Michael completed his Ph.D in the summer of 2014 on the subject of non-centrosymmetric superconductors and related systems, under the supervision of Prof. Geetha Balakrishnan and Prof. Don Paul.

Michael now works as a Tenure Track Professor in the Centre for Correlated Matter at Zhejiang University, China.

Publications:

'Investigations of the superconducting states of noncentrosymmetric LaPdSi₃ and LaPtSi₃', M. Smidman, A. D. Hillier, D. T. Adroja, M. R. Lees, V. K. Anand, R. P. Singh, R. I. Smith, D. M. Paul and G. Balakrishnan, Physical Review B, 89, 094509 (2014).

'Neutron scattering and muon spin relaxation measurements of the noncentrosymmetric antiferromagnet CeCoGe₃', M. Smidman, D. T. Adroja, A. D. Hillier, L. C. Chapon, J. W. Taylor, V. K. Anand, R. P. Singh, M. R. Lees, E. A. Goremychkin, M. M. Koza, V. V. Krishnamurthy. D. M. Paul and G. Balakrishnan, Physical Review B, 88, 134416 (2013).

'Crystal growth of the non-centrosymmetric superconductor Nb_0.18Re_0.82', R. P. Singh, M. Smidman, M. R. Lees, D. M. Paul and G. Balakrishnan, Journal of Crystal Growth, 361, pp 129 – 131 (2012).

'Is CeCoSi₃ a superconductor?', M. Smidman, R. P. Singh, M. R. Lees, D. M. Paul, D. T. Adroja and G. Balakrishnan, Journal of Physics: Conference Series, 391, 012068 (2012).

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Thesis Title: Superconducting and magnetic properties of non-centrosymmetric systems - PDF (35.0 MB) (awarded a Physics Department Thesis Prize for 2015).

Abstract

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 μ_B/Ce along the c axis was predicted. However, a magnetic moment of 0.405 μ_B/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 T_c = 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 Nb_0.18Re_0.82 have been investigated and are discussed.

 

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