Keywords

Superconductivity, Multi-band, Resistivity, Specific heat.

Abstract

Topologically protected quantum computation is getting great interest due to its potential use in creating fault-tolerant quantum computers. Using quasiparticle excitations known as non-Abelian anyons, which obey non-Abelian braiding statistics, is the way to perform topological quantum computation. A topological superconductor has protected gapless states on its boundaries while maintaining a superconducting gap in the bulk, and predicted to host Majorana fermions which are non-Abelian anyons. Although several theoretical models propose systems with non-Abelian anyons, px + ipy wave superconductors in which vortices exhibit non-Abelian braiding statistics is one of the most realistic physical systems. Realizing physical systems with non-Abelian anyons is one of the frontiers in recent theoretical and experimental research works. Therefore, in this thesis, we studied predicted superconducting topological nodal-line semimetals to realize possible topological superconductivity, flat energy bands and the effect of spin orbit coupling, in the normal and superconducting states.

To investigate the topological nature and superconductivity, we synthesized Sn0.15NbSe1.75 single crystals using the self-flux method and studied the normal and superconducting state properties of Sn0.15NbSe1.75 (Tc = 9.5 K) using soft-point-contact spectroscopy. We observe asymmetric double peaks in the normal state differential conductance dI/dV due to the Fano resonance as a result of quantum interference between two distinct tunneling paths of transmitting electrons into flat energy bands and dispersive bands. Hybridization between these bands below the hybridization temperature Thyb = 23 K is realized according to a phenomenological double Fano resonance model. A pseudogap below a characteristic temperature TPG = 6.8 K is also observed due to the hybridization between these bands. We observe an unusual linear in T behavior in the upper critical field from 0.4Tc to 0.01Tc, indicating a possible exotic superconducting state. Our results indicate the presence of surface flat energy bands and hybridization between the surface flat bands and the bulk bands in Sn0.15NbSe1.75.

To clarify the effect of spin orbit coupling on superconductivity, we studied the normal and superconducting state properties of Pb1-xSnxTaSe2 (0 ≤ x ≤ 0.23) single crystal synthesized by chemical vapor transport technique. Substituting Pb with Sn enhances the superconducting transition temperature Tc up to 5.1 K and also significantly increasing impurity scattering in Pb1-xSnxTaSe2. The normalized specific heat jump at Tc for x = 0 and 0.018, exceeds the Bardeen-Cooper-Schrieffer (BCS) predicted value of 1.43 for the weak-coupling superconductor. This observation indicates the possible strong-coupling superconductivity in undoped and slightly Sn-doped PbTaSe2. A single-gap model cannot explain the observed specific heat jump at Tc that is smaller than the BCS value of 1.43 for moderately Sn-doped samples. A two-gap model excellently reproduces the observed specific heat data of moderately Sn-doped (x = 0.08 and 0.15) PbTaSe2. Our observations suggest that the multiband effect increases the effective electron-phonon coupling strength, giving rise to an enhancement in Tc of Pb1-xSnxTaSe2.

Completion Date

2025

Semester

Spring

Committee Chair

Nakajima, Yasuyuki

Identifier

DP0029333

Document Type

Dissertation/Thesis

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