Enrico Faulhaber

Distinguishing s(+/-) and s(++) electron pairing symmetries by neutron spin resonance in superconducting NaFe0.935Co0.045As

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechanism for superconductivity. For iron pnictide superconductors, the s(+/-)-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies (E <= 2 Delta) below T-c. On the other hand, the s(++)-pairing symmetry expects a broad spin excitation enhancement at an energy above 2 Delta below Tc. Although the resonance has been observed in iron pnictide superconductors at an energy below 2 Delta consistent with the s(+/-)-pairing symmetry, the mode has also been interpreted as arising from the s++-pairing symmetry with E <= 2 Delta due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E = 7 meV in SC NaFe0.935Co0.045As (T-c = 18 K). On warming towards Tc, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitation spectra within the s(+/-) and s++-pairing symmetries, we conclude that the ground-state resonance in NaFe0.935Co0.045As is only consistent with the s(+/-) pairing, and is inconsistent with the s(++)-pairing symmetry.

Normal-State Hourglass Dispersion of the Spin Excitations in FeSexTe1-x

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe0.4Te0.6 and essentially nonsuperconducting (NSC) FeSe0.45Te0.55. In contrast with BaFe2-x(Co,Ni)xAs2, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe0.4Te0.6 are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe0.45Te0.55, we argue that the observed incommensurate spin excitations in FeSe(1-x)Tex are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.

Effect of the in-plane magnetic field on the neutron spin resonance in optimally doped FeSe0.4Te0.6 and BaFe1.9Ni0.1As2 superconductors

Electrospinning is a convenient and versatile method for fabricating different kinds of one-dimensional nanostructures such as nanofibres, nanotubes and nanobelts. Environmental parameters have a great influence on the electrospinning nanostructure. Here we report a new method to fabricate hafnium oxide (HfO2) nanobelts. HfO2 nanobelts were prepared by electrospinning a sol-gel solution with the implementation of heating and subsequent calcination treatment. We investigate the temperature dependence of the products by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive x-ray (EDX) spectroscopy. The heating temperature of spinning ambient is found to be crucial to the formation of HfO2 nanobelts. By tuning the temperature, the morphological transformation of HfO2 from nanowires to nanobelts was achieved. It was found that the rapid evaporation of solvent played an important role in the formation process of HfO2 nanobelts. It is shown that nanobelts can only be obtained with the temperature higher than 50 degrees C and they are in the high quality monoclinic phase. A possible growth mechanism of the nanobelts based on phase separation is proposed. The enhanced photoluminescence (PL) of HfO2:Eu3+ nanobelts is also illustrated.

Magnetic phases in CeCu2(Si1-xGex)2CeCu2(Si1-xGex)2 near the tetracritical point

We report on neutron diffraction experiments on single crystals of CeCu2(Si1-xGex)2CeCu2(Si1-xGex)2 with x=0.18x=0.18, 0.45. The aim was to investigate the magnetic order in the vicinity of the tetracritical point at x≈0.25x≈0.25. The magnetic structure factors of the observed antiferromagnetic satellite peaks at lowest temperatures scale onto each other for both samples. This indicates identical magnetic low-TT structures below and above x≈0.25x≈0.25 and suggests no change in the itinerancy of the magnetic order.

Polarized neutron scattering on the triple-axis spectrometer PANDA: First results

PANDA is a triple-axis spectrometer utilizing the cold neutron source of the German research reactor FRM II in Garching. Its typical applications comprise studies of spin dynamics and lattice dynamics, magnetic excitations and magnetic structures. In the past years, PANDA has earned its reputation by excellent performance in these fields operating in conventional, unpolarized mode. To complement this normal mode of operation, a full polarization analysis of the scattered neutrons was highly desired and, therefore, was recently implemented. First tests to characterize the different spectrometer components for the polarization analysis mode were performed with neutron spin directions perpendicular to the scattering plane, utilizing static vertical guide fields between sample, Heusler monochromator, spin flipper and Heusler analyzer. Finally, a fully automated setup of split coils around the sample space was implemented to allow for longitudinal polarization analysis for arbitrary configurations. Different neutron spin polarization directions were successfully investigated, showing polarizations of 92% or higher.

Spin-wave and electromagnon dispersions in multiferroicMnWO4as observed by neutron spectroscopy: Isotropic Heisenberg exchange versus anisotropic Dzyaloshinskii-Moriya interaction

High resolution inelastic neutron scattering reveals that the elementary magnetic excitations in multiferroic MnWO4 consist of low energy dispersive electromagnons in addition to the well-known spin-wave excitations. The latter can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1) meV. Two electromagnon branches appear at lower energies of 0.07(1) meV and 0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric coupling and persist in both, the collinear magnetic and paraelectric AF1 phase, and the spin spiral ferroelectric AF2 phase. These excitations are associated with the Dzyaloshinskii-Moriya exchange interaction, which is significant due to the rather large spin-orbit coupling.

Distinguishing S-plus-minus and S-plus-plus electron pairing symmetries by neutron spin resonances in superconducting Sodium-Iron-Cobalt-Arsenic (transitional temperature = 18 Kelvin)

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechansim for superconductivity. For iron pnictide superconductors, the s{sup {+-}}-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies (E {le} 2{Delta}). Although the resonances have been observed for various iron pnictide superconductors, they are broad in energy and can also be interpreted as arising from the s{sup ++}-pairing symmetry with E {ge} 2{Delta}. Here we use inelastic neutron scattering to reveal a sharp resonance at E = 7 meV in the SC NaFe{sub 0.935}Co{sub 0.045}As (T{sub c} = 18 K). By comparing our experiments with calculated spin-excitations spectra within the s{sup {+-}} and s{sup ++}-pairing symmetries, we conclude that the resonance in NaFe{sub 0.935}Co{sub 0.045}As is consistent with the s{sup {+-}}-pairing symmetry, thus eliminating s{sup ++}-pairing symmetry as a candidate for superconductivity.