Effects of niobium doping on the charge density wave and electronic correlations in the kagome metal Cs(V1−xNbx)3Sb

The transport and optical properties of Nb-doped $\mathrm{Cs}({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}$)${}_{3}{\mathrm{Sb}}_{5}$ with $x=0.03$ and 0.07 have been investigated and compared with those of undoped ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. Upon Nb doping, the charge density wave (CDW) transition temperature ${T}_{\text{CDW}}$ is suppressed, and the superconducting temperature ${T}_{c}$ rises. The residual resistivity ratio decreases with Nb doping, suggesting an increase of disorder. For all compounds, the optical conductivity in the pristine phase reveals two Drude components (D1 and D2). The substitution of Nb causes an increase of D1 alongside a reduction of D2 in weight, which implies a change of the Fermi surface. The total Drude weight is reduced with increasing Nb content, signifying an enhancement of electronic correlations. Below ${T}_{\text{CDW}}$, while the optical conductivity clearly manifests the CDW gap in all materials, the gapped portion of the Fermi surface shrinks as the Nb content grows. A comprehensive analysis indicates that the change of the Fermi surface, the enhancement of electronic correlations, the shrinkage of the removed Fermi surface by the CDW gap, and the increase of disorder may all have a considerable impact on the interplay between the CDW and superconductivity in $\mathrm{Cs}({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}$)${}_{3}{\mathrm{Sb}}_{5}$.