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Physics Procedia 49 (2013) 109 – 117 1875-3892 © 2013 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of the Department of Physics, School of Science and Humanities, Kongu Engineering College doi:10.1016/j.phpro.2013.10.017 ScienceDirect Spintronics Materials: Nanostructures and Devices (SMND-2011) Effect of off-stoichiometry at the Fe-site in FeSe 0.5 Te 0.5 superconductor Anil K. Yadav*, Ajay D. Thakur & C. V. Tomy Departmetment of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India Abstract Excess iron in Fe 1+x Se superconductor plays a crucial role in the observation of superconductivity in this material. We have investigated the effect of off-stoichiometry at the Fe site in Fe 1+x Se 0.5 Te 0.5 (x = 0.01, 0.01 and 0.03) superconductor through magnetization, electrical transport and specific heat measurements. Our results indicate an improvement in the superconducting properties in Fe-deficient compound compared to the stoichiometric or Fe-rich compounds, in contrast to what is observed in the parent compound, FeSe. Keywords: Off-stoichiometry, SDW, Fermi temperature. * Corresponding author. Tel.: +91-22-25764552; fax: +91-22-25767552. E-mail address: [email protected]. Available online at www.sciencedirect.com © 2013 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of the Department of Physics, School of Science and Humanities, Kongu Engineering College Open access under CC BY-NC-ND license. Open access under CC BY-NC-ND license.
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Page 1: Available online at ScienceDirect › download › pdf › 82558085.pdf · Anil K. Yadav*, Ajay D. Thakur & C. V. Tomy Departmetment of Physics, Indian Institute of Technology Bombay,

Physics Procedia 49 ( 2013 ) 109 – 117

1875-3892 © 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Department of Physics, School of Science and Humanities, Kongu Engineering Collegedoi: 10.1016/j.phpro.2013.10.017

ScienceDirect

Spintronics Materials: Nanostructures and Devices (SMND-2011)

Effect of off-stoichiometry at the Fe-site in FeSe0.5Te0.5 superconductor

Anil K. Yadav*, Ajay D. Thakur & C. V. Tomy Departmetment of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India

Abstract

Excess iron in Fe1+xSe superconductor plays a crucial role in the observation of superconductivity in this material. We have investigated the effect of off-stoichiometry at the Fe site in Fe1+xSe0.5Te0.5 (x = 0.01, 0.01 and 0.03) superconductor through magnetization, electrical transport and specific heat measurements. Our results indicate an improvement in the superconducting properties in Fe-deficient compound compared to the stoichiometric or Fe-rich compounds, in contrast to what is observed in the parent compound, FeSe. © 2013 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SMND-2011 Keywords: Off-stoichiometry, SDW, Fermi temperature.

* Corresponding author. Tel.: +91-22-25764552; fax: +91-22-25767552. E-mail address: [email protected].

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Department of Physics, School of Science and Humanities, Kongu Engineering College

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

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110 Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117

1. Introduction

Different families of Fe-based superconductors were discovered, after the report of superconductivity in F-doped LaFeAsO1-xFx (Tc 26 K) in 2008 [1]. These superconductors can be divided in four categories according to their chemical composition [2]. All of them exhibit tetragonal structure at room temperature and show spin density wave (SDW) anti-ferromagnetic transition at lower temperatures [3]. Among these, the selenium based compound Fe1+xSe (denoted as Fe-11) has the simplest crystal structure and chemical composition. Fe-11 materials are of great interest to the research community due to its resemblance to the other FeAs based compounds as well as interesting superconducting properties. The parent compound, FeSe, stabilizes in the tetragonal phase and exhibits superconductivity only with excess Fe (1-2%) at the Fe site [4]. Fang et.al., studied the evolution of superconductivity and the phase diagram of the ternary Fe(Se1-xTex)0.82 (0 ≤ x ≤ 1.0) system and reported that the superconducting transition temperature increased as x is increased, reached a maximum near x = 0.5, and decreased with further increase in x with no superconducting phase in the end member, FeTe0.82 [5]. Sales et.al., studied the growth and characterization of large single crystals of Fe1+ySe1-xTex and reported that bulk superconductivity can be observed only for crystals with x values near 0.5 and nominal y values [6]. Liu et.al., investigated the role of excess Fe at the interstitial sites of (Se,Te) layers in the Fe1+y(Te,Se) compounds and observed that the excess Fe not only suppressed superconductivity but also led to weak charge-carrier localization [8]. We have investigated the effect of Fe stoichiometry in the compounds, Fe1+xSe0.5Te0.5 (x = 0.01, 0.01 and 0.03). Our results show that the superconducting properties are improved if Fe has a deficiency at the Fe site, in contrast to what is observed in Fe1+xSe.

2. Sample Preparation and Characterizations Single crystal samples of Fe1+xSe0.5Te0.5 (x = 0.01, 0.01 and 0.03) were prepared by

solid state reaction method. Powders of Fe (99.9%), Se (99.99%) and Te (99.999%) were mixed with nominal stochiometry, sealed in a quartz tube and heated at 600ºC for 24 hours. Reground samples were then sealed in a quartz tube and heated to 950ºC. After 24 hours soaking, the charge was cooled slowly at a rate of 2ºC/h down to 600ºC. The furnace was then switched off and the crystals in the form of platelets were separated. A small part of the crystal was powdered to carry out X-ray diffraction measurements. DC magnetization measurements were performed using a vibrating sample magnetometer (PPMS-VSM, Quantum Design, USA). Electrical transport measurements using the four probe technique, heat capacity measurements using the relaxation method and thermal transport measurements were performed using the PPMS (Quantum Design, USA). 3. Results and Discussion

X-ray diffraction patterns of all the samples are shown in Fig. 1. All the peaks could be indexed to the tetragonal PbO-type crystal structure and P4/nmm space group. Only a negligible amount of impurity phase of Fe7Se8 was observed which is marked with an asterisk in Fig. 1.

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Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117 111

Fig. 1. X-ray powder diffraction patterns for Fe1+xSe0.5Te0.5 (x = 0.01, 0.01 and 0.03) samples.

DC magnetization measurements (zero field cooled (ZFC) and field cooled (FC)) were performed on all the crystals in a field of 10 Oe and are shown in Fig. 2(a). The superconducting transition temperature (Tc) is observed at 14.2 K, 10.5 K and 7.6 K for x =

0.01, 0.01 and 0.03 crystals, respectively. With increasing Fe at the Fe site in these crystals Tc gets suppressed more, which is the reverse to the case in Fe1+xSe compounds.

The transition width also broadens with increasing Fe in the stoichiometry. The inset panel shows the variation of Tc with Fe in the samples. Figure 2(b) shows the hysteresis loops for all the three crystals at 2 K. Hysteresis loop is better (and hence higher critical currents) for the Fe-deficient sample.

(a)

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112 Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117

(b)

Fig. 2(a). DC magnetization as a function of temperature at 10 Oe. Tc is suppressed for excess Fe content samples as shown in the inset panel. (b) Magnetic hysteresis loops at 2 K. Broadening of loops collapses for larger Fe content samples

(a)

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Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117 113

(b)

Fig. 3(a). Resistance vs Temperature at zero field. Inset panel shows the upper critical field values with temperature at mid transition temperature. (b) C/T plots vs temperature for the Fe1+xSe0.5Te0.5 (x = 0.01, 0.01 and 0.03) samples. Inset panel shows the electronic contribution to heat capacity for the x = 0.01 sample. Electrical transport measurements with temperature in zero field from 2 to 300 K are shown in Fig. 3(a). The onset transition temperatures are observed to be 16.2 K, 14.5 K and 13.0 K for x =

0.01, 0.01 and 0.03, respectively. Upper critical field at zero temperature (Hc2(0)) was calculated using Werthamer-Helfand-Hohenberg (WHH) formula [8],

μ0Hc2(0) = -0.693μ0 (dHc2/dT )TcTc (1)

The Hc2(0) values deduced using the mid transition temperature criteria (i.e., 50% of the normal resistance at the onset), are 91 T, 48 T and 25 T for x = 0.01, 0.01 and 0.03, respectively. Using these Hc2(0) values, the corresponding values of μ0Hc2/kBTc come out to be 6.2 T/K, 3.8 T/K and 2.4 T/K for x = 0.01, 0.01 and 0.03, respectively which are much higher than the Pauli limit (1.8 T/K) [9]. These high values of μ0Hc2/kBTc indicate the unconventional nature of superconductivity in these compounds. The coherence length (ξ) was deducted using the Ginzberg-Landau (GL) expression [9],

ξ = (Ф0 / 2πμ0 Hc2)1/2 (2)

where 0 = 2.07 10 7 Oe cm2. The coherence length at the mid transition temperature was calculated to be 18.9 Å, 26.1 Å and 35.9 Å for x = 0.01, 0.01 and 0.03, respectively.

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114 Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117

Low temperature heat capacity (HC) measurements were performed on all the three Fe1+xSe0.5Te0.5 (x = 0.01, 0.01 and 0.03) crystals to check the bulk nature of the superconductivity (see Fig. 3(b)). Only x = 0.01 crystal showed clear heat capacity jump at Tc. Crystals with x = 0.01 and 0.03 showed clear metallic-type behaviour in heat capacity at low temperatures whcih can be described by the Debye expression [6],

C = γT +BT3+CT5 (3)

with γ = 36.3 mJ/mole-K2. Debye temperature D was calculted using the expression

ΘD = (1944/B)1/3 (4)

which gave D as 143 K [6]. To get an approximate heat capacity jump for the x = 0.01 sample, the lattic contribution BT3+CT5 for x = 0.01 sample was subtracted from the total heat capacity data for x = 0.01 and is shown in the inset of Fig. 3(b).

(a)

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Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117 115

(b)

Fig. 4(a). κ with temperature from 2 to 350 K plot. Inset panel shows the enlarged view of κ between 2 to 20 K. (b) S with temperature from 2 to 350 K. Inset panel shows the enlarged view of S/T with temperature for Fe1+xSe0.5Te0.5 (x = 0, -0.01, 0.01 and 0.03) crystals. In Fig. 4(a), the thermal conductivity (κ) of Fe1+xSe0.5Te0.5 crystals are plotted as a function of temperature from 2 to 350 K. Stoichiometric sample exhibits the largest κ. It is known that in Fe-11 compounds, κ is dominated by phonons. The main scattering mechainism for phonons are the charge carriers and structural defects, while intrinsic phonon-phonon scattering is dominant in clean materials[10]. This implies that the smaller κ values can be attributted to crystallographic disorder, perhaps due to Fe-off stoichiometry [10]. At higher temperatures, the mean free path is reduced due to scattering of the carriers with phonons which results in a small variation of κ with temperature. The change of slope in κ, indicating the superconducting transition, is quite evident in the x = 0.01 and x = 0 samples (see the inset of Fig. 4(a)), while the Fe excess samples do not show any such jump. The Tc values extracted from the κ measurements (at the change in slope) is 13.2 K and 12.8 K, respectively for the x = 0 and x =

0.01 samples, which correspond well with the values obtained from the resisitivity measurements. Figure 4(b) shows the temperature dependent Seebeck coefficient (S) measurements at zero field for Fe1+xSe0.5Te0.5 (x = 0.01, 0.01 and 0.03). The S value of stoichiometric sample is positive at

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116 Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117

higher temperatures, crosses the zero line at 102 K and becomes negative below this temperature, which is consistent with the reported behaviour for this compound [11, 12]. However, the S values for all off-stoichiometric samples are negative at all temperatures, indicating electrons as majority charge carriers. Below Tc, S is zero due to the participation of charge carriers in forming the Cooper pairs. As the temperature increases the charge carriers migrate from one end to other end and accumulate there. Maximum S value reaches 28, 34, 24 and19 μV/K for x = 0, 0.01, 0.01 and 0.03, respectively. The inset of Fig. 4(b) shows S/T as a function of temperature at low temperatures. The extrapolated values of S/T to zero-temperature are 1.67, 2.23, 1.38 and 0.95 μV/K2 for x = 0, 0.01, 0.01 and 0.03, respectively. One can deduce the Fermi temperature (TF) using the expression, S/T = (±π2kB)/(2eTF) (5) where, kB is the Boltzmann’s constant, e is the electron charge, and n is the carrier density [13]. The obtained values for TF are 254, 190, 309 and 445 K for x = 0, 0.01, 0.01 and 0.03, respectively. 4. Conclusions

In summary, we have shown that the Fe-deficiency improves the superconducting properties in Fe1+xSe0.5Te0.5 compounds, even though the superconducting phase is Fe-rich in the parent compound, FeSe. The transition temperature is found to be suppressed with increase in off-stoichiometry. A clear jump in heat capacity, indicating bulk superconductivity, is observed only in the Fe deficient crystals. Thermal conductivity is found to get suppressed in the off-stoichiometric compounds. Thermopower measurements show that the off-stoichiometric compounds contain only one type of majority carrier (electrons), where as the charge carrier density showed temperature dependence in the stoichiometric compound. Fermi temperature TF, calculated from the Seebeck coefficient measurements, showed that the Fermi temperature increased with excess iron while TF decreased with deficiency of iron. Acknowledgements CVT would like to acknowledge the Department of Science and Technology for partial support through the project IR/S2/PU-10/2006. ADT would like to acknowledge the Institute Post Doctoral Fellowship at the Indian Institute of Technology, Bombay for partial support. AKY would like to acknowledge CSIR for financial support. References [1] Kamihara, Y.,Watanabe, T., Hirano, M., Hosono, H., 2008. Iron-based layered superconductor LaO1-xFxFeAs (x=0.05-0.12) with Tc = 26 K. Journal of the American Chemical Society 130, 3296. [2] Lumsden, M.D., Christianson, A.D., 2010. Magnetism in Fe-based superconductors. arXiv 1004, 1969v1. [3] Chen, Luo, J.L., Wang, N.L., Dai. P.C., 2008. Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems. Nature 453, 899–902. [4] McQueen, T.M., Huang, Q., Ksenofontov, V., Felser, C., Xu, Q., Zandbergen, H., Hor et.

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Anil K. Yadav et al. / Physics Procedia 49 ( 2013 ) 109 – 117 117

al., 2009. Extreme sensitivity of superconductivity to stoichiometry in Fe1+xSe. Physical Review B 79, 014522. [5] Fang, M.H., Pham, H.M., Qian, B., Liu, T.J., Vehstedt, E.K., Liu, Y., Spinu, L., Mao, L.Q., 2008. Superconductivity close to magnetic instability in Fe(Se1−xTex)0.82. Physical Review B 78, 224503. [6] Sales, B.C., Sefat, A.S., McGuire, M.A., Jin, R.Y., Mandrus, D., 2009. Bulk superconductivity at 14 K in single crystals of Fe1+yTexSe1−x. Physical Review B 79, 094521. [7] Liu, T.J., Ke, X., Qian, B., Hu, J., Fobes, D., Vehstedt et al., 2009. Charge-carrier localization induced by excess Fe in the superconductor Fe1+yTe1−xSex. Physical Review B 80, 174509. [8] Werthamer, N.R., Helfand, E., Hohenberg, P.C., 1966. Temperature and Purity Dependence of the Superconducting Critical Field, Hc2. III. Electron Spin and Spin-Orbit Effects. Phys. Rev 147, 295-302. [9] Yadav, C.S., Paulose, P.L., 2009. Upper critical field, lower critical field and critical current density of FeTe0.60Se0.40 single crystal. New Journal of Physics 11,103046. [10] Tropeano, M., Pallecchi, I., Cimberle, M.R., Ferdeghini, C., Lamura et al., 2010. Transport and superconducting properties of Fe-based superconductors: a comparison between SmFeAsO1−xFx and Fe1+yTe1−xSex , Supercond. Sci. Technol 23, 054001. [11] Lindén, J., Libäck, J.-P., Karppinen, M., Rautama, E.L., Yamauchi, H., 2010. Observation of lattice softening at Tc in the FeSe0.5Te0.5 superconductor, Solid State Communications 151, 130-134 [12] Craco1, L., Laad, M.S., 2010. Theory of Normal State Pseudogap Behavior in FeSe1−xTex, arxiv 1001, 3273. [13] Pourret, A., Malone, L., Antunes, A.B., Yadav, C.S., Paulose, et al., 2010. Thermoelectric response of Fe1+yTe0.6Se0.4: evidence for strong correlation and low carrier density. arXiv 1010, 1484.


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