Boosting Superconducting Properties of Fe(Se, Te) via Dual

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Surfaces, Interfaces, and Applications

Boosting Superconducting Properties of Fe(Se, Te) via Dual Oscillation Phenomena Induced by Fluorine Doping Jixing Liu, Sheng-Nan Zhang, Meng Li, Lina Sang, Zhi Li, Zhenxiang Cheng, Weiyao Zhao, Jianqing Feng, Cheng-Shan Li, Pingxiang Zhang, Shi Xue Dou, Xiaolin Wang, and Lian Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02469 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Applied Materials & Interfaces

Boosting Superconducting Properties of Fe(Se, Te) via Dual Oscillation Phenomena Induced by Fluorine Doping Jixing Liu1, 2, 3, +, Shengnan Zhang2, +, Meng Li3, Lina Sang3, Zhi Li3, Zhenxiang Cheng3, Weiyao Zhao3, Jianqing Feng2, Chengshan Li1, 2, *, Pingxiang Zhang1 ,2, Shixue Dou3, Xiaolin Wang3, **and

Lian Zhou1, 2

1 School of Material Science and Engineering, Northeastern University, Shenyang, 110819, China; 2 Superconducting Materials Research Center, Northwest Institute for Nonferrous Metal Research, Xi’an, 710016, China; 3 Institute for Superconducting and Electronic Materials, Faculty of Engineering, Australian Institute for Innovative Materials, University of Wollongong, NSW 2500, Australia; * Corresponding author: [email protected]; ** Corresponding author: [email protected]; + These authors contributed equally to this work. KEYWORDS.

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iron-based superconductor; Fe(Se, Te); chemical doping; melting process; interface effect; flux pinning.

ABSTRACT. Fluorine doped Fe(Se, Te) has been successfully synthesized using the melting method. A dual oscillation effect was found in F-doping sample, which combined both the microstructural oscillation and the chemical compositional oscillation. The microstructural oscillation could be attributed to alternate growth of tetragonal -Fe(Se, Te) and hexagonal -Fe(Se, Te) which formed a pearlite-like structure, and led to the enhancement of l flux pinning due to the alternating distributed non-superconducting -Fe(Se, Te) phase. The chemical compositional oscillations in -Fe(Se, Te) phase was owing to the inhomogeneously distributed Se and Te, which changes the pinning mechanism from surface pinning in undoped sample to  pinning in 5% F-doped one. As a result, the critical current, upper critical field and thermally-activated-flux-flow activation energy of FeSe0.45Te0.5F0.05 were enhanced by 7, 2 and 3 times, respectively. Our work revealed the physical insights of F-doping rendering high performance Fe(Se, Te) superconductors, and inspired a new approach to optimize superconductivities in iron-based superconductors through phase and element manipulations.

Introduction


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Superconducting materials have a great marketing prosperity and scientific meaning for solving the current energy crisis based on their application in many fields, such as energy storage, zeroresistance current transport and even controllable nuclear fusion generation1-3. Among the existed superconducting materials, iron-based superconductor (IBS) is one of promising candidates for next-generation practical superconducting applications due to its high critical temperature, high upper critical field and low anisotropy4-7. Recently, Fe(Se, Te) superconductors have drawn great attention for the merits of simplest lattice and non-toxic material among all IBSs8-10. For practical applications, the critical temperature (Tc), critical current density (Jc) and upper critical field (Hc2) are the most essential factors, since they represent the inferior limitation of working condition. It has been reported that the Tc of Fe(Se, Te) could be enhanced above 77 K by lowering the dimensionality11-12. Even in bulk systems, the Tc of ~15 K is suitable for the application under liquid helium environment13. In addition to the relatively high Tc, the Jc ~105 A/cm2 at 30 T in Fe(Se, Te) coated conductor has been achieved, which is desirable for ultrahigh magnetic field working conduction14. Meanwhile, the Hc2 of above 45 T has been reported, which offers a possibility for application of Fe(Se, Te) under the B >15 T at 4.2 K as a substitution for traditional low temperature superconductors, including NbTi and Nb3Sn15-16. Hence, it is imperative to explore facile synthesis of high-performance Fe(Se, Te) superconductor as well as understand relevant physical insights to meet the massive marketing requirement. Fabrication of wires or tapes with traditional powder in tube process should be a promising way to realize the industrial fabrication and application of Fe(Se, Te). However, the main restriction of



Fe(Se, Te) from application is that the intrinsic Jc of wires and tapes remain only about 103 A/cm2, which is lower than those well-studied superconductors (~105 A/cm2)17-20. The intergrain weaklinks caused by the low density of superconducting filaments, the excess Fe in Fe(Se, Te) lattice and poor flux pinning ability of Fe(Se, Te) polycrystalline are responsible for the inferior Jc21-24. In the past decade, tremendous efforts have been made and various promising tactics have been addressed to enhance the Jc in Fe(Se, Te), such as adopting high energy ball milling aided sintering or incorporating low melting point elements (Ag/Sn) to strengthen the interaction at grain, or applying post-annealing treatment to eliminate the interstitial Fe25-30. Moreover, high pressure treatment and irradiation can both trigger more pinning centers to enhance the pinning ability, but the procedures are usually expensive and complex31-33. Whereas, chemical doping is a facile and effective method to tune the density of state of IBS near the Fermi level and manipulate Jc34-36. Meanwhile, the introduced dopants acting as effective pinning centers could maintain the large Jc to high magnetic field37-38. In this study, the effect of F-dopes on the superconductivity, microstructure, and phase evolution in Fe(Se, Te) system have been explored for the first time. The superconductivities of undoped and F-doped samples are then systematically characterized, where the relationships between microstructures and superconductivities are highlighted and elucidated. In FeSe0.45Te0.5F0.05 bulks, dual oscillations, namely structural oscillation and chemical compositional oscillation have both been observed, which solve an inherent problem in traditional fabrication of Fe(Se, Te) superconductors, namely the inhomogeneous distribution of phase separation39-40, thus changes


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the flux pinning mechanism from surface pinning to Δκ pinning. As a result, enhanced critical current, upper critical field and thermally activated flux flow activation energy of FeSe0.45Te0.5F0.05 are all obtained. Experimental Methods High purity elemental Fe (99.99%), Se (99.999%), Te(99.999%) and FeF2 (99%) powders (from Alfa Aesar ) were weighed with the designed stoichiometric molar ratio of Fe/Se/Te/F = 1.00:0.50x:0.50:x (x=0, 0.01, and 0.05) in glove box under the atmosphere of Ar and ground for 30 min in agate mortar to achieve high uniformity. The powders after mixing were cold pressed into pellets of 10 mm in diameter and 2.0 mm in thickness under a uniaxial pressure of 10 MPa. Then the pellets were sealed in double-walled evacuated quartz tubes separately. Finally, the assemblies were heated up to 900 oC for 48 h, then cooled down to 500 oC with a dwell time of 3 h, and then slowly cooled down to room temperature. The crystalline phases were identified by X-ray diffractometer (XRD, Bruker D8 Advance) with Cu K 1.542 Å) radiation at room temperature. Polarized light optical microscope (PLOM, Leica DM6000), scanning electron microscope (SEM, JEOL-7500A), electron back-scattered diffraction (EBSD, FEI Helios G3) and scanning transmission electron microscope (STEM, JEOL ARM200F) were used to examine the morphology and phase distribution of sintered bulks. The temperature dependences of magnetization, resistivity, and Hall coefficient were measured by a physical property measurement system (PPMS, Quantum Design), separately.



Results and discussions

Figure 1 (a) XRD patterns of sintered bulks, (b) Hall coefficient RH vs. temperature, (c) Temperature dependence of magnetization and (d) Resistivity vs. temperature with different F doping content. Blue solid line represents undoped sample, red solid line represents 0.01F-doped sample and green solid line represents 0.05F-doped sample. The inset in panel (a) shows the enlarge (101) peak of -Fe(Se, Te) at about 28o. The inset in panel (c) is the table of Tc value in different F doping content sample.


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The XRD patterns of the obtained bulks with different F element content are plotted in Figure 1(a). It can be found that the major phase of all the three samples is tetragonal β-Fe(Se, Te). Meanwhile, hexagonal phase -Fe(Se, Te) is also visible in all these samples. According to the Fe-Se phase diagram, the 50 atom% of Fe could not only form the β-Fe(Se, Te) but lead to the formation of Fe(Se, Te) phase, which has also been verified by previous studies41-42. No impurity involving F doping was observed, suggesting a complete reaction of FeF2. By enlarging the (101) peak of βFe(Se, Te) phase as shown in the inset of Figure 1(a), it is noticed that the (101) peak shifts to high degree with increasing F content, indicating that the F could probably have incorporated into the β-Fe(Se, Te) unit cells. The lattice parameters, a and c, decrease from a=3.796 Å, c=6.062 Å in undoped sample to a=3.774 Å, c=5.970 Å in F = 0.05 sample, respectively. Considering the smaller ionic radius of F- comparing with that of Se2-, the lattice contraction phenomenon also verifies that F substitute into the lattice of β-Fe(Se, Te). More details of the lattice data are listed in Supporting information (SI) Table S1 and S2. Figure 1(b) shows the temperature dependence of the Hall coefficient, RH, for the Fe(Se, Te) with different F doping content samples. The RH values for all three samples are temperature independence above 100 K. When the temperature decreases below 100 K, obviously different behavior can be observed between undoped and F-doped samples. In undoped sample, RH gradually increases with decreasing temperature showing an upturn at low temperature. In the Fdoped samples, RH values keep its nearly temperature independence above 60 K, followed by a sudden decrease, and finally change sign from positive to negative below 30 K. This suggests that



the dominant conduction mechanism changes from hole conduction in the undoped sample to electron conduction in F-doped sample as a result of the substitution of F at Se sites, which introduces extra electron to the system. Combining the XRD and Hall results, it can be concluded that the F atoms have entered into the β-Fe(Se, Te) lattices, and related electrical properties have been modified. The temperature dependence of field-cooled (FC) and zero-field-cooled (ZFC) magnetization in 10 Oe between 5 and 20 K for all the samples are plotted in Figure 1(c). All the samples perform a similar Tc of 14.4 K. An inset table in Figure 1(c) shows the Tc values vs. different F doping content for each sample. The Tc (obtained using the criteria of 10% and 90% of the ZFC magnetization) value is about 1.3 K for 0.05F-doped sample, which is the smallest among all the FeSe0.5Te0.5 polycrystalline bulks and close to the one of the best quality single crystal43. The temperature dependent electrical resistivity is shown in Figure 1(d). Similar Tc (onset) value of ~14.7 K can be obtained for all these samples, which is consistent with the Tc values obtained by magnetization measurement. Combining the results of both M-T and -T curves, the doping of F into Fe(Se, Te) system has slightly influence on Tc, but it greatly decreases the ΔTc value, suggesting a improving quality of Fe(Se, Te) polycrystalline with F doping.


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Figure 2 PLOM images of (a) undoped and (b) 0.05F-doped sample; EBSD phase distribution maps of (c) undoped and (d) 0.05F-doped sample; (e-k) STEM-HAADF images of 0.05F-doped sample. The red dash lines in panel (a) and (b) represent the grain boundaries.


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In order to determine the microstructure evolution with F doping, we have conducted the PLOM, EBSD and STEM characterization for both undoped and F=0.05 doped samples, with the typical images being shown in Figure 2. The microstructures of undoped sample are observed with PLOM as shown in Figure 2(a) with different contrast areas, which is due to the phase segregation phenomenon during the melting process as reported in many previous reports44-45. Interestingly, a pearlite-like layer by layer structure with the average period of ~ 10 m is found in Figure 2(b) with F doping, suggesting that F doping can change the phase formation dynamic during melting process. In order to distinguish the different phases and its distribution, EBSD mapping are performed on Figure 3(c-d) and the details of EBSD characterization are shown in SI Figure S1 and S2. The existence of both -Fe(Se, Te) phase and -Fe(Se, Te) phase can be observed in all the samples. Most of the -Fe(Se, Te) are crystallized into small particles locating at the grain boundary of big -Fe(Se, Te) grains in the undoped sample. However, a layer-by-layer heterostructure of -Fe(Se, Te) and -Fe(Se, Te) is found in the 0.05F-doped bulk, corresponding to the pearlite-like structure in PLOM image in Figure 2(b). It is well known that the pearlite structure in steel can combine the plasticity of ferrite and the hardness of the Fe3C phase to achieve a better mechanical performance. Therefore, it can be expected that the pearlite-like structure in F-doped sample can combine the advantages of these two phases, which are the superconducting properties of -Fe(Se, Te) and the enhancement of the flux pinning due to the appearance of Fe(Se, Te). In order to look into more details of microstructures in the F doping sample, the STEM high-angle



annular dark-field (STEM-HAADF) images with energy-dispersive X-ray spectroscopy (EDXS) mapping of the -Fe(Se, Te) area in 0.05F-doped sample are shown in Figure 2(e-k). It is found that another heterostructure with much small period of ~20-500 nm is formed due to the different Se/Te ratio in superconducting -Fe(Se, Te) phase. At higher magnification shown in Figure 2(j) and (k), the layered structure with different Se/Te ratio and even smaller period can also be observed, which seems like a pseudo-fractal structure. It is expected that the Se/Te ratio oscillation can greatly influence the superconducting coherence length and penetration depth of intrinsic Fe(Se, Te) system, which may change the flux pinning mechanism. Therefore, it can be deduced that the F doping can significantly change the phase evolution dynamic and modify the final microstructure and chemical component in Fe(Se, Te) system. On one hand, F doping can tune the eutectoid reaction of -Fe(Se, Te) and -Fe(Se, Te) phases to form a pearlite-like oscillation structure. On the other hand, another chemical compositional oscillation due to the different ratio of Se/Te in -Fe(Se, Te) has also been induced with F doping and a pseudo-fractural structure with different periods has been formed.


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Figure 3 (a-c) Resistivity vs. temperature in different magnetic field and (d) Hc2 and Hirr vs. temperature for the samples with different F doping content. In the panel of (d), the solid symbol represents Hc2 and the half-solid symbol represents Hirr.

The influence of observed oscillations of both structure and chemical component on the superconducting properties, such as Jc, Hc2 and flux pinning force have been systematically studied. The temperature dependent electrical resistivity under various magnetic fields up to 8 T are shown in Figure 3(a), (b) and (c) for undoped, 0.01F-doped, and 0.05F-doped sample, respectively. The Tc gradually shifts towards the lower temperature as field increases with the rate of 0.06 K/T for 0.05F-doped, 0.09 K/T for 0.01F-doped, and slight faster 0.12 K/T for undoped sample, respectively. Hence, it can be concluded that the vortex-liquid state region is narrower when samples are doped by F element. The Hc2 at T=0 K (Hc2(0)) is determined by the conventional Werthamer-Helfand-Hohenberg (WHH) equation, i.e., Hc2(0) = -0.693Tc(dHc2/dT)T=Tc for all the criteria. The estimated Hc2(0) for 90% criteria of (T) (onset) is 68.0 T for undoped, 110.1 T for



0.01F-doped and 140.7 T for 0.05F-doped sample, respectively. In Figure 3(d), the solid symbols are the extrapolation to the Ginzburg-Landau equation Hc2(T) = Hc2(0)(1-t2/(1+t2)), where t = T/Tc is the reduced temperature. These upper critical field value for F-doped sample is much higher than that of the undoped one. The irreversible field Hirr is determined using 10% criteria of the normal state of (T) under various magnetic fields, and the Hirr(0) for undoped sample is about 14.3 T, while for 0.05F-doped sample is nearly 48.3 T. To further determine other superconducting parameters, the Ginzburg-Landau coherence length ξ(0) is calculated by taking the values of Hc2(0) as Hc2(0) = Φo/2πξ(0)2 where Φo = 2.0678 × 10−15 T·m2 is the flux quantum. The ξ(0) is estimated to be 22.0 Å for the undoped sample, 17.3 Å for 0.01F-doped and 15.3 Å for 0.05F-doped sample, respectively. The higher Hc2, Hirr and the smaller ξ(0) values with F doping suggest that F doping can tune the electrical properties to enhance the coupling of Cooper-Electron-Pairs via inducing dual oscillation.


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Figure 4 (a-c) Arrhenius plots of the undoped, 0.01F-doped, and 0.05F-doped samples at different fields, respectively; and (d) Field dependence of U0 for the samples with different F doping content.

For further description upon the (T)-H behavior of the F-doped samples, the thermally activated flux flow (TAFF) plots i.e., ln verses Temperature at various fields for undoped, 0.01F-doped, and 0.05F-doped samples are shown in Figure 4(a), (b) and (c), respectively. According to TAFF theory, ln verses Temperature graph in the TAFF region is described with Arrhenius relation that is given by equation ln(T, H) = ln0(H) − U0(H)/kBT, where ln0(H) is temperature dependent constant, U0(H) is called TAFF activation energy and kB is Boltzmann’s constant46. From the equation, it is clearly seen that in TAFF region, ln vs 1/T graph would be linearly fitted with magnetic fields. All the linearly fitted extrapolated lines are intercepted at a certain temperature, which nearly equals to the Tc coming around 15.0 K, 14.5 K, and 14.9 K for undoped, 0.01Fdoped, and 0.05F-doped samples, respectively. Figure 4(d) shows the magnetic field dependency of thermally activation energy for Fe(Se, Te) bulks with different F doping content. Magnetic field



dependence of thermally activation energy follows the power law i.e., U0(H) = K × H−, where K is a constant and  is field dependent constant. For undoped sample, field dependent constant α = 0.13 when H < 1 T and  = 0.55 for H > 1 T. While for 0.01F-doped sample,  = 0.08 for H < 1 T,  = 0.43 for 1 < H < 6 T, and  = 1.02 for H > 6 T. Besides, for 0.05F-doped sample,  = 0.11 when H < 1 T,  = 0.48 for 1 < H < 6 T, and α = 0.57 for H > 6 T, where the double or triple linearity can be explained by the transition from a single-vortex pinning to a collective-vortex pinning. The U0 values can be calculated as 1180, 2340 and 2993 K for the undoped, 0.01F-doped, and 0.05F-doped samples at zero field, respectively. It should be noted that the U0 value for the 0.05F-doped sample is ~3 times of the undoped sample, which is one of the better values than those in other reported FeSe0.5Te0.5 polycrystalline bulks and comparable to the values in single crystals37.


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Figure 5 Jc vs. field at (a) 5 K and (b) 9 K; (c) Plots of the normalized pinning force (f = Fp/Fpmax) vs. h = H/Hmax and (d) Normalized measured Jc vs. t = T/Tc at 0 T for the samples with different F doping content.

The critical current density Jc at different temperature of 5 K and 9 K are calculated from the magnetic hysteresis loops (provided in SI Figure S3) using Bean model and plotted in Figure 5(a-b). The values of Jc of all the samples are about 2×104 A/cm2 at 5 K in self-field, which is much larger than the reported values. Furthermore, the field dependence of Jc values becomes much weaker with the field even up to 8 T with increasing F content, which are enhanced from 2.3 times at 5 K to 6.9 times at 9 K under the field of 8 T. Therefore, the larger value of Jc and weaker dependence with field and temperature all suggest a stronger flux pining property with F doping. In order to lessen the uncertainty during the pinning analysis, the pinning force Fp(H) (Fp(H) = Jc(H) × H) data are rescaled by h*= H/Hmax (Hmax is the magnetic field when Fp reaches the maximum) as widely adopted47-48. The scaling of f*(h) data can be determined by the following



equations, 1/3f*(h)= h2(1-2h/3) (for  pinning), 4/9f*(h)= h(1-h/3)2 (for normal point pinning), 16/25f*(h)= h1/2(1-h/5)2 (for surface pinning). Figure 5(c) shows the scaled data for the samples with different F doping content. It is obviously observed that the experimental data follow the case of surface pinning well in undoped sample, which is consistent with previous reports48. It is suggested that the hexagonal -Fe(Se, Te) particles exhibit less effects comparing with the grain boundary of -Fe(Se, Te) on the flux pinning mechanism in the undoped sample. While, the dominant pinning mechanism changes with F doping. In 0.01F-doped sample, the experimental data can be fitted into point pining for low magnetic field, but  pinning at high field. Moreover, the pinning type is  pinning in 0.05F-doped sample at both low and high field. The reason for the change of pinning mechanism can be related to the microstructure evolution as analyzed above. Considering that the appearance of  pinning should be attributed to the change of GinsbergLandau factor  (), the formation of chemical compositional oscillation in -Fe(Se, Te) phase due to the inhomogeneously distributed Se and Te should be the main reason for the change of flux pinning mechanism and the enhancement of flux pinning properties. To gain further insight into the doping effect on the pining mechanism in F-doped samples, the experimental results have also been analyzed by collective pinning theory49. There are two predominant mechanism of core pinning, i.e. δl pinning, which comes from spatial variation in the charge carrier mean free path and δTc pinning due to randomly distributed spatial variation in Tc. According to the theoretical approach proposed by Griessen et al.49, Jc(t)/Jc(0)∝(1-t2)5/2(1+t2)-1/2 in the case of l pinning, while for Tc pinning, Jc(t)/Jc(0)∝(1-t2)7/6(1+t2)5/6, where t=T/Tc. Figure


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5(d) shows the comparison between the experimental Jc values at 0 T for the samples with different F doping content. For the undoped sample, the curve follows well with the l pinning, which is due to the appearance of hexagonal -Fe(Se, Te) phase as small particles at grain boundary. While, for both the 0.01F and 0.05F doped samples, the curves agree well with the l pinning at low field due to the structural oscillation of forming superconducting tetragonal layer by nonsuperconducting hexagonal layer, and change to the combination of l and Tc pinning at high field attributed to different Se/Te ratio areas in -Fe(Se, Te) with different superconducting properties. Conclusions Fe(Se, Te) superconducting bulks with different F doping content were fabricated based on melting process. The doping of fluorine into Fe(Se, Te) lattices has been confirmed by both XRD analysis and Hall coefficient measurement. Meanwhile, the F doping greatly varied the phase evolution dynamic in Fe(Se, Te) system, thus two different oscillation phenomena appeared. One is the structural oscillation, which is formed by the alternative growth of -Fe(Se, Te) and -Fe(Se, Te) to form a pearlite-like oscillation structure, thus enhanced the l pining mechanism. The other is the chemical compositional oscillation, which is due to the different chemical compositional distribution of Se and Te to form a pseudo-fractural structure with different periodic length in Fe(Se, Te) and lead to the enhancement of  pining and Tc pinning. All the superconducting properties changed with F doping obviously. Comparing with the undoped samples, the Jc, Hc2 and U0 are significantly enhanced by up to 7 times, 2 times and 3 times in the 0.05F-doped sample,



respectively, with little change in Tc. Therefore, it could be deduced that fluorine doping is a promising way to improve the superconducting properties of Fe(Se, Te) system. Supporting Information. Structural data obtained by refinement for (001) and (101) peaks of -Fe(Se, Te) phase with different F doping content; Structure data obtained by refinement for (102) peaks of -Fe(Se, Te) phase and phase content between tetragonal and hexagonal with different F doping content; EBSD Forescatter diodes (FSD) mixed images and Inverse pole figure (IPF) images with different orientation of undoped and 0.05F-doped sample; Magnetic hysteresis loops of different F doping content at 5 K and 9 K. ACKNOWLEDGMENT This research was supported by funding from the National ITER Program of China (No. 2015GB115001) and the International Cooperative Project in Shaanxi province (No. 2018kw-055). Xiaolin Wang acknowledges support from the Australian Research Council (ARC) through an ARC Discovery Project (DP130102956), ARC Professorial Future Fellowship project (FT130100778) and the Taishan Scholars Program of Shandong Province. Zhi Li and Zhenxiang Cheng acknowledge support from through an ARC Discovery Project (DP170104116). Jixing Liu is grateful to the China Scholarship Council (CSC) for providing his Ph. D scholarship. REFERENCES


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Boosting Superconducting Properties of Fe(Se, Te) via Dual

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