Thermodynamic and Transport Properties of Iron-Pnictide Superconductors under hydrostatic (Hegas) pressure

• Antragsteller: Professor Dr. Michael Lang
• Fachliche Zuordnung: Experimentelle Physik der kondensierten Materie
• Förderung: Förderung von 2013 bis 2016
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 237626901

Among the various iron-arsenide-based superconductors, members of the AFe2As2 (A = Ba, Sr, Ca), the so-called 122 family, have become model systems for exploring superconductivity in this new class of high-Tc superconductors. This is partly related to the fact that for these systems large, high-quality, and homogeneous crystals can be grown. The (temperature vs chemical doping) phase diagrams of these materials have been explored extensively for unravelling the interplay of superconductivity with the nearby magnetic and structural orders – an aspect which has become a major theme of research in this class of materials.In this project we propose to study the interplay of superconductivity with the nearby magnetic and structural orders on selected members of the 122 family, especially Ca(Fe1- xCox)2As2, Ba(Fe1-xCox)2As2, Ca1-xSrxFe2As2, and Ba1-xKxFe2As2, via thermodynamic measurements under hydrostatic (He-gas) pressure. The work is motivated by our recent discovery, that for properly heat-treated crystals of Ca(Fe1-xCox)2As2 with x = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic, superconducting, and non-magnetic collapsed tetragonal states can be accessed all in one sample with reasonably small and truly hydrostatic pressure.Through the combination of various thermodynamic (magnetic susceptibility, highresolution thermal expansion, ultrasonic) and transport (resistivity) measurements, all under truly hydrostatic (He-gas) pressure, we will address among others the following aspects: (i) The conditions under which superconductivity does or does not coexist with structural/magnetic order in the 122 family. (ii) Comparative investigations on the structural/magnetic fluctuations on systems showing first- and second-order phase transitions into the low-temperature orthorhombic/antiferromagnetic (o/afm) phase. (iii) The possibility of a tricritical point in the T-P phase diagram where the o/afm and superconducting phase transition lines meet. (iv) Detailed investigations on the interrelation of superconductivity and the occurrence of the collapsed-tetragonal phase on the high-pressure side. (v) Study of the elastic behavior upon pressure tuning across the phase diagram; comparison with the anomalous behaviour reported in the literature for materials with varying doping levels.

• DFG-Verfahren: Schwerpunktprogramme
• Teilprojekt zu: SPP 1458: Hochtemperatursupraleitung in Eisenpniktiden
• Beteiligte Person: Privatdozent Dr. Bernd Wolf