Leggett’s mode in Mg1−x AlxB2

Ponomarev, Ya. G.; Kuzmichev, S. A.; Mikheev, M. G.; Sudakova, M. V.; Tchesnokov, S. N.; Hoai, Hoang Van; Булычев, Б. М.; Maksimov, Evgenii G.; Krasnosvobodtsev, S. I.

doi:10.1134/s0021364007010092

Cited by 20 publications

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“…Similar energy was obtained in tunneling SIS contact studies, extracted from a fine structure caused by a resonant excitation of Leggett plasmons mode by Josephson supercurrent [39,40]. According to theory [56], that energy was not exceeded the doubled small gap, and evolved as ω 2 L ∼ ∆ σ · ∆ π within nearly full range of aluminum concentration and T c = 6 − 41 K [39,40]. Here in Sm-1111, we reproducibly observe up to k = 4 equidistant satellites, which evidences their bosonic origin.…”

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confidence: 69%

“…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%

“…(6) Unlike magnesium diborides [39,40], one cannot attribute the observed bosonic mode as Leggett plasma mode [56]. Firstly, several theoretical studies shown that Leggett plasmons are unobservable in iron pnictides [62,63].…”

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confidence: 99%

“…Boson absorption is nearly impossible, due to the lack of excited bosons at low temperatures. When the bosonic mode has a particular energy ε 0 , one should observe satellite dips beyond the SGS at bias voltages [34,39,40,48] …”

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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: 69%

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confidence: 99%

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confidence: 99%

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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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“…Inelastic neutron scattering studies have reported optical phonon modes at ∼35, 55, 75, 85 and 100 meV 28 , while Raman scattering reports a single, broad asymmetric peak at 75meV, attributed to the E 2g mode, as well as a sharp peak at 2∆ σ = 12meV due to a pair breaking excitation 29 . As a bonus, due to its multi-gap nature, MgB 2 also contains another exotic collective mode that can couple to the electrons: the Leggett mode [21][22][23][24][25] . This mode is a longitudinal fluctuation corresponding to equal and opposite displacements of the two condensates.…”

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Strong interaction between electrons and collective excitations in the multiband superconductorMgB2

et al. 2015

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We use a tunable laser ARPES to study the electronic properties of the prototypical multiband BCS superconductor MgB 2 . Our data reveal a strong renormalization of the dispersion (kink) at ∼65 meV, which is caused by coupling of electrons to the E 2g phonon mode. In contrast to cuprates, the 65 meV kink in MgB 2 does not change significantly across T c . More interestingly, we observe strong coupling to a second, lower energy collective mode at binding energy of 10 meV. This excitation vanishes above T c and is likely a signature of the elusive Leggett mode.

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