27 research outputs found
Electron-doping evolution of the low-energy spin excitations in the iron arsenide BaFeNiAs superconductors
We use elastic and inelastic neutron scattering to systematically investigate
the evolution of the low-energy spin excitations of the iron arsenide
superconductor BaFe2-xNixAs2 as a function of nickel doping x. In the undoped
state, BaFe2As2 exhibits a tetragonal-to-orthorhombic structural phase
transition and simultaneously develops a collinear antiferromagnetic (AF) order
below TN = 143 K. Upon electron-doping of x = 0.075 to induce bulk
superconductivity with Tc = 12.3 K, the AF ordering temperature reduces to TN =
58 K.We show that the appearance of bulk superconductivity in
BaFe1.925Ni0.075As2 coincides with a dispersive neutron spin resonance in the
spin excitation spectra, and a reduction in the static ordered moment. For
optimally doped BaFe1.9Ni0.1As2 (Tc = 20 K) and overdoped BaFe1.85Ni0.15As2 (Tc
= 15 K) superconductors, the static AF long-range order is completely
suppressed and the spin excitation spectra are dominated by a resonance and
spin-gap at lower energies. We determine the electron-doping dependence of the
neutron spin resonance and spin gap energies, and demonstrate that the
three-dimensional nature of the resonance survives into the overdoped regime.
If spin excitations are important for superconductivity, these results would
suggest that the three-dimensional character of the electronic superconducting
gaps are prevalent throughout the phase diagram, and may be critical for
superconductivity in these materials
Effect of Li-deficiency impurities on the electron-overdoped LiFeAs superconductor
We use transport, inelastic neutron scattering, and angle resolved
photoemission experiments to demonstrate that the stoichiometric LiFeAs is an
intrinsically electron-overdoped superconductor similar to those of the
electron-overdoped NaFe1-xTxAs and BaFe2-xTxAs2 (T = Co,Ni). Furthermore, we
show that although transport properties of the stoichiometric superconducting
LiFeAs and Li-deficient nonsuperconducting Li1-xFeAs are different, their
electronic and magnetic properties are rather similar. Therefore, the
nonsuperconducting Li1-xFeAs is also in the electron overdoped regime, where
small Li deficiencies near the FeAs octahedra can dramatically suppress
superconductivity through the impurity scattering effect.Comment: 5 figures,5 page
In-plane magnetic field effect on the neutron spin resonance in optimally doped FeSeTe and BaFeNiAs superconductors
We use inelastic neutron scattering to study the effect of an in-plane
magnetic field on the magnetic resonance in optimally doped superconductors
FeSeTe ( K) and BaFeNiAs
( K). While the magnetic field up to 14.5 Tesla does not change the
energy of the resonance, it particially suppresses and the corresponding
superconductivity-induced intensity gain of the mode. However, we find no
direct evidence for the field-induced spin-1 Zeeman splitting of the resonance.
Therefore, it is still unclear if the resonance is the long-sought
singlet-triplet excitation directly coupled to the superconducting electron
Cooper pairs.Comment: 5 pages, 4 figures, The first two wrong figures are correcte
Coexistence and competition of the short-range incommensurate antiferromagnetic order with superconductivity in BaFe2-xNixAs2
Superconductivity in the iron pnictides develops near antiferromagnetism, and
the antiferromagnetic (AF) phase appears to overlap with the superconducting
phase in some materials such as BaFe2-xTxAs2 (where T = Co or Ni). Here we use
neutron scattering to demonstrate that genuine long-range AF order and
superconductivity do not coexist in BaFe2-xNixAs2 near optimal
superconductivity. In addition, we find a first-order-like AF to
superconductivity phase transition with no evidence for a magnetic quantum
critical point. Instead, the data reveal that incommensurate short-range AF
order coexists and competes with superconductivity, where the AF spin
correlation length is comparable to the superconducting coherence length.Comment: 7 pages, 5 figures, 1 tabl
A minimum single-band model for low-energy excitations in superconducting KFeSe
We propose a minimum single-band model for the newly discovered iron-based
superconducting KFeSe. Our model is found to be numerically
consistent with the five-orbital model at low energies. Based on our model and
the random phase approximation, we study the spin fluctuation and the pairing
symmetry of superconducting gap function. The spin excitation
and the pairing symmetry are revealed. All of the results can
well be understood in terms of the interplay between the Fermi surface topology
and the local spin interaction, providing a sound picture to explain why the
superconducting transition temperature is as high as to be comparable to those
in pnictides and some cuprates. A common origin of superconductivity is
elucidated for this compound and other high-T materials.Comment: 5 pages, 4 figure
Effect of Pnictogen Height on Spin Waves in Iron Pnictides
We use inelastic neutron scattering to study spin waves in the antiferromagnetic ordered phase of iron pnictide NaFeAs throughout the Brillouin zone. Comparing with the well-studied AFe2As2 (A=Ca, Sr, Ba) family, spin waves in NaFeAs have considerably lower zone boundary energies and more isotropic effective in-plane magnetic exchange couplings. These results are consistent with calculations from a combined density functional theory and dynamical mean field theory and provide strong evidence that pnictogen height controls the strength of electron-electron correlations and consequently the effective bandwidth of magnetic excitations