LAUSR

We investigate electronic properties of hole-doped cuprates ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ with the quasi-one-dimensional two-leg-ladder structure. We succeeded in extending the solubility limit of Li in ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ from $x=0.12$ to $x=0.60$ by using the high-pressure synthesis technique. The antiferromagnetic transition temperature rapidly decreases with increasing Li content from 94 K at $x=0$ to 7.5 K at $x=0.30$, and takes an almost constant value (3--6 K) at $x\ensuremath{\ge}0.35$. The antiferromagnetic order still exists even at $x=0.60$, where the formal valence of Cu is as large as $+2.30$. The temperature dependence of the specific heat suggests the finite contribution of the electronic specific heat at $x=0.20--0.60$, which is consistent with high valence of Cu. Nevertheless, the temperature dependence of resistivity shows a variable range hopping behavior in the whole $x$ ranges, and the insulating behavior survives under the pressure up to 2.9 GPa. This peculiar behavior is owing to the disorder originating from the intersite atom exchanges due to the similar ionic radius of cations in ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$. Nonmagnetic ions of ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Li}}^{+}$ are introduced into the ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ planes of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$, resulting in the localization of doped hole carriers.