Mott transition and collective charge pinning in electron doped Sr2IrO4

K. Wang, N. Bachar, J. Teyssier, W. Luo, C. W. Rischau, G. Scheerer, A. de la Torre, R. S. Perry, F. Baumberger, D. van der Marel
2018 Physical review B  
We studied the in-plane dynamic and static charge conductivity of electron doped Sr2IrO4 using optical spectroscopy and DC transport measurements. The optical conductivity indicates that the pristine material is an indirect semiconductor with a direct Mott-gap of 0.55 eV. Upon substitution of 2% La per formula unit the Mott-gap is suppressed except in a small fraction of the material (15%) where the gap survives, and overall the material remains insulating. Instead of a zero energy mode (or
more » ... e peak) we observe a soft collective mode (SCM) with a broad maximum at 40 meV. Doping to 10% increases the strength of the SCM, and a zero-energy mode occurs together with metallic DC conductivity. Further increase of the La substitution doesn't change the spectral weight integral up to 3 eV. It does however result in a transfer of the SCM spectral weight to the zero-energy mode, with a corresponding reduction of the DC resistivity for all temperatures from 4 to 300 K. The presence of a zero-energy mode signals that at least part of the Fermi surface remains ungapped at low temperatures, whereas the SCM appears to be caused by pinning a collective frozen state involving part of the doped electrons.
doi:10.1103/physrevb.98.045107 fatcat:xuowqrx3yzdqle4wjsyyrkvuxm