Pseudogap and Strong Pairing Induced by Incipient and Shallow Bands in the Quasi-Two-Dimensional KCa${}_2$Fe${}_4$As${}_4$F${}_2$

Abstract

The optical properties of KCa${}_2$Fe${}_4$As${}_4$F${}_2$ (K12442, $T_c=33.5$~K) and KCa${}_2$(Fe${}_{0.95}$Ni${}_{0.05}$)${}_4$As${}_4$F${}_2$ (Ni-K12442, $T_c=29$~K) have been examined at a large number of temperatures. For both samples, a nodeless superconducting gap is clearly observed in the optical conductivity at 5~K. The superconducting gap $\Delta \simeq 8.7$~meV ($2\Delta /k_{\text{B}}{T}_c\simeq 6.03$) in K12442, pointing towards strong-coupling Cooper pairs, but in sharp contrast, $\Delta \simeq 4.6$~meV ($2\Delta /k_{\text{B}}{T}_c\simeq 3.68$) in Ni-K12442, which agrees with the BCS weak-coupling pairing state. More intriguingly, below $T^{\ast }\simeq 75$~K, the optical conductivity of K12442 reveals a pseudogap that smoothly evolves into the superconducting gap below $T_c$, while no such behavior is detected in the electron-doped Ni-K12442. The comparison between the two samples hints that the pseudogap and strong-coupling Cooper pairs in K12442 may be intimately related to the shallow and incipient bands. We provide arguments supporting a preformed pairing mechanism of the pseudogap, but at the moment a magnetic scenario can not yet be excluded.

8 pages, 5 figures, Comments are welcome and appreciated

References in corpus (30)

• Unconventional pairing originating from disconnected Fermi surfaces in superconducting LaFeAsO${}_{1-x}$F${}_x$}
• Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2
• Origin of the spin density wave instability in AFe${}_2$As${}_2$ (A=Ba, Sr) as revealed by optical spectroscopy
• Probing superconducting energy gap from infrared spectroscopy on a Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ single crystal with T${}_c$=37 K
• Anisotropy of the Optimally-Doped Iron Pnictide Superconductor Ba(Fe0.926Co0.074)2As2
• Pseudogap from ARPES experiment: three gaps in cuprates and topological superconductivity
• Superconductivity in KCa${}_2$Fe${}_4$As${}_4$F${}_2$ with Separate Double Fe${}_2$As${}_2$ Layers
• Evolution of the optical spectrum with doping in iron pnictides Ba(Fe1-xCox)2As2
• Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS-BEC crossover
• Gate-controlled BCS-BEC crossover in a two-dimensional superconductor
• Observation of strong electron pairing on bands without Fermi surfaces in LiFe1-xCoxAs
• Quantum vortex core and missing pseudogap in the multi-band BCS-BEC-crossover superconductor FeSe
• Synthesis, anisotropy, and superconducting properties of LiFeAs single crystal
• Superconductivity-induced optical anomaly in an iron arsenide
• Neutron spin resonance in a quasi-two-dimensional iron-based superconductor
• Single-crystal growth and extremely high H_c2 of 12442-type Fe-based superconductor KCa_2Fe_4As_4F_2
• Persistence of superconducting condensate far above the critical temperature in YBa${}_2$(Cu,Zn)${}_3$O${}_y$ revealed by $c$-axis optical conductivity measurements for several Zn concentrations and carrier doping levels
• Optical conductivity of iron-based superconductors
• Enhanced critical temperature, pairing fluctuation effects, and BCS-BEC crossover in a two-band Fermi gas
• Ultrafast Optical Spectroscopy Evidence of Pseudogap and Electron-Phonon Coupling in an Iron-Based Superconductor KCa${}_2$Fe${}_4$As${}_4$F${}_2$
• FeTe${}_{0.55}$Se${}_{0.45}$: a multiband superconductor in the clean and dirty limit
• Charge Density Wave order and electron-phonon coupling in ternary superconductor Bi${}_2$Rh${}_3$Se${}_2$
• Strong Pauli paramagnetic effect in the upper critical field of KCa${}_2$Fe${}_4$As${}_4$F${}_2$
• Pseudogap Behavior of the Nuclear Spin-lattice Relaxation Rate in FeSe Probed by ${}^{77}$Se-NMR
• Band-selective clean- and dirty-limit superconductivity with nodeless gaps in the bilayer iron-based superconductor CsCa${}_2$Fe${}_4$As${}_4$F${}_2$
• Muon spin rotation and infrared spectroscopy study of magnetism and superconductivity in Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$
• Optical observation of spin-density-wave fluctuations in Ba122 iron-based superconductors
• Comprehensive study of out-of-plane transport properties in BaFe${}_2$As${}_2$: Three-dimensional electronic state and effect of chemical substitution
• Investigation of Precursor Superconducting State in YBa${}_2$Cu${}_3$O${}_{7-\delta }$ through In-plane Optical Spectroscopy
• Carrier localization due to local magnetic order induced by magnetic impurities in Ba(Fe${}_{1-x}$TM${}_x$)${}_2$As${}_2$ (TM = Mn and Cr) as seen via optical spectra

Cited by in corpus (4)

• Quasi two-dimensional nature of high-Tc superconductivity in iron-based (Li,Fe)OHFeSe
• Evidence of electron interaction with an unidentified bosonic mode in superconductor CsCa${}_2$Fe${}_4$As${}_4$F${}_2$
• Interplay of electron-phonon coupling, pseudogap, and superconductivity in CsCa${}_2$Fe${}_4$As${}_4$F${}_2$ studied using ultrafast optical spectroscopy
• Decoupled interband pairing in a bilayer iron-based superconductor evidenced by ultrahigh-resolution ARPES