Penetration depth studies of unconventional superconductors

• Main Author: Sujith, Kunniniyil Sudheesh
• Other Authors: Chia Ee Min, Elbert
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Physics::Electricity and magnetism
• Online Access: https://hdl.handle.net/10356/150687
• Institution: Nanyang Technological University

A tunnel diode oscillator based system is used to measure the London penetration depth of unconventional superconductors. With noise levels less than 4 parts in 10^9, high resolution measurements of penetration depth can be made. Sample temperatures as low as 350 mK can be achieved using our helium-3 cryostat based setup. Information about the superfluid density and the superconducting order parameter, including the symmetry of the superconducting gap can be obtained from this data. In this thesis, I present penetration depth data on single crystals of PdTe2, FeS, pure and doped CeCoIn5, and MoTe2 intercalated with ionic liquid. For the topological superconductor candidate PdTe2, we nd that the bulk is fully gapped with an isotropic s-wave like gap with moderate coupling. For FeS, our data indicates multi-gap superconductivity with the superfluid density preferring a s + d gap model. The heavy fermion superconductor CeCoIn5 exhibit power-law behaviour with exponent n = 1:5 at low temperatures, which is indicative of a d-wave gap with line nodes as expected. Doping with Sn or Cd suppresses superconductivity and the power law behaviour changes to n > 2. This, along with the superfluid density suggest an evolution of superconducting gap upon doping. Our measurements on ionic liquid intercalated MoTe2, which is a organic-inorganic hybrid superconductor, show power-law dependence hinting the presence of line nodes in the order parameter. A dirty d-wave model fits the data well at low temperatures.