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Title:Pairing symmetry, competing orders, and quantum criticality in iron based superconductors via London penetration depth measurements
Author(s):Salovich, Nicholai
Director of Research:Giannetta, Russell W.
Doctoral Committee Chair(s):Chiang, Tai-Chang
Doctoral Committee Member(s):Giannetta, Russell W.; Stack, John D.; DeMarco, Brian L.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
London Penetration Depth
Iron based superconductors
pairing symmetry
quantum criticality
Abstract:The temperature dependence of the magnetic penetration depth (λ) has been measured in single crystals of BaFe2As2 that have been driven into superconductivity by several different kinds of dopants, specifically potassium, cobalt, and phosphorous. In (Ba{0.6}K{0.4})Fe2As2 the low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap Δ{min}/k^B T^C≈ 1. At very high levels of heavy ion irradiation (a column-column separation of 10 nm) a T^2 power law was observed below T^C/3, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. This is evidence for an s{+−} pairing state. In Ba(Fe{1-x}Co^x)^2As^2 an aluminum coating procedure was employed to extract the zero-temperature value of the in-plane penetration depth λ{ab}(0) as a function of the cobalt concentration x, as it was varied through both the underdoped and overdoped regions of the phase diagram. A pronounced increase in λ{ab}(0) was found as the doping value was decreased below the optimal level. This is evidence for direct competition between the itinerant antiferromagnetic phase and superconductivity that region of the phase diagram. In BaFe^2(As{1-x}P^x)^2 the same aluminum coating procedure was employed to measure λ{ab}(0) as a function of phosphorous doping. A sharp peak in the penetration depth was found at optimal doping, where the superconducting transition temperature reaches a maximum. This may arise from quantum fluctuations associated with a quantum critical point buried beneath the superconducting dome.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Nicholai William Salovich
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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