STUDY OF SUPERCONDUCTING PROPERTIES OF MgB2 AND Fe-CHALCOGENIDE SUPERCONDUCTORS

Abstract

Ever since the discovery of superconductivity in Hg by Kamerlingh Onnes in 1911, it has been on the frontier of innovation in science and technology. Till 1970s the highest achievable superconducting transition temperature (Tc) remained at 23 K (in Nb3Ge), and some theorists proclaimed that higher Tc was impossible to attain. This empirical claim was shattered with the discovery of high-Tc superconductivity in copper oxide-based ceramic superconductors (current record is ~160 K under pressure) giving new horizons in Tc and also the richness of interesting phenomena like d-wave pairing symmetry, pseudogaps, stripes, and exotic pairing mechanisms. However, these complex materials required complex processing techniques to achieve industrial production of superconducting wires and devices. The practical application of both these types of superconductors requires cumbersome cooling systems that are expensive and impractical for many applications. Physicists have been working meticulously to achieve superconductivity at feasible and technically relevant temperatures. Another problem is that the secret behind what causes the high-Tc superconductivity is yet to be unlocked. Once the nature of this phenomenon is laid bare, then full attention towards finding ways to raise the critical temperature of superconductors can be focused. Since the discovery of superconductivity in MgB2 in 2001, it has been the focus of great activity. It possesses a combination of many favorable characteristics, like chemical simplicity, high Tc among known binary superconductors, absence of weak links, low anisotropy, high critical current density (Jc) (approaching 106A/cm2 at low fields and temperatures) and high upper critical field (Hc2) (the values exceeding that of existing industrial standards of Nb3Sn and Nb-Ti) making it quite promising for industrial applications. However it’s Tc is much lower than the record 160 K, but almost double the previous record in Nb3Ge. And this temperature can be achieved by either liquid hydrogen or by using fairly inexpensive closed-cycle refrigerators. Recent studies indicate that its superconducting properties can be enhanced by judicious addition of impurities. Chemical doping and inducing fine grain-size have proved useful to improve superconducting properties at high magnetic fields. Doping of numerous elements or compounds have been tried of which the most effective have been C, carbohydrates, SiC, B4C, rare-earth oxides, graphene etc. Among these, graphene is found to be most effective in improving high-field superconducting properties. More recently, a major breakthrough in high-Tc superconductivity research was observed with the discovery of superconductivity in Fe-based superconductors with the highest Tc reaching about 56 K in GdFeAsO1-xFx. These materials resemble cuprates in their structure and in the fact xi that part of their phase diagram shows strong magnetic behavior. Likewise cuprates, the superconductivity in them develops when magnetism is suppressed by doping. The Fe-based superconductors are layered compounds with stacking of FeAs/FeSe layers. These layers may be intercalated with various atoms or molecules to modulate the lattice constants or change the crystal structure, leading to diverse members of this family. This gives an opportunity to carry out a detailed study which may yield a clear picture of how they operate, and guide the way to a high-Tc material with desirable properties. Among the members of Fe-based superconductors, FeSe has got the simplest structure, with a single layer of FeSe which is considered essential for superconductivity in this family and it has less toxic Se in place of As. Thus, this material offers an important test ground to study superconductivity in Fe-based superconductors. FeSe goes superconducting at Tc = 9 K. Various substitutions have been done at the Fe and Se sites to observe the doping effects on superconductivity. Highest Tc (~ 15 K) has been found for 50% Te doping at the Se-site. It has been found that the stoichiometry has an important influence on superconductivity in this material. The thesis is divided into two main parts. In the first part (Chapters -3 and 4), we have focused on improving the superconducting properties (Jc, Hc2 and Hirr) of MgB2 without affecting the Tc adversely. For this, we have used graphene oxide, ferrocene, rare-earths and citric acid for addition. In the second part (Chapters- 5 and 6) of the thesis, we have presented a systematic study of doped and undoped polycrystalline and single crystalline FeTe/Se samples. The thesis comprises of seven chapters. Chapter-1 contains the basic introduction and survey of the field, describing crystal structures and superconducting properties of MgB2 and Fe-based superconductors. In chapter-2, various experimental techniques used to characterize the structure and superconducting properties are described. The techniques include X-ray diffraction (XRD) for checking phase purity and identification; Fourier transform infra-red (FTIR) spectroscopy used to identify and analyze chemicals; X-ray photoelectron spectroscopy (XPS) to analyze elemental composition and its chemical state; field emission scanning electron microscopy (FE-SEM) in secondary electron imaging mode to study the surface morphology and energy dispersive X-ray (EDX) spectroscopy for chemical analysis. Basic working principles of vibrating sample magnetometer (VSM), superconducting quantum interference device (SQUID) used to study the magnetization in the sample and four-probe method to measure the resistivity in the samples are also briefly described. Further, the sample preparation techniques – solid state reaction method and self-flux method to grow single crystalline samples are illustrated. xii The first section of chapter-3 describes the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB2. The polycrystalline samples have been synthesized via solid state reaction route with compositions MgB2 + x wt% of GO (x = 0, 1, 2, 3, 5, 7 and 10) by sintering at ~ 850°C in reducing atmosphere of Ar/H2 (9:1). The XRD results confirm the formation of the MgB2 phase in all samples together with trace of an MgO impurity phase. These also show substitution of carbon for boron, but in the present case the amount of actual C substituting for B is very small as compared to other C-sources. A substantial improvement in the Jc(H) has been observed in the entire magnetic field range (0 – 7 T) for samples x = 1, 2 and 3 as compared to the undoped sample. In addition to Jc(H), marginal improvements in the Hc2 and Hirr have been observed for the doped samples (x = 1, 2 and 3) with respect to pristine MgB2. Furthermore, an interesting result of the present investigation is that there is no significant change in Tc up to a doping level of 10 wt%. The possible mechanisms of flux pinning have been described and discussed. In the Second section of chapter-3, the effects of ferrocene (FeC10H10) addition on the superconducting properties of polycrystalline MgB2 using transport and magnetic measurements are reported. An addition of FeC10H10 up to 2 wt% has shown enhanced Jc in the entire magnetic field region without affecting much the Tc. At 10 K, with respect to a pristine MgB2 sample, Jc has improved by a factor of 6.55 in an applied field of 6 T for 1 wt% ferrocene. An improvement in Hc2 and Hirr has also been observed up to 2 wt% addition of ferrocene. The value of Hc2(0) as obtained using the Ginzburg-Landau (GL) - theory fit of the experimental data increases by almost 2 T for 2 wt% FeC10H10 added MgB2 as compared to the pristine samples. From XPS, we observed that Fe is present in sample in the form of ferromagnetic oxides, Fe3O4 and γ-Fe2O3. These ferromagnetic inclusions provide efficient pinning centres to improve Jc(H) behavior. The flux pinning mechanisms present in the ferrocene added samples are described and discussed. Chapter- 4 is again divided into two sections. In the first section, polycrystalline samples with compositions MgB2 + 3wt% GO + x wt% C6H8O7 (x = 0, 5 and 10) have been synthesized to study the effect of combined addition of graphene oxide (GO) and citric acid (C6H8O7) on superconducting properties of MgB2. XRD studies confirm the formation of hexagonal crystal structure of MgB2 with space group P6/mmm. We observe that the addition of GO in the sample improves the grain connectivity and consequently enhances field dependence of Jc significantly with no substantial change in Tc. However, there is only a marginal improvement in Hc2 and Hirr. With the combined addition of GO and citric acid, the Jc(H), Hc2 and Hirr improve substantially, with enhanced flux pinning as compared to the pristine MgB2 and GO added MgB2 samples. xiii Further, in the second section of chapter-4, the effects of addition of (i) GO, (ii) a series of rare-earth oxides, REO (RE = La, Sm, Eu, Gd, Tb and Ho) and (iii) a mixture of GO and REO on the superconducting properties of MgB2, have been studied. All the samples have been prepared following the standard solid-state reaction route. We have used an optimum value of 1 wt% REO and 3 wt% GO for addition on the basis of previous studies. XRD studies confirm the formation of hexagonal crystal structure of MgB2 with small amounts of REBx (x = 4 and 6) and MgO impurity phases in all the synthesized samples. We observe that Jc and Hc2 (0) improve significantly in the REO-added and GO-added samples with no significant change in Tc. A substantial enhancement in Jc(H) and Hc2(0) is observed with the GO + REO addition in MgB2. The different flux pinning mechanisms in all the samples are studied and described in the chapter. In chapter-5, we have synthesized and studied polycrystalline Fe-chalcogenide superconductors. This chapter is also divided into two sections. In the first section, we have studied the effect of Fe composition on the superconducting properties (Tc, Hc2 and Hirr) of FeSe1/2Te1/2. The polycrystalline samples have been prepared via solid state reaction route with nominal compositions FexSe1/2Te1/2 (x = 0.95, 1.00, 1.05 and 1.10). The XRD results show the presence of tetragonal α-FeSe phase with the P4/nmm space group symmetry in all the samples. The zero resistance temperatures, Tczero, measured in zero magnetic field, have been found to be 10.0, 12.4, 12.3 and 11.7 K for x = 0.95, 1.00, 1.05 and 1.10, respectively. The values of Hc2(0) estimated from Werthamer-Helfand-Hohenberg (WHH) theory are 121.3 T, 142.8 T, 82.7 T and 79.3 T for x = 0.95, 1.00, 1.05 and 1.10, respectively. Below Tc, at all temperatures both Hc2 and Hirr have higher values for sample x= 1.00 as compared to other samples. The possible reasons for the variation of superconducting properties with Fe composition (x) have been described and discussed in this chapter. In the second section, we have studied the effect of doping Sb and Si at the Se-site of FeSe0.9 on the superconducting properties, such as Tc, Hc2 and irreversibility field (Hirr). The polycrystalline samples have been synthesized via two step solid state reaction route with nominal compositions Fe[Se1-x(Sb/Si)x]0.9 (x = 0.0, 0.05, 0.10, 0.15 and 0.20). The XRD results show the presence of tetragonal α-FeSe phase in all the samples. The highest superconducting onset temperatures, Tconset ~ 9.42 K and 9.20 K, respectively, for Si and Sb doped samples have been found for x =0.05. The values of Hc2(0) estimated from WHH and GL- theories are found to follow the same trends and maximum Hc2(0) is found for the composition x = 0.10 for both the Si and Sb doped samples. The Hirr and activation energy, U0, have also been calculated to study the vortex motion behavior of the samples. A clear cut correlation between Hirr and Uo has been found. xiv In chapter-6, we have carried out a systematic study on single crystalline Se-substituted FeTe1-xSex ( x = 0, 0.15, 0.25, 0.35, 0.40 and 0.50) samples. All the single crystalline samples are grown using the self-flux method. The elemental composition and phase purity of the samples is analyzed using EDX and powder XRD techniques. The superconducting properties of all the samples are studied using resistivity measurements. We have observed highest Tc and Hc2 for the sample FeTe0.50Se0.50. Chapter-7 presents the summary of the work presented in the thesis through chapters 3 to 6. The overall comments and future prospects have also been included in this chapter. xv