1202.4152 (C. Stock et al.)

Spin fluctuations and superconductivity in powders of Fe_1+xTe_0.7Se_0.3
as a function of interstitial iron concentration    [PDF]

C. Stock, E. E. Rodriguez, M. A. Green

Using neutron inelastic scattering, we investigate the role of interstitial
iron on the low-energy spin fluctuations in powder samples of
Fe_{1+x}Te_{0.7}Se_{0.3}. We demonstrate how combining the principle of
detailed balance along with measurements at several temperatures allows us to
subtract both temperature-independent and phonon backgrounds from S(Q,\omega)
to obtain purely magnetic scattering. For small values of interstitial iron
(x=0.009(3)), the sample is superconducting (T_{c}=14 K) and displays a spin
gap of 7 meV peaked in momentum at wave vector q_{0}=(\pi,\pi) consistent with
single crystal results. On populating the interstitial iron sites, the
superconducting volume fraction decreases and we observe a filling in of the
low-energy magnetic fluctuations and a decrease of the characteristic wave
vector of the magnetic fluctuations. For large concentrations of interstitial
iron (x=0.048(2)) where the superconducting volume fraction is minimal, we
observe the presence of gapless spin fluctuations at a wave vector of
q_{0}=(\pi,0). We estimate the absolute total moment for the various samples
and find that the amount of interstitial iron does not change the total
magnetic spectral weight significantly, but rather has the effect of shifting
the spectral weight in Q and energy. These results show that the
superconducting and magnetic properties can be tuned by doping small amounts of
iron and are suggestive that interstitial iron concentration is also a
controlling dopant in the Fe_{1+x}Te_{1-y}Se_{y} phase diagram in addition to
the Te/Se ratio.

View original: http://arxiv.org/abs/1202.4152