The thesis work presented here emphasizes important aspects of crystal growth and the influence of chemical substitution in Fe-As superconductors. High temperature solution growth technique is one of most powerful and widely used technique to grow single crystals of various materials. The biggest advantage of high temperature solution growth technique is the, possibility of growing single crystals from both congruently and incongruently melting materials. Solution growth technique has the potential to control high vapour pressures, given the fact that, in Fe-based superconductors elements with high vapour pressure like As, K, Li and Na have to be handled during the crystal growth procedure. In this scenario high temperature solution growth is the best suitable growth technique to synthesize sizable homogeneous single crystals.
Using self-flux high temperature solution growth technique, large centimeter-sized high quality single crystals of BaFe2As2 were grown. This pristine compound BaFe2As2 undergoes structural and magnetic transition at TS/N=137 K. By suppressing this magnetic transition and stabilizing tetragonal phase with chemical substitution, like Co-doping and Na-doping, bulk superconductivity is achieved. Superconducting transitions of as high as Tc = 34 K with Na substitution and Tc = 25 K with Co-doping were obtained. A combined electronic phase diagram has been achieved for both electron doping with Co and hole doping with Na in BaFe2As2.
Single crystals of LiFe1−xCoxAs with x = 0, 0.025, 0.05 and 0.075 were grown by a self-flux high temperature solution growth technique. The charge doping in LiFeAs is achieved with the Co-doping in Fe atoms. The superconducting properties investigated by means of temperature dependent magnetization and resistivity revealed that superconductivity is shifted to lower temperatures and with higher amount of charge carriers superconductivity is killed.
Single crystals of KFe2As2 were grown with two different fluxes, namely, FeAs-flux and KAs-flux. The superconducting transition is found to be at 3.8K in both the crystals. The influence of doping with selected elements like Na, Rh, Co and Cr has been investigated systematically in KFe2As2 single crystals. With Na-doping at the K-site, yield (K1−xNax)Fe2As2; superconductivity is suppressed to lower temperatures.
Substitution of Co and Cr at Fe site, yield K(Fe0.95Co0.05)2As2, K(Fe0.95Cr0.05)2As2 superconductivity is rapidly killed. Single crystals of (Ba0.6Eu0.4)(Fe1−xCox)2As2 with x = 0, 0.05, 0.1, 0.15 and 0.2 were grown with solution growth technique using Fe-As flux and investigated with several physical measurements.
The growth conditions are highly optimized to grow flux free large single crystals especially in case of BaFe2As2 family. The high quality of the crystals were revealed by several physical properties, for e.g. single crystals of Ba(Fe1−xCox)2As2 are of the highest quality which was confirmed by the magnetic ac susceptibility which showed a very sharp superconducting transition.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:26248 |
| Date | 08 November 2012 |
| Creators | Aswartham, Saicharan |
| Contributors | Büchner, Bernd, Felser, Claudia, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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