Effects of doping on charge-density waves in layer compounds

Electrical, magnetic, and thermal properties of $1T-\mathrm{T}\mathrm{a}{\mathrm{S}}_2$ and $1T-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ doped with cations (Ti, Zr, Hf, V, Nb), $1T-\frac{{\mathrm{Nb}}_{1-x}{\mathrm{Ti}}_x{\mathrm{S}}_2}{\mathrm{Se}}$, and $1T-\mathrm{T}\mathrm{a}{\mathrm{S}}_{2-x}{\mathrm{Se}}_x$ are reported, with particular emphasis on the effects of doping on the charge density wave (CDW) observed in the parent compounds. Cation doping is found to slightly suppress the onset temperature $T_0$ of the CDW, but to suppress much more rapidly the lock-in to the commensurate state at $T_d$. Beyond a doping level of $\approx 10\%$ the commensurate state no longer exists. The anion disorder in $1T-\mathrm{T}\mathrm{a}{\mathrm{S}}_{2-x}{\mathrm{Se}}_x$, on the other hand, does not suppress transitions toward the commensurate state at any $x$. An anomalous increase of the low-temperature resistivity is observed for some compounds at certain doping levels. This increase appears to be due to both a reduction in carrier density and a reduction in mobility. The data are qualitatively consistent with recent ideas concerning the role of electronegativity in lock-in of the CDW to the commensurate state, and recent Landau models of the CDW state.