Near coincidence of metal-insulator transition and quantum critical fluctuations: Electronic ground state and magnetic order in Fe${}_{1-x}$Co${}_x$Si

We present a detailed study of the electronic and magnetic ground state properties of Fe${}_{1-x}$Co${}_x$Si using a combination of macroscopic and microscopic experimental techniques. From these experiments we quantitatively characterize the metal-insulator transition and magnetic/non-magnetic quantum phase transition occurring at low doping levels in Fe${}_{1-x}$Co${}_x$Si. From our study, we find a surprising closeness of the critical composition of the metal-insulator transition at $x_{\mathrm{M}\mathrm{I}\mathrm{T}}=0.014$ and the quantum phase transition at $x_{\mathrm{L}\mathrm{R}\mathrm{O}}\sim 0.024-0.031$. It suggests that these effects are cooperative and depend on each other.