Pairing symmetry, competing orders, and quantum criticality in iron based superconductors via London penetration depth measurements

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Salovich, Nicholai

Contributors dc:contributor

• Giannetta, Russell W.
• Chiang, Tai-Chang
• Stack, John D.
• DeMarco, Brian L.

• Superconductivity
• London Penetration Depth
• Iron based superconductors
• pairing symmetry
• quantum criticality

Statement dc:rights

• Copyright 2013 Nicholai William Salovich

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/46664

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/46664