Magnetic hour-glass dispersion and its relation to high-temperature superconductivity in iron-tuned Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$

摘要

High-temperature superconductivity remains arguably the largest outstandingenigma of condensed matter physics. The discovery of iron-basedhigh-temperature superconductors has renewed the importance of understandingsuperconductivity in materials susceptible to magnetic order and fluctuations.Intriguingly they show magnetic fluctuations reminiscent of the superconducting(SC) cuprates, including a 'resonance' and an 'hour-glass' shaped dispersion,which provide an opportunity to new insight to the coupling between spinfluctuations and superconductivity. Here we report inelastic neutron scatteringdata on Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$ using excess iron concentration to tunebetween a SC ($y=0.02$) and a non-SC ($y=0.05$) ground states. We findincommensurate spectra in both samples but discover that in the one thatbecomes SC, a constriction towards a commensurate hourglass shape develop wellabove $T_c$. Conversely a spin-gap and concomitant spectral weight shift happenbelow $T_c$. Our results imply that the hourglass shaped dispersion is mostlikely a pre-requisite for superconductivity, whereas the spin-gap and shift ofspectral weight are consequences of superconductivity. We explain thisobservation by pointing out that an inwards dispersion towards the commensuratewave-vector is needed for the opening of a spin gap to lower the magneticexchange energy and hence provide the necessary condensation energy for the SCstate to emerge.