Electronic, structural, and magnetic properties of single crystalline pnictides: Thermodynamic studies down to ultra-low temperatures

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We will apply thermodynamic and magnetic studies down to 10 mK in order to investigate the nature of superconductivity and the interplay of magnetism, structure, and electronic properties in Fe-based superconductors. For investigating superconductivity, we will perform measurements of the specific heat at very low temperatures 10 mK – 4 K in magnetic fields up to 9T by means of an existing calorimeter and a newly designed magnetic thermometry-based calorimeter which will be realized. The specific heat studies will be complemented by magnetic susceptibility and heat transport measurements in the same temperature regime. Both in-plane and out-of-plane doped materials will be addressed which are delivered by our SPP partners, i.e. single-crystalline K1- xNaxFe2As2, LiFeAs, K(Fe,M)2As2 and Na(Fe,M)As with M = Rh, Ru, Co,Ni Cr, Mn. In addition, the interplay of electronic, magnetic, and structural degrees of freedom will be studied by thermal expansion experiments at 2 – 300K. Here, in addition to Na(Fe,M)As, LaFeAsO single crystals and polycrystalline doping series RFeAsO1-xFx will be studied, with a particular focus on the tetragonal phase and nematic magnetic short-range order.