Magnetic form factor of SrFe2_2As2_2

Neutron diffraction measurements have been carried out to investigate the magnetic form factor of the parent SrFe2As2 system of the iron-based superconductors. The general feature is that the form factor is approximately isotropic in wave vector, indicating that multiple d-orbitals of the iron atoms are occupied as expected based on band theory. Inversion of the diffraction data suggests that there is some elongation of the spin density toward the As atoms. We have also extended the diffraction measurements to investigate a possible jump in the c-axis lattice parameter at the structural phase transition, but find no detectable change within the experimental uncertainties

Evolution of the bulk properties, structure, magnetic order, and superconductivity with Ni doping in CaFe2-xNixAs2

Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray diffraction have been used to characterize the properties of single crystalline CaFe2-xNixAs2 as a function of Ni doping for x varying from 0 to 0.1. The combined first-order structural and magnetic phase transitions occur together in the undoped system at 172 K, with a small decrease in the area of the a-b plane along with an abrupt increase in the length of the c-axis in the orthorhombic phase. With increasing x the ordered moment and transition temperature decrease, but the transition remains sharp at modest doping while the area of the a-b plane quickly decreases and then saturates. Warming and cooling data in the resistivity and neutron diffraction indicate hysteresis of ~2 K. At larger doping the transition is more rounded, and decreases to zero for x=0.06. The susceptibility is anisotropic for all values of x. Electrical resistivity for x = 0.053 and 0.06 shows a superconducting transition with an onset of nearly 15 K which is further corroborated by substantial diamagnetic susceptibility. For the fully superconducting sample there is no long range magnetic order and the structure remains tetragonal at all temperature, but there is an anomalous increase in the area of the a-b plane in going to low T. Heat capacity data show that the density of states at the Fermi level increases for x > 0.053 as inferred from the value of Sommerfeld coefficient. The regime of superconductivity is quite restrictive, with a maximum TC of 15 K and an upper critical field Hc2=14 T. Superconductivity disappears in the overdoped region.Comment: 14 pages, 12 figures. Submitted to Phys. Rev.

Short-range incommensurate magnetic order near the superconducting phase boundary in Fe(1+d)Te(1-x)Se(x)

We performed elastic neutron scattering and magnetization measurements on Fe(1.07)Te(0.75)Se(0.25) and FeTe(0.7)Se(0.3). Short-range incommensurate magnetic order is observed in both samples. In the former sample with higher Fe content, a broad magnetic peak appears around (0.46,0,0.5) at low temperature, while in FeTe(0.7)Se(0.3) the broad magnetic peak is found to be closer to the antiferromagnetic (AFM) wave-vector (0.5,0,0.5). The incommensurate peaks are only observed on one side of the AFM wave-vector for both samples, which can be modeled in terms of an imbalance of ferromagnetic/antiferromagnetic correlations between nearest-neighbor spins. We also find that with higher Se (and lower Fe) concentration, the magnetic order becomes weaker while the superconducting temperature and volume increase.Comment: Version as appeared in PR

First-order nature of the ferromagnetic phase transition in (La-Ca)MnO_3 near optimal doping

Neutron scattering has been used to study the nature of the ferromagnetic transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the naturally occurring O-16 can be replaced with the O-18 isotope. Small angle neutron scattering on the x=0.3 single crystal reveals a discontinuous change in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1. Strong relaxation effects are observed for this domain scattering, for the magnetic order parameter, and for the quasielastic scattering, demonstrating that the transition is not continuous in nature. There is a large oxygen isotope effect observed for the T_C in the polycrystalline samples. For the optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5 K at 90% O-18 substitution. The temperature dependence of the spin-wave stiffness is found to be identical for the two samples despite changes in T_C. Hence, T_C is not solely determined by the magnetic subsystem, but instead the ferromagnetic phase is truncated by the formation of polarons which cause an abrupt transition to the paramagnetic, insulating state. Application of uniaxial stress in the x=0.3 single crystal sharply enhances the polaron scattering at room temperature. Measurements of the phonon density-of-states show only modest differences above and below T_C and between the two different isotopic samples.Comment: 13 pages, 16 figures, submitted to Phys. Rev.

Neel to Spin-Glass-like Phase Transition versus Dilution in Geometrically Frustrated ZnCr_{2-2x}Ga_{2x}O_4

ZnCr2O4 undergoes a first order spin-Peierls-like phase transition at 12.5 K from a cubic spin liquid phase to a tetragonal Neel state. Using powder diffraction and single crystal polarized neutron scattering, we determined the complex spin structure of the Neel phase. This phase consisted of several magnetic domains with different characteristic wave vectors. This indicates that the tetragonal phase of ZnCr2O4 is very close to a critical point surrounded by many different Neel states. We have also studied, using elastic and inelastic neutron scattering techniques, the effect of nonmagnetic dilution on magnetic correlations in ZnCr_{2-2x}Ga_{2x}O_4 (x=0.05 and 0.3). For x=0.05, the magnetic correlations do not change qualitatively from those in the pure material, except that the phase transition becomes second order. For x= 0.3, the spin-spin correlations become short range. Interestingly, the spatial correlations of the frozen spins in the x=0.3 material are the same as those of the fluctuating moments in the pure and the weakly diluted materials

Legacy Vehicle Fuel System Testing with Intermediate Ethanol Blends

The effects of E10 and E17 on legacy fuel system components from three common mid-1990s vintage vehicle models (Ford, GM, and Toyota) were studied. The fuel systems comprised a fuel sending unit with pump, a fuel rail and integrated pressure regulator, and the fuel injectors. The fuel system components were characterized and then installed and tested in sample aging test rigs to simulate the exposure and operation of the fuel system components in an operating vehicle. The fuel injectors were cycled with varying pulse widths during pump operation. Operational performance, such as fuel flow and pressure, was monitored during the aging tests. Both of the Toyota fuel pumps demonstrated some degradation in performance during testing. Six injectors were tested in each aging rig. The Ford and GM injectors showed little change over the aging tests. Overall, based on the results of both the fuel pump testing and the fuel injector testing, no major failures were observed that could be attributed to E17 exposure. The unknown fuel component histories add a large uncertainty to the aging tests. Acquiring fuel system components from operational legacy vehicles would reduce the uncertainty

Spectroscopy with random and displaced random ensembles

Due to the time reversal invariance of the angular momentum operator J^2, the average energies and variances at fixed J for random two-body Hamiltonians exhibit odd-even-J staggering, that may be especially strong for J=0. It is shown that upon ensemble averaging over random runs, this behaviour is reflected in the yrast states. Displaced (attractive) random ensembles lead to rotational spectra with strongly enhanced BE2 transitions for a certain class of model spaces. It is explained how to generalize these results to other forms of collectivity.Comment: 4 pages, 4 figure