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Register a new userRobust spin fluctuations and $spm$ pairing in the heavily electron doped iron-based superconductors
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We theoretically study the spin fluctuation and superconductivity in La1111 and Sm1111 iron-based superconductors for a wide range of electron doping. When we take into account the band structure variation by electron doping, the hole Fermi surface originating from the $d_{X^2-Y^2}$ orbital turns out to be robust against electron doping, and this gives rise to large spin fluctuations and consequently $spm$ pairing even in the heavily doped regime. The stable hole Fermi surface is larger for Sm1111 than for La1111, which can be considered as the origin of the apparent difference in the phase diagram.
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Based on an effective two-band model and using the fluctuation-exchange (FLEX) approach, we explore spin fluctuations and unconventional superconducting pairing in Fe-based layer superconductors. It is elaborated that one type of interband antiferromagnetic (AF) spin fluctuation stems from the interband Coulomb repulsion, while the other type of intraband AF spin fluctuation originates from the intraband Coulomb repulsion. Due to the Fermi-surface topology, a spin-singlet extended s-wave superconducting state is more favorable than the nodal $d_{XY}$-wave state if the interband AF spin fluctuation is more significant than the intraband one, otherwise vice versa. It is also revealed that the effective interband coupling plays an important role in the intraband pairings, which is a distinct feature of the present two-band system.
Multiband systems, which possess a wide parameter space, allow to explore a variety of competing ground states. Bright examples are the Fe-based pnictides and chalcogenides, which demonstrate metallic, superconducting, and various magnetic phases. Here I discuss only one of the many interesting topics, namely, spin fluctuations in metallic multiband systems. I show how to calculate the effect of itinerant spin excitations on the electronic properties and formulate a theory of spin fluctuation-induced superconductivity. The superconducting state is unconventional and thus the system demonstrates unusual spin response with the spin resonance feature. I discuss its origin, consequences, and relation to experimental observations. Role of the spin-orbit coupling is specifically emphasized.
Heavily electron-doped iron-selenide (HEDIS) high-transition-temperature (high-$T_{rm{c}}$) superconductors, which have no hole Fermi pockets, but have a notably high $T_{rm{c}}$, have challenged the prevailing $s$$_pm$ pairing scenario originally proposed for iron pnictides containing both electron and hole pockets. The microscopic mechanism underlying the enhanced superconductivity in HEDIS remains unclear. Here, we used neutron scattering to study the spin excitations of the HEDIS material Li$_{0.8}$Fe$_{0.2}$ODFeSe ($T_{rm{c}}$ = 41 K). Our data revealed nearly ring-shaped magnetic resonant excitations surrounding ($pi$, $pi$) at $sim$ 21 meV. As the energy increased, the spin excitations assumed a diamond shape, and they dispersed outward until the energy reached $sim$ 60 meV and then inward at higher energies. The observed energy-dependent momentum structure and twisted dispersion of spin excitations near ($pi$, $pi$) are analogous to those of hole-doped cuprates in several aspects, thus implying that such spin excitations are essential for the remarkably high $T_{rm{c}}$ in these materials.
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental results indicate the close relationship between the AFMSFs and the SC from the hole-doped state to heavily electron-doped state, which shed light on a unique SC phase emerged in the heavily electron-doped regime being formally equivalent to non-SC compound Ba(Fe0.5Co0.5)Fe2As2.
We present a theoretical understanding of the superconducting phase diagram of the electron-doped iron pnictides. We show that, besides the Fermi surface nesting, a peculiar motion of electrons, where the next nearest neighbor (diagonal) hoppings between iron sites dominate over the nearest neighbor ones, plays an important role in the enhancement of the spin fluctuation and thus superconductivity. In the highest $T_c$ materials, the crossover between the Fermi surface nesting and this prioritized diagonal motion regime occurs smoothly with doping, while in relatively low $T_c$ materials, the two regimes are separated and therefore results in a double dome $T_c$ phase diagram.
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