DCMPMS Seminars

Investigation of doping dependent interplay between magnetic and superconducting order in BaFe2-xCoxAs2 single crystals

by Dr. Biplab Baq (Indian Institute of Technology, Kanpur)

Tuesday, August 22, 2017 from to (Asia/Kolkata)
at AG80
Description
After the discovery of high temperature superconductivity in iron based pnictide compounds, an enormous amount of research activities in both experimental and theoretical condensed matter physics were stimulated. The parent compound of this class of materials is generally non-superconducting, rather it shows semi-metallic properties and exhibited structural and magnetic transitions. The magnetic transition in the parent compound corresponds to a transition from a high temperature (T) paramagnetic to a low T long range antiferromagnetic spin density wave state. The effect of doping in these compounds was found to suppresses magnetic and structural transition temperatures as well as destroys the long range magnetic ordering. As the long range magnetic order was destroyed there was an onset of superconductivity and the superconducting transition temperature (Tc) exhibited a domed shaped behavior with doping concentration. The most fascinating property of the doping phase diagram of iron pnictide compound is that the coexistence of magnetic and superconducting order within a certain regime of the phase diagram, especially in the underdoped regime. Now, what is the nature of this magnetic order in the coexisting regime? Does the magnetic order coexist with superconductivity at other doping levels and how the presence of magnetic order affects the superconducting order? Based on this, using bulk and local magnetization techniques, we have done a detailed doping dependence study to observe the interplay between superconductivity and magnetic correlations with doping in BaFe2-xCoxAs2 single crystals (underdoped, optimally doped and overdoped) [1]. The local magnetization properties have been measured using high sensitivity magneto-optical imaging technique. Our study reveals a very inhomogeneous superconducting state due to presence of a spontaneous positive magnetic response in all our samples. We have found that the positive magnetic response in our samples survives even in the normal state of the samples and appears due to presence of a short range magnetic order (magnetic fluctuations). On application of magnetic field we find an unusual suppression in the magnetic response along with an enhancement in the diamagnetic shielding (superconducting) response in our samples. We show that the nature of the superconducting state is very inhomogeneous due to presence of the short range magnetic order. The novelty of our work is that for the first time we have showed presence of superconducting fluctuation response above the bulk Tc. We have found that the strength of the short range magnetic order, and superconducting fluctuations are the strongest for the optimally doped sample (which also possess the highest Tc) and weakest for the over doped sample. We believe our results provide evidence of interplay between magnetic and superconducting order in iron pnictide superconductors. Theoretical studies have proposed that the magnetic correlations may provide the required pairing glue for the cooper pairs in these compounds. Hence, following the correlation between the strength of the magnetic and fluctuations we believe in our samples the short range magnetic order may meditate the superconducting fluctuations above Tc. 
1. Biplab Bag, K. Vinod, A. Bharathi, & S. S. Banerjee, Observation of anomalous admixture of superconducting and magnetic fractions in BaFe2-xCoxAs2 single crystals, New J. Phys. 18, 063025 (2016)