Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe 0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained

Bellingeri, E.

Bonavolontà, C.

De, Lisio, C.

Ferdeghini, C.

Kazumasa, I.

Kurth, F.

Laviano, F.

Pallecchi, I.

Parlato, L.

Pepe, G.P.

Putti, M.

Ummarino, G.A.

Valentino, M.

English

Repositorium für Naturwissenschaften und Technik

Journal of Physics: Conference SeriesOPEN ACCESSFemtosecond spectroscopy in a nearly optimallydoped Fe-based superconductors FeSe0.5Te0.5and Ba(Fe1−xCox)2As2/Fe thin filmTo cite this article: C Bonavolontà et al 2014 J. Phys.: Conf. Ser. 507 012004 View the article online for updates and enhancements.Related contentMagnetic measurements based onmagneto-optical Kerr effect on pnictideBa(Fe1x Cox)2As2/Fe thin film-Hydrostatic pressure-induced hugeenhancement of critical current densityand flux pinning in Fe1x Co x Se0.5Te0.5single crystals-Anomalous Scaling Relations and PairingMechanism of the Fe-basedSuperconductors-This content was downloaded from IP address 194.95.157.36 on 26/10/2020 at 11:01Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and Ba(Fe1-xCox)2As2/Fe thin filmC Bonavolontà1,2, L Parlato1,2, C de Lisio1,2, M Valentino1,2, GP Pepe1,2, I  Kazumasa3, F Kurth3, E Bellingeri4 , I Pallecchi4 , M Putti4, C Ferdeghini4,  G A Ummarino5, F Laviano51Department of Physics, University of Naples “Federico II”, Italy 2CNR-SPIN, Piazzale Tecchio 80, 80125 Naples, Italy3 IFW Dresden, P. O. Box 270116, 01171 Dresden, Germany4CNR-SPIN, Corso Perrone 24, 16152 Genova, Italy5Dipartimento di Fisica and CNISM, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy E-mail: bonavolo@na.infn.it Abstract: Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and  optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides  both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained. 1.IntroductionThe recent discovery of superconductivity with transition temperature Tc up to 55 K in iron–pnictide systems [1,2]] has sparked enormous interest in this class of materials. Investigations on this new class of superconductors have revealed a lot of similarities to cuprates, for instance, layered structures, a multiband nature, high upper critical fields and a short coherence length. A common phase diagram for Fe–pnictides suggests the existence of an antiferromagnetic spin density-wave (SDW) ground state that is stabilized by Fermi-surface nesting [3] as well as by strong antiferromagnetic (AFM) spin interactions along the Fe-square-diagonal [4,5]. The coexistence of superconductivity and magnetism in the phase diagram suggests that magnetism plays an important role in determining the superconducting properties and in forming electron Cooper pairs. Then, the importance of the spin degrees of freedom for the superconducting pairing interaction becomes immediately apparent, and  investigating the Fe-pnictide compounds from the point of electron-phonon and spin-charge interactions is therefore beneficial for understanding the nature of the superconducting coupling in the doped compounds.  Time-resolved spectroscopy is considered an efficient and useful technique to study the nature of the electronic excitations in superconductors [2] and more recently also in iron pnictides [3,4]. This technique gives information on the low-lying electronic structures, and potentially on the pseudo-gap, due to its capability to distinguish between scattering processes characterized by different relaxation dynamics [5]. In particular, the transient reflectivity change as a function of the time 11th European Conference on Applied Superconductivity (EUCAS2013) IOP PublishingJournal of Physics: Conference Series 507 (2014) 012004 doi:10.1088/1742-6596/507/1/012004Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1delay (in the ps range) between the pump and the probe beams hitting the same region of the sample is clearly dependent on the electron relaxation processes which restore the equilibrium. Pump-probe fast spectroscopy has been already proposed for estimating the electron-boson constant λ in novel pnictides materials as in the case of BaFe0.92Co0.08As2 (λ = 0.12) [6],  BaFe2As2 [7] (λ = 0.15), and SmFeAsO [8]  (λ = 0.18). This technique can be considering complementary to other conventional techniques (i.e. point contact spectroscopy) to evaluate the superconducting energy gap.   In this work we use a time-resolved spectroscopy on FeSe0.5Te0.5 (Fe-11) thin films and optimally Co-doped BaFe2As2 (Ba-122) epitaxial thin films grown on Fe-buffered MgO(001) substrates to infer about the value of the energy gap in the quasi-particle density of states and the strength of the electron-boson interaction which characterize the superconducting state.  2. Materials and experimental set upThe Fe(Se,Te) films were deposited on single crystal lanthanum aluminates (001) substrates in a ultra high vacuum pulsed laser deposition (PLD) system using as a target a bulk polycrystal with a sharp resistive transition with an onset at T=16.2 K. further experimental details about the sample fabrication are found in [9]. The thin Fe(Se0.5Te0.5) films has a thickness of about ≈ 150 nm and the critical temperature of the film was about 19K.  The Co-doped Ba-122/Fe bilayers on MgO substrate were prepared by pulsed laser deposition under ultra high vacuum condition (base pressure of 10-9 mbar). The nominal Co content was x=0.08 in Ba(Fe1-xCox)2As2. The respective layer thickness of Co-doped Ba-122 and Fe are 100 nm and 20 nm. In-situ reflection high energy electron diffraction confirmed that both, Fe and Co-doped Ba-122, were grown epitaxially with smooth surface. The detailed films preparation can be found in Ref. [10]. The critical temperature, Tc, defined as 90% of the normal state resistance was Tc=26.3K, Time resolved spectroscopic measurements were performed using a standard pump-probe technique, based on mode-locked Ti:sapphire laser, delivering pulses at 82 MHz repetition rate, with 810 nm center wavelength, 100 fs duration. The sample was mounted on a cold finger in a temperature controlled liquid helium continuous flow optical cryostat, operating down to 5K. Further details on experimental details can be found in [11]. The photoinduced normalized reflectivity change ∆R/R was measured using a weaker probe pulse (1:20 the ratio between probe and pump beam) at a variable delay time, and as a function of the temperature. The pump fluences were about 5 µJ/cm2 and 1.6µJ/cm2 for the Fe-11 and Ba-122 sample, respectively. The different fluences are chosen because of the different signal to noise ratio of the signals due to the  different samples surface reflectivity. 3. Results and discussionIn figure 1 the normalized reflectivity change ∆R/R as a function of the delay time between the pump and probe pulse at different temperatures is reported, in the case of Fe(Se,Te) and Ba-122/Fe thin films, respectively.  Figure 1. (a) The photoinduced reflectivity ∆R/R vs delay time at different temperatures related to the FeSeTe compound; (b) The ∆R/R vs the delay time at temperatures ranging from 20K to 180K, measured in Ba-122/Fe. The black solid lines in figure 1(a)-(b) represent the single and double-exponential fit curves used to estimate the relaxation time for Fe(Se,Te) and Ba-122/Fe, respectively. (a) (b)-2 0 2 4 6 80,00,51,01,52,02,5∆R/R(*10-3)Delay Time (ps) 20K 24,5K 28K 40K 60K 100K 140K 180K-2 0 2 4 6 8 10 12 140,00,20,40,60,81,0 ∆R/R (*10-3)Delay Time (ps) 12 K 16 K 18 K 30 K11th European Conference on Applied Superconductivity (EUCAS2013) IOP PublishingJournal of Physics: Conference Series 507 (2014) 012004 doi:10.1088/1742-6596/507/1/0120042To estimate the relaxation time τ the experimental curves have been fitted by a single and double-exponential function (see the solid line in figures 1(a) and (b)) for the Fe(Se,Te) and Ba-122/Fe thin films, respectively. Since for both Fe-pnictides samples the relaxation time (in particular the slowest time for the Ba-122/Fe thin film) shows a peak close to the critical superconducting temperature Tc, as reported in figures 2(a) and (b), the electron relaxation time data were analyzed according to a temperature dependence of quasiparticle recombination time in a superconductor with a temperature-dependent energy gap ∆(T) below Tc as described by the equation (1) [12]: ⎪⎭⎪⎬⎫⎪⎩⎪⎨⎧⎟⎟⎠⎞⎜⎜⎝⎛+∆∆=−∆−1)(22 )(ln)(TkTRBeTBTAτ (1) where A and B are physical constant related both to phonon characteristic energies and the normal density of states N(0) of the Fe-11 and Ba-122 compound, and ∆(T)=∆0[1-(T/Tc)]0.5 is the temperature dependence of the superconducting energy gap.     Figure 2. The relaxation time τ as a function of temperature for the sample with TC=19.1 K and thickness of 150nm. The inset in figure 2(a) shows the fitting procedure applied to the decay time for T<TC. The solid line in figure 2(a) represents the linear dependence between the relaxation time and the temperature starting from T=30K. (b) relaxation time τslow extracted from double-exponential decay fits of ∆R/R data in Ba-122/Fe.  In the case of FeSeTe sample, the fitting procedure, showed in the inset of figure 2(a), applied to experimental points recorded at T <Tc, gives ∆0=4.1 ± 0.4meV for the superconducting energy gap extrapolated at T=0K, a value which corresponds to 2∆0/kBTc = 4.9 ±0.5. This  is in agreement with similar determinations already reported by specific heat [13] and Scanning Tunneling spectroscopy measurements [13]. The pre-factor A≈2.9 ± 0.1 10-14 eV s cm-1 in eq.(1) can be written also as follows [14], A=h ωph/12Γph  where ωph is a typical phonon frequency, Гph is its characteristic phonon line-width. According to Subedi et al [14] we can assume Гph =2cm-1, and then estimate the characteristic phonon frequency ωph=32.4±0.4 cm-1= 4.0±0.1 meV.  Moreover, using a linear approximation for τ vs T behavior also at T>80K, according to[15]:  ><= 23 ωλπτhTkB      (2) where λ<ω2> is the second moment of the Eliashberg function, α2F(ω). The equation is expected to be valid for energy higher than ħω0, where ω0 is the characteristic Eliashberg –function phonon frequency. Looking at figure 2(a), in the range T>50K, and assuming a similar slope also at larger temperature, we obtain λ<ω2>= 10±2 meV2, which corresponds to an electron-phonon coupling constant λ∼0.6±0.1 (calculated using ωph previously obtained from the parameter A). We consider this feature only a remarkable element of consistency even if we are not completely within the range of validity of the proposed theory. Moreover, the same results can be obtained using a two-temperature model, as reported in a previous work [16].  For the Ba-122/Fe sample, from the fitting analysis using eq.(1), the value of superconducting gap ∆0=3.67 ± 0.13meV that corresponds to 2∆0/kBTc = 3.4 ± 0.1 has been evaluated. In previous time-resolved studies on SmFeAs(O,F) [17,18] and Ba-122 [7,18] iron pnictides superconductors, 2∆0/kBTc ranging from 3 to 3.7 have been estimated. Our result is in agreement with the superconducting gap value estimated with other techniques [6,7,19]. In addition to evaluate the 0 10 20 30 40 50 60 70 800.00.20.40.60.81.01.21.412 14 16 18 200.00.30.60.91.21.5relaxation time (ps)Temperature (K)relaxation time (ps)Temperature (K)(a) 18 20 22 24 262,02,53,03,54,04,5relaxation Time (ps)Temperature (K)(b)11th European Conference on Applied Superconductivity (EUCAS2013) IOP PublishingJournal of Physics: Conference Series 507 (2014) 012004 doi:10.1088/1742-6596/507/1/0120043electron-phonon coupling constant for the Ba-122/Fe, we use again the value of the fit parameter in eq.(1), namely A =22.2e-15 eV s cm-1 to calculate the phonon frequency ωph, where Γ= 4cm-1 [6]. In this way ωph=127.2 cm-1=15.77 meV and considering λ〈ω2〉∼50meV2 from inelastic neutron data[20], the electron-phonon coupling constant is about λ=0.18. This results is in agreement with that obtained for Co optimally doped Ba-122 and SmFeAsO compounds [20,21]. 4. ConclusionsIn conclusion, we report femtosecond spectroscopy measurements in a nearly optimally doped Fe-based superconductor FeSe0.5Te0.5 with Tc≈19K and Ba(FeCo)As with Tc ≈26K. Our results indicate the existence of temperature dependence of quasiparticle recombination time in the superconducting state and a temperature-dependent energy gap ∆(T) at T<Tc. The fitting procedure gives for the Fe(Se,Te) sample ∆0=4.1±0.4meV for the superconducting energy gap, and an electron-phonon constant λ≈0.6, while for the Ba-122/Fe ∆0=3.67±0.13meV and λ≈0.18. These results are in very good agreement with values of the superconducting energy gap and electron-phonon coupling constant obtained with other techniques (i.e. Angle Resolved Photoemission Spectroscopy (ARPES)) and predicted by theoretical models [6,19,21] applied to the pnictides compounds.  Acknowledgments This work was supported by European Project “IRONSEA”, Grant Agreement nr. 283141. 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Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and Ba(Fe 1-xCox)2As2/Fe thin film

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Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and Ba(Fe 1-xCox)2As2/Fe thin film

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