Non-centrosymmetric Superconductivity and Magnetism in the Presence of Broken Symmetries

Abstract

Non-centrosymmetric (NC) superconducting and magnetic compounds have been synthesized and investigated using magnetic, specific heat, and transport measurements, as well as by neutron scattering and quantum oscillations. The crystal structures of NC compounds are defined by the lack of an inversion center. In NC superconductors, a finite antisymmetric spin orbit coupling originating from broken inversion symmetry results in unconventional Cooper pairing. Instead of a single spin channel, the order parameter is a mixture of spin-singlet and spin-triplet states. For NC magnetic compounds, the antisymmetric and isotropic spin interactions compete, leading to a helical ground state. We have studied the NC superconductor Re6Zr through measurements of the magnetic, transport, and thermal properties in polycrystalline form as well as via electronic structure calculation. We observed a bulk superconducting transition at temperature Tc~6.7 K. From the magnetic, magneto-transport, and heat capacity measurements, we extracted the critical field and other superconducting parameters of Re6Zr. The upper critical field and thermal conductivity measurements indicate a relatively weak to moderate contribution from a triplet component to the order parameter, while the heat capacity and London penetration depth measurements favor a fully gapped superconductor. The results suggest a complex superconducting behavior with the possibility of point nodes in the superconducting order parameter. We have also studied NC superconductor BiPd via quantum oscillations in its magnetization in an effort to probe the possible topological states of this compound. The search for topological superconductors (TSCs) is now at the forefront of condensed matter physics. TSCs are characterized by a full superconducting gap in the bulk and robust topologically protected gapless edge or surface states, which are Andreev bound states composed of Majorana fermions. Due to the presence of a Dirac point at 0.7 eV below the Fermi level of the surface in BiPd, topological surface states have been observed. The bulk superconductivity appears to be complex with the possible realization of vortex core states, suggesting the order parameter is unconventional. We observed clear oscillations in the magnetization of BiPd using the de Haas-van Alphen (dHvA) effect. Several pieces of a complex multi-sheet Fermi surface have been identified. Among those, a 40-T pocket was found to be three dimensional and anisotropic in nature. From the temperature dependence of the amplitude of the oscillations, the cyclotron effective mass was found to be (0.18±0.01)me. Further analysis showed a non-trivial π-Berry phase associated with the 40-T pocket, which strongly supports the presence of