Center for Correlated Electron Systems

Seminar

Spin dynamics study on heavily electron-doped iron-selenide superconductors

July 20, 2020l Hit 297
Date : December 3, 2019 16:00 ~ 17:30
Speaker : Dr. Jitae Park (Technical University of Munich)
Location : Bldg. 19, Rm. 210

Heavily electron-doped iron-selenide (HEDIS) superconductors with a relatively high Tc , including monolayer FeSe thin ?lm (Tc > 50K), alkali-metal intercalated 122-type FeSe (Tc  32K), and newly discovered phase-pure Li0.8 Fe0.2 OHFeSe (Tc  41K), possess a peculiar electronic structure di?erent from typical Fe-based superconductors: No hole-like Fermi surface at the center of Brillouin zone. Such feature immediately challenges the sign-reversed s-wave (s± ) pairing symmetry, which was originally proposed for Fe-based superconductors based on presence of both electron and hole Fermi pockets. The exact physical origin of a notably high transition temperature in the HEDIS without the hole Fermi pocket is not yet understood. Our recent inelastic neutron scattering studies on magnetic ?uctuations in a series of sulfur-doped superconducting 122-FeSe and deutrated Li0.8 Fe0.2 ODFeSe (11111) single crystals revealed the magnetic resonant mode in both systems, con?rming that the superconducting gap function still contains sign-reversal structure. In the 122-FeSe, the magnetic resonant mode appears at (π,0.5π) wave vector and below 2 times of superconducting energy gap magnitude, but the resonant mode gradually smears out into the particle-hole continuum upon increase of sulfur-doping, indicating a possible change of the pairing channel. In 11111, nearly ring-shaped resonant excitations have been found at (π,0.625π) and at the energy of 21 meV below Tc , and high-energy spin excitations show the twisted  dispersion  across    60  meV  towards  the  (π,π)  similar  to  those  of  hole-doped  cuprates.   In  this talk,  we  will  discuss  physical  implication  on  our  experimental  observations.Heavily electron-doped iron-selenide (HEDIS) superconductors with a relatively high Tc , including monolayer FeSe thin ?lm (Tc > 50K), alkali-metal intercalated 122-type FeSe (Tc  32K), and newly discovered phase-pure Li0.8 Fe0.2 OHFeSe (Tc  41K), possess a peculiar electronic structure di?erent from typical Fe-based superconductors: No hole-like Fermi surface at the center of Brillouin zone. Such feature immediately challenges the sign-reversed s-wave (s± ) pairing symmetry, which was originally proposed for Fe-based superconductors based on presence of both electron and hole Fermi pockets. The exact physical origin of a notably high transition temperature in the HEDIS without the hole Fermi pocket is not yet understood. Our recent inelastic neutron scattering studies on magnetic ?uctuations in a series of sulfur-doped superconducting 122-FeSe and deutrated Li0.8 Fe0.2 ODFeSe (11111) single crystals revealed the magnetic resonant mode in both systems, con?rming that the superconducting gap function still contains sign-reversal structure. In the 122-FeSe, the magnetic resonant mode appears at (π,0.5π) wave vector and below 2 times of superconducting energy gap magnitude, but the resonant mode gradually smears out into the particle-hole continuum upon increase of sulfur-doping, indicating a possible change of the pairing channel. In 11111, nearly ring-shaped resonant excitations have been found at (π,0.625π) and at the energy of 21 meV below Tc , and high-energy spin excitations show the twisted  dispersion  across    60  meV  towards  the  (π,π)  similar  to  those  of  hole-doped  cuprates.   In  this talk,  we  will  discuss  physical  implication  on  our  experimental  observations.

 

Host: Prof. Je-Geun Park