Observation of Multiple Superconducting Gaps in the Infrared Reflectivity Spectra of Ba(Fe0.9Co0.1)2As2 ∗ Yu. A. Aleshchenko, A. V. Muratov, and V. M. Pudalov Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia E. S. Zhukova and B. P. Gorshunov Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991 Russia and Moscow Institute of Physics and Technology, Dolgoprudnyi, Moscow region, 141700 Russia 2 1 0 F. Kurth and K. lida 2 Institut fu¨r Metallische Werkstoffe, Leibniz-Instilut fu¨r Fesik¨orper- und Werkstoffforschung Dresden, D-01171 Dresden, Germany n (Date: 20 October 2011) a J The results of infrared reflectivity measurements for the iron-based high-temperature supercon- 1 ductorBa(Fe0.9Co0.1)2As2 arereported. Thereflectivityisfoundtobeclosetounityatfrequencies 1 ω lower than 2∆/h (2∆ is the superconducting gap and h is Planck’s constant). This is evidence for the s+/− or s+/+ symmetry of the superconducting order parameter in the studied compound. ] n TheinfraredreflectivityspectraofBa(Fe0.9Co0.1)2As2 manifest openingofseveralsuperconducting o gaps at temperatures lower than critical Tc. c - r Studies of the recently discovered iron-based high- periments including observations of the SC energy gap p u temperature superconductors (HTSC) [1] and compar- with s−type symmetry [14, 15] and with nodes at the s ison of their properties with those for cuprate HTSC Fermi surface, where the order parameter changes sign, t. provide information necessary to understand the mech- i.e., with the d−type symmetry [8, 16–18]. a anisms of high-temperature superconductivity. The en- m The infrared (IR) spectroscopy is a direct technique ergy gap, 2∆, in the spectrum of quasi-particle excita- to gain information on the energy spectrum of charge - tions is one of the most important parameters of a su- d carriers in superconductors. Contrary to angle-resolved perconductor. For iron-based compounds, experimental n photoemission spectroscopy (ARPES) and tunnel spec- o studies of this parameter, as well as the mechanisms of troscopy, a relatively thick layer of material contributes c superconductivity,arehamperedbythemultibandstruc- to the IR reflectivity, thus making the observed prop- [ tureoftheirenergyspectrum. TheBrillouinzoneinthese erties similar to those of the bulk material. However, materials contains two hole bands at the Γ point and 2 studiesofironpnictidesbythelattertechniquefacewith v two electron bands at the X(0, ±π) points. Therefore, obstaclesbecausetypicalmagnitudeofthesuperconduct- 6 we can expect the existence in this material of several ing gaps (at T ∼20−55 K) lies in the far-IR and tera- c 3 superconducting (SC) condensates with different energy hertz(THz)ranges. TheefficiencyofstandardIRFourier 6 gaps [2–4]. spectrometersinthesespectralrangesdecreasesdramat- 4 The complexity of the energy spectrum in iron pnic- . ically. Inthispaper,wereportstudiesoftheIRreflectiv- 2 tides, the difference in quality and phase composition of 1 the earlier studied samples, significant experimental er- ityspectraofBa(Fe0.9Co0.1)2As2 filminthe wide ranges of wavelengths and temperatures. The high optical ef- 1 rors, as well as the different sensitivity of various ex- ficiency of the Bruker IFS 125HR Fourier spectrometer 1 perimental techniques to the different energy gaps may : and the high sensitivity of its detectors enabled us to v be responsible for wide scattering in the measured ra- overcome the above obstacles and to perform measure- i tios of the SC energy gap to the critical temperature, X ments even in the far-IR range. 2∆/k T = 1.6 − 10 (k is the Boltzmann constant) ar [d5e–r12p]Ba.racTmheeterreilniabthleesdeatmaBatoenriathlseasryemamlseotrnyotofavtahielaborle- mmT2he(LBaa,(SFre)0(.A9Cl,oT0a.1))O2A3ss2ubfisltmrawteabsydpepuolsseidteldasoenrdae4po×si9- yet. Within the framework of the most widely discussed tiontechnique, where the Ba(Fe0.9Co0.1)2As2 targetwas s+/− model of superconductivity [13] it is assumed that ablated with KrF laser radiation with a 248 nm wave- length under ultra-high vacuum [19]. The film had a the SC condensates of the electron and hole bands in mirror-like surface with the rms roughness less than 12 iron-based HTSC possess the s−type order parameters nm,asmeasuredbyatomicforcemicroscopy(AFM).The with the Bardeen-Cooper-Schrieffer(BCS)-like tempera- film thickness, d = 90 nm, was monitored in situ by a ture dependence and with the opposite sign. However, quartz balance, and finally measured by AFM and ellip- different conclusions have been reported in various ex- sometry. The phase purity of the film was checked by X-ray diffraction and energy-dispersive spectroscopy. Standard four-probe dc-method was used to measure ∗ [email protected] resistivity at the superconducting transition. From the 2 Ourmeasurementsrevealanew featureat∼900cm−1 5 K 1,0 (see Fig. 1) in the reflectivity spectra, whose origin is Ba(Fe0.9Co0.1)2As2 unknown yet. In the region below 30 cm−1 one can see interference fringes arising due to re-reflections within the cryostat windows. This instrument effect does not vity 0,8 20 K hamper the analysis of the spectra. cti 100 K 200 K Refle 300 K beTrshebeulpotwur1n0i0n0thcme r−e1fleicsticvaituyseRd(ωb)yfoitrinthereawntavcehnaurmge- carriers, i.e., electrons and holes in various bands of 0,6 Ba(Fe0.9Co0.1)2As2. As temperature decreases from 300 to 100 K, the reflectivity R(ω) shows a strong tem- perature dependence (Fig. 1). At the same time, the 0,4 spectra measured in the temperature interval 30−5 K 19 100 1000 10000 nearlycoincideintheregionoflargewavenumbers,above -1 Wavenumber (cm ) 300 cm−1. The difference between them appears only FIG.1. InfraredreflectivityspectraoftheBa(Fe0.9Co0.1)2As2 in the region of small wavenumbers. One can see that film. at temperatures lower than that of the superconducting transition for Ba(Fe0.9Co0.1)2As2, the reflectivity of the film reachesnearly unity. This providesa convincing ev- resistivity onset at 22 K with a transition width of 2 K idenceofopeningofthe superconductingenergygapdue we estimated T =20 K. The IR reflectivity spectra with to formation of the SC condensate. c a frequency resolution of 1 cm−1 were measured in the The opening of the superconducting gap in the elec- 14000-8 cm−1 range, using a gold mirror as a reference. tron density of states at T < T is the basic feature of c The liquid-nitrogen cooled InSb, HgCdTe photodetec- the superconducting state. In this case, for the super- tors,andthe liquid-heliumcooledsiliconbolometerwere conducting state with the isotropic SC energy gap (the used as detectors in the near-IR, middle-IR, and far-IR s−type symmetry of the order parameter), the reflectiv- region, respectively. For measurements in the temper- ity should reach values quite close to 100 % at temper- ature range 5−300 K, the sample was placed into the ature 5 K ≪ T in the region of wavenumbers smaller OptistatCF−V (Oxford Instruments) cryostat with ZnSe than 2∆/hc (c cdenotes the speed of light). The shape and polyethylene windows. Wedged windows made of of the R(ω) spectra in Fig. 1 appears to be nearly flat TPX plastic were used for measurements in the far IR in the region of wavenumbers smaller than 60 cm−1 and range. To improve the signal-to-noise ratio, we made up resembles that for the superconducior with s−type pair- to120runs,eachoftenmeasurements,withasubsequent ing. For wavenumbers above 2∆/hc , the reflectivity averaging. decreases resulting in the formation of the character- Figure 1 shows the IR reflectivity spectra of istic peak in the frequency dependence of the ratio of Ba(Fe0.9Co0.1)2As2 film measured at temperatures 300, reflectivities in the superconducting and normal states, 200,100,20,and5K.Thesmallfilmthicknessandmod- R(T < T )/R(T ≥ T ). The decrease in the amplitude c c erateconductivityhamperedcalculatingtheconductivity of this peak in the region of small wavenumbers is due and permittivity spectra by the Kramers- Kronig proce- to absorption by the superconducting material at ener- dure. However,someimportantconclusionscanbemade giessmallerthanthe superconductinggap. In the multi- from the straightforward analysis of the IR reflectivity band superconductor, the corresponding features associ- spectra. ated with different SC gaps coincide, thus hindering the Totally,the measuredIRspectraagreewellwiththose determination of the gap magnitudes. measured earlier for the Ba(Fe0.9Co0.1)2As2 films from Figure 2 is a semilogarithmicplotof the frequency de- the same batch [20]. This is also true for the far IR pendence of the R(T)/R(30 K) ratio, where R(T) is the range where the reflectivity spectra were calculated in reflectivity of the Ba(Fe0.9Co0.1)2As2 film at tempera- [20] from the results of direct measurements of THz op- tures T = 5, 20 and 100 K. The dashed lines (drawn tical conductivity and permittivity. The wide band seen at the right gentle wing of the peak) show for clarity in Fig. 1 in the wavenumber range above 1000 cm−1 apiecewise-linearapproximationofthe normalizedspec- results from the interband transitions with the maxima truminthefarIRregion. Onecannoticethepronounced closeto4400and20800cm−1 [20]. Thedipinthe region kinksinthefrequencydependenceofR(5K)/R(30K)at of 1000 cm−1 can be explained by the resonance absorp- ∼ 43 cm−1 and 23.5 cm−1 as well as a weaker feature tion. The similar feature was observed earlier in the IR at 29 cm−1. The peak with the discussed features is reflectivityspectraforBa(Fe1−xCox)2As2[12,21,22]and absent in the R(20 K)/R(30 K) and R(5 K)/R(30 K) alsoforundopedBaFe2As2 [23,24]. Itmaybeassociated dependences, i.e. for T > Tc. This allows us to in- with an intraband transition [25, 26]. A number of the terpret the above features as the manifestation of the relativelynarrowfeaturesintheregionof100-1000cm−1 superconducting gaps, 2∆ with the energies of 5.3 meV isduetothephononsinthe(La,Sr)(Al,Ta)O3 substrate. (43cm−1),3.6meV(29cm−1)and2.9meV(23.5cm−1). 3 1,02 ductivity in the studied Ba(Fe0.9Co0.1)2As2 sample cor- T = 5 K responds to the “dirty” limit, i.e., the carriersscattering Ba(Fe0.9Co0.1)2As2 ratesatisfiesthe relationship1/τ ≥2∆. Inthis case,the spectral weight of the condensate in the IR reflectivity spectraandintheopticalconductivityisdistributedover K) a wide spectrum region, however, a large portion of the 30 T = 20 K R( condensateisconcentratedbelowtheenergyofthe order T)/ 1,00 of 2∆. In the “clean” limit, nearly all spectral weight R( associated with the condensate lies below 2∆, therefore atenergiesabout2∆nodiscerniblechangesareobserved across the SC transition. T = 300 K The measured SC gaps 2∆ = 2.9, 3.6, and 5.3 meV correspond to the 2∆/k T ratio ≈ 1.6, 2.0, and 2.9, 0,97 B c 19 100 750 respectively. These values fall into the range of reported -1 Wavenumber (cm ) data 2∆/kBTc = 1.6−10 for Ba(Fe1−xCox)2As2 [5–12]. FIG.2. InfraredreflectivityspectraoftheBa(Fe0.9Co0.1)2As2 To the best of our knowledge, the results reported here filmtakenatvarioustemperaturesnormalizedtothescectrum arethefirstobservationofmultiplesuperconductinggaps measured at 30 K. in the Ba(Fe0.9Co0.1)2As2 compound. To summarize, we performed IR spectroscopic mea- surements on a thin film of the superconducting Forthesuperconductorwiths−typesymmetryoftheor- derparameter,the correspondingfeaturesshouldappear Ba(Fe0.9Co0.1)2As2 iron pnictide with Tc = 20 K in the wide spectral and temperature ranges. The behavior of as steps in the R /R dependence. However, the finite s n theIRreflectivityspectrainthefarIRrangeatT <T is temperature of measurements and the superposition of c indicative of the s−type pairing in the studied material. the features related to different gaps result in smearing The IR reflectivity spectra at temperatures below T re- of the anticipated steps. It is worthy of note that in c veal the existence of three superconducting gaps 2∆ = our (BG, FK, KI) previous measurements, made by ter- 2.9,3.6 and 5.3 meV (2∆/k T ≈ 1.6, 2.0 and2.9). The ahertz spectroscopy technique [20] on the samples from B c manifestation of the SC gaps in the IR spectra signifies the same batch, the optical conductivity was found to vanish at about 30 cm−1 and at T = 5 K, the fact that the “dirty” limit in the studied material. evidences for opening of the SC gap. 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