Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

Shanygina, T. E.; Ponomarev, Ya. G.; Kuzmichev, S. A.; Mikheev, M. G.; Tchesnokov, S. N.; Omel’yanovskii, O. E.; Sadakov, A. V.; Eltsev, Yu.; Dormidontov, A S; Pudalov, V. M.; Usoltsev, A. S.; Хлыбов, Е. П.

doi:10.1134/s0021364011020111

Cited by 21 publications

(32 citation statements)

References 36 publications

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“…In a case of a multi-gap superconductor several independent SGSs corresponding to the each gap should be observable (see Figs.2-5). Such two distinct SGSs were reported earlier at Andreev spectra of Mg(Al)B 2 break-junctions [33,34,35], LaO 0.9 F 0.1 FeAs [24], GdO(F)FeAs [36,37,38], and FeSe [39]. The main advantage of a symmetrical SNS-contact is that at any T < T C the gap energy can be defined directly from the bias voltages of the peculiarities at the dI/dV -curve making use of the formula (1).…”

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confidence: 78%

“…For the large gap, our experimental data lead to ratio 2∆ L /k B T C = (4.6 ÷ 5.6) which exceeds the standard one 3.52 for single-gap BCS superconductors in the weak coupling limit. At the same time, the BCS-ratio for the small gap 2∆ S /k B T C ≪ 3.52 indicates the superconductivity induced by interband coupling with the "driving" bands (characterized by the large gap ∆ L ) in the bands with ∆ S due to k-space (internal) proximity effect [51] between two condensates, which resembles the situation in Mg(Al)B 2 [33,34,35], and some Fe-based superconductors [24,36,37,38,39].…”

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Investigation of LiFeAs by means of “break-junction” technique

et al. 2012

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In our tunneling investigation using Andreev superconductor -normal metal -superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap ∆L = (2.5 ÷ 3.4) meV and the small gap ∆S = (0.9 ÷ 1) meV at T = 4.2 K for the T local C ≈ (10.5 ÷ 14) K (the contact area critical temperature which deviation causes the variation of ∆L). The BCS-ratio is found to be 2∆L/kBTC = (4.6 ÷ 5.6), whereas 2∆S/kBTC ≪ 3.52 results from induced superconductivity in the bands with the small gap.The new class of superconducting rare-earth oxypnictides [1] is still not completely understood and therefore requires further investigation. The layered LiFeAs (111-system) [2] is one of the few stoichiometric Febased pnictides which shows neither magnetic nor structural transition but becomes superconducting at 18 K [3,4]. Band-structure calculations [5,6,7] show the Fermi surfaces for 111-system to be comprised of quasitwo-dimensional (2D) hole cylinders centered at the Γ-point and electron ones at the M-point of the first Brillouin zone that can be considered as two effective bands (so called minimal two-band model) [8,9]. The total density of states at the Fermi level is formed mainly by Fe 3d-states [10,11,12,13,14]. As was shown in [15], the superconducting transition temperatures T C for different types of iron-based superconductors correlate with the total density of states at the Fermi level. This fact and the strong Fe isotope effect, which was reported by [16] supports the phonon-mediated coupling importance [17] in these compounds [15,18]. The electron-phonon coupling is enhanced by an extended van Hove singularity [19] which was shown for iron pnictides and, in particular, for LiFeAs [20]. In this work, we present an investigation of the superconducting properties of LiFeAs single crystals by means of Andreev spectroscopy of superconductor -normal metal -superconductor (SNS) contacts, and the corresponding superconducting gaps.The LiFeAs single crystals were obtained by self-flux method. The synthesis and investigation of the composition and properties are detailed in [21]. A mixture of small lumps of the Li metal and powders of Fe and As in a molar ratio of Li : Fe : As = 3 : 2 : 3 was 1) e-mail: kuzmichev@mig.phys.msu.ru placed into an alumina crucible. All work on the reactive mixture preparation was carried out in a dry box under argon atmosphere. The crucible was inserted into a niobium container, which was welded in argon at 1.5 atm. The sealed Nb container was enclosed in a quartz ampoule. The sample was heated up to 1363 K, kept at this temperature and slowly cooled down to 873 K. At this temperature ampoule was extracted from the furnace and cooled in open air. The LiFeAs single crystals in the form of thin plates with lateral dimensions of (12 ± 6) × (12 ± 6) × (0.1 ± 0.05) mm 3 were separated from flux mechanically. According to the XRD, EDX, ICP MS, a...

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Investigation of LiFeAs by means of “break-junction” technique

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“…Under such cleavage, a layered sample exfoliates along the ab-planes; as a result, it contains two cryogenic clefts separated by a weaklink. For Fe-based superconductors of the 1111, and 111 families [12,26,28,29,30,31], the current-voltage characteristic (CVC) and the dynamic conductance correspond to a ballistic [32] high-transparent SnS-Andreev contact [33,34,35]. The break-junction technique enables the formation of clean cryogenic clefts, true 4probes connection, thus preventing overheating of the contact area, and easy mechanical readjustment.…”

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Estimation of Intraband and Interband Relative Coupling Constants from Temperature Dependences of the Order Parameter for Two-Gap Superconductors

Kuzmichev

Kuzmicheva

Tchesnokov

et al. 2016

J Supercond Nov Magn

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We present temperature dependences of the large and the small superconducting gaps measured directly by SnS-Andreev spectroscopy in various Fe-based superconductors and MgB 2 . The experimental ∆ L,S (T ) are well-fitted with a two-gap model based on Moskalenko and Suhl system of equations (supplemented with a BCS-integral renormalization). From the the fitting procedure, we estimate the key attribute of superconducting state -relative electron-boson coupling constants and eigen BCS-ratios for both condensates. Our results evidence for a driving role of a strong intraband coupling in the bands with the large gap, whereas interband coupling is rather weak for all the superconductors under study.

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“…7. ªÊÏÇÓÇÐÐÞÇ ÊÐÂÚÇÐËâ ØÂÓÂÍÕÇÓËÔÕËÚÇÔÍËØ ÐÂÒÓâÉÇÐËÌ V n 2D LY S aen Ä ÊÂÄËÔËÏÑÔÕË ÑÕ 1an LY S AEÎâ ÚÇÕÞÓÈØ SNS-ÍÑÐÕÂÍÕÑÄ, ÒÓËÅÑÕÑÄÎÇÐÞØ ÏÇÕÑAEÑÏ "break-junction" ÒÓË ÅÇÎËÇÄÑÌ ÕÇÏÒÇÓÂÕÖÓÇ [50].…”

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V L Ginzburg and the development of experimental work on high-temperature superconductivity at LPI: 'iron superconductors'

Pudalov¹,

Omel’yanovskii²,

Хлыбов³

et al. 2011

Usp. Fiz. Nauk

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Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

Cited by 21 publications

References 36 publications

Investigation of LiFeAs by means of “break-junction” technique

Investigation of LiFeAs by means of “break-junction” technique

Estimation of Intraband and Interband Relative Coupling Constants from Temperature Dependences of the Order Parameter for Two-Gap Superconductors

V L Ginzburg and the development of experimental work on high-temperature superconductivity at LPI: 'iron superconductors'

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