The nematic phase transition in Fe-based superconductors (FeSCs) has been a topic under intensive investigation. So far it is commonly accepted that the structural transition from tetragonal (C4) to orthorhombic (C2) symmetry in FeSCs has an electronic nematic origin due to the unusual anisotropy in resistivity, optical conductivity and orbital occupancy observed above the structural transition. However, recent studies of (Ba, Eu)Fe2(As1-xPx)2 by magnetic torque measurements show the existence of a “true” nematic transition well above the commonly accepted structural/nematic transition .Controversies about this “true” nematic phase transition arise as residue strains or external applied fields are known to break C4 symmetry and render the structural transition merely a crossover. We performed high resolution AC micro-calorimetry and SQUID magnetometry measurements of BaFe2(As1-xPx)2 (x=0, 0.3) to investigate the various phase transitions and to explore the “true” nematic phase transition. The advantageous design of our membrane calorimeter allows us to perform high resolution studies of the thermodynamic phase transitions without any symmetry breaking fields. Our results suggest that there is not a second order “true” nematic phase transition in BaFe2(As1-xPx)2 even though the Ginzburg-Landau model used to fit the magnetic torque data indicates that the expected thermal anomaly should be within our experimental resolution. In addition to the above, we present specific heat and magnetization studies of Ba1-xNaxFe2As2 in search of the recent discovered emergent reentrant C2 to C4 symmetry SDW transition in this series of compound. Our results indeed locate a new phase transition in Ba0.74Na0.26Fe2As2 at 45K. However, the absence of the conventional SDW transition at around 80K in our data leaves doubt about the exact nature of this new phase transition. We also systematically studied the effects of heavy ion irradiation (HII) on the anisotropy of YBCO single crystals by angular rotation specific heat measurements. We found that the anisotropy of YBCO decreases by approximately a factor of two with an irradiation dose of 6T (matching field). The dependence of anisotropy on irradiation doses agrees well with the prediction from a simple phenomenological model that takes into account the anisotropic scattering caused by columnar defects created in HII.
Luo, Xu,
"Thermodynamic Studies of Phase Transitions and Emerging Orders in Unconventional Superconductors"
(2015).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z09W0CVK
