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Title: Penetration depth studies of cuprate and iron-based superconductors
Author: Serafin, Alessandro
ISNI:       0000 0004 2718 3070
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2011
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Using a high resolution susceptometer based on a self-resonant tunnel diode circuit, the temperature dependence of the London penetration depth λ(T) was measured in the cuprate superconductor YBa2Cu4O8 and in two iron-based superconductor materials (LaFePO,FeSe0.5 Te0.5). The first project investigated the role of CuO chains in the underdoped high- Tc cuprate YBa2CU4O8 in relation to the anisotropy observed in the in-plane components of the superfluid density [1][2]. The components of λ(T) relative to the three crystallographic axises were measured. An in-plane anisotropic response was revealed by a clear enhancement of the superfluid density along the chain direction (b-axis) at low temperature. A small DC magnetic field applied perpendicular to the CuO chain revealed a strong suppression of this extra screening current density. In terms of coupling between plane and chain bands, the result can be interpreted within a proximity-model with superconductivity induced in the intrinsically normal chains by the superconducting planes through the "proximity-effect' [3] [4] In the second project we studied the symmetry of the order parameter in iron- based superconductors. The linear T dependence of λ observed in LaFePO strongly suggested the presence of nodes in the gap function. The linearity evolved into a T2 dependence upon isovalent substitution of Y atoms on the La site supporting the conclusion of the existence of nodes where the gap changes sign. In contrast λ(T) in FeSe0.5Te0.5 followed a higher power-law T" with n=2.2±O.1. Such behaviour can be taken as evidence for strong pair breaking within a sign-changing pairing state either with or, more likely according to other experimental results (thermal conductivity in particular), without nodes. Heat-capacity measurements were also performed on this material revealing a field dependence close to the transition which resembles the high- Tc cuprates behaviour with evidences of strong thermal fluctuations. Interestingly such fluctuations appear to have unusual anisotropic effects in FeSe0.5Te0.5.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available