Figure 1
(a) Schematic in-plane spin structure of the nonsuperconducting FeTe, where the solid and hollow arrows represent two sublattices of spins which can be either parallel or antiparallel [
6]. Upon substitution of Se for Te to form
, the static long-range AF order is suppressed, and the system display strong spin excitations at incommensurate positions near
as shown in (b). The incommensurate scattering only appear at positions (
,
) and (
,
). Our transverse scans are along the scan direction
, and the scan along the incommensurate position that is perpendicular to scan-
is marked as scan-
. (c),(d) Schematic diagram of Fermi surfaces near
and
points from results of recent photoemission experiments [
33,
34]. (e) In a multiband itinerant picture, quasiparticle excitations from the
band to the
band can give rise to the upper branch of the hourglass dispersion as shown in the solid red lines. The lower branch of the dispersion is then a consequence of the excitations from the
band to the
band. (f) Experimental determination of the spin excitation dispersions in the normal (open red circles) and SC (black filled squares) states of
. A full spin gap opens below
at 1.5 K. The magnitude of
marks the energy below which intensity of spin excitations decrease below
, whereas
indicates the resonance energy. The dispersion curves are obtained by fitting two Gaussians on linear backgrounds through transverse scans in Figs. 2, 3. We note that the incommensurability at 2 meV at 20 K is obtained by fitting the difference between the 20 K data and 1.5 K data. The horizontal error bars are the fitted errors of the incommensurability. (g) Hourglass dispersion in the NSC
at 4 and 15 K.
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