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Title: Structural transition and phase separation studies for superconducting material designing
Tutor: Bianconi, Antonio
Keywords: superconductivity
heterogeneous materials
phase separation in complex materials
structural transition
Issue Date: 19-Dec-2011
Abstract: The recent discovery of high-temperature superconductivity (HTS) in the iron pnictides [1] has provided a new class of materials for the search of the physical mechanism that allows a quantum coherent condensate to avoid the effects of temperature decoherence. The discovery of HTS in $LaO_{1-x}FeAsF_{x}$ was soon followed by the discovery of a different class of superconducting iron-based compounds, among which the quaternary systems “1111” (REFeAsO, RE stands for rare earth) and ternary "122" $(AFe_{2}As_{2}$, A stands for Ba, Ca, Sr, etc.) systems are the most investigated. Superconductivity arises in the 1111 systems by the partial fluorine substitution for the oxygen, or by the creation oxygen defects. On the other hand, substitution of Fe by K, Ni, etc. is found to result in the superconducting transition in the 122 systems. Interestingly, in both cases, the bulk superconductivity appears by the suppression of the long-range magnetic ordering of the parent compounds. The parent compounds, which do not show superconductivity (SC), show a structural transition from the room temperature tetragonal to the low temperature orthorhombic structure. In the 1111 systems, the structural and magnetic transitions do not coincide, whereas the structural and magnetic transitions coincide in the 122 systems. The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO (La-1111) and SmFeAsO (Sm-1111) single crystals measured by high-resolution x-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm) polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single-crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature-dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the $AFe_{2}As_{2}$ (A=Ca, Sr, Ba) “122” systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 (55 K) versus 122 (35 K) systems and implies the surface reconstruction in 1111 single crystals. Superconductivity has been induced by oxygen defects in the fluorite spacers in iron-based oxypnictides $Sm[O_{1-x}]FeAs$, $Nd[O_{1-x}]FeAs$ and $Pr[O_{1-x}]FeAs$ with the maximum critical temperature being 43, 51.9 and 52 K, respectively. Oxygen defects induce electron doping of the Fe derived bands crossing the Fermi level and the electronic/magnetic structure is very sensitive to different atomic substitutions and rare-earth (RE) ions. The change of the RE atomic radius induces a change of the elastic misfit strain between the superconducting Fe layers and the intercalated layers, like in cuprates, and diborides. Superconductivity, with transition temperature, $T_{c}$, above 30 K, has been reported also in a new discovered family of Fe-based high temperature superconductors (HTS) $A_{x}Fe_{2-y}Se_{2}$. These systems are made of iron chalcogenide FeSe molecular layers, intercalated by A=K,Cs,Rb,Tl, (Tl,Rb), (Tl,K) spacer layers, providing the more recent practical realization of metal heterostructures at atomic limit, as cuprates and pnictides high temperature superconductors. These $A_{x}Fe_{2-y}Se_{2}$ chalcogenide superconductors show both high temperature superconductivity and magnetism. In these compounds, one may tune the interplay of superconductivity and magnetism by changing the Fe-vacancy order and the superlattice misfit strain. The open question answered in this thesis is whether there is a co-existence of magnetism and superconductivity in the same spatial region or these phenomena occur in different spatial regions dictated by the phase separation. In order to understand the complex behaviour of superconductors and to answer this key question, advanced synchrotron radiation focusing down to a size of 300 nm has been used to visualize nanoscale phase separation in the $K_{0.8}Fe_{1.6}Se_{2}$ superconducting system using scanning nano-focus single crystal X-ray diffraction. The results show an intrinsic phase separation in $K_{0.8}Fe_{1.6}Se_{2}$ single crystals at T$<$ 520 K, revealing coesistance of; i) a magnetic phase characterized by an expanded lattice with superstructures due to Fe vacancy ordering and; ii) a non-magnetic phase with an in-plane compressed lattice. The spatial distribution of the two phases at 300 K shows a frustrated or arrested nature of the phase separation. The space resolved imaging of the phase separation permitted us to provide a direct evidence of nano phase domains smaller than 300 nm and different micron size regions with percolating magnetic or non-magnetic domains forming a multiscale complex network of the two phases. The phase separation appears on multiple scales from micron-scale to nano-scale in cuprates, diborides and pnictides. Several theories have described the complex phase separation as an intrinsic feature of all known HTS. The multilayer architecture, a common feature of cuprates, diborides, pnictides and chalcogenides, underlines the relevance of lattice effects for high-temperature superconductivity. The structure of the iron pnictide superconductors is made of a superlattice of $[FeAs ]^{-Q+\delta}_{\infty}$ with Q = 1, layers intercalated by spacers (oxide layers like $[LnF_{y}O_{1-y}]^{+Q-\delta}_{\infty}$ or $[LnFO_{1-y}]^{+Q-\delta}_{\infty}$ in the “1111” family or metallic atomic layers $[ (A^{2+}_{1-x}B^{1+}_{x})_{1/2} ]^{+Q-\delta}_{\infty}$ in the “122” family, and therefore they represent practical realizations of a ‘heterostructure at the atomic limit’ (HsAL) that was described to be the essential material architecture for the emergence of HTS. All of them contain a characteristic layer of FeAs layers made of a tetrahedral network (with As atoms located at the apical sites of the tetrahedron) corresponding to the featured planar $CuO_{2}$ plane in cuprates and honey-comb 2D lattice of boron in diborides. In pnictides and chalcogenides, EXAFS and XANES measurements have shown the local lattice fluctuations like in cuprates. These results have recently been applied to design a new heterostructure at the atomic limit for functional materials. Infact, at the end of this thesis we propose a novel class of heterostructures made of alternate layers of pnictides and cuprates to produce an High-Temperature multiband Superconductor (HTMS) material which have additional microscopic degrees of freedom. These HTMS are multilayers involving alternate layer components from cuprate and iron-arsenide superconductor families. In particular, we provide a specific example of a composite system involving two basic components, $Nd_{2}CuO_{4}$ and NdOFeAs. The idea of such a HTMS is motivated by the striking similarities in the structural and electronic (as evidenced by the Nd $L_{3}$-XANES) properties of the $Nd_{2}CuO_{4}$ and NdOFeAs systems. The electronic properties and the XANES calculations for this novel HTMS are presented to highlight the importance of such a superstructure which offers further scope for the material manipulation and design of high temperature superconductors.
Research interests: Complex Materials, Superconductivity

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