYP hybride functional formalism using a 6–31G(d) Gaussian type molecular-orbital (MO) basis set. With increasing polarity of the Si interface bonds, the highest occupied MO (HOMO)-lowest unoccupied MO (LUMO)-gaps depend less on quantum confinement but more on the nature of the interface. The HOMO and LUMO positions are a function of the anion/functional group intended to model different dielectric matrices hosting the Si quantum dot (QD). We discuss the influence of the Si interface bond termination on charge transfer and local breakdown of the MO symmetry. Relating these observations to the ratio of interface anions to Si core atoms, we estimate a size range below which the electronic structure of Si QDs is determined exclusively by the nature of the interface bonds. For Si cores consisting of ≥10 Si atoms, the interface governs the electronic structure with quantum confinement competing for covalent and weak polar interface terminations or being only a secondary effect for strong polar interface terminations. We estimate that the interface has a major impact on the electronic structure of Si QDs in Si₃N₄, SiO₂, or fluorides to a Si core size of ca. 1330 atoms.

DC and ac magnetization, resistivity, specific-heat, and neutron-diffraction data reveal that stoichiometric LaFePO is metallic and non-superconducting above T=0.35 K, with γ=12.5mJ / mol K². Neutron-diffraction data at room temperature and T=10 K are well described by the stoichiometric, tetragonal ZrCuSiAs structure, and show no signs of structural distortions or long-range magnetic ordering to an estimated detectability limit of 0.07μ_B/Fe. We propose a model based on the shape of the iron-pnictide tetrahedron that explains the differences between LaFePO and LaFeAsO, the parent compound of the recently discovered high-Tc oxyarsenides, which, in contrast, shows both structural and spin-density wave transitions.

The magnetic and crystal structures of the α-NaMnO₂ have been determined by high-resolution neutron powder diffraction. The system maps out a frustrated triangular spin lattice with anisotropic interactions that displays two-dimensional spin correlations below 200 K. Magnetic frustration is lifted through magneto-elastic coupling, evidenced by strong anisotropic broadening of the diffraction profiles at high temperature and ultimately by a structural phase transition at 45 K. In this low-temperature regime a three-dimensional antiferromagnetic state is observed with a propagation vector k=(1 / 2,1 / 2,0).

Critical exponents closely matching those of the N=2 chiral universality class have been obtained for the layered magnetic system cobalt glycerolate using muon spin relaxation. This class was originally introduced to represent geometrically frustrated triangular stacked-layer XY magnets with chiral noncollinear spin structures. Since the present magnetic system is a canted XY system without geometrical frustration or chiral degeneracy, the results indicate that the order parameter for canting in this system plays a similar role to the chiral order parameter in the geometrically frustrated systems, strongly suggesting that both types of noncollinear system share the same universality class.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

Rietveld refinement of powder neutron diffraction data, combined with group theory considerations, is used to determine the magnetic structures of the binary metal dicyanamide, M^II[N(CN)₂]₂ where M=Ni, Co, Fe, Mn, Ni_0.5Co_0.5, and Ni_0.5Fe_0.5. Compounds with M=Mn or Fe show a canted antiferromagnetic arrangement of spin oriented in the ab crystallographic plane, with antiparallel components of the two sublattices along the a axis and parallel along the b axis. Symmetry considerations forbid an additional moment, whether compensated or not, to be present along the c axis. The compounds with fewer unpaired electrons (Co and Ni) are ferromagnets, with all moments oriented along the c axis. The mixed composition of Ni_0.5Co_0.5 displays the same collinear ferromagnetic structure as its parent compounds. However, the composition with M=Ni_0.5Fe_0.5, whose parent compounds show different magnetic behavior, does not exhibit long-range magnetic ordering down to 1.7 K. Magnetostriction was observed for the ferromagnets for which we investigated the variable temperature powder neutron diffraction. The cobalt-rich compounds show more pronounced effects, consistent with their increasing magnetocrystalline anisotropy.

Magnetic susceptibility and zero-field muon spectroscopy (ZF-μ⁺SR) measurements of monoclinic Nb₁₂O₂₉ are reported. The magnetic susceptibility shows a broad maximum at around 12 K with no divergence between the zero-field cooled and field-cooled susceptibility, implying low-dimensional antiferromagnetic ordering. Application of the Bonner-Fischer model, applicable for a uniform S=1/2 linear Heisenberg chain, gives an exchange energy of J/k=12.71 K and substantially reduced g=0.556. In this magnetically dilute system, the observation of an oscillating signal in the ZF-μ⁺SR experiments is a clear and unambiguous signature of static magnetic correlations. We argue that this is due to an incommensurate arrangement of the Nb⁴⁺ electronic moments, similar to a spin-density wave. This phase sets in below T_f≈10 K and produces spontaneous muon-spin precession associated with two distinct internal local fields that saturate at 39.3(6) and 138(1) G at 60 mK.

The rare-earth pyrochlore material Gd₂Ti₂O₇ is considered to be an ideal model frustrated Heisenberg antiferromagnet with additional dipolar interactions. For this system there are several untested theoretical predictions of the ground state ordering pattern. Here we establish the magnetic structure of isotopically enriched ¹⁶⁰Gd₂Ti₂O₇, using powder neutron diffraction at a temperature of 50 mK. The magnetic structure at this temperature is a partially ordered, noncollinear antiferromagnetic structure, with propagation vector k=1/21/21/2. It can be described as a set of “q=0” ordered kagomé planes separated by zero interstitial moments. This magnetic structure agrees with theory only in part, leaving an interesting problem for future research.

The development and initial results of a method for the determination of differential cross sections for electron scattering by molecular oxygen are described. The method has been incorporated into an existing package of computer programs which, given spectroscopic factors, dissociation energies and an energy-loss spectrum for electron-impact excitation, determine the differential cross sections for each electronic state relative to that of the elastic peak. Enhancements of the original code were made to deal with particular aspects of electron scattering from O₂, such as the overlap of vibrational levels of the ground state with transitions to excited states, and transitions to levels close to and above the dissocation energy in the Herzberg and Schumann-Runge continua. The utility of the code is specifically demonstrated for the “6-eV states” of O₂, where we report absolute differential cross sections for their excitation by 15-eV electrons. In addition an integral cross section, derived from the differential cross section measurements, is also reported for this excitation process and compared against available theoretical results. The present differential and integral cross sections for excitation of the “6-eV states” of O₂ are the first to be reported in the literature for electron-impact energies below 20 eV.

Zero field muon spin relaxation (μSR) has been used to study the magnetic properties of n=2 Ruddlesden-Popper phases Sr₂RMn₂O₇, where R=Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho. The results show that the size of the lanthanide ion is crucial in determining the magnetic state and dynamics of the system. Because muons are implanted throughout the bulk of the sample, impurity phases contribute only according to their volume fraction. Hence in the case of biphasic samples the data are dominated by the majority phase. Although none of our samples has a ferromagnetic ground state, colossal magnetoresistance (CMR) is observed over a wide temperature range, 4 K<T<150 K, for both the Pr and Nd compounds. The μSR results show that the magnetic transition in both these samples is broad. Ordered, but fluctuating, regions form at ∼150 K, the reported onset of CMR, with the fluctuation rates gradually decreasing with temperature. Even at 5 K, fluctuations are still observed. The ferromagnetic double exchange between Mn ions becomes weaker as the size of the lanthanide ion decreases. Sr₂SmMn₂O₇ shows weak clustering at a much reduced temperature of 30 K whereas Sr₂EuMn₂O₇ shows spin-glass-like behavior. For all lanthanide ions smaller than Eu no long range magnetic ordering of the spins is observed and the observed relaxation rates follow an activated dependence. The technique allows us to extract the effective activation energy associated with the magnetic fluctuations of the lanthanide moments in samples with R=Sm, Eu, Gd, Tb, Dy, and Ho. CMR is only observed where μSR measurements show a broad magnetic transition associated with fluctuations. We therefore believe that these fluctuating ordered regions are responsible for the extended temperature regime in which CMR has been observed in these nonferromagnetic n=2 Ruddlesden-Popper phases.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

The muon anomalous magnetic moment has been measured in a new experiment at Brookhaven. Polarized muons were stored in a superferric ring, and the angular frequency difference, ω_a, between the spin precession and orbital frequencies was determined by measuring the time distribution of high-energy decay positrons. The ratio R of ω_a to the Larmor precession frequency of free protons, ω_p, in the storage-ring magnetic field was measured. We find R  =  3.707 220(48)×10^-3. With μ_μ/μ_p  =  3.183 345 47(47) this gives a_μ⁺  =  1 165 925(15)×10^-9 ( ±13 ppm), in good agreement with the previous CERN measurements for μ⁺ and μ^- and of approximately the same precision.

We report muon-spin-rotation (μSR) measurements on a series of compounds with composition La_2-xSr_xNiO_4+δ with strontium doping level x between 0 and 1. A magnetic transition is found in all the samples studied, which occurs at a composition-dependent temperature, T_M. Below T_M clear precession signals are observed in zero applied magnetic field for all x, indicating the existence of at least short-range magnetic order on a time scale longer than 10^-8 s. Above T_M the magnetic correlation times decrease by several orders of magnitude. At x≈0.33, we find peaks in both T_M and the zero-temperature staggered magnetization, an observation which we attribute to the higher degree of localization of the holes at this doping level. The measurements of T_M as a function of x extend the determination of Néel temperatures by previous neutron diffraction and μSR measurements.

Neutron powder diffraction techniques have been used to refine the crystal and magnetic structures of the Ruddlesden Popper (RP) compounds Sr_2-xNd_1+xMn₂O₇ (x=0.0, 0.1), which show colossal magnetoresistance below 100 K. Samples of both compositions have been shown to be biphasic, although in each case both phases are of the RP type and have very similar structural parameters. Only one phase in each sample shows long range magnetic order. For x=0.0, the Mn cations and the Nd cations in the rocksalt layers of the structure are antiferromagnetically ordered in a noncollinear magnetic structure below 28 K; the Nd cations within the perovskite blocks do not show long range magnetic order. In the temperature range 28⩽T/K⩽137, only the Mn cations are ordered. The decoupling of the Nd cations is accompanied by a rotation of the Mn moments from a direction close (φ=27°) to the z axis into the xy plane. A similar temperature dependence is seen in the case x=0.1, although the ordered components of the cation moments are much reduced and the low temperature phase has a collinear magnetic structure with the moments aligned along z. The magnetotransport properties of these samples are discussed in the light of their crystal and magnetic structures. © 1996 The American Physical Society.

We report muon-spin-relaxation measurements on a series of compounds with composition La_2-xSr_xNiO_4+δ where the net hole concentration x+2δ is greater than 0.4. A magnetic transition is found in all the samples studied which occurs at a composition-dependent temperature T_M. Below T_M clear precession signals are observed in zero applied magnetic field, indicating the existence of at least short-range magnetic order on a time scale greater than 10^-8 s. Above T_M the correlation times decrease by several orders of magnitude. The measurements of T_M as a function of the net hole concentration extend the determination of Néel temperatures by previous neutron diffraction measurements to higher doping levels.