Investigations into unconventional superconductors through the use of small angle neutron scattering

Cameron, Alistair (2014). Investigations into unconventional superconductors through the use of small angle neutron scattering. University of Birmingham. Ph.D.

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Abstract

Here we present an investigation into the vortex lattice of KFe\(_2\)As\(_2\) (KFA), BaFe\(_2\) (As\(_1\)\(_-\)\(_x\)P\(_x\))\(_2\) (BFAP) an YBa\(_2\)Cu\(_3\)O\(_7\) (YBCO) by small angle neutron scattering (SANS). KFA and BFAP are members of the pnictide group of superconductors which, since the discovery of an iron-based superconductor in 2006 (Kamihara et al., 2006), have become perhaps the most investigated class of superconductor in recent years. As a relatively new class of superconductor there is a great deal of interest in clarifying the fundamental mechanism in these materials which leads to superconductivity. Here, through the study of the magnetic vortex lattice, we search for information about the pairing state of two members of the 122 group of the pnictide superconductors, finding strong evidence for a nodal gap structure in both materials. In contrast, YBCO is a member of the cuprate family of superconductors, discovered in 1987 (Wu et al., 1987) shortly after the discovery of high temperature superconductivity a year earlier (Bednorz and Muller, 1986). The cuprate superconductors, so named for the copper-oxide layers which form their common building block, have been under continuous investigation since their discovery, and here we present the first microscopic study of vortex matter in fields of up to 16.7 T. We observe the continuation of a field dependent vortex lattice structure which was observed in measurements at lower fields (White et al., 2009), and strong indications of a static Debye-Waller effect arising from disorder in the vortex lattice. At high field, vortex lattice melting is observed at a temperature and field which corresponds to a 1st order melting transition observed in heat capacity measurements (Roulin et al., 1998), and above the melting transition no SANS signal from the vortex liquid is observed on the time-scale of our measurements.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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