Effect of Te-doping on the superconducting characteristics of FeSe single crystal

Journal of Alloys and Compounds 809 (2019) 151851

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Effect of Te-doping on the superconducting characteristics of FeSe single crystal Rui Mao a, Zhaofeng Wu b, 1, Zeying Wang a, Zihan Pan a, Meihua Xu c, *, Zhihe Wang b, d, ** a

2011 College, Nanjing Tech University, Nanjing, 211816, China Department of Physics, Nanjing University, Nanjing, 210093, China c School of Physical and Mathematical Sciences, Nanjing Tech University, Nanjing, 211816, China d Center for Superconducting Physics and Materials, Collaborative Innovation Center of Advanced Microstructures and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 May 2019 Received in revised form 28 July 2019 Accepted 12 August 2019 Available online 13 August 2019

FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals were prepared by the chemical reaction sealed in a small quartz tube with high vacuum. The angular dependence of resistance was measured in different magnetic field at a constant temperature, and scaled by the anisotropic Gingzburg-Landau theory, respectively, for these single crystals. The field dependences of critical temperature and flux pinning energy were obtained from resistance versus temperature curves measured in various magnetic fields perpendicular and parallel to the c-axis. The critical current density and the flux pinning force were calculated by the Bean model from the magnetization loops. The Te-doping had an obvious effect on the critical temperature Tc, the critical current density Jc and the flux pinning energy U; but there was a weak effect on the anisotropy of FeSe1-xTex single crystals. The flux pinning characteristics were also discussed. © 2019 Elsevier B.V. All rights reserved.

Keywords: FeSe1-xTex single crystals Anisotropy Flux pinning

1. Introduction In February 2008, Kamihara et al. [1] first observed superconductivity in LaOFeAs with superconducting transition temperature Tc up to 26 K. Then the superconducting transition temperature was rapidly raised to 51 K after Nd(O1-xFx)FeAs was discovered [2], exceeding the limit of critical temperature predicted by McMillan's formula. However, many research groups flinched due to the highly toxic As. Later, Hsu et al. [3] reported the discovery of superconductivity at 8 K in nontoxic a-FeSe compounds. The Tc of FeSe can rise to 14 K when 50~60% of Se was substitute for Te [4]. Although several papers reported the flux pinning properties for FeSe0.5Te0.5 and FeSe0.4Te0.6 single crystals [5,6], respectively, but the effect of Te-doping on the superconducting characteristics is not clear up to now. It was well known that cuprate superconductors have high

* Corresponding author. ** Corresponding author. Department of Physics, Nanjing University, Nanjing, 210093, China. E-mail addresses: [email protected] (M. Xu), [email protected] (Z. Wang). 1 Present address: Department of Mathematics and Physics, Nanjing Institute of Technology, Nanjing, 211167, China. https://doi.org/10.1016/j.jallcom.2019.151851 0925-8388/© 2019 Elsevier B.V. All rights reserved.

superconducting transition temperature, layered structure, short coherence length, strong thermal fluctuation, etc. Therefore, the vortex physics was extremely rich, which has led to unprecedentedly prosperous development on the vortex physics. Meanwhile, many new concepts and phenomena, such as collective vortex creep, vortex glass, first-order vortex transitions, vortex melting, the second-peak effect of magnetization, etc., have been proposed or discovered [7]. In the iron-based superconductors, many experimental studies have showed that they also have a layered structure, high critical temperature and large upper critical field. So, the vortex physics in iron-based superconductors looks quite like that of the cuprate superconductors and can be understood with the thermally activated flux model [8]. But its anisotropy parameter only about 2e3 is much smaller than that in the cuprate superconductors. Therefore, we have grown FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals and studied the effect of Te-doping on vortex physics roundly. In this paper, we report the angular dependence of resistance under different magnetic field at the same temperature to get the anisotropic parameter g for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals. The temperature dependence of resistance was measured at several magnetic fields perpendicular and parallel to the c-axis. Moreover, the critical current density was obtained from the

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isothermal magnetization loop. We also give the flux pinning properties near Tc for the FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals. 2. Preparation and experimental equipment The FeSexTe1ex (x ¼ 0.5, 0.6 and 0.7) single crystals were prepared by the chemical reaction of the elements (the purity of Fe, Te and Se powders is 99.99%, 99.999% and 99.999%, respectively) in the stochiometric composition. The fabrication process of FeSexTe1ex single crystals is similar to Ref. [9]. The powders were mixed in a glove box and sealed into a small quartz tube with high vacuum. The sealed quartz tube was heated to 1110  C and kept for 72 h, and then cooled down to 750  C at a rate of 6  C h1. Next, the sealed quartz tube was cooled down to 420  C and kept for 70 h. Finally, the sealed quartz tube was quenched in water. The asgrown single crystal with shine flat surface is easily cleaved. The structure of the sample was detected by X-ray diffraction (XRD) at room temperature with a standard Cu-anode powder diffractometer (Siemens D5000). The electric transport was performed on a commercial physical property measurement system (PPMS-9 T, Quantum Design) using a standard four probe configuration. The gold wires of diameter 0.05 mm were pasted on the sample surface with silver paste. The distance of two gold wires is about 1 mm. The sample size used in transport meaurement is about 420.6 mm3 for FeSe0.5Te0.5, 3.61.20.22 mm3 for FeSe0.4Te0.6, and 5.62.80.05 mm3 for FeSe0.3Te0.7. After the transport meaurement, these samples were cut into the sizes of 0.90.720.07 mm3 for FeSe0.5Te0.5, 1.00.60.22 mm3 for FeSe0.4Te0.6, and 2.72.20.05 for FeSe0.3Te0.7 to measure its magnetization. The magnetization measurement was also carried out on the PPMS with a vibrating sample magnetometer (VSM). The magnetic field is applied along the c-axis.

Fig. 2. Temperature dependence of resistance from 225 to 9 K at 0 T for FeSe0.5Te0.5 and FeSe0.3Te0.7 single crystals. The insets are enlarged plots near the superconducting transition temperature Tc.

We can find that the resistance of both samples decreases rapidly down to 0 near 14 K, which is consistent with the results reported in Refs. [4,5]. But the resistance in the normal state has a different behavior. For the FeSe0.5Te0.5 sample, the resistance increases with the decreasce of temperture above 165 K and changes to decrease, manifesting that a crossover from semiconducting behavior to metallic behavior occurred near 165 K. The change is caused by structural phase transition with magnetic phase transition [10,11]. However, the resistance of FeSe0.3Te0.7 single crystal only displays semiconducting behavior, namely the resistance increases continuously before superconducting transition. This result may imply the conducting carriers were localized or hopped. The onset and end of superconducting transition temperature Tonset and Tend are defined as 90% and 10% of normal state resisc c tance Rn. To see the superconducting transition clearly, we enlarged them, as shown in the inset of Fig. 2. Here, Tonset and Tend of FeSc c

3. Results and analysis The XRD pattern of FeSexTe1ex (x ¼ 0.3, 0.4, 0.5) compounds is shown in Fig. 1. Compared with the standard card, we see that all peaks are corresponding to the (00l) peaks, indicating that the caxis of the crystal is perpendicular to the cleaved surface. From Fig. 1 we can also observe that the (00l) peak shifted to the smaller angle with the ratio x decreasing from 0.5 to 0.3. The shifting is related to the lattice expansion, because the ionic radius of Te is larger than that of Se. The lattice parameters are obtained by the Bragg formula, namely c ¼ 0.60284 nm for FeSe0.5Te0.5, 0.60651 nm for FeSe0.4Te0.6 and 0.61335 nm for FeSe0.3Te0.7, respectively. The result is accordance with the previous reports [4e6,9]. Fig. 2 shows the temperature dependence of resistance from 225 K to 9 K for typical FeSe0.5Te0.5 and FeSe0.3Te0.7 single crystals.

and Tend e0.5Te0.5 sample are about 15.07 K and 14.08 K, while Tonset c c of FeSe0.3Te0.7 sample are about 13.45 K and 10.86 K, The superconducting width DT for FeSe0.5Te0.5 and FeSe0.3Te0.7 samples are about 0.99 K and 2.59 K, respectively, larger than that for cuprate single crystals. We summarize all such data in Table 1. In order to study the effect of Te-doping on the anisotropy of FeSe1-xTex single crystals, we have measured the angular dependence of resistance at a fixed temperature near Tc in various fields and scaled them by the anisotropic Ginzburg-Landau theory. According to the anisotropic Ginzburg-Landau theory [12], the mixed state resistance is connected with applied effective field HGL effect ðqÞ. So the angular dependence of resistance R in different magnetic field H at a constant temperature can be scaled to R  HGL effect ðqÞ curve [13]. The scaling equation is

1=2  2 2 2 HGL effect ðqÞ ¼ H cos q þ g sin q

(1)

where g is the anisotropy parameter of single crystals and q is the angle between the magnetic field and the c-axis. Fig. 3 shows a typical scaled resistance curve by the anisotropic

Table 1 Parameters Tonset , Tend and DT for FeSexTe1ex (x ¼ 0.3, 0.4, 0.5) single crystals. c c

Fig. 1. X-Ray Diffraction (XRD) patterns for FeSe1-xTex (x ¼ 0.5, 0.6, 0.7) single crystals.

FeSe0.5Te0.5

FeSe0.4Te0.6

FeSe0.3Te0.7

Tonset (K) c

15.07

13.59

13.45

(K) Tend c DT (K)

14.08

12.09

10.86

0.99

1.50

2.59

R. Mao et al. / Journal of Alloys and Compounds 809 (2019) 151851

Fig. 3. Field dependence of resistance at 14 K scaled by the anisotropic GingzburgLandau theory 14 K for a typical FeSe0.5Te0.5 single crystal. The inset is the angular dependence of resistance in FeSe0.5Te0.5 sample at 14 K with various magnetic fields.

Ginzburg-Landau theory for FeSe0.5Te0.5 single crystal. The inset of Fig. 3 is the angular dependence of resistance measured at 14 K in different magnetic field. From these R- q curves, we can see a peak at 0 and a dip at 90 , indicating that there is an anisotropic behavior near Tc in the crystal. Similar result was also observed in FeSe0.4Te0.6 and FeSe0.3Te0.7 single crystals. By scaling, we get the anisotropic parameter g of FeSe0.5Te0.5, FeSe0.4Te0.6 and FeSe0.3Te0.7 is about 2.8, 2.7 and 2.6, espectively. These small values manifest that although FeSe has a layered structure along the c-axis, but the anisotropic parameter g is natural small. For the purpose of further study about the anisotropy of upper critical field, the field dependence of resistance was measured in various magnetic fields up to 9 T parallel and perpendicular to the c-axis for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals. Here, we just display the temperature dependence of resistance for FeSe0.5Te0.5 single crystal in Fig. 4. As shown in the Fig. 4, these R-T

Fig. 4. Temperature dependence of resistance near Tc at several fixed fields for a FeSe0.5Te0.5 single crystal. (a) H// c-axis; (b) H ⊥ c-axis.

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curves show an obvious anisotropy. Although the superconducting transition temperature Tc decreases gradually with the increase of magnetic field for two configurations, but the superconducting width DT for H//c-axis is larger than that for H//ab-plane, manifesting the vortex motion is anisotropic. With the increase of the magnetic field, the superconducting width becomes larger and larger, and the tail dragging phenomenon occurs at low temperature. Lee et al. [14] suggested that the increase of superconducting width and tail dragging phenomenon may result from the thermal fluctuation. From these R-T curves, we can get the values of Tonset and Tend c c for each curve according to above criteria. In addition, the temperature corresponding to 1%Rn is generally taken as the standard of the irreversible line Tirr [15]. The magnetic field corresponding is called the irreversible field Hirr. So, the mixed state phase diagrams of FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) samples were shown in Fig. 5. Taking the irreversible field as the cut-off point, the superconductor can be divided into the flux solid state and the flux liquid state. To obtain the field dependences of critical temperature Tonset , Tend and c c

Fig. 5. Field dependence of special temperature determined by different criteria of R/ Rn ¼ 90%, 10% and 1%, respectively. (a) FeSe0.5Te0.5, (b) FeSe0.4Te0.6 and (c) FeSe0.3Te0.7. The insets are double logarithmic plots to get the parameters n for H-T lines.

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Tirr, we replot these data in double logarithmic diagram according to the formula:

HðTÞ ¼ Hð0Þð1  T=Tc Þn

(2)

The results were given in the insets of Fig. 5. As seen in the insets of Fig. 5, the relationship between the critical field Hc2(T) and 1 T= Tc has a good linear, indicating the filed dependences of critical temperature follows the formula (2). From the H - Tonset lines, the c upper critical fields Hc2(0) and the parameter n corresponding to FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals are 238.05 T and 0.91, 218.69 T and 1.18, and 286.87 T and 1.13 for H//c-axis; 223.51 T and 1.08, 90.97 T and 1.12, and 128.02 T and 1.27 for H//ab-plane, respectively. Apparently, the upper critical field Hc2(0) is higher than that of conventional low temperature superconductors, indicating its greater application potential [16,17]. According to the thermally activated flux flow model [8], the part of less than 10% Rn should satisfy the following equation

R ¼ R0 expð  U0 =kB TÞ

(3)

where kB is Boltzmann Constant, and U0 is effective pinning energy. The pinning energy can be easily obtained by the Arrhenius relation U0 ðHÞ ¼ 

dlnR . dð1=TÞ

Fig. 6 shows the relationship between lnR and 1/T of FeSe0.5Te0.5 sample with magnetic field up to 9 T parallel and perpendicular to the c-axis. Visibly, lnR and 1/T has a linear relationship when the resistance is less than 10% Rn as seen these straight lines. Therefore, we get the effective pinning energy from the absolute slope of lnR and 1/T curve less than 10% Rn for each field. Fig. 7 shows the field dependence of the effective pinning energy U for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals. The effective pinning energy U0 for H//ab-plane is stronger than that for H//c-axis, indicating the flux pinning is anisotropic. From Fig. 7, we can find that the field dependence of the effective pinning energy obeys a power law

Fig. 7. Field dependence of the effective flux pinning energy for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals. Those thin black lines are the fitting lines via the exponential law, U0(H) ~ Ha.

U0(H) ~ Ha with a crossover at H of 1 T. The crossover is also appeared in iron-pnictide superconductor and cuprate superconductor [18,19]. Yeshurun and Malozemoff [19] argued that the crossover phenomena is related with the flux pining characteristics. In the low field region of H < 1 T, the pinning energy and magnetic field present a very weak power relationship. It is believed that the flux pinning in the low field region is likely to be single flux pinning [20,21]. With the increase of the magnetic field, more flux lines enter the superconductor, and the flux spacing becomes smaller, thus resulting in collective flux pinning [22]. Here, we point out the difference between our results and literature. The parameter a is almost 0.15 below 1 T and about 0.50 above 1 T. All of parameter a was shown in the figure. Therefore, we think that the flux pinning is dominated by single flux pinning below 1 T and the collective flux pinning above 1 T. 3.1. An intuitive model of flux pinning energy [23] predicts

1 n Uf m0 H2c x an0 2

Fig. 6. Relationship curves between lnR and 1/T at several magnetic fields for FeSe0.5Te0.5 sample. (a) H// c-axis; (b) H ⊥ c-axis. Those thin black lines are the linear fitting via the formula (3).

(4)

where 12m0 H2c is the condensation energy per unit volume, and the volume has some power of the average intervortex spacing ao ~ (F0/ B)1/2. The exponent a for the activation energy U0(H) ~ Ha is expected to be a ¼ ð3  mÞ=2, where m ¼ 0e3. The 1imiting form m ¼ 0 corresponds to the expression for core pinning of independent vortices. The planar defects are responsible for a ¼ 1/2, while the linear defects are responsible for a ¼ 1. Therefore, the flux pinning in the high field region could be the collective flux pinning of the surface defect and 3 D volume core in FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals. In addition, from the figure, we can see that the pinning energy of three samples are in different magnitude orders at low field. As the Te doping increases, the pinning energy decreases sharply. We think that the Te doping may produce larger nonsuperconducting regions in the crystal. The size of defect is larger than its coherent length. Thus, the effective pinning center becomes less, and the pinning energy decreases with the increase of Te doping. Meanwhile, Te doping should reduce its critical current density. To understand the effect of Te-doping on the superconducting characteristics for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals, we have measured the zero field cooling (ZFC) and field cooling (FC) curves at 0.01 T, and the magnetization loops at several temperatures. The ZFC and FC curves at 0.01 T for FeSe0.5Te0.5 and FeSe0.4Te0.6 samples are shown in Fig. 8. The ZFC and FC curves of both samples show that the magnetization rate of both samples nearly

R. Mao et al. / Journal of Alloys and Compounds 809 (2019) 151851

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Fig. 8. ZFC and FC curves at magnetic field of 0.01 T for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals.

has temperature-independent characteristics above the superconducting transition temperature, which indicates that the samples are in Pauli paramagnetic state. Furthermore, it is can be seen from that the magnetization superconducting volume of FeSe0.5Te0.5 sample is larger than that of FeSe0.4Te0.6 sample. The magnetization of FeSe0.5Te0.5 sample is also steeper than that of FeSe0.4Te0.6 sample. Therefore, with the doping of Te, the superconductivity of FeSe1-xTex was reduced. There is a bifurcation on the ZFC and FC curves at a characteristic temperature Tirr. Above this characteristic temperature, the two curves coincide, so the magnetization process is reversible. However, below this temperature, the two curves do not coincide, and the magnetization process is irreversible. Simultaneously, below the temperature Tirr, the Lorentz force is not large enough due to the pinning effect of the lattice defect, so the flux flow is irreversible. Conversely, above the temperature Tirr, the flux flow is reversible. From Fig. 8, we get the irreversible temperature Tirr at H ¼ 0.01 T is about 13.95 K for FeSe0.5Te0.5 sample and about 11.71 K for FeSe0.4Te0.6 sample. Fig. 9 displays the magnetic hysteresis loops measured at several magnetic fields parallel to the c-axis up to 7 T for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals, respectively. Visibly, the magnetic hysteresis loops are all symmetrically distributed, which indicate that the superconducting current is mainly the volume current rather than the surface shielding current. The samples should be volume superconducting, and the flux pinning should be the volume pinning, rather than the surface pinning caused by the surface shielding current. With the increase of temperature, the magnetic hysteresis loop is narrower indicating that the critical current density is decrease; meanwhile the pinning force becomes weak. As can be seen from Fig. 9, for FeSe0.4Te0.6 sample, the magnetic hysteresis loops show a bulge at the middle magnetic field, namely the second-peak effect, which reflects the abnormal increase of critical current with the increase of magnetic field. The magnetic hysteresis loop near Tc is like that of ferromagnetic materials, which has also drawn attention to the coexistence of superconductivity and ferromagnetism in our samples. According to the Bean critical state model [24], we calculated the magnetic critical current density Jc using the following formula

DM Jc ¼ 20   a a 1  3b

(5)

where DM ¼ Mþ  M is the width of the magnetic hysteresis loops and the unit of DM is A/m. Mþ and M represent the values of rise and decline of magnetic hysteresis loops. a and b represent the length and width of samples. To escape the effect of excessive Fe

Fig. 9. Magnetic hysteresis loops at several temperatures with magnetic field parallel to the c-axis. (a) FeSe0.5Te0.5, (b) FeSe0.4Te0.6 and (c) FeSe0.3Te0.7 single crystals. Inset of Fig. 9c displayed the background magnetic hysteresis loop for FeSe0.3Te0.7 single crystals.

hysteresis on the critical current density Jc. we choose the magnetic hysteresis loop near Tc as a background. As an example, we show the background magnetic hysteresis loop for FeSe0.3Te0.7 single crystal in the inset of Fig. 9c. Fig. 10 shows the field dependence of the critical current density Jc at various temperatures for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals, respectively. From Fig. 10, we can see that the critical current density Jc is about 1.44106 A/cm2 (4 K, 0 T) for FeSe0.5Te0.5 and 1.1106 A/cm2 (5 K, 0 T) for FeSe0.4Te0.6 much larger than that of undoped FeSe single crystal [25]. Such large current density manifests that our single crystals are high quality with good superconductivity. However, for FeSe0.3Te0.7, the critical current density Jc is about 2.1103 A/cm2 (4 K, 0 T) very lower than that of undoped FeSe single crystal. This result implies that Te-doping caused lattice distortion, leaded to defect generation, and enhanced the flux pinning energy and the critical current density. As the Te doping increases, it may produce larger nonsuperconducting regions in the crystal and the superconducting volume decreased. The effective flux pinning energy and the critical current density decrease sharply. From Fig. 10, we can see that the field dependence of the critical current density follows a power law, Jc f Hm, in the middle magnetic field for FeSe0.5Te0.5 and FeSe0.4Te0.6, and an exponent relation Jc f e-kH above 0.2 T for FeSe0.3Te0.7. In the higher magnetic field region, the field dependence of the critical current density

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Fig. 11. Scaling diagram of normalized pinning force density Fp/Fmax and reduced p magnetic field h near Tc for FeSe0.4Te0.6 single crystal. The Solid lines are the fitting curves. Inset: Temperature dependence of the parameter p.

Fig. 10. Field dependence of the critical current density calculated by Bean model at several temperatures. (a) FeSe0.5Te0.5, (b) FeSe0.4Te0.6 and (c) FeSe0.3Te0.7 single crystals.

displays an obvious second-peak (or “fishtail”) effect for FeSe0.4Te0.6, a weak second-peak effect for FeSe0.5Te0.5, and no secondpeak effect for FeSe0.3Te0.7. The difference of Jc ~ H relation further manifests that there is a strong flux pinning in x ¼ 0.5 and 0.6 single crystals, and a weak flux pinning in x ¼ 0.7 single crystal. The second-peak effect was found in cuprate superconductors [26,27], iron-pnictide superconductors [15,28,29] and iron-chalcogenide [30,31]. Wei et al. [26] suggested that a change of the elastic properties of the vortex lattice is likely to induce the second-peak effect, the phonon-like harmonic fluctuation of vortex lattice plays an important role in the second-peak effect as well. Pramanik et al. [29] argued that the second-peak effect is likely to result from the vortex lattice structural transition from hexagonal system to square system. According to the Dew-Hughes model [32], there is a scaling rule for the relationship of the flux pinning force and the applied magnetic field, namely

Fp ¼ Ahp ð1  hÞq

Fp/Fmax and reduced magnetic field h. The solid lines are fitting p curves with different p and same q ¼ 3.15. From Fig. 11, we can see that the Fp-H curves at 11, 10 and 9 K are very closed together. The temperature dependence of p is given in the inset of Fig, 11. Compared p, q parameters and peak of pinning force density with the Dew-Hughes model, we think that the flux pinning centers are dominant by the normal surface pinning. In order to understand the effect of Te-doping on the superconducting characteristics of FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) sing crystals, we show typical parameters in Fig. 12. First, the anisotropy parameter g near Tc decreases a little from 2.8 to 2.3, indicating that the intrinsic pinning induced by layered structure in cuprate superconductors is not obvious in iron-based superconductors. So, the flux properties are worth studying. Second, the superconducting transition temperature Tc decreases with the increase of Te-doping, manifesting that there is optimum when the stoichiometric ratio of doped Se and Te is near 1:1. In the end, the critical current density Jc of approximate 1.44106 A/cm2 in FeSe0.5Te0.5 sample decreases with the doped of Te, which indicates the most effective lattice distortion in FeSe0.5Te0.5 and indirectly reflects that FeSe0.5Te0.5 is closest to optimum. The critical current density to flux motion becomes weak with the pinning center decreasing because Te doping makes up more nonsuperconducting regions, leading to the weakening of the flux pinning energy. 4. Conclusion FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals were prepared by the chemical reaction sealed in a small quartz tube under vacuum. The scaling result of angular dependence of resistance near Tc showed that there is almost no influence of Te doping on the

(7)

In the expression, p and q are two parameters related to the pinning mechanism. h is reduced magnetic field which is defined as H/Hirr. Hirr is the irreversible magnetic field corresponding to the critical current density Jc decreasing to a low value. Here we choose 20 A/cm2 as a criterion to determine Hirr for FeSe0.4Te0.6 sample. Fig. 11 shows the relationship of normalized pinning force density

Fig. 12. Effect of Te-doping on the anisotropic parameter g, the critical temperature Tc and the critical current density Jc for FeSe1-xTex (x ¼ 0.5, 0.6 and 0.7) single crystals.

R. Mao et al. / Journal of Alloys and Compounds 809 (2019) 151851

anisotropy of FeSe1-xTex single crystals. The results of transport and magnetization measurements displayed that the superconducting transition temperature Tc decreases with the increase of Te-doping for x ¼ 0.5, 0.6 and 0.7. Meanwhile, the flikux pinning energy obtained from the thermally activated flux flow model and the critical current density calculated by the Bean model decreased rapidly with the increase of Te-doping for x ¼ 0.5, 0.6 and 0.7. The field dependence of flux pinning energy displayed that the flux pinning is dominated by the single flux pinning below 1 T and the collective flux pinning above 1 T. The parameters scaled by the Dew-Hughes model implied that the flux pinning centers are dominant by the normal surface pinning. The Te-doping closest to optimum could enhance the flux pinning energy and the critical current density. Overdoped Te produced more nonsuperconducting region and rapidly reduced the flux pinning energy and the critical current density. Competing financial interests The authors declare no competing financial interests.

[11]

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