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

TL;DR

This study shows that an hourglass-shaped magnetic dispersion in Fe-based superconductors likely preconditions superconductivity, with spectral weight shifts occurring as a consequence of the superconducting state, revealing insights into spin fluctuations' role.

Contribution

It demonstrates that hourglass-shaped magnetic dispersion precedes superconductivity in Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$, highlighting its potential role as a prerequisite for high-temperature superconductivity.

Findings

Hourglass-shaped dispersion develops above $T_c$ in superconducting samples.

Spin-gap and spectral weight shifts occur below $T_c$.

Inward dispersion towards the wave-vector is crucial for superconductivity.

Abstract

High-temperature superconductivity remains arguably the largest outstanding enigma of condensed matter physics. The discovery of iron-based high-temperature superconductors has renewed the importance of understanding superconductivity 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 spin fluctuations and superconductivity. Here we report inelastic neutron scattering data on Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$ using excess iron concentration to tune between a SC ($y=0.02$) and a non-SC ($y=0.05$) ground states. We find incommensurate spectra in both samples but discover that in the one that becomes SC, a constriction towards a commensurate hourglass shape develop well above $T_c$. Conversely a spin-gap and concomitant spectral weight shift happen below $T_c$. Our results imply that the hourglass shaped dispersion is most likely a pre-requisite for superconductivity, whereas the spin-gap and shift of spectral weight are consequences of superconductivity. We explain this observation by pointing out that an inwards dispersion towards the commensurate wave-vector is needed for the opening of a spin gap to lower the magnetic exchange energy and hence provide the necessary condensation energy for the SC state to emerge.