Unexpected weak spatial variation in the local density of states induced by individual Co impurity atoms in superconducting Na(Fe<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>Co<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mi>x</mml:mi></mml:msub></mml:math>)As crystals revealed by scanning tunneling spectro

In conventional s-wave superconductors, only magnetic impurities exhibit impurity bound states, whereas for an s± order parameter they can occur for both magnetic and non-magnetic impurities. Impurity bound states in superconductors can thus provide important insight into the order parameter. Here, we present a combined experimental and theoretical study of native and engineered iron-site defects in LiFeAs. Detailed comparison of tunneling spectra measured on impurities with spin fluctuation theory reveals a continuous evolution from negligible impurity bound state features for weaker scattering potential to clearly detectable states for somewhat stronger scattering potentials. All bound states for these intermediate strength potentials are pinned at or close to the gap edge of the smaller gap, a phenomenon that we explain and ascribe to multi-orbital physics.

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“…2(c)). The rather weak influence of cobalt defects on superconductivity is consistent with previous STM/STS reports in NaFeAs 19 .…”

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confidence: 91%

Impact of iron-site defects on superconductivity in LiFeAs

et al. 2016

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“…Both effects mentioned here will weaken the pair-breaking. This is quite similar to the case of Co dopants in Na(Fe 1−x Co x )As [20]. For the s ++ pairing, the interaction of the strongly screened impurity with the superconducting surface is rather weak.…”

Section: Discussionsupporting

confidence: 66%

“…The weakest bond in this crystal sits between the two adjacent Na layers, therefore the topmost layer after cleaving is the one with Na atoms. Beside the observed square lattice, one can see some rectangular blocks, which have already been identified as the Co dopants on the Fe layer [20]. For the Mn substitutions, it can only be identified with lower bias voltage during the scanning.…”

Section: Methodsmentioning

confidence: 98%

Spectroscopic signature of Kondo screening on single adatoms inNa(Fe0.96Co0.03Mn0.01)As

et al. 2014

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The electronic states of surface adatoms in Na(Fe 0.96 Co 0.03 Mn 0.01 )As have been studied by low-temperature scanning tunneling spectroscopy. The spectra recorded on the adatoms display both superconducting coherence peaks and an asymmetric resonance in a larger energy scale. The Fano-type line shape of the spectra points towards a possible Kondo effect at play. The apparent energy position of the resonance peak shifts about 5 meV to the Fermi level when measured across the critical temperature, supporting that the Bogoliubov quasiparticle is responsible for the Kondo screening in the superconducting state. The tunneling spectra do not show the subgap bound states, which is explained as the weak pair breaking effect given by the weak and broad scattering potential after the Kondo screening.

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“…[6] Scanning tunneling spectroscopy found a plethora of exotic atomicsized impurity-generated states, [7][8][9][10] NMR and neutrons observed clear evidence of glassy magnetic behavior, [11,12] and µSR discovered magnetic phases generated by non-magnetic disorder. [13,14] The resulting complex inhomogeneous phases and their properties in terms of thermodynamics and transport constitute an important open problem in the field.…”

mentioning

confidence: 99%

Unconventional Disorder Effects in Correlated Superconductors

2016

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The understanding of disorder has profoundly influenced the development of condensed matter physics, explaining such fundamental effects as, for example, the transition from ballistic to diffusive propagation, and the presence of quantized steps in the quantum Hall effect. For superconductors, the response to disorder reveals crucial information about the internal gap symmetries of the condensate, and thereby the pairing mechanism itself. The destruction of superconductivity by disorder is traditionally described by Abrikosov-Gorkov (AG) theory, [1,2] which however ignores spatial modulations and ceases to be valid when impurities interfere, and interactions become important. Here we study the effects of disorder on unconventional superconductors in the presence of correlations, and explore a completely different disorder paradigm dominated by strong deviations from standard AG theory due to generation of local bound states and cooperative impurity behavior driven by Coulomb interactions. Specifically we explain under which circumstances magnetic disorder acts as a strong poison destroying high-T c superconductivity at the sub-1% level, and when non-magnetic disorder, counter-intuitively, hardly affects the unconventional superconducting state while concomitantly inducing an inhomogeneous full-volume magnetic phase. Recent experimental studies of Fe-based superconductors (FeSC) have discovered that such unusual disorder behavior seem to be indeed present in those systems.For cuprates, heavy-fermions, and FeSC the study of disorder currently constitutes a very active line of research, motivated largely by the fact that these systems are made superconducting by "chemical disordering" (charge doping), but also boosted by controversies of the correct microscopic model, and a rapid development of local experimental probes. [3][4][5] Focusing on multiband FeSC, disorder studies have proven exceptionally rich and strongly material dependent.[6] Scanning tunneling spectroscopy found a plethora of exotic atomicsized impurity-generated states, [7][8][9][10] NMR and neutrons observed clear evidence of glassy magnetic behavior, [11,12] and µSR discovered magnetic phases generated by non-magnetic disorder. [13,14] The resulting complex inhomogeneous phases and their properties in terms of thermodynamics and transport constitute an important open problem in the field.Here, we present a theoretical study of correlationdriven emergent impurity behavior of both magnetic and nonmagnetic disorder in unconventional s± multi-band superconductors. For the case of magnetic disorder, we find that correlations anti-screen the local moment, and significantly enhance the inter-impurity RudermanKittel-Kasuya-Yosida (RKKY) exchange interactions by inducing non-local long-range magnetic order which operates as an additional competitor to superconductivity. This results in an aggressive T c suppression rate where superconductivity is wiped out by sub-1% concentrations of disorder. This mechanism explains the "poisoning effect" discovered in ...

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Unexpected weak spatial variation in the local density of states induced by individual Co impurity atoms in superconducting Na(Fe1−xCox)As crystals revealed by scanning tunneling spectro

Cited by 27 publications

References 39 publications

Impact of iron-site defects on superconductivity in LiFeAs

Impact of iron-site defects on superconductivity in LiFeAs

Spectroscopic signature of Kondo screening on single adatoms inNa(Fe0.96Co0.03Mn0.01)As

Unconventional Disorder Effects in Correlated Superconductors

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