Multigap Superconductivity in GdFeAsO0.88 Evidenced by SnS-Andreev Spectroscopy

Shanygina, T. E.; Kuzmichev, S. A.; Mikheev, M. G.; Ponomarev, Ya. G.; Tchesnokov, S. N.; Eltsev, Yu.; Pudalov, V. M.; Sadakov, A. V.; Usoltsev, A. S.; Хлыбов, Е. П.; Kulikova, L. F.

doi:10.1007/s10948-013-2155-y

Cited by 15 publications

(22 citation statements)

References 30 publications

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“…Similar measurements of the Andreev spectra within the range 4.2 K ≤ T ≤ T C local give temperature dependence of both gaps in various iron-based superconductors. As an example, Δ L (T) (solid symbols) and Δ S (T) (open symbols) for Sm-based oxypnictide (triangles and squares) and Gd-based one (circles) [69][70][71][72][73][74] are presented in Fig. 4.…”

Section: Resultsmentioning

confidence: 99%

“…4, and Refs. [64,[67][68][69][70]72,74,75]). To analyze the measured temperature dependencies, we fitted them using two-gap system of equations by Moskalenko and Suhl [50,52] with renormalized BCSintegral (shown by the solid lines in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…Therefore, the bias voltage for any peculiarities (caused by bulk properties of the sample) in CVC and the dynamic conductance should be N times larger (where N is the number of junctions in the stack) for array contact in comparison with that of single ScS-contact. Such array contacts were for the first time observed on cuprates [76][77][78][79], and then on other layered superconductors [67][68][69][70][71][72]74,75]. In the Andreev mode (i.e.…”

Section: Methodsmentioning

confidence: 99%

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Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of $ \Delta_{\rm L, S}$ with $ T_{\rm C}$

Kuzmicheva¹,

Kuzmichev²,

Mikheev³

et al. 2014

Phys.-Usp.

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IntroductionUnexpected discovery in 2006 [1] of the first layered Fe-based high-temperature superconductor (HTSC) -LnOFePn (where Ln -lanthanide, Pn -pnictide; hereafter referred to as "1111") becomes a key issue of modern solid state physics. Since 2008, the class of iron-based superconductors has much expanded: several families of iron pnictides and chalcogenides have been synthesized [2][3][4]. The crystal structure of oxypnictides recalls that of cuprates and is in fact a stack of the superconducting Fe-As layers alternating along the c-direction with spacers, the nonsuperconducting oxide blocks, Ln-O. In spite of the pronounced layered structure and anisotropic physical properties, the electron subsystem in Fe-based superconductors is less quasi-two-dimensional in comparison with that in cuprate HTSC, because the height of the Fe-As blocks exceeds the thickness of the Cu-O planes, whereas the distance between superconducting blocks in iron-based superconductors is significantly less than that in cuprates. The latter seems to be a reason [5] why the critical temperature of Fe-based superconductors, though being as high as T C ≈ 57.5 K [6] still does not reach the cuprate one. Superconductivity in novel materials emerges with the suppression of spin density wave ground state under doping of the superconducting Fe-As layers or under external pressure [7]. The key distinction from cuprates, however, is the multiband nature of newly-discovered superconductivity in iron-based materials. Band structure calculations showed (for a review, see [8]) the coexistence of the electron and the hole quasi-two-dimensional bands in these compounds, whereas the Fermi surfaces consist of slightly warped along the c-direction cylinders (near Γ and M points), where several superconducting condensates could form. Contrary to the observation of the strong isotope effect [9], an early theoretical study showed [10] that the high temperature superconductivity in Fe-based compounds could not to be based on the electron-phonon interaction solely. Although the latter plays an important role, it is incapable [10] to explain the observable values of T C in the framework of the Eliashberg theory [11]. Taking into account the nesting of the Fermi surface sheets along the Γ-M-direction, the vicinity of the antiferromagnetic state [12,13], and the appearance of the experimentally observed [14] peak of dynamic spin susceptibility ("magnetic resonance"), Mazin et al. suggested the theoretical description of the mechanism of superconductivity in iron pnictides and chalcogenides based on sign-changing (in different bands) order parameter -the so-called s ± -model. The simplest initial model considers two isotropic order parameters (in the electron and the hole bands, correspondingly), which have equal amplitudes but are in antiphase (i.e. having opposite signs). The strong interband interaction mediated by spin fluctuations along the Γ-M-direction plays the key role in this model, whereas the intraband electron-phonon coupling is an order of magnit...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of $ \Delta_{\rm L, S}$ with $ T_{\rm C}$

Kuzmicheva¹,

Kuzmichev²,

Mikheev³

et al. 2014

Phys.-Usp.

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“…4 lie slightly lower than standard single-band BCStype (depicted by dashed lines). This deviation is typical for all 1111 we studied [16,17,22], as well as for other multigap superconductors [21,23,24,25,26,27,28], and may be caused by the presence of a second condensate with a small gap, and a nonzero interband coupling (k-space proximity effect [29]). The gap temperature dependence can be fully described by a system of gap equations (with renormalized BCS-integral) by Moskalenko and Suhl [30,31,32] comprising a 2 × 2 matrix of coupling constants λ ij .…”

mentioning

confidence: 76%

Intrinsic Multiple Andreev Reflections in Layered Th-Doped Sm1−x Th x OFeAs

Kuzmicheva

Kuzmichev

Tchesnokov

et al. 2015

J Supercond Nov Magn

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Layered oxypnictide Sm 1−x Th x OFeAs (Sm-1111) is an ideal candidate to be probed by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy. Using the classical "break-junction" technique, we formed ballistic Andreev arrays of identical S-n-Scontacts, where S is superconductor, and n is a layer of normal metal. For T < T C , the I(V) curve shows an excess-current and a subharmonic gap structure (SGS): a set of sharp dI(V)/dV-dips at positions which depend on the superconducting gap value, the number of junctions in the array, and the natural subharmonic order, thus manifesting the effect of intrinsic multiple Andreev reflections. Here we present the I(V) and dI(V)/dV with up to 4 SGS dips for Andreev arrays formed in optimally doped Sm-1111 with critical temperatures T C ≈ 49 K, as well as in underdoped samples with T C ≈ 37 K. We show that a number of Andreev subharmonics facilitates the determination of the superconducting gap with a better accuracy. In this study, we used polycrystalline Sm 1−x Th x OFeAs samples with nearly optimal Th-doping and T C ≈ 49 K, and underdoped samples with T C ≈ 37 K. The highquality polycrystallites possessing a single superconducting phase were synthesized under high pressure, as detailed in [3,4]. Superconducting properties of Sm-1111 samples were probed by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy [5], to use this method, we applied the "break-junction" technique [6]. The sample consisting of a thin rectangular plate of 4 × 2 × 0.2 mm 3 was attached to a spring sample holder by a liquid In-Ga alloy. Then the sample holder was cooled down to T = 4.2 K, where a slight mechanical curving produced a cryogenic cleavage, thus creating two superconducting banks separated with a weak link (ScS-contact, c -constriction). In oxypnictides, the constriction usually acts as a thin normal metal, making it possible to observe multiple Andreev reflections in a ballistic 8,9,10]. In a clean classical Andreev mode, it causes a currentvoltage characteristic (CVC) with an excess current at low bias voltages ("foot") and a subharmonic gap structure (SGS) in the dynamic conductance [8,9,10]. SGS presents a series of dI(V)/dV-minima at certain positions V n = 2∆/en, n = 1, 2, . . . The typical diameter of a ballistic SnS-contact in oxypnictides was estimated using Sharvin's formula: a = 10 ÷ 30 nm [11,12].A layered sample exfoliates along the ab-planes with the formation of steps and terraces along the c-direction.

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“…In our experiments we observed quite the opposite: in both single crystals and polycrystalline samples of similar compounds a typical resistance of ScS contact was reproduced, and the singularities of the dynamic conductance became sharper with increasing m [71]. Note that the position and shape of the singularities along the dI(V)/dV spectra (caused by bulk effects such as gap and phonon singularities) are reproduced when scaling the bias of the electric potential by a natural number m in order to normalize the conductance features to a single junction spectrum [36,51,66,68,69,71,74,75,77], and coincide with the single contact characteristics. Similar data were obtained for single crystals of layered superconductors [30,67,72,76,81].…”

Section: On the Possibility Of Creating Break Junctions In Polycrystamentioning

confidence: 99%

“Break-junction” technique in application to layered superconductors (Review Article)

Kuzmichev

Kuzmicheva

2016

Low Temperature Physics

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Here presented a systematic study of superconductor-constriction-superconductor contacts realized by a break-junction technique in layered superconductors. Depending on the constriction transparency the tunneling and SnS-Andreev spectroscopies could be used, for the direct determination of values of superconducting gaps, characteristic BCS-ratios and gap temperature dependences in cuprate superconductors, magnesium diboride, novel pnictides and chalcogenides. Basing on these data, one can estimate the gap anisotropy magnitude as well as values of electron-boson coupling constants. We discuss the advantages and difficulties of the break-junction technique and demonstrate this method is powerful enough for high-resolution investigation of optical phonon modes in high-temperature superconducting cuprates and for creating of the contacts with the selective transparence in Mg 1-x Al x B 2 compounds.In memory of Ya. G. Ponomarev, professor, our teacher, warrior, and outstanding experimenter. In memory of S. N. Tchesnokov, mentor, friend, and extraordinary inventor

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Multigap Superconductivity in GdFeAsO0.88 Evidenced by SnS-Andreev Spectroscopy

Cited by 15 publications

References 30 publications

Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of $ \Delta_{\rm L, S}$ with $ T_{\rm C}$

Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of $ \Delta_{\rm L, S}$ with $ T_{\rm C}$

Intrinsic Multiple Andreev Reflections in Layered Th-Doped Sm1−x Th x OFeAs

“Break-junction” technique in application to layered superconductors (Review Article)

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