The study of vortex state across the phase diagram in single crystals of Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2}}$ using current voltage characteristics.*

Utilizing the current voltage (I-V) characteristic measurement, we investigate the vortex state of Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2}}$ single crystals across a wide doping range: under-doped (x $=$ 0.042 and 0.056), near optimally-doped (x $=$ 0.06 and 0.072), and the over-doped (x $=$ 0.1). We compare the nature of the I-V characteristic curves obtained in our measurements with those of conventional type II superconductors. Using our data we could find a direct relationship between the critical current and flux-flow resistivity over the whole doping range studied. The implications of the comparison between the I-V curves of a conventional type II superconductor and Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2}}$ will be discussed. We will also comment on the observed relationship between the critical current and the flux-flow resistivity for the Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2}}$ crystals studied.