Physical properties of iron-based superconductors probed by low-temperature specific-heat measurements

In this thesis, specific heat, magnetic susceptibility and resistivity studies on the iron-pnictide superconductors LiFeAs, NaFe1−xCoxAs, AFe2As2 (A = K, Ca, Ba), M1−xNaxFe2As2 (M = Ca, Ba), and Ca(Fe1−xCox)2Fe2As2 are presented, from which different intrinsic physical properties are resolved. The combined first-order spindensity wave/structural transition which occurs in the parent compounds of the 122 pnictide systems is shown to gradually shift to lower temperature for low doping levels. Upon higher doping, this transition is completely suppressed and simultaneously, superconductivity appears at lower temperature. In contrast, the phase diagram in Ca(Fe1−xCox)2Fe2As2 (0 _ x _ 0.20) is shown to exhibit a pronounced region of coexistence of magnetism and superconductivity. Further important results reported in this work concern the electronic properties and superconducting-gap characteristics. In LiFeAs, the zero-field temperature dependence of the electronic specific heat can be well described by two s-wave gaps, whose magnitudes are in agreement with ARPES results. Our gap analysis in KFe2As2, Ca0.32Na0.68Fe2As2, and Ba0.65Na0.35Fe2As2 single crystals also implies the presence of two s-wave-like gaps. The magnetic phase diagram of LiFeAs and KFe2As2 for magnetic fields along both principal orientations has been constructed and an anisotropy of Hc2(T) of 3 and 5, respectively, has been obtained.