Scrutinizing the double superconducting gaps and strong coupling pairing in (Li1−xFex)OHFeSe

Du, Zuliang; Xiong, Yan; Lin, Hai; Fang, Delong; Du, Gonghuan; Xing, Jie; Yang, Huan; Zhu, Xiyu; Wen, Hao

doi:10.1038/ncomms10565

Cited by 84 publications

(113 citation statements)

References 37 publications

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“…Interestingly, recent experiments propose that an s-wave state is realized in FeSe thin films [42]. We also find that, when the SOC and/or ISB energy scales are larger than the energy scale associated with the mismatch between the two electron pockets, a nearly isotropic gap appears at the electron pockets, whose angular dependence agrees with ARPES and STM measurements in FeSe thin films [26,43] and intercalated Li 1−x (OH) x FeSe [44]. …”

supporting

confidence: 86%

“…4). Interestingly, recent ARPES experiments in monolayer FeSe observe gap maxima at θ = ±π/4 [56], whereas STM measurements in the intercalated Li 1−x (OH) x FeSe compound report gap minima at θ = ±π/4 [44]. Besides SOC, the inversion-symmetry breaking (ISB) at the interface of thin films also lifts the degeneracy between s-wave and d-wave in the case of FeSe thin films.…”

Section: Microscopic Modelmentioning

confidence: 99%

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Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling

Kang

Fernandes

2016

Phys. Rev. Lett.

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The origin of the high-temperature superconducting state observed in FeSe thin films, whose phase diagram displays no sign of magnetic order, remains a hotly debated topic. Here we investigate whether fluctuations arising due to the proximity to a nematic phase, which is observed in the phase diagram of this material, can promote superconductivity. We find that nematic fluctuations alone promote a highly degenerate pairing state, in which both s-wave and d-wave symmetries are equally favored, and Tc is consequently suppressed. However, the presence of a sizable spin-orbit coupling or inversion symmetry-breaking at the film interface lifts this harmful degeneracy and selects the s-wave state, in agreement with recent experimental proposals. The resulting gap function displays a weak anisotropy, which agrees with experiments in monolayer FeSe and intercalated Li1−x(OH)xFeSe.In most iron-based superconductors (FeSC), superconductivity is found in close proximity to a magnetically ordered state, suggesting that magnetic fluctuations play an important role in binding the Cooper pairs [1][2][3][4]. Indeed, the fact that the Fermi surface of these materials is composed of small hole pockets and electron pockets separated by the magnetic ordering vector led to the proposal of a sign-changing s +− wave state, in which the gap function has different signs in the hole and in the electron pockets. However, the recent observation of superconductivity over 70 K in monolayer FeSe brought new challenges to the field [5][6][7][8][9][10][11][12][13][14]. In contrast to the standard FeSC, no long-range magnetic order is observed in thin films or even bulk FeSe [15], and the Fermi surface of monolayer FeSe consists of electron pockets only [6,7,11,16]. Since T c in monolayer FeSe is the highest among all FeSC, the elucidation of its origin is a fundamental step in the search for higher T c in these systems.One of the proposed scenarios to explain the dramatic ten-fold increase of T c in monolayer FeSe with respect to the 8 K value in bulk FeSe [17] was the strong coupling to an optical phonon mode of the SrTiO 3 (STO) substrate [16,18,19], which is manifested by replica bands observed in ARPES [16]. Although such a coupling can certainly enhance T c [20][21][22][23][24], recent experiments indicate that the STO substrate may not be essential to achieve the high-T c state. In particular, T c up to 40 K was observed in electrostatically-gated films of FeSe with different thickness grown both on STO and MgO substrates [25]. Similar values of T c were reported in FeSe coated with potassium [26,27] and in the bulk sample Li 1−x (OH) x FeSe [28,29], which consists of intercalated FeSe layers. In common to all these systems is the fact that their Fermi surface consists of electron pockets only, suggesting that doping by negative charge carriers plays a fundamental role in stabilizing the high-T c state.Importantly, recent experiments in K-coated bulk FeSe [26] revealed that, besides shifting the chemical potential, electron-doping also su...

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supporting

confidence: 86%

Section: Microscopic Modelmentioning

confidence: 99%

Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling

Kang

Fernandes

2016

Phys. Rev. Lett.

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“…24,25 However, scanning tunneling spectroscopy (STS) studies show two distinct features in the conductance spectra, suggesting the presence of multiple gaps. 26,27 Meanwhile the in-plane superfluid density obtained from muon-spin rotation (µSR) measurements is consistent with either one or two gaps, but very different behavior is seen in the out-of-plane component, which shows a much more rapid drop of the superfluid density with temperature. 28 Furthermore, inelastic neutron scattering (INS) measurements give evidence for the presence of a spin resonance peak, 29,30 consistent with a sign change of the order parameter across the Fermi surface, while a lack of a sign change was suggested from an STS study, on the basis of quasi-particle interference (QPI) results and the effect of impurities.…”

Section: 15mentioning

confidence: 55%

“…We note that there have been conflicting reports about the nature of the gap structure of (Li 1−x Fe x )OHFeSe, with only a single gap being resolved from ARPES measurements 24,25 , while two gaps are found from STS 26,27 and the in-plane superfluid density from µSR measurements is compatible with both single-gap and two-gap models. 28 The gap values we obtain from fitting the in-plane superfluid density are smaller than those reported from previous measurements, particularly in the case of the smaller gap.…”

mentioning

confidence: 66%

“…2(a) that ∆λ(T ) for the #A samples decreases with decreasing temperature before flattening below about 0.1T c , indicating a nodeless gap structure in (Li 1−x Fe x )OHFeSe with a lack of low energy excitations, which is consistent with previous results. [24][25][26][27][28] Furthermore when fitted with a power law dependence ∆λ(T ) ∼ T n , exponents of n = 2.83 and n = 2.46 are obtained for samples #A-1 and #B-1 respectively. In the case of nodal superconductors, n = 1 for line nodes and n = 2 for point nodes is generally anticipated.…”

Section: Resultsmentioning

confidence: 99%

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Probing the superconducting gap structure of (Li1−xFex)OHFeSe

et al. 2017

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We report measurements of the London penetration depth [∆λ(T )] of the recently discovered ironbased superconductor (Li1−xFex)OHFeSe, in order to characterize the nature of the superconducting gap structure. At low temperatures, ∆λ(T ) displays nearly temperature independent behavior, indicating a fully open superconducting gap. We also analyze the superfluid density ρs(T ) which cannot be well accounted for by a single-gap isotropic s-wave model but are consistent with either two-gaps, a model for the orbital selective s × τ3 state or anisotropic s-wave superconductivity.

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Synthesis, phase stability, structural, and physical properties of 11‐type iron chalcogenides

Rößler

Koz

Wirth

et al. 2016

Physica Status Solidi (b)

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This article reviews recent experimental investigations on two binary Fe-chalcogenides: FeSe and Fe1+yTe. The main focus is on synthesis, single crystal growth, chemical composition, as well as on the effect of excess iron on structural, magnetic, and transport properties of these materials. The structurally simplest Fe-based superconductor Fe1+xSe with a critical temperature Tc ≈ 8.5 K undergoes a tetragonal to orthorhombic phase transition at a temperature Ts ≈ 87 K. No longrange magnetic order is observed down to the lowest measured temperature in Fe1+xSe. On the other hand, isostructural Fe1+yTe displays a complex interplay of magnetic and structural phase transitions in dependence on the tuning parameter such as excess amount of Fe or pressure, but it becomes a superconductor only when Te is substituted by a sufficient amount of Se. We summarize experimental evidence for different competing interactions and discuss related open questions.

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Scrutinizing the double superconducting gaps and strong coupling pairing in (Li1−xFex)OHFeSe

Cited by 84 publications

References 37 publications

Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling

Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling

Probing the superconducting gap structure of (Li1−xFex)OHFeSe

Synthesis, phase stability, structural, and physical properties of 11‐type iron chalcogenides

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