Investigation of Energy Gap Structure of Strongly Correlated Superconductors Using Linear and Nonlinear Magnetic Penetration Depth Measurements

  • Joe Wilcox

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


In the field of unconventional superconductivity, identifying the pairing mechanism is one of the biggest challenges facing the condensed matter community. The magnetic penetration depth has been well established as a powerful method of probing the gap structure, which is closely related to the pairing mechanism. In this thesis, we present measurements of the magnetic penetration depth to very low temperature in three low-Tc unconventional compounds.
Historically, the prototypical heavy fermion compound and unconventional superconductor CeCu2Si2 was identified as a nodal d-wave superconductor. Recent measurements of the specific heat found a surprising absence of quasi-particle excitations that would be expected in such a pairing state. We present penetration depth measurements down to 50 mK that also indicate the absence of nodal quasiparticles, ruling out a nodal d-wave state in favour of a fully-gapped state and requiring a re-evaluation of heavy Fermion superconductivity in this compound.
In the iron-based superconductors, KFe2As2 is the end member of the Ba1-xKxFe2As2 series. Uniquely, KFe2As2 remains superconducting unlike other typical end members. Due to the complex evolution of the Fermi surface topology in this system, suggestions have been made for both an extended s+- state or a nodal d-wave state. Experimental evidence also suggests nodes in this compound, though disagreement surrounds the symmetry. We present penetration depth measurements down to 50 mK in samples in a pristine state and after intentionally exposing to air. This creates a change in the response at low temperature that is consistent with accidental nodes or deep minima, ruling out a nodal d-wave state. In addition, we present novel nonlinear measurements that are also consistent with a gap structure with deep minima.
One of the key predictions regarding the nodal d-wave superconducting state is the nonlinear Meissner effect. Despite extensive work in a number of cuprate superconductors, this effect has yet to be convincingly observed. We present novel nonlinear measurements in CeCoIn5, a superconductor with a well established dx2-y2 pairing state. The observed response is consistent with predictions from the Yip-Sauls theory, confirming for the first time the existence of this effect.
In addition, a novel technique for measuring the lower critical field under hydrostatic pressure is presented. This technique utilises an array of Hall bars to locally probe the magnetic induction in proximity to a superconducting sample, all located within the sample space of a piston cylinder cell. The technique is validated through measurements of Hc(T) in lead under pressure. We also present preliminary measurements in under-doped YBa2Cu3O7-x.
Date of Award7 May 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SponsorsEngineering and Physical Sciences Research Council
SupervisorAntony Carrington (Supervisor)

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