INVESTIGATIONS ON SYNTHESIS, STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF Fe(Te, Se) SUPERCONDUCTORS

Abstract

Since the discovery of superconductivity in Hg by H. K. Onnes in 1911, a huge progress has been made with the discovery of new conventional as well as unconventional superconducting materials with enhanced superconducting transition temperature (TC). Till 1986, the highest TC achieved was close to 23 K in Nb3Ge compound. In 1986, Bednorz and Muller discovered superconductivity in Sr/Ba doped La2CuO4 with TC close to 35 K. This was a remarkable discovery and triggered extensive research with the aim to find new superconductors with higher transition temperature. Thus, after this discovery, superconductivity was unpredictably found one after another in cuprate superconductors such as YBa2Cu3O7 (TC = 92 K), Bi2Sr2Ca2Cu3O10 (TC = 110 K), Tl2Ba2Ca2Cu3O10 (TC = 125 K) and HgBa2Ca2Cu3O8 (TC = 135 K). The cuprates superconductors have been studied extensively due to rich variety of many interesting phenomena such as d-wave pairing symmetry, stripes, pseudo gaps, and the exotic pairing mechanism. The discovery of Fe-based superconductors (IBS) with high transition temperature in 2008 built a foundation of new era of superconductivity after Cu age. In these compounds the highest TC ~ 56 K has been observed in 1111-type GdFeAsO1-xFx superconductor at ambient pressure. In these materials, the coexistence of superconductivity and magnetism provides the promising interesting physics and the wider variety of compounds with multiband electronic structure which offer the hope for understanding the mechanism behind superconductivity. The Fe-based superconductors have a layered structure consisting of FeCh (Ch=Se, Te) or FePn (Pn=As, P) plane which is similar to the CuO2 plane of Cu-based superconductors and this plane is considered essential for the occurrence of superconductivity in these materials. The stacking of another layer in between these FeCh/FePn layers is responsible for the change in the behaviour of superconductivity due to the modulation of crystal structure and anion height which leads to a diverse family of Fe-based superconductors (IBS). Among the IBS family, the As free 11-type FeSe superconductor has the simplest crystal structure with the stacking of the FeSe layers along c-direction without any intermediate layers. The simplest crystal structure of 11- compounds makes them the best candidate to study and to understand the mechanism behind the superconductivity in these high temperature unconventional superconductors. FeSe at the ambient pressure shows superconducting behaviour close to TC = 8.5 K. The change in the internal pressure with the doping of Te (50%) at the Se site leads to enhancement in TC up to ~15 K. In the single layered FeSe superconducting thin films, the TC above 100 K has also been achieved. iii Moreover, the superconductivity in these materials is highly influenced by the doping which indirectly leads to the change in the chemical pressure. The Fe(Te, Se) superconductors have attracted much attention due to their immensely high critical current density (JC) and upper critical field (HC2(0)). The JC ~ 1 MA/cm2 at self-field for thin films and JC ~1×105 A/cm2 at higher magnetic fields (~30 T) for bulk superconducting samples of composition FeTe0.5Se0.5 have been observed. The high upper critical fields HC2(0) > 100 T at 0 K in bulk Fe(Te,Se) superconducting samples have been achieved. The capability to carry high current at high magnetic fields makes these superconductors the perfect candidate for practical applications. Also, among the iron based superconductors, 11- type Fe(Te, Se) compounds are interesting to understand the vortex properties due to their simplest crystal structure and less toxic Se, however very few studies have been performed on the vortex phase of Fe(Te, Se). In the present thesis work single crystals, thin films and polycrystalline samples of Fe(Te,Se) superconductors have been synthesized with the aim to study structural and superconducting properties (TC, HC2(0), U0, vortex phase diagram etc.). The present thesis is divided mainly into seven chapters. In Chapter 1, the basic introduction, literature survey and the properties of Fe- based superconductors, especially 11-type Fe(Te, Se) superconductors, have been described. Chapter 2 describes the methodologies adopted for the synthesis of single crystalline, polycrystalline and thin films of Fe (Te, Se) superconductors. A brief description of the working principles and operation of the characterization techniques used in the thesis work have been presented in this chapter. The characterization techniques include X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Field Emission Scanning Electron Microscope (FESEM), Energy dispersive spectroscopy (EDX), Superconducting Quantum Interference Device (SQUID) and four probe method. Chapter 3 deals with the study of structural and superconducting properties of FeTe1-xSex single crystals. The single crystalline samples of nominal compositions FeTe0.88 and FeTe1-xSex (x=0.10, 0.20, 0.40 and 0.60) have been synthesized using the self-flux method to study the effect of Se content (x) on the structural and superconducting properties of the grown crystals. Enhancement in the superconducting properties, such as transition temperature (TC), upper critical field (HC2(0)), activation energy (U0) and vortex phase diagram, have been observed with the increasing Se content. The values of upper critical field (HC20) have been estimated using the Ginzburg Landau (GL) theory iv as well as by Werthamer-Helfand-Hohenberg (WHH) model and found highest value of HC2(0) for x=0.60 single crystalline sample. The values of coherence length (ξ(0)) have been estimated using the calculated HC2(0) values for all the superconducting samples. The activation energy of the flux flow has been estimated using the Arrhenius relation as well as by modified thermally activated flux flow (TAFF) model. The field dependence of U0 shows the transition from the single vortex pinning regime to the collective vortex pinning regime at magnetic field ~ 2T. Furthermore, the modified TAFF model has been used to study the two dimensional (2D) and three dimensional (3D) behaviour of vortex state for all superconducting samples. It has been observed that the single crystalline FeTe0.80Se0.20 and FeTe0.60Se0.40 show 3D behaviour while FeTe0.40Se0.60 crystal shows 2D behaviour. The vortex phase diagrams of all the superconducting samples have been obtained by analyzing the magnetotransport data. The vortex phase diagrams reveal the transitions from unpinned vortex liquid - pinned vortex liquid state and vortex liquid - vortex glass state. Below the critical temperature (T*), the critical exponent (s) is in good agreement with the ‘q’ values for the 2D/3D behaviour of vortex states of all the superconducting samples. Chapter 4 comprises the study of Fe variation effect on the superconducting properties of FexTe0.60Se0.40 single crystalline samples. The single crystals of nominal compositions FexTe0.60Se0.40 (x=0.970, 0.985, 1, 1.015 and 1.030) have been synthesized using the self-flux method to study the effect of Fe variation (x) on the structural and superconducting properties of the grown crystals. The modification in the superconducting properties, such as TC, HC2(0), U0 and the vortex phase diagram have been observed with the variation of Fe content in the samples. The maximum TC has been obtained for FeTe0.60Se0.40 single crystalline sample. The values of upper critical field (HC20) have been estimated using the Ginzburg Landau (GL) theory and Werthamer-Helfand-Hohenberg (WHH) model. The activation energy has been calculated using the conventional as well as the modified TAFF model. From both the analytical methods, the field dependent U0 values reveal the crossover from the single vortex pinning to the collective vortex pinning regime. Furthermore, the fitting of the activation energy reveals the presence of planer defects in the whole magnetic field regions obtained via both the analytical methods. From the modified TAFF model, the fitting parameter q reveals the 3D vortex behaviour for all the superconducting single crystalline samples. The vortex phase diagrams have been estimated using the magnetotransport data where the transition from the unpinned vortex liquid - pinned vortex liquid and the vortex liquid - vortex glass have been observed. From the fitting, the critical exponent (s) prominently shows 3D behaviour which is in good agreement with the ‘q’ values below T*, for all the synthesized superconducting samples. v Chapter 5 embodies the study of vortex glass - liquid transition in superconducting Fe(Te, Se) thin films of different thicknesses grown on different substrates using pulse laser deposition (PLD) technique. The polycrystalline target of composition Fe1.05Te0.50Se0.50 has been used to grow films on (100) oriented magnesium oxide (MgO), LaAlO3 (LAO), SrTiO3 (STO) and Yattria stabilized Zirconia (YSZ) substrates. The polycrystalline target material of Fe1.05Te0.50Se0.50 has been prepared using the standard solid state reaction route. The XRD results reveal the growth of (00l) oriented thin films on all the substrates. The onset of superconducting transition temperatures ( onset C T ) at 0 T magnetic field, are found highest for the thin films deposited on MgO substrate. The thickness of the grown thin films has been estimated using cross-sectional FESEM imaging. The upper critical field HC2(0) values have been calculated by Ginzburg Landau theory as well as by Werthamer-Helfand- Hohenberg (WHH) model and the corresponding coherence lengths have also been estimated. The thermally activated energy (TAE) has been obtained using conventional Arrhenius law as well as by modified TAFF theory. For both the models, the TAEs of vortices show a crossover at magnetic field ~ 2 T corresponding to the transition from the single vortex pinning regime to the collective vortex pinning regime. Based on the analysis of the field dependence of TAE, the planer/point defect dominating magnetic field regions have been identified. The analysis of the magnetotransport data with the modified TAFF model reveals the 3D/2D behaviour of vortices for all used substrates in both the thickness based thin films. Moreover, in the vortex phase diagram a narrow vortex melting region and the vortex glass - liquid phase transition below the upper critical field line have been observed. Chapter 6 contains the study of effect of Co doping on the superconducting and structural properties of iron chalcogenides Fe (Te, Se). The polycrystalline samples of compositions Fe1.01- xCoxTe0.52Se0.48 (x = 0, 0.03, 0.05 and 0.06) have been synthesized by solid state reaction route. The phase identification has been done with the help of XRD results. XPS has been done at the surface of the samples to study the electronic structure of the material. The temperature dependent resistivity measurements show that the superconducting transition temperature (TC) decreases with the increasing doping concentration of Co. From the magnetoresistance measurements, the superconducting parameters like upper critical field and corresponding coherence lengths have been calculated and it has been observed that values of HC2(0) decrease with the increasing content of Co. The highest upper critical field values have been found in the undoped sample. Using the Ginzburg Landau theory and Werthamer-Helfand-Hohenberg (WHH) model, the estimated values of HC2(0) vi have been obtained and are found higher as compared to the previously reported values of HC2(0) for polycrystalline iron chalcogenide superconductors. From the broadening of superconducting transitions, the values of activation energy (U0) of the thermally activated vortices at different magnetic fields are calculated using the conventional Arrhenius law. It has been observed that U0(H) shows a weak field dependence in low fields (< 2 T), while for higher fields (> 2T) it shows strong field dependence, suggesting the crossover from the single to collective vortex pinning as magnetic field exceed 2T. Chapter 7 describes the summary of entire work reported in the chapters 3-6. The future prospects of the thesis work have also been included in this chapter.