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Superconducting properties of the <i>s</i> <sup>?</sup>-wave state: Fe-based superconductors
Bang, Yunkyu,Stewart, G R IOP 2017 Journal of Physics, Condensed Matter Vol.29 No.12
<P>Although the pairing mechanism of Fe-based superconductors (FeSCs) has not yet been settled with consensus with regard to the pairing symmetry and the superconducting (SC) gap function, the vast majority of experiments support the existence of spin-singlet sign-changing <I>s</I>-wave SC gaps on multi-bands (<img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn003.gif'/>-wave state). This multi-band <img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn004.gif'/>-wave state is a very unique gap state <I>per se</I> and displays numerous unexpected novel SC properties, such as a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear magnetic resonance signals, various Volovik effects in the specific heat (SH) and thermal conductivity, and anomalous scaling behaviors with a SH jump and condensation energy versus <I>T</I> <SUB>c</SUB>, etc. In particular, many of these non-trivial SC properties can easily be mistaken as evidence for a nodal-gap state such as a <I>d</I>-wave gap. In this review, we provide detailed explanations of the theoretical principles for the various non-trivial SC properties of the <img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn005.gif'/>-wave pairing state, and then critically compare the theoretical predictions with experiments on FeSCs. This will provide a pedagogical overview of to what extent we can coherently understand the wide range of different experiments on FeSCs within the <img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn006.gif'/>-wave gap model.</P>
Superfluid density of the ±s-wave state for the iron-based superconductors
Editions de Physique 2009 Europhysics letters Vol.86 No.4
<P>We study the superfluid density of the ±<I>s</I>-wave state of the minimal two-band model for the Fe-based superconductors and its evolution with impurity concentration. We show that the impurity scattering of the strong-coupling limit induces the self-energy of a generic form ImΣ<SUB><I>imp</I></SUB>(ω)≈<I>i</I>γ+<I>i</I>βω beyond a critical impurity concentration Γ<SUB><I>imp</I></SUB>>Γ<SUB><I>crit</I></SUB>. This form of ImΣ<SUB><I>imp</I></SUB>(ω) causes the temperature dependence of the superfluid density [ρ<SUB><I>s</I></SUB>(<I>T</I>)−ρ<SUB><I>s</I></SUB>(0)]≈−γ<I>T</I><SUP>2</SUP>-β<I>T</I><SUP>3</SUP>. Combined with the full gap behavior of ρ<SUB><I>s</I></SUB>(<I>T</I>) for lower impurity concentration Γ<SUB><I>imp</I></SUB><Γ<SUB><I>crit</I></SUB>, the ±<I>s</I>-wave state produces a continuous evolution of Δλ(<I>T</I>): exponentially flat →<I>T</I><SUP>3</SUP>→<I>T</I><SUP>2</SUP> with increasing impurity concentration that is consistent with the measurements of the Fe pnictide superconductors such as <I>M</I>-1111 (<I>M</I>=La, Nd, Sm, Pr) and Ba-122 with various dopings, except LaFePO which shows Δλ(<I>T</I>)∝<I>T</I><SUP>1.2</SUP> at low temperatures by a recent experiment. Our results also demonstrate that the density of states (DOS) measured by thermodynamic properties and the DOS measured by transport properties can in general be different.</P>
The origin of the condensation energy scaling of iron-based superconductors
Editions de Physique 2016 Europhysics letters Vol.115 No.2
<P>The relation between the condensation energy (CE) and T-c of a phase transition reveals a fundamental nature of the transition. In view of this, the recent experimental observation of the non-BCS scaling relation of the CE vs. T-c (Delta E similar to T-c(3.5)) with about forty different samples of the Fe-based superconductors (Xing J. et al., Phys. Rev. B, 89 (2014) 140503) was intriguing and strongly hinted at a non-BCS pairing mechanism. In this paper, we have studied the CE and Tc of the multiband BCS model and found that the observed anomalous scaling relation Delta E similar to T-c(3.5) is well reproduced by the two-band BCS model paired by a dominant repulsive interband interaction (V-inter > V-intra > 0). Our result implies that this seemingly non-BCS-like scaling behavior of Delta E similar to T-c(3.5), on the contrary to the common expectations, 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. Copyright (C) EPLA, 2016</P>
Optical properties of optimally doped single-crystal Ca8.5La1.5(Pt3As8)(Fe2As2)5
Seo, Yu-il,Choi, Woo-Jae,Kimura, Shin-ichi,Bang, Yunkyu,Kwon, Yong Seung American Physical Society 2017 Physical Review B Vol.95 No.9
<P>We have measured the reflectivity of the optimally doped Ca8.5La1.5(Pt3As8)(Fe10As10) single crystal (Tc = 32.8K) over the broad frequency range from 40 cm(-1) to 12 000 cm-1 and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low-frequency region (<1000 cm(-1)) in the normal state (80 K < T <= 300 K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low-frequency Drude spectra develops a pseudogap (PG) hump structure at around 100 cm(-1) and continuously evolves into the fully opened superconducting (SC) gap structure belowTc. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above Tc into the SC gap structure below Tc. The extracted two SC gap sizes are Delta S = 4.9 meV and Delta L = 14.2 meV, suggesting Ca8.5La1.5(Pt3As8)(Fe10As10) as a multiple-gap superconductor with a mixed character of the weak-coupling and strong-coupling superconductivity.</P>