Evidence of a Spin Resonance Mode in the Iron-Based Superconductor<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Ba</mml:mi><mml:mn>0.6</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="bold">K</mml:mi><mml:mn>0.4</mml:mn></mml:msub><mml:msub><mml:mi>Fe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>As</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>from Scanning Tunneling Spectroscopy

Shan, Lei; Gong, Jing; Wang, Yonglei; Shen, Bing; Hou, Xiaochen; Ren, Cong; Li, Chunhong; Yang, Huan; Wen, Hao; Li, Shiliang; Dai, Pengcheng

doi:10.1103/physrevlett.108.227002

Cited by 66 publications

(79 citation statements)

References 48 publications

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“…In Nd-1111, the Eliashberg function extracted from d 2 I(V)/dV 2 [14] well matches the calculated one, and the phonon density of states [44]. Scanning tunneling spectroscopy (STS) studies with Ba 0.6 K 0.4 Fe 2 As 2 revealed a fine structure attributed to a coupling of quasiparticles with a bosonic mode near 14 meV [43]. In contrast, in STS probe with SmFeAsO 1−x F x [42], the minimum of the dip-hump structure was attributed to a sign of a resonance spin mode within the energy range ε res = 2 − 8 meV.…”

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confidence: 72%

“…More recent theoretical studies showed that in framework of both s ++ and s ± models the dynamic spin susceptibility has a feature near ∆ L + ∆ S energy, a sharp peak related to spin resonance in s ± state, or spectral density enhancement above ∆ L + ∆ S in s ++ state [25,32]. Tunneling contact probes could provide information about electron-boson interaction [33][34][35][36][37][38][39][40][41][42][43]. For tunneling normal metal -insulator -superconductor (NIS) junction, the derivative d 2 I/dV 2 of dynamic conductance spectrum at bias voltages V > ∆/e represents a spectral function of electron-boson interaction [41].…”

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confidence: 99%

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Evidence of a multiple boson emission in Sm _1−x Th _x OFeAs

Kuzmichev

Kuzmicheva

Zhigadlo

2017

EPL

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-We studied a reproducible fine structure observed in dynamic conductance spectra of Andreev arrays in Sm1−xThxOFeAs superconductors with various thorium concentrations (x = 0.08-0.3) and critical temperatures Tc = 26-50 K. This structure is unambiguously caused by a multiple boson emission (of the same energy) during the process of multiple Andreev reflections. The directly determined energy of the bosonic mode reaches ε0 = 14.8 ± 2.2 meV for optimal compound. Within the studied range of Tc, this energy as well as the large ∆L and the small ∆S superconducting gaps, nearly scales with critical temperature with the characteristic ratio ε0/kBTc ≈ 3.2 (and 2∆L/kBTc ≈ 5.3, correspondingly) resembling the expected energy ∆L + ∆S of spin resonance and spectral density enhancement in s ± and s ++ states, respectively.Fe-based superconductors Sm 1−x Th x OFeAs belong to the oxypnictide family (so called 1111), and have rather simple crystal structure, resembling the stack of superconducting FeAs blocks alternating with Sm 1−x Th x O spacers along the c-direction [1]. Under electron doping, the T c varies in the wide range, reaching 54 K at x ≈ 0.3 nominal concentration [1,2]. Band-structure calculations [3] showed the density of states at the Fermi level formed mainly by iron 3d states. For this reason, the (Sm,Th) substitution affecting the spacer structure barely seems not changing the underlying pairing mechanism [4]. The Fermi surface consists of tubular sections, electron-like near the M point of the first Brillouin zone, and hole-like near the Γ point, both with no significant k z anisotropy [3,5].The majority of theoretical and experimental studies [3][4][5][6][7][8] suppose two superconducting condensates developing below T c . Earlier we reported the scaling between both gaps (∆ L -large gap, ∆ S -small gap) and T c , keeping 2∆ S /k B T c ≈ 1.2 − 1.6 and 2∆ L /k B T c = 5. 0−5.7 [4,9-11]. Similar 2∆ L /k B T c was obtained in literature for Sm-1111 in point-contact probes [12,13], and for various other 1111 [14][15][16][17][18][19][20]. Both BCS-ratios diverge from the weakcoupling BCS prediction due to a strong coupling in the "driving" bands where the large gap is developed, and a k-space proximity effect with the "driven" ∆ S bands. The pairing p-1

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confidence: 72%

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confidence: 99%

Evidence of a multiple boson emission in Sm _1−x Th _x OFeAs

Kuzmichev

Kuzmicheva

Zhigadlo

2017

EPL

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“…More recently, the method has been also applied to (Li 1−x Fe x )OHFe 1−y Zn y Se with only electron Fermi surface pockets 74 . For the 122 doped systems, such an experiment has not been yet performed; however, one would expect that such a test is feasible in these compounds as well, given the availability of the high-quality STM data 75,76 . Here we show that the QPI patterns contain information on the phase difference between the order parameters of different bands for the case when it is not 0 or π and even if one of the bands is incipient.…”

Section: Stm-signatures Of S + Is-statementioning

confidence: 99%

s+is superconductivity with incipient bands: Doping dependence and STM signatures

et al. 2017

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Motivated by the recent observations of small Fermi energies and comparatively large superconducting gaps, present also on bands not crossing the Fermi energy (incipient bands) in iron-based superconductors, we analyze the doping evolution of superconductivity in a four-band model across the Lifshitz transition including BCS-BEC crossover effects on the shallow bands. Similar to the BCS case, we find that with hole doping the phase difference between superconducting order parameters of the hole bands change from 0 to π through an intermediate s + is state, breaking time-reversal symmetry (TRS). The transition, however, occurs in the region where electron bands are incipient and chemical potential renormalization in the superconducting state leads to a significant broadening of the s + is region. We further present the qualitative features of the s + is state that can be observed in scanning tunneling microscopy (STM) experiments, also taking incipient bands into account.

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“…While evidence that self-energy effects due to electron-boson-coupling phenomena are occurring in iron-based materials abounds [26][27][28][29][30][31][32] , a direct comparison between a theoretical prediction with realistic band/gap structure that distinguishes effects of coupling to AFSF from those due to coupling to E g -phonons generating the orbital fluctuations, has not been achieved. Here, by combining new theoretical insight into QPI discrimination between Σ(k,ω) from resonant-AFSF and Σ(k,ω) due to alternative scenarios, together with novel QPI techniques designed to visualize the Σ(k,ω)…”

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Identifying the 'fingerprint' of antiferromagnetic spin fluctuations in iron pnictide superconductors

et al. 2014

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We show that all the measured phenomena comprise the predicted QPI "fingerprint" of a self-energy due to antiferromagnetic spin-fluctuations, thereby distinguishing them as the predominant electron-boson interaction. 1The microscopic mechanism for Cooper pairing in iron-based high-temperature superconductors has not been identified definitively [1][2][3] . Among the complicating features in these superconductors is the multiband electronic structure (see Fig. 1a). However, it is believed widely that the proximity to spin order [1][2][3][4][5] 2Each type of electron-boson interaction should produce a characteristic electronicrepresenting its effect on every non-interacting electronic state k with momentum ħk and energy ħω. Thus, the interacting Green's function obtained by first visualizing scattering interference patterns in real-space (r-space) images of the tip-sample differential tunneling conductance dI/dV(r,ω=eV)≡g(r,ω) using spectroscopic-imaging scanning tunneling microscopy, and then Fourier transforming g(r,ω) to obtain the power spectral density g(q,ω) 11 . The g(q,ω) can then be used to reveal the electron dispersion k(ω) because elastic scattering of electrons from −k(ω) to +k(ω) results in high intensity at q(ω)=2k(ω) in g (q,ω). Sudden changes in the energy evolution k(ω) due to Σ(k,ω) can then be determined, in principle 19 , using such data. 3In a conventional single band s-wave superconductor with isotropic energy gap magnitude Δ, it has been well-established that coupling to an optical phonon with frequency Ω can lead to a renormalization of the electronic spectra at energy Δ+Ω (ħ=1)due to a singularity in the momentum independent self-energyThis classic case is illustrated in Fig. 1c,d through a model spectral function A(k,ω)∝ImG(k,ω) and the associated density of states N(ω)=∫ dk A(k,ω).In Fig. 1c, the "free" dispersion of a hole-like band is represented by the red dashed line, while the renormalized dispersion k(ω) due to Σ(ω) is highlighted by the locus of maxima in A(k,ω). These effects can be understood from the conservation of energy and momentum during scattering processes (Fig. 1b), where the flat dispersion of an optical phonon presents constraints only on energy without any momentum dependence. 4In developing our new approach to "fingerprinting" different electron-boson interactions using QPI, we use the realization that the kinematic constraints for a multiband electronic system coupled to resonant AFSF with a sharp momentum structure should result in a strongly momentum-dependent (anisotropic) self-energy. This is because, given a fermionic dispersion ) , ( n k k ω for different bands n and a spectrum of spin fluctuations whose intensity is strongly concentrated at (Q,Ω), the renormalization due to the self-energy at a point ) , ( n k k ω will be most intense when that point can be connected to another pointThis is the constraint from conservation of both energy and momentum in the electron- (2) can be satisfied and thus where the strongest self-energy effect due to coupling to A...

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Evidence of a Spin Resonance Mode in the Iron-Based SuperconductorBa0.6K0.4Fe2As2from Scanning Tunneling Spectroscopy

Cited by 66 publications

References 48 publications

Evidence of a multiple boson emission in Sm _1−x Th _x OFeAs

Evidence of a multiple boson emission in Sm _1−x Th _x OFeAs

s+is superconductivity with incipient bands: Doping dependence and STM signatures

Identifying the 'fingerprint' of antiferromagnetic spin fluctuations in iron pnictide superconductors

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Evidence of a Spin Resonance Mode in the Iron-Based SuperconductorBa0.6K0.4Fe2As2from Scanning Tunneling Spectroscopy

Cited by 66 publications

References 48 publications

Evidence of a multiple boson emission in Sm 1−x Th x OFeAs

Evidence of a multiple boson emission in Sm 1−x Th x OFeAs

s+is superconductivity with incipient bands: Doping dependence and STM signatures

Identifying the 'fingerprint' of antiferromagnetic spin fluctuations in iron pnictide superconductors

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Product

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Evidence of a multiple boson emission in Sm _1−x Th _x OFeAs

Evidence of a multiple boson emission in Sm _1−x Th _x OFeAs