Normal and superconducting state properties of Cu-doped FeSe single crystals

We report the evolution of the physical properties of FeSe single crystals with Cu substitution. We show that introducing Cu suppresses bulk superconductivity quickly, much faster than the decrease in structural transition temperature, and further doping Cu induces a metal-insulator transition. In contrast, zero-field $ab$-plane resistivity ${\ensuremath{\rho}}_{xx}$ exhibits an unusual temperature dependence ${\ensuremath{\rho}}_{xx}(T)\ensuremath{\sim}A{T}^{n}$, with $n\ensuremath{\sim}$ 1, that is almost unchanged with the variation of Cu content. This result implies that magnetic fluctuations in FeSe are insensitive to the Cu substitution. The field dependence of the Hall resistivity of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$ with $xg$ 0 shows a positive slope in the low-temperature region which can be ascribed to the relatively higher hole mobility than the electron one. Therefore, the low-concentration Cu dopant as a strong scattering source can lead to the significant decrease in electron mobility which is detrimental to superconductivity, but it has minor effects on the carrier densities of electrons and holes as well as the shapes of the Fermi surfaces. Correspondingly, the structural transition and magnetic fluctuations in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$ change slowly with $x$.