Spin resonance peak in Fe-based superconductors with unequal gaps

Korshunov, M. M.; Shestakov, V. A.; Togushova, Yu.N.

doi:10.1103/physrevb.94.094517

Cited by 21 publications

(28 citation statements)

References 41 publications

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“…22) These features provide strong evidence in favor of the sign-changing s ± -pairing symmetry in CaKFe 4 As 4 . 44) This result is further supported by our RPA calculations, as described below.…”

Section: )supporting

confidence: 81%

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe₄As₄

et al. 2017

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The dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe 4 As 4 was investigated by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations. Powder INS measurements show that the spin resonance at Q res = 1.17(1) Å −1 , corresponding to the (π, π) nesting wave vector in tetragonal notation, evolves below T c . The characteristic energy of the spin resonance E res = 12.5 meV is smaller than twice the size of the superconducting gap (2∆). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe 4 As 4 is the s ± symmetry.Various iron-based superconducting materials with different crystal structures have been investigated to transcend our understanding of the nature of their pairing mechanism, [1][2][3][4] as the crystal structure is believed to have a strong relationship with the superconducting transition temperature T c . 5,6) Recently, new-structure-type iron-based superconducting materials AeAFe 4 As 4 (where Ae = Ca, Sr, or Eu and A = K, Rb, or Cs) were reported. 7-9)The AeAFe 4 As 4 family exhibits T c = 32-37 K, which is relatively high compared with other stoichiometric iron-based superconductors. [10][11][12][13][14] As Ae and A layers stack alternatively between Fe 2 As 2 layers owing to the large difference between their ionic radii, AeAFe 4 As 4 belongs to the P4/mmm space group, which is different from I4/mmm in hole-doped (Ba 1−x K x )Fe 2 As 2 systems. 7, 15) AeAFe 4 As 4 has high T c at stochiometric composition and a unique crystal structure, providing us with a great opportunity to theoretically and experimentally investigate iron-based superconductivity in the absence of substitutional disorder.CaKFe 4 As 4 undergoes superconductivity below T c ≃ 35 K without other phase transitions for 1.8 ≤ T ≤ 300 K. [15][16][17] The magnetic ground state of the normal state in CaKFe 4 As 4 is paramagnetic. 16,18) Various measurements 15,16,[18][19][20][21] indicated that CaKFe 4 As 4 shows behaviors similar to optimally or slightly overdoped (Ba 1−x K x )Fe 2 As 2 . Angle-resolved photoemission spectroscopy (ARPES) measurements supported by density functional theory calculations indicated that the Fermi surface consists of three-hole pockets at the Γ point and two electron pockets at the M point. 22,23) This multiband nature implies multi-gap superconductivity in CaKFe 4 As 4 . Two superconducting gaps which are nodeless and isotropic were, indeed, observed by scanning tunneling microscopy, 20) optical conductivity, 24) penetration depth, 21) muon spin rota- * k iida@cross.or.jp tion, 18) and 75 As nuclear magnetic resonance (NMR) 25) measurements. The largest superconducting gaps (∆ hole = 13 meV and ∆ electron = 12 meV) for quasi-two-dimensional hole and electron p...

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Section: )supporting

confidence: 81%

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe₄As₄

et al. 2017

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“…The estimated boson energy at T → 0 is less than the indirect gap ε 0 < ∆ L (0) + ∆ S (0), and, in accordance with theory 18,19 , supports a spin-exciton nature of the observed boson. The energy which similarly fulfills the resonance condition was extracted by us earlier in other 1111 compounds within a wide range of critical temperatures 46,47 .…”

Section: Resonant Coupling With a Bosonic Modesupporting

confidence: 83%

“…We determined the magnitudes of the two distinct superconducting order parameters and their temperature dependence, estimated intrato interband coupling strength imbalance, and eigen parameters of both superconducting condensates (to be realized excluding interband coupling). A resonant coupling with a characteristic bosonic mode was observed, with the energy more than ∆ L and less than indirect gap (∆ L + ∆ S ) at T → 0 which satisfies the theoretical condition 18,19 .…”

Section: Introductionmentioning

confidence: 54%

Superconducting order parameter and bosonic mode in hydrogen-substituted NdFeAsO0.6H0.36 revealed by multiple-Andreev-reflection spectroscopy

2019

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Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied ballistic superconductor -normal metal -superconductor (SnS) contacts in layered oxypnictide superconductors NdFeAsO0.6H0.36 with critical temperatures Tc = 45 − 48 K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap ∆L ≈ 10.4 meV, and a possible small gap ∆S ≈ 1.8 meV, and their temperature dependence. Additionally, a resonant coupling with a characteristic bosonic mode was observed. The boson energy at 4.2 K, ε0 = 10.5 − 11.0 meV being less than the indirect gap (∆L < ε0 < ∆L + ∆S).

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“…At T c ∼ 50 K, ε 0 reaches ≈ 15 meV (120 ± 20 cm −1 ) and resembles that determined by us earlier for GdO 1−x F x FeAs with similar T c ). One cannot attribute the observed bosonic resonance with Leggett mode or optic phonon mode, nonetheless the ε 0 is close to the expected position of the the energy of spin resonance peak in s ± state [32] or the enhanced spectral density maximum in s ++ state [25]. * * *…”

mentioning

confidence: 61%

“…In s ++ approach, imaginary part of dynamic spin susceptibility demonstrates a smeared maximum rather than peak [22,25,26]. 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].…”

mentioning

confidence: 99%

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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Spin resonance peak in Fe-based superconductors with unequal gaps

Cited by 21 publications

References 41 publications

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe₄As₄

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe₄As₄

Superconducting order parameter and bosonic mode in hydrogen-substituted NdFeAsO0.6H0.36 revealed by multiple-Andreev-reflection spectroscopy

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

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Spin resonance peak in Fe-based superconductors with unequal gaps

Cited by 21 publications

References 41 publications

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe4As4

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe4As4

Superconducting order parameter and bosonic mode in hydrogen-substituted NdFeAsO0.6H0.36 revealed by multiple-Andreev-reflection spectroscopy

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

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Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe₄As₄

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe₄As₄

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