Non-centrosymmetric superconductors (NCS) and related compounds have been studied using magnetic, specific heat and transport measurements as well as by neutron scattering and muon spin relaxation/rotation (μSR). The crystal structures of NCS lack inversion symmetry and in the presence of a finite antisymmetric spin orbit coupling, the Cooper pairs are a mixture of spin-singlet and spin-triplet states. In particular, the cerium based NCS have been reported to display unconventional superconductivity. Two different approaches for studying NCS are used. Firstly, the ground states of materials in the CeTX3 (T = transition metal, X = Si or Ge) family have been studied. CeCoGe3 is an antiferromagnet at ambient pressures and becomes superconducting at p > 4.3 GPa and was studied using inelastic neutron scattering (INS), muon spin relaxation/rotation (μSR), neutron diffraction and magnetic susceptibility measurements. The crystal electric fields (CEF) were studied using INS and magnetic susceptibility and the CEF scheme was evaluated. From this a ground state magnetic moment of 1.01 μB/Ce along the c axis was predicted. However, a magnetic moment of 0.405 μB/Ce along the c axis was observed in single crystal neutron diffraction measurements, indicating a reduced magnetic moment due to hybridization between the cerium f-electrons and the conduction band. The INS response was compared to the isostructural CePdSi3, CePtSi3 and CeRuSi3. The former two order antiferromagnetically and the Kondo temperatures were evaluated from the quasielastic scattering. CeRuSi3 is non-magnetic and there is a broad peak in the magnetic scattering at 59 meV. Another approach is to study weakly correlated NCS to look for evidence of unconventional behaviour. In particular, systems where the spin-orbit coupling can be varied by the substitution of heavier atoms into non-centrosymmetric positions were considered. LaPdSi3 and LaPtSi3 are superconductors with Tc = 2.65 and 1.52 K respectively and crystallize in the same crystal structure as the CeTX3 compounds. Magnetization, specific heat and μSR measurements reveal that both compounds are weakly coupled, fully gapped s-wave superconductors but LaPdSi3 is a type-I material while LaPtSi3 is type-II with a Ginzburg-Landau parameter of 2.49. The superconducting properties of single crystals of Nb0.18Re0.82 have been investigated and are discussed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:632893 |
| Date | January 2014 |
| Creators | Smidman, M. |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/64119/ |
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