Transport and magnetic properties of a Mott-Hubbard system whose bandwidth and band filling are both controllable: R1−xCaxTiO3+y/2

Abstract

Transport and magnetic properties of ${R}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{TiO}}_{3+y/2}$ have been systematically investigated varying the one-electron bandwidth $(W)$ and the band filling $(n=1\ensuremath{-}\ensuremath{\delta}),$ which can be controlled by the $R$-dependent lattice distortion and by the Ca content $x$ and/or oxygen offstoichiometry $y$ $(\ensuremath{\delta}=x+y),$ respectively. The end compound $R{\mathrm{TiO}}_{3}$ is a ${3d}^{1}$ Mott-Hubbard insulator and its charge-gap magnitude increases with decreasing ionic radius of $R,$ i.e., an increase of electron correlation $(U/W)$ in proportion with $(U/W)\ensuremath{-}{(U/W)}_{c},$ where ${(U/W)}_{c}$ is the critical value for the (hypothetical) $n=1$ Mott transition. Such a Mott insulator is transformed to a correlated metal by substitution of $R$ with Ca (hole doping), and the nominal hole concentration required for the insulator-metal transition $({\ensuremath{\delta}}_{c})$ increases in proportion with $(U/W)\ensuremath{-}{(U/W)}_{c}.$ Concerning magnetism, $R{\mathrm{TiO}}_{3}$ with $R=\mathrm{La},$ Pr, Nd, and Sm, shows the antiferromagnetic ordering and its N\'eel temperature ${(T}_{N})$ decreases with smaller $R.$ ${T}_{N}$ also decreases with Ca doping, but remains finite up to the metal-insulator phase boundary. On the basis of these results, electronic phase diagrams are derived for a series of titanates as an electron-correlated system with changes of two parameters, i.e., the strength of electron correlation and band filling. Possible origins of the insulating state with finite hole doping are also discussed in terms of the kinetic energy of doped carriers in the Mott-Hubbard insulator.