Abstract
We report neutron-scattering studies on static magnetic orders and spin excitations in the Fe-based chalcogenide system ${\text{Fe}}_{1+\ensuremath{\delta}}{\text{Se}}_{x}{\text{Te}}_{1\ensuremath{-}x}$ with different Fe and Se compositions. Short-range static magnetic order with an in-plane wave vector near the (0.5,0) (using the two-Fe unit cell), together with strong low-energy magnetic excitations is found in all nonsuperconducting samples for Se doping up to 45%. When the static order disappears and bulk superconductivity emerges, the spectral weight of the magnetic excitations shifts to the region of reciprocal space near the in-plane wave vector (0.5, 0.5), corresponding to ``collinear'' spin correlations. Our results suggest that there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in this system. Excess Fe appears to be important for stabilizing the magnetic order that competes with superconductivity.
Highlights
Since the discovery of the first high-temperature superconductor in the 1980s, there has been a continuing effort to understand the origin of high-Tc superconductivity
Short-range static magnetic order with an in-plane wave vector near the0.5,0͒ ͑using the two-Fe unit cell, together with strong low-energy magnetic excitations is found in all nonsuperconducting samples for Se doping up to 45%
The spin resonance observed in the superconducting compositions of this family has been found to occur at the same0.5,0.5͒ in-plane wave vector,29–37 as in the Fe pnictides,19–23 but rotated 45° from the ordering wave vector of the parent Fe1+␦ Te compound
Summary
Since the discovery of the first high-temperature superconductor in the 1980s, there has been a continuing effort to understand the origin of high-Tc superconductivity. A “spin resonance” has been observed in the 122 system and in the 1111 system by inelastic neutron scattering, showing a sharp increase in the magnetic scattering intensity at the “resonance” energy when the system goes into the superconducting phase For the pnictides, both the static magnetic order in the parent compound and the resonance in the superconducting compounds occur around the in-plane wave vector0.5,0.5͒ ͑using the two-Fe unit cell, suggesting a “collinear” or “C-type” AF structuresee Fig. 1͑a͒.11,25,26. The substitution of Se for Te, which induces superconductivity, does not directly modify the density of electrons in the conduction bands Despite these differences, the spin resonance observed in the superconducting compositions of this family has been found to occur at the same0.5,0.5͒ in-plane wave vector, as in the Fe pnictides, but rotated 45° from the ordering wave vector of the parent Fe1+␦ Te compound. There is a clear difference between the two families: the local magnetic correlations in the cuprate system are always based on a G-type antiferromagnetic configuration, even when spatial segregation occurs. The versatility of spin configurations in the Fe-based superconductor families is very different and extremely interesting
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