Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime

We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO₀.₈₅, having a critical temperature Tc=55 K. Rather specific dependence Δ*(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corr...

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Datum:	2011
Hauptverfasser:	Solovjov, A.L., Svetlov, V.N., Stepanov, V.B., Sidorov, S.L., Tarenkov, V.Yu., D’yachenko, A.I., Agafonov, A.B.
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Veröffentlicht:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2011
Schriftenreihe:	Физика низких температур
Schlagworte:	Сверхпроводимость, в том числе высокотемпературная
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spelling	irk-123456789-1186172017-05-31T03:09:09Z Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime Solovjov, A.L. Svetlov, V.N. Stepanov, V.B. Sidorov, S.L. Tarenkov, V.Yu. D’yachenko, A.I. Agafonov, A.B. Сверхпроводимость, в том числе высокотемпературная We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO₀.₈₅, having a critical temperature Tc=55 K. Rather specific dependence Δ(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, Δ(T) decreases linearly down to the temperature TAFM ≈ 133 K. This last peculiarity can likely be attributed to the antiferromagnetic (AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara theory developed for the AFM superconductors. 2011 Article Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime / A.L. Solovjov, V.N. Svetlov, V.B. Stepanov, S.L. Sidorov, V.Yu. Tarenkov, A.I. D’yachenko, A.B. Agafonov // Физика низких температур. — 2011. — Т. 37, № 7. — С. 703–707. — Бібліогр.: 37 назв. — англ. 0132-6414 PACS: 774.25.−q, 74.40.–n, 74.78.Fk, 74.70.−b http://dspace.nbuv.gov.ua/handle/123456789/118617 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	Сверхпроводимость, в том числе высокотемпературная Сверхпроводимость, в том числе высокотемпературная
spellingShingle	Сверхпроводимость, в том числе высокотемпературная Сверхпроводимость, в том числе высокотемпературная Solovjov, A.L. Svetlov, V.N. Stepanov, V.B. Sidorov, S.L. Tarenkov, V.Yu. D’yachenko, A.I. Agafonov, A.B. Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime Физика низких температур
description	We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO₀.₈₅, having a critical temperature Tc=55 K. Rather specific dependence Δ(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, Δ(T) decreases linearly down to the temperature TAFM ≈ 133 K. This last peculiarity can likely be attributed to the antiferromagnetic (AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara theory developed for the AFM superconductors.
format	Article
author	Solovjov, A.L. Svetlov, V.N. Stepanov, V.B. Sidorov, S.L. Tarenkov, V.Yu. D’yachenko, A.I. Agafonov, A.B.
author_facet	Solovjov, A.L. Svetlov, V.N. Stepanov, V.B. Sidorov, S.L. Tarenkov, V.Yu. D’yachenko, A.I. Agafonov, A.B.
author_sort	Solovjov, A.L.
title	Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime
title_short	Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime
title_full	Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime
title_fullStr	Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime
title_full_unstemmed	Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime
title_sort	possibility of local pair existence in optimally doped smfeaso₁₋х in pseudogap regime
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate	2011
topic_facet	Сверхпроводимость, в том числе высокотемпературная
url	http://dspace.nbuv.gov.ua/handle/123456789/118617
citation_txt	Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime / A.L. Solovjov, V.N. Svetlov, V.B. Stepanov, S.L. Sidorov, V.Yu. Tarenkov, A.I. D’yachenko, A.B. Agafonov // Физика низких температур. — 2011. — Т. 37, № 7. — С. 703–707. — Бібліогр.: 37 назв. — англ.
series	Физика низких температур
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fulltext	© A.L. Solovjov, V.N. Svetlov, V.B. Stepanov, S.L. Sidorov, V.Yu. Tarenkov, A.I. D’yachenko, and A.B. Agafonov, 2011 Fizika Nizkikh Temperatur, 2011, v. 37, No. 7, p. 703–707 Possibility of local pair existence in optimally doped SmFeAsO1–х in pseudogap regime A.L. Solovjov, V.N. Svetlov, and V.B. Stepanov B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 47 Lenin Ave., Kharkov 61103, Ukraine E-mail: solovjov@ilt.kharkov.ua S.L. Sidorov, V.Yu. Tarenkov, and A.I. D’yachenko A. Galkin Institute for Physics and Engineering of the National Academy of Sciences of Ukraine 72 R. Luxemburg, Donetsk 83114, Ukraine A.B. Agafonov Institut für Festkörperphysik, Leibniz Universität, Hannover Appelstr. 2, D-30167 Hannover, Germany Received January 10, 2011 We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO0.85, having a critical temperature Tc = 55 K. Rather specific dependence Δ(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, Δ(T) decreases linearly down to the temperature TAFM ≈ 133 K. This last pe- culiarity can likely be attributed to the antiferromagnetic (AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara theory developed for the AFM superconductors. PACS: 74.25.−q Properties of superconductors; 74.40.–n Fluctuation phenomena; 74.78.Fk Multilayers, superlattices, heterostructures; 74.70.−b Superconducting materials other than cuprates. Keywords: high-Tc superconductivity, FeAs-based superconductors, pseudogap, local pairs. 1. Introduction Despite of considerable efforts devoted to the study of superconducting pairing mechanism in the new FeAs-based high-Tc superconductors (HTS’s) its physical nature still remains uncertain. However, there is growing evidence that it is of presumably magnetic type and all members of the iron arsenide RFeAsO1–xFx family, where R is a lanthanide, are characterized by the long-range (nonlocal) magnetic correlations [1]. It is well known that upon electron or hole doping with F substitution at the O site [2–4], or with oxy- gen vacancies [5,6] all properties of parent RFeAsO com- pounds drastically change and evident antiferromagnetic (AFM) order has to disappear [7–9]. However, the compari- son with the present SmFeAsO1–xFx superconductors points towards an important role of low-energy spin fluctuations that emerge on doping away from an antiferromagnetic state which is of spin-density wave (SDW) type [10,11]. Thus, below 150 K the AFM fluctuations, being likely of spin wave type, are believed to affect noticeably the properties of RFeAsO1–xFx systems [1,10,11]. As shown by many studies [10–13] the static magnetism persists well into the super- conducting regime of ferropnictides. Besides, it was recently shown theoretically that antiferromagnetism and supercon- ductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state [1,13,14]. In SmFeAsO1–xFx highly disordered but static magnet- ism and superconductivity both are found to exist in the wide range of doping and prominent low-energy spin fluctu- ations are observed up to the highest achievable doping le- A.L. Solovjov, V.N. Svetlov, V.B. Stepanov, S.L. Sidorov, V.Yu. Tarenkov, A.I. D’yachenko, and A.B. Agafonov 704 Fizika Nizkikh Temperatur, 2011, v. 37, No. 7 vels where Tc is maximal [10]. The analysis of the muon asymmetry [11] demonstrates that the coexistence of mag- netism and superconductivity must be nanoscopic, i.e., the two phases must be finely permeated over a typical length scale of few nm. Recently reported results on peculiar mag- netic properties of LaFeAsO0.85F0.1 at TAFM ≈ 135 K [15] are likely due to this two-phase structure. The relation between the SDW and superconducting or- der is a central topic in the current research on the FeAs- based high-Tc superconductors. However, a nature of the complex interplay between magnetism and superconduc- tivity in FeAs-based HTS’s is still rather controversial. As a result, extremely complicated phase diagrams for differ- ent FeAs-based high-Tc systems [4,12–14] and especially for SmFeAsO1–xFx [3,16–19] are reported. For all these HTS’s adequately wide temperature region is found in which the superconductivity coexists with SDW regime. For SmFeAsO1–xFx in a zero magnetic field this tempera- ture region ranges from approximately x = 0.1 up to x = = 0.18 [3,10]. As a result, sufficiently peculiar normal state behavior of the system upon T diminution is expected in this case [3,12–14] when x is, let say, 0.15, as it is in our sample [20]. To shed more light on the problem, in our previous study [20] the fluctuation conductivity (FLC) and ( ),TΔ referred to as a pseudogap (PG), derived from resistivity measure- ments on SmFeAsO0.85 polycrystal with Tc ≈ 55 K were analyzed. As expected, the temperature behavior of FLC was found to be rather similar to that observed for YBCO films with nearly optimal oxygen doping [21], whereas ( )TΔ has demonstrated several peculiar features [20]. In this contribution we venture to explain found ( )TΔ pecu- liarities in terms of Machida–Nokura–Matsubara (MNM) [22] theory developed for AFM superconductors as well as to compare results with Babaev–Kleinert (BK) theory [23] considering superconductors with different charge carrier density nf. 2. Results and discussion To begin with the pseudogap analysis, at first the FLC in SmFeAsO0.85 polycrystal with Tc ≈ 55 K has been tho- roughly analyzed [20]. The FLC is a part of a common excess conductivity ( ) ( ) ( )NT T T′σ = σ −σ which is usual- ly written as ( ) [ ( ) ( )] / [ ( ) ( )] .N NT T T T T′σ = ρ −ρ ρ ρ (1) Here ( )xx Tρ = ρ is the measured resistivity, and ( ) 1/ ( )N NT T aT bρ = σ = + determines the resistivity of a sample in the normal state extrapolated towards low tem- peratures. At the PG temperature (175 1)T = ± K the lon- gitudinal resistivity ( )xx Tρ demonstrates a pronounced downturn from its linear dependence at higher tempera- tures, thus resulting in the appearance of the excess con- ductivity. The excess conductivity ( )T′σ as a function of the re- duced temperature which is defined as ln ( / )mf cT Tε = ≈ ( ) /mf mf c cT T T≈ − is plotted in Figs. 3 and 4 (see Ref. 20) in a double logarithmic scale. Here mf cT ≈ 57 K is the mean-field critical temperature [21]. It was shown that the conventional fluctuation theories by Aslamasov–Larkin (AL) [24] and Hikami–Larkin (HL) [25] well fit the expe- rimental data in the temperature region relatively close to Tc. The result suggests the interband pairing mechanism as dominant one in SmFeAsO0.85, as it was theoretically dis- cussed in Ref. 26. It should be also noted that in the HL theory only the Maki–Thompson (MT) fluctuation contri- bution was used [20]. The MT–AL (2D–3D) crossover is distinctly seen in the ( )T′σ dependence as T approaches Tc [20]. Taking into account the crossover temperature T0 ≈ 58.5 K and the distance between As layers in conducting As–Fe–As planes d ≈ 3.05 Å, the coherence length along the c axis 1/2 0( ) (1.4 0.005)с cТ dξ = ε = ± Å was determined [20]. The coherence length ( )с Тξ is an important parameter of the PG analysis [21]. 2.1. Pseudogap analysis To analyse PG we assume that the excess conductivity ( )T′σ at the temperatures mf cT T T<< << arises as a re- sult of the paired fermions organization in the form of the local pairs (strongly bound bosons (SBB)) [21,27], which satisfy the Bose–Einstein condensation (BEC) theory [28–32]. Upon temperature decrease the local pairs trans- form into fluctuating Cooper pairs as T approaches mf cT [21]. The conventional fluctuation theories describe expe- riment only up to Tc0 ≈ 69 K [20]. Unfortunately, there is still no fundamental theory to describe the experimental curve in the whole PG region. Nevertheless, the equation for σ'(ε) has been proposed in Ref. 21 as 2 * 4 *0 0 [1 ( / )] exp (– / ) ( ) , 16 (0) 2 sinh (2 / )c c c A e T T T− Δ′σ ε = ξ ε ε ε (2) where A4 is a numerical factor which has the same mean- ing as a C-factor in the FLC theory. In this case the fact is important that Eq. (2) contains PG in an explicit form. Be- sides, the dynamics of pair-creation and pair-breaking be- low T has been taken into account in order to correctly describe experiment [21]. To find coefficient A4 the curve, calculated with Eq. (2), has to fit the ( )′σ ε data in the region of 3D AL fluctuations near Tc [20,21]. All other parameters in Eq. (2) directly come from resistivity and FLC analysis. As it was shown in Ref. 20 the curve constructed using Eq. (2) with parameters 0 0.616,cε = (0) 1.405сξ = Å, 57mf cT = K, T = 175 K, A4 = 1.98 and ( ) / с BТ kΔ = 160 K= describes the experimental data well in the whole temperature interval of interest. Possibility of local pair existence in optimally doped SmFeAsO1–х in pseudogap regime Fizika Nizkikh Temperatur, 2011, v. 37, No. 7 705 Solving Eq. (2) for Δ we obtain [21] 2 * 4* *0 0 [1 ( / )] ( ) ln . ( )(16 (0) 2 sinh (2 / )c c c A e T T T T T − Δ = ′σ ξ ε ε ε (3) Here ( )T′σ is the experimentally measured values of the excess conductivity in the whole temperature range from T down to .mf cT All other parameters are the same as de- signated above. As all the parameters, including ( ),T′σ are determined from the experiment, ( )TΔ can be calculated according to Eq. (3) and plotted now as shown in Fig. 5 of Ref. 20. Unfortunately, the value of ( )cTΔ and in turn the ratio 2 ( ) /с B сТ k ТΔ in the FeAs-based HTS’s remain uncer- tain. At present it is believed that SmFeAsO1–x has two superconducting gaps, i.e., Δ1(0) ≈ 6 meV (~ 70 K) and Δ2(0) ≈ (14…21) meV [~ (160…240) K] [33]. Besides we think that ( ) ~ (0)сТΔ Δ [21,34]. That is why, four curves are finally plotted in Fig. 5 (Ref. 20) with ( ) /с BТ kΔ = = 160 K (2 ( ) /с B сТ k ТΔ ~ 5.82 close to strongly coupled limit), 140 K (2 ( ) /с B сТ k ТΔ ~ 5.0), 120 K (2 ( ) / ~ 4.36)с B сТ k ТΔ and 100 K (2 ( ) / 3.63с B сТ k ТΔ ∼ close to weakly coupled BCS limit) from top to bottom, re- spectively. Naturally, different values of coefficients A4 cor- respond to each curve whereas the other parameters men- tioned above remain unchangeable. It was found [20] that at T T≤ the ( )TΔ starts to in- crease rapidly as it was observed for YBCO films with different oxygen concentration [21]. However, an unex- pected sharp decrease of ( )TΔ at Ts ≈ 147 K was re- vealed as clearly illustrate Fig. 1 as well as Fig. 5 in Ref. 20. Usually Ts is treated as a temperature at which a structural tetragonal–orthorhombic transition occurs in SmFeAsO. In the undoped FeAs compounds it is also ex- pected to be a transition to SDW ordering regime [7–9]. Below Ts the pseudogap ( )TΔ drops linearly down to TAFM ≈ 133 K which is attributed to the AFM ordering of the Fe spins in a parent SmFeAsO [7,35]. Below TAFM the slop of the ( )TΔ curves apparently depends on the ( )cTΔ [20]. Found ( )TΔ behavior is believed to be explained in terms of the MNM theory (Fig. 1) [22] developed for the AFM superconductors, in which the AFM ordering with a wave vector Q may coexist with the superconductivity. In the MNM theory the effect of the AFM molecular field ( ) ( \| )Q Q Fh T \| h << ε on the Cooper pairing was studied. It was shown that below TN the BCS coupling parameter ( )TΔ is reduced by a factor [1 const \| ( )\| / ]Q Fh T− ⋅ ε due to the formation of energy gaps of SDW on the Fermi sur- face along Q. As a result, the effective attractive interac- tion (0),ğN or, equivalently, the density of states at the Fermi energy ,Fε is diminished by the periodic molecular field, that is, (0) (0)[1 ( )] .ğN gN m T= −α (4) Here ( )m T is the normalized sublattice magnetization of the antiferromagnetic state and α is a changeable parame- ter of the theory. Between Tc and (N c NT T T> is assumed) the order parameter is that of the BCS theory. Since below NT the magnetization ( )m T becomes nonvanishing, (0)ğN is weakened that results in turn in a sudden drop of ( )TΔ immediately below .NT As ( )m T saturates at lower temperatures, ( )TΔ gradually recovers its value with in- creasing the superconducting condensation energy (Fig. 1, solid curves). This additional magnetization ( )m T was also shown to explain the anomaly in the upper critical field Hc2 just below NT observed in studying of RMo6S8 (R = Gd, Tb, and Dy) [22]. However, predicted by the theory decrease of ( )TΔ at ,NT T≤ was only recently ob- served in AFM superconductor ErNi2B2C with Tc ≈ 11 K and NT ≈ 6 K, below which the SDW ordering is believed to occur in the system [36]. The result evidently supports the prediction of the MNM theory. Our results are found to be in a qualitatively agreement with the MNM theory as shown in Fig. 1, where the data for ( ) /с BТ kΔ = 130 and 135 K are compared with the MNM theory. The curves are scaled at / cT T = 0.7 and demonstrate rather good agreement with the theory below / cT T = 0.7. The upper scale is / .T T Both shown ( )TΔ dependencies suggest the issue that just ( ) 133 KcTΔ = would provide the best fit with the theory. Above / cT T = 0.7 the data evi- dently deviate from the BCS theory. It seems to be reasona- ble seeing SmFeAsO0.15, as well as any ferropnictide, to be not a BCS superconductor. It is important to emphasize that in our case we observe the particularities of ( )TΔ in the PG state, i.e., well above Tc, but just at Ts, below which the SDW ordering in parent SmFeAsO should occur. It seems to be somehow surprising as no SDW ordering in optimally doped Fig. 1. * max( ) /TΔ Δ in SmFeAsO0.85: ( ) /с BТ kΔ = 130 K (□); 135 K (○). Solid curves correspond to MNM theory with differ- ent ~ 1 / [ (0)] :gNα 0.1 ( ),1α = 0.2 ( ),2 0.3 (3), 0.6 (4), 1.0 (5); TN/Tc = 0.7 [22]. 0 0.2 0.4 0.6 0.8 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.0 1.0 T T/ T T/ c 1 2 3 4 5 5 A.L. Solovjov, V.N. Svetlov, V.B. Stepanov, S.L. Sidorov, V.Yu. Tarenkov, A.I. D’yachenko, and A.B. Agafonov 706 Fizika Nizkikh Temperatur, 2011, v. 37, No. 7 SmFeAsO0.15 is expected. On the other hand, the AFM fluctuations (low-energy spin fluctuations) should exist in the system as mentioned above. At the singular tempera- ture Ts these fluctuations are believed to enhance the AFM in the system likely in form of SDW. After that, in accor- dance with the MNM theory scenario, the SDW has to suppress the order parameter of the local pairs as shown by our results. Thus, the results suggest the existence of paired fermions in the PG region, which order parameter is appar- ently suppressed by the AFM fluctuations at .sT T≤ These fermions have most likely to exist in the form of the local pairs (SBB) [21]. To justify the issue the relation ** max( ) /TΔ Δ as a function of /T T ( / cT T in the case of the theory) is plotted in Fig. 2. The dots and circles represent the stu- died SmFeAsO0.85 with ( ) / 160 Kс BТ kΔ = and the data for YBCO film with 87.4 K,cT = respectively [21]. The solid and dashed curves display the results of the Ba- baev–Kleinert (BK) theory [23] developed for the super- conducting systems with different charge carrier density nf. For the different curves the different theoretical para- meter 0 / (0)x = μ Δ is used, where μ is a chemical poten- tial. Curve 1, with 0 10,x = + gives the BCS limit. For curve 2 the value 0 2x = − is taken, for curve 3 parameter 0 5.x = − Finally curve 4 with 0 10x = − represents the BEC limit, which corresponds to the systems with low nf in which the SBB must exist [28–32]. As well as in YBCO film the max( ) /TΔ Δ in SmFeAsO0.85 evidently corres- ponds to the BEC limit suggesting the local pairs presence in the FeAs-based superconductor. Below max( ) /TΔ Δ ≈ 0.4 both experimental curves demonstrate the very similar slope suggesting the BEC–BCS transition from local pairs to the fluctuating Cooper pairs found for the YBCO films with different oxygen concentration as temperature approaches Tc [21]. But, naturally, no drop of ( )TΔ is observed for the YBCO film (Fig. 2) as no antiferromagnetism is expected in this case. This fact accentuates the AFM nature of the ( )TΔ linear reduction below Ts in SmFeAsO0.85 found in our experiment. 3. Conclusion Analysis of the pseudogap ( )TΔ in the FeAs-based su- perconductor SmFeAsO0.85 with Tc = 55 K based on the systematic study of the excess conductivity ( )T′σ [20] has been performed. Rather specific temperature dependence of the ( )TΔ was found (Figs. 1 and 2). The more striking re- sult is the pronounced decrease of ( )TΔ below Ts ≈ 147 K. As a rule, Ts is treated as a temperature at which a structural tetragonal–orthorhombic transition occurs in SmFeAsO [7–9]. In accordance with recent results [3,16–18] it is expected to be a transition to SDW ordering regime in the undoped Fe–As compounds too. Below Ts the pseudogap ( )TΔ is linear down to TAFM ~ 133 K, which is attributed to the antiferro- magnetic ordering of the Fe spins in SmFeAsO [6–9]. Note that no such peculiarities of ( )TΔ are observed in the su- perconducting state of SmFeAsO1–xFx [37] as no prono- unced antiferromagnetism in SC state of the FeAs-based compounds is expected [1–9]. Found ( )TΔ reduction can be qualitatively explained in the framework of the MNM theory [22], which predicts the suppression of the superconducting order parameter in AFM superconductors. But we have to emphasize that we observe the ( )TΔ reduction in the PG state, i.e., well above Tc. The finding suggests the presence of paired fer- mions in SmFeAsO0.85 in the PG region, the order parame- ter of which ( )TΔ is apparently suppressed by the en- hanced AFM fluctuations (spin waves) in accordance with the MNM theory. At the same time no unusual drop of ( )TΔ is observed for the YBCO film (Fig. 2) as no anti- ferromagnetism is expected in this case. This fact is to jus- tify the AFM nature of the found ( )TΔ reduction in SmFeAsO0.85. As it is clearly seen in Fig. 2, the ratio * max( ) /TΔ Δ in SmFeAsO0.85 at high temperatures evidently corresponds to the BEC limit. It seems to be reasonable as in FeAs-based compounds nf is found to be at least an order of magnitude less than in conventional metals [17]. Thus, we may conclude that paired fermions should exist in the PG temperature re- gion of the FeAs-based superconductor SmFeAsO0.85. Most likely they should appear in the form of local pairs (strongly bound bosons), as it was found for the YBCO films with different oxygen concentration [21]. Thus, the local pair presence seems to be the common feature of the PG forma- tion in both cuprates and FeAs-based HTS’s. It has to be emphasized that recently reported phase di- agrams [3,16–19] apparently take into account a complexi- ty of magnetic subsystem in SmFeAsO1–xFx and are in much more better agreement with our experimental results. 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Possibility of local pair existence in optimally doped SmFeAsO₁₋х in pseudogap regime

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