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Register a new userImpurity-induced smearing of the spin resonance peak in Fe-based superconductors
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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.
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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 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.
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.
We study the effect of strong spin-orbit coupling (SOC) on bound states induced by impurities in superconductors. The presence of spin-orbit coupling breaks the $mathbb{SU}(2)$-spin symmetry and causes the superconducting order parameter to have generically both singlet (s-wave) and triplet (p-wave) components. We find that in the presence of SOC the spectrum of Yu-Shiba-Rusinov (YSR) states is qualitatively different in s-wave and p-wave superconductor, a fact that can be used to identify the superconducting pairing symmetry of the host system. We also predict that in the presence of SOC the spectrum of the impurity-induced bound states depends on the orientation of the magnetic moment $bf{S}$ of the impurity and, in particular, that by changing the orientation of $bf{S}$ the fermion-parity of the lowest energy bound state can be tuned. We then study the case of a dimer of magnetic impurities and show that in this case the YSR spectrum for a p-wave superconductor is qualitatively very different from the one for an s-wave superconductor even in the limit of vanishing SOC. Our predictions can be used to distinguish the symmetry of the order parameter and have implications for the Majorana proposals based on chains of magnetic atoms placed on the surface of superconductors with strong spin-orbit coupling.
We show that only a few percentage of Sn doping at the Ba site on BaFe$_2$As$_2$, can cause electronic topological transition, namely, the Lifshitz transition. A hole like d$_{xy}$ band of Fe undergoes electron like transition due to 4% Sn doping. Lifshitz transition is found in BaFe$_2$As$_2$ system around all the high symmetry points. Our detailed first principles simulation predicts absence of any Lifshitz transition in other 122 family compounds like SrFe$_2$As$_2$, CaFe$_2$As$_2$. This work bears practical significance due to the facts that a few percentage of Sn impurity is in-built in tin-flux grown single crystals method of synthesizing 122 materials and inter-relationship among the Lifshitz transition, magnetism and superconductivity.
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