Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Physics and Astronomy

First Advisor

Paul C. Canfield

Second Advisor

Sergey L. Bud'ko


BaFe2As2 single crystal undergoes strongly coupled tetragonal to orthorhombic / paramagnetic to antiferromagnetic phase transitions at 134 K and can become superconducting under doping. To study the correlations between the structural / magnetic phase transitions and the superconductivity, seven transition metal electron

doped series : Ba(Fe1-xTMx)2As2 (TM=Co, Ni, Cu, Co / Cu, Rh and Pd) were grown and characterized by microscopic, transport and thermodynamic measurements.

It was found that the structural and magnetic phase transitions (upper transitions) at 134 K in pure BaFe2As2 are monotonically suppressed and increasingly separated in these doped series; in addition, superconductivity can be stabilized with a superconducting temperature rising from 0 K to a maximum and then being suppressed back to 0 K again with doping. The analysis of the temperature-doping concentration (T-x) and temperature-extra electrons (T-e) phase diagrams for all these series shows whereas the upper phase transitions are suppressed in a grossly similar manner with increasing doping concentration for all these dopants, the superconducting domes act quite differently, scaling with e: the number of extra electrons added by the doping, at least on the overdoped side of the superconducting dome. Further examination of the structural, magnetic phase transition temperatures (Ts and Tm) and the superconducting temperature,Tc, in the underdoped regime for these series reveals an essentially linear relation between Ts / Tmand Tc, which indicates a clear correlation between the suppression of structural / magnetic phase transitions and the stabilization of superconductivity. These observations lead to the understanding that the suppression of the upper (structural / magnetic ) phase transitions is a necessary but not a sufficient condition for the occurrence of low temperature superconductivity. There exists a superconducting window (a limited range of the number of extra electrons added), inside which superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough.


Copyright Owner

Ni Ni



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165 pages

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