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Title: Micro-calorimetric studies of superconductivity in BaFe2(AS1-xPx)2 and YBa2Cu4O8
Author: Walmsley , Philip
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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Specific heat measurements are presented on the unconventional superconductors BaFe2(Asl-xP xb YBa2Cu408 and Lio.gM060 17 · The development of a novel highprecision AC micro-calorimeter is also presented. An inexpensive and simple to fabricate SiN membrane-based AC micro-calorimeter was developed and successfully calibrated to an accuracy of 1 % between 6 K and 200 K. An appropriate thermal model was developed and the device shown to be capable of resolving transitions in heat capacity with an absolute accuracy of 5%, with good resolution of transitions down to 10 pJ /K. The total heat capacity of a sample at any given temperature can be determined to within 10%. Specific heat measurements of the supel'conducting transition in BaFe2(Asl_xP xh show that the quantum critical fluctuations renormalising the electron effective mass are robust to both temperature and magnetic field and that inter-band scattering is dominant giving equal renormalisation across the Fermi surface. The upper critical field has been shown to contradict theoretical predictions through a lack of enhancement around the quantum critical point, possibly due to mixing of antiferromagnetism in the vortex cores with the superconducting state. With reference to lower critical field measurements, the condensation energy is used to infer a strongly renormalised vortex core energy that indicates a direct enhancement of the superconducting state due to quantum criticality. The heat capacity of YBa2Cu408 was studied with magnetic fields applied along each of the orthogonal crystallographic directions. An enhancement in the density of states along the chain direction of 0.52 mJ mol- 1 K-2 was found in relatively low fields «1 T) consistent with the quenching of the superfluid density in the unhybridised region of the chain Fermi surface. Two values of the electron effective mass are deduced, mi = 3.9 ± OAme and m2 = 3 ± 0.3me, in excellent agreement with quantum oscillations. A large increase in the electron effective mass is seen on approach to the pseudogap end point up to a value of m* ~ 9.9me by analysing published data in a similar fashion. The condensation energy of Lio.gM06017 was determined and with reference to magnetoresistance data shows a breaching of any reasonable Pauli-limiting field thereby invoking an unconventional superconducting state. The superconducting critical temperature, the normal state specific heat, and the size of the superconducting transition are found to loosely correlate between different samples. Hall effect measurements on the same samples reveal that the strong upturn seen in the Hall effect cannot be a result of carrier loss thus refuting a density wave gap. A long-localisation scenario is proposed as an alternative explanation. This study also suggests that the sample dependence in Lio.gM06017 is a doping effect at all temperatures, likely due to the removal of electrons by excess oxygen. The Hall coefficient is observed to change sign at low temperatures and moderate fields.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available