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Register a new userRole of interactions in the energy of the spin resonance peak in Fe-based superconductors
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We consider the spin response within the five-orbital model for iron-based superconductors and study two cases: equal and unequal gaps in different bands. In the first case, the spin resonance peak in the superconducting state appears below the characteristic energy scale determined by the gap magnitude, $2Delta_L$. In the second case, the energy scale corresponds to the sum of smaller and larger gap magnitudes, $Delta_L + Delta_S$. Increasing the values of the Hubbard interaction and the Hunds exchange, we observe a shift of the spin resonance energy to lower frequencies.
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The spin resonance peak in the iron-based superconductors is observed in inelastic neutron scattering experiments and agrees well with predicted results for the extended s-wave ($s_pm$) gap symmetry. On the basis of four-band and three-orbital tight binding models we study the effect of nonmagnetic disorder on the resonance peak. Spin susceptibility is calculated in the random phase approximation with the renormalization of the quasiparticle self-energy due to the impurity scattering in the static Born approximation. We find that the spin resonance becomes broader with the increase of disorder and its energy shifts to higher frequencies. For the same amount of disorder the spin response in the $s_pm$ state is still distinct from that of the $s_{++}$ state.
We study the spin resonance in superconducting state of iron-based materials within multiband models with two unequal gaps, $Delta_L$ and $Delta_S$, on different Fermi surface pockets. We show that due to the indirect nature of the gap entering the spin susceptibility at the nesting wave vector $mathbf{Q}$ the total gap $tildeDelta$ in the bare susceptibility is determined by the sum of gaps on two different Fermi surface sheets connected by $mathbf{Q}$. For the Fermi surface geometry characteristic to the most of iron pnictides and chalcogenides, the indirect gap is either $tildeDelta = Delta_L + Delta_S$ or $tildeDelta = 2Delta_L$. In the $s_{++}$ state, spin excitations below $tildeDelta$ are absent unless additional scattering mechanisms are assumed. The spin resonance appears in the $s_pm$ superconducting state at frequency $omega_R leq tildeDelta$. Comparison with available inelastic neutron scattering data confirms that what is seen is the true spin resonance and not a peak inherent to the $s_{++}$ state.
We study the spin resonance peak in recently discovered iron-based superconductors. The resonance peak observed in inelastic neutron scattering experiments agrees well with predicted results for the extended $s$-wave ($s_pm$) gap symmetry. Recent neutron scattering measurements show that there is a disparity between longitudinal and transverse components of the dynamical spin susceptibility. Such breaking of the spin-rotational invariance in the spin-liquid phase can occur due to spin-orbit coupling. We study the role of the spin-orbit interaction in the multiorbital model for Fe-pnictides and show how it affects the spin resonance feature.
Motivated by the recent experiment of the non-BCS scaling relation of the condensation energy $Delta CE$ vs. $T_c$ ($Delta CE sim T_c ^{beta}, betaapprox 3.5$) [PRB 89 140503 (2014)] for the Fe-based superconductors, we studied the CE and $T_c$ of the multiband BCS superconductors. We showed that the experimentally observed anomalous scaling relation $Delta CE sim T_c ^{3.5}$ is well reproduced by the two-band BCS superconductor paired by a dominant interband interaction ($V_{inter} > V_{intra}$). Our result implies that this seemingly non-BCS-like scaling behavior, on the contrary to the common expectation, is in fact a strong experimental evidence that the pairing mechanism of the Fe-based superconductors is genuinely a BCS mechanism, meaning that the Cooper pairs are formed by the itinerant carriers glued by a pairing interaction.
The exact superconducting phase of K2-xFe4+ySe5 has yet conclusively decided since its discovery due to its intrinsic multiphase in early material. In an attempt to resolve the mystery, we have carried out systematic structural studies on a set of well controlled samples with exact chemical stoichiometry K2-xFe4+xSe5 (x=0~0.3) that are heat-treated at different temperatures. Our investigations, besides the determination of superconducting transition, focus on the detailed temperature evolution of the crystalline phases using high resolution synchrotron radiation X-ray diffraction. Our results show that superconductivity appears only in those samples been treated at high enough temperature and then quenched to room temperature. The volume fraction of superconducting transition strongly depends on the annealing temperature used. The most striking result is the observation of a clear contrast in crystalline phase between the non-superconducting parent compound K2Fe4Se5 and the superconducting K2-xFe4+ySe5 samples. The x-ray diffraction patterned can be well indexed to the phase with I4/m symmetry in all temperature investigated. However, we need two phases with similar I4/m symmetry but different parameters to best fit the data at temperature below the Fe-vacancy order temperature. The results strongly suggest that superconductivity in K2-xFe4+ySe5 critically depends on the occupation of Fe atoms on the originally empty 4d site.
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