Structural and electronic properties of La0.2Sr0.8MnO3 and Ca3Co2O6 : X-ray spectroscopic studies

The first part of my talk, deals with the interplay between the electronic and structural transition in La_0.2Sr_0.8MnO_3 compound using Mn K-edge x-ray absorption and photoemission spectroscopic studies. The transport and specific heat measurements exhibit unusual evolution and signature of first order phase transition at ~ 265 K. This transition marks the onset of metal to insulator, cubic to tetragonal and paramagnetic to C-type antiferromagnetic (AFM) transitions. Mn K-edge extended x-ray absorption fine structure (EXAFS) studies results reveal distortion in the MnO6 octahedra, even in the cubic phase and a remarkable evolution of distortion across the phase transition. These results manifest the importance of fluctuations in Mn 3d orbital occupancy in their electronic properties. Our high resolution ultraviolet photoemission studies reveal a pseudo gap at room temperature, then opens up a hard gap in the C-type AFM phase via the formation of a soft gap around the transition temperature region. Despite being an insulator, this material exhibit linear specific heat. Our results indicate signature of an amorphous phase presumably magnetic in nature coexisting with the long range ordered phase. I will also be discussing some of our recent core level photoemission spectroscopic results carried out on this system. 

The second part my talk deals with the origin of the precursor effect associated to magnetic phase transition in quasi one dimensional system, Ca_3Co_2O_6. Experimental results reveal unusual changes in the Co-O bond lengths in CoO_6 units nucleating at a temperature T*, where the precursor effect occurs. The corresponding Debye-Waller factors representing disorder effect exhibit anomalous evolution across T*. These results reveal unique link between the local structural changes and the precursor effect that needs to be considered in the understanding of various phase transitions.