Improvement of superconducting properties in Fe1+xSe0.5Te0.5 superconductor by Cr-substitution

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Improvement of superconducting properties in Fe1 þ xSe0.5Te0.5 superconductor by Cr-substitution Anil K. Yadav a,b,n, Anup V. Sanchela a, C.V. Tomy a a b

Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India Department of physics, Indian Institute of Science Bangalore, Bengaluru 560012, India

art ic l e i nf o

a b s t r a c t

Article history: Received 4 November 2014 Received in revised form 7 January 2015 Accepted 12 January 2015 communicated by T. Kimura

Enhancement of superconducting transition temperature (Tc) of parent superconductor, Fe1 þ xSe, of ‘Fe-11’ family by Cr-substitution for excess Fe has been motivated us to investigate the effect of Cr-substitution in optimal superconductor of Fe1 þ xSe0.5Te0.5 at Fe site. Here, we report structural, magnetic, electrical transport, thermal transport and heat capacity properties of Cr-substitute compounds. x-ray diffraction measurement confirms the substitution of Cr-atoms in host lattice. Magnetic and electrical transport measurements are used to explore the superconducting properties where Crsubstituted compounds show improvement in superconducting diamagnetic fraction with same Tc as undoped one. Heat capacity measurement confirms the bulk superconducting properties of compounds. Thermopower measurement characterizes the type of charge carriers in normal state. & 2015 Published by Elsevier Ltd.

Keywords: Iron chalcogenide Substitution Thermopower Strong correlation

1. Introduction Iron chalcogenide materials (‘Fe-11’ family) of Fe-based superconductors have got a special attention due to its simplest crystal structure and stoichiometry. It can also be considered as prototype material to explore the mechanism of superconductivity of these newly discovered superconductors because its non-toxic property. Parent compounds of ‘Fe-1111’ and ‘Fe-122’ family of Fe-based superconductors, posses high superconducting transition temperatures (Tc), exhibit superconductivity by doping of charge carriers, however, parent compound of ‘Fe-11’ family shows lowest (Tc) ( 8 K) but without doping of any charge carriers [1,2]. Superconductivity in parent compound (FeSe) of ‘Fe-11’ materials is very much sensitive towards the stoichiometry variation [3] and application of external pressure [4–6]. In stoichiometry form, FeSe is not a superconductor while turn into a superconductor with doping of 1 wt% excess Fe and also completely suppressed with doping of 3 wt % excess Fe [3]. However, maximum (Tc) of this family is obtained 37 K with the application of 7 GPa external pressure into Fe1.01Se compound [4]. Fe-based superconductors are also categorized as multiband superconductor as its band gap energy changes in magnitude at Fermi surfaces with direction [1,7,8].

n Corresponding author at: Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India . E-mail address: [email protected] (A.K. Yadav).

Chemical pressure (doping or substitution) is the alternate way to enhance the Tc of parent superconductor rather than external pressure. Foremost, Wu et al. have studied the substitution effect of various chemical elements (Al, Ti, V, Cr, Mn, Co, Ni, Cu, Ga, In, Ba and Sm, x Z10 wt%) in place of excess Fe in Fe1 þ xSe but none of elements help to enhance in Tc [9]. Only substitution of S or Te in FeSe at Se-site is found to be enhancing the Tc [10]. At ambient pressure, the optimal Tc of FeSe is achieved  14.5 K by substitution of 50% Te for Se [11–14]. Our group has also reported the enhancement of Tc  11.2 K by 2 wt% of Cr-substitution in Fe1 þ xSe for excess Fe [15,16]. Except for this, we have also extended our study of nominal transition metal substitution for excess Fe in which we found that only higher atomic radius atom support to enhance the Tc [17]. There are also many reports about effort to enhance the Tc of optimal superconductor, FeSe0.5Te0.5, by chemical pressure [18–20]. Zhang et al. [18] and Gawrlyluk et al. [19] have studied the substitution of various transition and nontransition metals in Fe1  xTMxSe0.5Te0.5 but none of substituted element supports to enhance the Tc. Gunther et al. have reported improvement of volume fraction with 2 wt% of Mn substitution for Fe in FeTe0.5Se0.5 compound [20]. Getting motivation from our previous study of substitution of Cr for excess Fe in Fe1 þ xSe compound [15], in this paper, we have extended our study and reported superconducting as well as some physical properties of Cr-substituted compounds in optimal superconductor Fe1 þ xTe0.5Se0.5 at Fe-site by magnetization, electrical transport, thermal transport and heat capacity measurements.

http://dx.doi.org/10.1016/j.ssc.2015.01.009 0038-1098/& 2015 Published by Elsevier Ltd.

Please cite this article as: A.K. Yadav, et al., Solid State Commun (2015), http://dx.doi.org/10.1016/j.ssc.2015.01.009i

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2. Experimental techniques All polycrystalline samples, Fe1 xCrxTe0.5Se0.5 (x¼ 0, 0.01, 0.02) were synthesized via conventional a solid state reaction method in two step heat process. Starting elements Fe (99.99%), Se (99.999%), Te (99.99%) and Cr (99.9%) of high purity were taken in desired stoichiometry from Alfa Easar and homogenized in an agate/mortar. Homogenized powder were filled in evacuated quartz tubes and sealed. Sealed quartz tubes were loaded in a furnace for first heating at 450 1C and held at this temperature for 24 h following with furnace off. After first heating, partial reacted samples were again re-grounded and filled in double wall quartz tube for second heating. In this heating, all samples were heated at high temperature of 960 1C with soak time of 24 h followed by furnace off. Powder x-ray diffraction (XRD) measurements were performed in a Philips X'Pert PRO diffractometer using of Cu-Kα radiation for confirmation of phase purity. Energy dispersive x-ray measurement (EDAX) was used for identification of substituted elements. DC magnetization measurements were performed in physical property measurement system (PPMS) of Quantum Design with the attachment of a vibrating sample magnetometer (VSM). Resistivity, heat capacity and thermal transport measurements were carried out in resistivity, heat capacity and thermal transport options of PPMS, respectively.

3. Results and discussions 3.1. Structure analysis Fig. 1 (a) shows the x-ray diffraction patterns of Fe1 xCrxTe0.5Se0.5 (x¼0, 0.01, 0.02) samples along with refinement spectrum for x¼ 0 sample. Intensity of maximum intensity peak (101) is found to be increased (due to substitution of higher atomic radius of Cr atom compare to Fe atom) as percentage of Cr-substitution increases in undoped sample. This can be taken as confirmation of Cr-substitution into the host lattice. Rietveld refinement has been performed on powder XRD in FullProf Suit software using reference patterns of

Fig. 1. (Color online) (a) XRD patterns of Fe1  xCrxSe0.5Te0.5 (x¼ 0, 0.01, 0.02) polycrystalline samples. β-FeSe tetragonal phase peaks are indexed with corresponding (h, k, l) values and impurity phase from hexagonal peaks are marked with asterisk. (b) Variation of lattice parameters (a (red curve), c (green curve)) with Cr concentration in the host lattice.

tetragonal symmetry P4/nmm for β-FeSe phase and hexagonal symmetry P63/mmc for hexagonal impurity phase. Crystal structure of samples x¼0.0 and 0.01 were perfectly refined by using only tetragonal phase as reference, however, x¼0.02 sample was perfectly refined by using both reference patterns. Intensity peaks related to β-FeSe phase are indexed with corresponds (h, k, l) values and peaks from hexagonal impurity phase are marked with asterisk in Fig. 1 (a). Refined lattice parameters are listed in Table 1. There are nominal changes in lattice parameters of Cr-compounds compare to undoped one (see Fig. 1(b)). Lattice parameters (a and c) are found to be increased with increasing Cr-concentration into host lattice. Secondary phase formation starts to grow as Cr-substitution rises above 1 wt% (see Table 1). All substituted elements into host lattice were found in EDAX analysis.

3.2. Confirmation of superconducting properties Fig. 2 shows the temperature dependence of zero field cooled (ZFC) and field-cooled (FC) dc magnetization curves at H ¼10 Oe. Superconducting onset transitions temperatures (Ton c ) from magnetization are observed (at deviation of ZFC and FC curves from zero magnetization line) to be  13.9 K,  14.6 K and 14.2 K for x¼ 0, 0.01 and 0.02 samples, respectively (see enlarge view of Fig. 2). The Tc of all three samples are approximately the same and also similar with previously reported Fe(Se, Te) based superconductors [12,14,21]. Superconducting transition width (ΔTc r 1.8 K) is observed narrower for the x¼ 0.01 as compared to x ¼0 ( 6 K) that indicates improvement in quality of sample due to Crsubstitution. The purpose of making x ¼0.02 sample is to see the enhancement of Tc and diamagnetic shielding fraction but both properties are found to be suppressed as compared to the x¼ 0.01 sample, therefore further measurements are not performed in this Table 1 Rietveld refinement results of Fe1  xCrxSe0.5Te0.5 (x¼ 0, 0.01, 0.02) samples. Lattice parameters are determined using FullProf Suit software with the help of β-FeSe tetragonal phase (P4/nmm space group) and α-FeSe hexagonal phase (P63/mmc space group) as reference patterns. Atoms Wyck. positions and fractional coordinates in tetragonal structure are, Fe, Cr: 2a QUOTE (3/4, 1/4, 0) and Se, Te: 2c QUOTE (1/4, 1/4, z). Sample Occu. Fe

Occu. Cr

Occu. Se

Occu. Te

a, b (Å)

c (Å)

Tet. Hex. (W%) (W%)

x ¼0 1. 00 x ¼0.01 0.98 x ¼0.02 0.975

0. 00 0. 01 0. 02

0.48 0.52 0.49

0.52 0.48 0.51

3.800 6. 038 100 0 3.799 6. 044 100 0 3.803 6. 070 67.5 32.5

GOF

2.90 3.32 2.40

Fig. 2. (Color online) Temperature dependence of zero field cooled (ZFC) and field cooled (FC) magnetization curves at H¼ 10 Oe for Fe1  xCrxSe0.5Te0.5 (x ¼ 0, 0.01 and 0.02) samples. Enlarged view shows the Tc of compounds.

Please cite this article as: A.K. Yadav, et al., Solid State Commun (2015), http://dx.doi.org/10.1016/j.ssc.2015.01.009i

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Fig. 3. (Color online) (a) Isothermal M–H loops for Fe1  xCrxSe0.5Te0.5 (x¼ 0, 0.01) at 2 K and 7 K and (b) corresponding Jc vs H curves.

sample. Fig. 3(a) shows magnetic hysteresis curves with fields between 7 70 kOe at 2 K and 7 K for x¼ 0 and 0.01 samples. The hysteresis width is found to be broaden for x ¼0.01 compare to x ¼0 at same temperature. Fig. 3(b) displays the critical current density (Jc) plots as function of field where Cr-substitute sample posses larger Jc. The Jc was extracted using Bean's critical state formula, Jc ¼20ΔM/a(1-a/3b), where ΔM ( ¼Mup  Mdown) is the hysteresis width between the Mup (field decreasing) and Mdown (field increasing) of loop, a and b (b 4a) are the sample dimensions [22]. Thus Cr-substitute sample is better from application point of view as it transports larger current. Main panel of Fig. 4(a) shows the temperature dependent resistivity measurements at zero fields from 2 K to 300 K. Resistivity curve for x ¼0 shows a non-metallic behavior, similar to the previously reported Fe(Se,Te) systems [23–25]. However, nature of resistivity curve for x ¼0.01 is found to be different, its resistivity curve changes slop around 200 K, above this resistivity varies slowly and below this decreases sharply with temperature. Similar results have been seen in stoichiometry dependent resistivity of Fe (Se, Te) compounds [26]. Superconducting onset transition temρ peratures (Ton; ) from resistivity are observed to be 14.0 K and c 15.7 K for x ¼0 and 0.01 samples, respectively (see inset of Fig. 4 (a)). Superconducting transition widths (ΔTc) in resistivity has been found sharper for x ¼0.01 (  1.6 K) compare to x ¼0 ( 4 K) as similar to magnetic behavior. In order to estimate the upper critical fields (Hc2(0)), we have performed resistivity measurements at different applied magnetic fields for both samples (see Fig. 4(b) and (c)). The Tc gradually shifts towards the lower temperature as field increases with the rate of 0.045 K/kOe for x ¼0.01 and slight faster 0.08 K/kOe for x ¼0 sample. Insets of Fig. 4(b) and (c) show the H–T phase diagrams at three transmid ition temperatures (Ton and Toff c , Tc c , define as 90% ρn, 50% ρn and

Fig. 4. (Color online) Resistivity measurements (ρ(T)) as function of temperature for Fe1  xCrxSe0.5Te0.5 (x ¼ 0, 0.01) samples: (a) from 2 K to 300 K in main panel and inset shows onset of Tc . The main panels of (b) and (c) show the field dependent ρ mid (T) plots at low temperatures at three transition temperatures (Ton and Toff c , Tc c ) (see text). Insets (b) and (c) depict Hc2 vs T phase diagram at three transition temperatures.

Table 2 Superconducting parameters of Fe1  xCrxSe0.5Te0.5 (x ¼0, 0.01) polycrystalline samples determined from the electrical transport and Seebeck measurements (see text). Compounds x ¼0

x ¼0.01

Ton;ρ c Tmid;ρ c Toff;ρ c Ton;ρ c Tmid;ρ c Toff;ρ c

Tc (K)

dHc2/dT

Hc2(0) (kOe)

ξ(Å)

S/T

TF (K)

Tc/TF

14.0 11.8 9.5 15.7 14.7 14.0

 3.8  2.8  2.6  6.7  4.5  4.0

332 237 171 1088 469 429

31.5 37.2 43.9 17.4 26.5 27.7

1.7

250

0.08

2.4

177

0.09

10% ρn, where ρn is normal state resistivity). Values of Hc2(0) are estimated using Werthamer–Helfand–Hohenberg expression [27],

μ0 Hc2 ð0Þ ¼  0:693μ0 T c dHc =dT


Tc

ð1Þ

where (dHc/dT)Tc is the slop of H–T curve at transition temperature. Superconducting parameters (dHc/dT)Tc, Hc2(0) and coherence length (ξ(0)) are listed in Table 2 for both the samples at three transition temperatures. These Hc2(0) values are found to be comparable with the reported Hc2(0) of FeSe0.6Te0.4 [21] and FeSe0.25Te0.75 [28] single crystals.

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conductivity (κph p l) increase. The κph increases continuously as l increases with lowering the temperature and at certain temperature where l becomes comparable with the grain size of compound, κph starts to suppress by following this relation κph p T3 up to the lowest possible temperature [37]. Fig. 5 (b) shows the plots of Seebeck coefficient (S) as function of temperature at H¼ 0 Oe from 2 K to 300 K. The sign of (S(T)) in normal state indicates the type of charge carriers that suggests undoped sample posses both types (electrons and holes) of charge carriers as its curve changes sign  180 K. This result is consistent with previously reported thermopower results of Fe(Se,Te) system [32,37], whereas only electrons are found as majority charge carriers for Cr-substitute sample as S(T) comprises negative sign in normal state. The absolute value of S(T) increases with lowering the temperature and attains maximum  22 μV/K (at  26 K) and  31 μV/K (at  27 K) for x ¼0 and 0.01, respectively. Below these temperatures, S(T) sharply decreases and becomes zero in superconducting state. Thermopower (S) originates from two components in any material: one is called as electron diffusion (Sd) component and another is known as phonon-drag (Sph) component [29,38]. Following Fermi-liquid theory, Behania et al. [39] have derived an expression that correlates diffusive component with temperature, where S should be linear in zero temperature limits and its magnitude defines strength of electronic correlations as in the case of specific heat, Ces/T ¼ γ. Similarly S can be linked to the Fermi temperature TF by expression [39]: S=T ¼ ð 7 π 2K B Þ=2eT F

Fig. 5. (Color online) Thermal transport properties of Fe1  xCrxSe0.5Te0.5 (x ¼0, 0.01): (a) κ vs T plots at zero field and enlarged view in inset from 2 K to 20 K. (b) S vs T plots between 2 K and 300 K in main panel and S/T vs T plots between 2 K and 30 K in inset. (c) Figure of merit (ZT) as function of temperature in main panel and derivation of Seebeck coefficient with T in inset (the variation of slop change marked around Debye temperature (ΘD)).

3.3. Thermal transport properties Fig. 5 (a) depicts thermal conductivity (κ) measurements as function of temperature at zero field between 2 K and 300 K in which κ(T) curves change slop at  45 K and  62 K for x¼0 and 0.01 in normal state, respectively. Around these temperatures, κ(T) behavior changes differently, above increases slowly towards the room temperature, however below decreases sharply. The overall temperature dependent nature of κ(T) curves for both the samples in normal state are found reminiscent with MgB2 superconductor [29]. The absolute value of κ at each temperature for Cr-sample is found less compared to undoped one. The reduction in κ for the Cr-substitute sample may be attributed due to the crystallographic disorder arising from the substitution of Cr-atoms. Similar reduction in κ is reported for Fe1 þ y(Se1  xTex) [30], SmFeAsO1  xFex [31] and Kx(Fe1  xSe2) [32] compounds after substitution of more iron in compounds. In low temperature regime, below Tc both κ(T) curves exhibit hump like features as shown in the inset of Fig. 5(a). Similar features have been seen in other superconductors such as hole-doped Ba1  xKxFe2As2 [33], unconventional CeCoIn5 [34] and high temperature YBaCu3O7  δ [35] superconductors. Typically this feature has been associated with strength of electron–phonon coupling in superconducting state. The physical origin of hump can be explained as: in superconducting state electrons form Cooper pairs and neither carry entropy nor do they scatter with phonon thus phonon are main source of heat transport [36]. As temperature decreases the electron–phonon scattering reduces thus phonon mean free path (l) and its associated phonon thermal

ð2Þ

where KB is the Boltzmann's constant and e is the electron charge. Inset of Fig. 5(b) shows the S/T versus T plots along with linear fitting curves. To get the zero temperature limit value, the linear fitting curves are extrapolated up to zero temperature which meets zero temperature line at S/T  1.7 μV/K2 and  2.4 μV/K2 and by using these values corresponding Fermi temperatures (TF) are estimated  250 K and  177 K with the help of Eq. (2) for x¼0 and 0.01, respectively (see Table 2). The ratio of the superconducting transition temperature and Fermi temperature (Tc/TF) characterizes the electron– phonon correlation strength in any superconductor and its value predicts the nature of superconductor (conventional or unconventional) [32]. Typically value of (Tc/TF) for unconventional superconductors, such as CeCoIn5 [40] and YBa2Cu3O6.67 [41], is found to be 0.1 while for conventional BCS-type superconductors (e.g., LuNi2B2C [42]), this ratio can be as low as 0.02 [41]. Compound KFe1 xSey is an example of weakly correlated iron superconductor, its ratio is found to be 0.04 [32]. The ratio (Tc/TF) for Fe1 þ xSe0.4Te0.6 superconductor [42] is found to be near 0.1 that indicates strong electronic correlation system. In our samples, this ratio is found to be  0.08 and 0.09 for the x¼0 and 0.01, respectively. These values of ratio suggest that our superconductors lie in strong electronic correlation systems. Fig. 5(c) shows the figure of merit (ZT¼S2/ρκ) plots as function of temperature. The ZT for x¼0.01 is nearly seven times higher than x¼0 at 30 K. Thus, Cr-substitution also improves figure of merit from thermoelectric material point of view in low temperature. 3.4. Heat capacity properties Fig. 6 shows temperature dependent heat capacity measurements for x ¼0, 0.01 and 0.02 at zero field from 2 K to 30 K. There is no such clear heat capacity jump observed for the x¼ 0 and 0.02 while x¼0.01 shows an obvious superconducting jump at Tc ( 14.7 K). This can be considering as a clear evidence for the improvement of bulk superconducting property via 1 wt% Crsubstitution. In low temperature, metal's heat capacity behavior can be explained by the Debye heat capacity equation [13]: C ¼ γ T þ BT 3 þcT 5

Please cite this article as: A.K. Yadav, et al., Solid State Commun (2015), http://dx.doi.org/10.1016/j.ssc.2015.01.009i

ð3Þ

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Acknowledgment AKY would like to thank CSIR, India for SRF Grant. References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 6. (Color online) Total heat capacity (C/T) vs T plots for Fe1  xCrxSe0.5Te0.5 (x ¼ 0, 0.01, 0.02) along with Debye fitting. Inset shows electronic heat capacity (Ces/T) vs T plot for x¼ 0.01 sample.

where γ, B and c are the coefficients of electronic, lattice and anharmonic impurities contribution, respectively. Eq. (3) perfectly fitted with x¼0 curve in normal state in low temperature regime that gives fitting parameters, γ ¼31 mJ/mole K2 and Debye temperature (ΘD)  144 K, which is estimated from equation ΘD ¼(1944/B)1/3 [43]. These values of fitting parameters are slightly lower than previously reported for FeSe0.5Te0.5 single crystals [14,25]. The electronic heat capacity (Ces) due to superconducting electrons are extracted by subtracting lattice contribution from total heat capacity for x¼0.01 using above fitting parameters. Inset of Fig. 6 shows Ces/T versus T plot which appears exponential in nature as predicted in the BCS theory [44]. The required energy 19mJ/mole K2 (see inset of Fig. 6) for Cooper pair formation has been estimated by subtracting superconducting and normal electrons heat capacities.

4. Conclusions In summary, all samples, Fe1 xCrxSe0.5Te0.5 (x¼0, 0.01, 0.02), are successfully grown via the solid state reaction method. Secondary phase formation begins as Cr-substitution increases more than 1 wt% into host lattice. Transition temperature of optimal compound unaltered while diamagnetic shielding fraction is found to be improved by Cr-substitution. A very high upper critical field (Hc2(0)  1080 kOe) is observed for 1 wt% Cr-substituted superconductor. The sign of Seebeck coefficient indicates electrons as majority charge carriers for Cr-substituted sample and both types of charge carriers for undoped one. The ratio Tc/TF indicates the strong electronic correlation superconducting systems for these superconductors. High heat capacity jump and hump like feature support bulk property of doped superconductor. Thus from above results, it can be concluded that Cr-substitution improve the superconducting property of FeSe0.5Te0.5 polycrystalline superconductor.

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