Spin gap and resonance at the nesting wave vector in superconducting FeSe_{0.4}Te_{0.6}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• Iron-based superconductors (IBS) exhibit complex phase diagrams with intertwined magnetic and superconducting orders.
• Understanding the relationship between spin fluctuations and superconductivity is crucial for designing high-temperature superconductors.
• FeSe_{0.4}Te_{0.6} is an IBS with a transition temperature (Tc) of 14 K, making it a model system for studying these phenomena.

Purpose of the Study:

• To investigate the nature of magnetic excitations in superconducting FeSe_{0.4}Te_{0.6} using neutron scattering.
• To identify the characteristic wave vectors of spin fluctuations and their relation to Fermi surface nesting.
• To characterize the spin resonance observed in the superconducting state and its connection to the pairing symmetry.

Main Methods:

• Inelastic neutron scattering experiments were performed on single crystals of FeSe_{0.4}Te_{0.6}.
• The magnetic excitation spectrum was measured across the superconducting transition temperature (Tc).
• Analysis focused on identifying spin fluctuation wave vectors and the energy-momentum characteristics of the spin resonance.

Main Results:

• Low-energy spin fluctuations were observed with a characteristic wave vector (1/2, 1/2, L), indicative of Fermi surface nesting.
• This wave vector differs from that observed in the magnetically ordered parent compound Fe_{1+y}Te.
• A spin resonance, with energy Omega_{0} \approx 6.51 meV and width Gamma \approx 1.25 meV, was detected in the superconducting state, developing from a normal state continuum.
• The resonance energy is approximately 5.3 times the superconducting critical temperature (k_{B}T_{c}).

Conclusions:

• The observed spin resonance is consistent with a bound state predicted for s_{+/-} superconductivity.
• The findings support the role of imperfect quasi-2D Fermi surface nesting in mediating superconductivity in FeSe_{0.4}Te_{0.6}.
• Neutron scattering provides a powerful tool for elucidating the interplay between magnetism and superconductivity in IBS.