Tunneling interstitial impurity in iron-chalcogenide-based superconductors

Abstract

A pronounced local in-gap zero-energy bound state (ZBS) has been observed by recent scanning tunneling microscopy (STM) experiments on the interstitial Fe impurity (IFI) and its nearestneighboring (nn) sites in FeTe0.5Se0.5 superconducting (SC) compound. By introducing a new impurity mechanism, the so-called tunneling impurity, and based on the Bogoliubove-de Gennes (BDG) equations we investigated the low-lying energy states of the IFI and the underlying Feplane. We found the peak of ZBS does not shift or split in a magnetic field as long as the tunneling parameter between IFI and the Fe-plane is sufficiently small and the Fe-plane is deep in the SC state. Our results are in good agreement with the experiments. We also predicted that modulation of spin density wave (SDW), or charge density wave (CDW) will suppress the intensity of the ZBS.

Nesting-induced local electronic patterns around a single As (Te, Se) vacancy in iron-based superconductors

“…This model also explained successful a series of STM experiments in iron-based superconductors, e.g. in-gap impurity bound states [12,23], the negative energy resonance peak in the vortex core [24,25], the 90°domain walls and anti-phase domain walls [26][27][28][29], the zero energy bound state induced by the interstitial excess Fe ions [30][31][32], etc, and especially repeated the phase diagram observed by nuclear magnetic resonance and neutron scattering experiments [33][34][35].…”

“…However, the coherence peaks and the resonance peaks are symmetric with respect to the bias voltage in figure 2(b). We note that the in-gap resonance peaks is irrelative to the phase of the superconducting order parameter, similar to those induced by interstitial excess Fe impurities in iron-based superconductor Fe(Te, Se) [30][31][32].Therefore, very different from nonmagnetic impurities on the Fe sites [12], such a ligand vacancy cannot be used to distinguish the s +− and s ++ pairing symmetries. Figure 3 shows the LDOS images at different U and ω under optimal electron doping for the s +− pairing symmetry in the Fe sublattice B with 31×31 sites due to quasiparticle interference.…”

“…Here we have calculated a square Fe lattice with N=500×500 unit cells, which is enough to ensure the accuracy of theoretical results. We have also employed the energy band parameters: t 1 =−0.5 eV, t 2 =−0.2 eV, t 3 =1.0 eV, and t 4 =−0.02 eV, which are same with the previous works [12,25,28,29,31,32,35]. We note that the electron-doped BaFe 2−x Co x As 2 has a superconducting energy gap Δ 0 =5.8 meV observed by the STM experiments [10,11,37].…”

Anomalous sharp peak in the London penetration depth induced by the nodeless-to-nodal superconducting transition in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

“…According to the fact of experimental measurements 7,12 , we extend the two-dimensional phenomenological model with two orbitals 31 to a three-dimensional model with three orbitals to study the superconducting electronic properties in isovalent-doped BaFe 2 (As 1−x P x ) 2 . The previous twodimensional model considered the effect of asymmetric arsenic atoms is appropriate to describe the experimental observations in ARPES and scanning tunnel microscope for the 122 family [32][33][34][35] . The calculated superfluid density is in qualitative agreement with the direct experimental measurement in films of Fe pnictide superconductors at low temperatures 36 .…”

Effects of spin-orbit coupling on zero-energy bound states localized at magnetic impurities in multiband superconductors

“…In other recent work, Ref. [17] has attempted to explain the same experiments 1 within a so-called 'tunneling impurity' formulation, in which the magnetic impurity is assumed to be coupled to the underlying Fe lattice only by hoping terms, but no exchange interaction, in spite of the fact that the impurity possesses a non-zero local magnetic moment. In this paper we show that the zero-energy bound states localized at magnetic impurities in sign changing s +− -wave superconductors (but not in sign unchanged s ++ -wave superconductors) can be surprisingly robust to perturbations such as Zeeman fields and variations in the impurity scattering strength in the presence of spin-orbit coupling.…”