Electron doping in the cubic perovskite ${\text{SrMnO}}_3$: Isotropic metal versus chainlike ordering of Jahn-Teller polarons

Single crystals of electron-doped ${\text{SrMnO}}_3$ with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only $1–2\mathrm{\%}$ electron doping by Ce substitution for Sr, a $G$-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distortion, accompanied by a quasi-one-dimensional $3z^2-r^2$ orbital ordering with the $C$-type antiferromagnetism. The self-organization of such dilute polarons may reflect the critical role of the cooperative Jahn-Teller effect that is most effective in the originally cubic system.

Figure 1

(Color online) Temperature $\left(T\right)$ dependence of (a) magnetization $\left(M\right)$ at 0.5 T and (b) resistivity $\left(\rho \right)$ at 0 T for ${\text{Sr}}_{1-x/2}{\text{Ce}}_{x/2}{\text{MnO}}_3$ $\left(0\le x\le 0.2\right)$ single crystals. The solid and dashed lines correspond to warming and cooling runs, respectively. The closed triangles and arrows in (a) denote the Néel temperatures of $G$-type and of $C$-type AFM phases, respectively. Inset to (a): $M$ versus magnetic field $\left(H\right)$ up to 7 T at 2 K for $x=0.01–0.04$.

Figure 2

(Color online) (a) $M$ at 0.5 T at 2 K and tetragonality $\left(c_{\text{p}}/a_{\text{p}}\right)$ at 25 K versus $x$ for ${\text{Sr}}_{1-x/2}{\text{Ce}}_{x/2}{\text{MnO}}_3$ $\left(0\le x\le 0.2\right)$, where $a_{\text{p}}$ and $c_{\text{p}}$ are the lattice constants defined in the perovskite subcell. (b) Electronic phase diagram as a function of $x$, based on the data for warming runs in Fig. 1. The transition temperatures of $G$-type $\left[T_{\text{N}}\left(\text{G}\right)\right]$, canted $G$-type $\left(T_{\text{CA}}\right)$, and $C$-type AFM $\left[T_{\text{N}}\left(C\right)\right]$ are indicated by closed circles, squares, and inverted triangles, respectively. The transition to OO phase is represented by closed triangles $\left(T_{\text{OO}}\right)$. The data for ${\text{Sr}}_{1-y}{\text{La}}_y{\text{MnO}}_3$ $\left(y=0.02,0.04\right)$ are indicated by the corresponding open symbols.

Figure 3

(Color online) (a) Synchrotron x-ray powder-diffraction profiles for ${\text{Sr}}_{1-x/2}{\text{Ce}}_{x/2}{\text{MnO}}_3$ $\left(0\le x\le 0.2\right)$ at 25 K. The indices are based on the cubic setting $\left(Pm\overline{3}m\right)$ and $a_{\text{p}}$ monotonously increases with increasing $x$. (b) Temperature profiles of tetragonality $\left(c_{\text{p}}/a_{\text{p}}\right)$ in warming runs.

Figure 4

(Color online) (a) $T$ profiles of $\rho$ at 0 T for single crystals of ${\text{Sr}}_{1-x/2}{\text{Ce}}_{x/2}{\text{MnO}}_3$ ($x=0.01–0.04$, solid lines) and ${\text{Sr}}_{1-y}{\text{La}}_y{\text{MnO}}_3$ ($y=0.02–0.04$, open circles). The closed and open triangles indicate $T_{\text{N}}\left(G\right)$ and $T_{\text{OO}}$, respectively. Inset: effective carrier number at 2 K as a function of $x$ (closed circle) and $y$ (open circle) deduced from Hall coefficient $R_{\text{H}}$. The dashed line corresponds to the nominal carrier density calculated from chemical composition. (b) Specific heat $C$ divided by $T$ is plotted against $T^2$ for $x=0.01$ and 0.02. (c) $T$ profiles of Seebeck coefficient $Q$. The solid and dashed lines in (c) indicate the calculated results using the Boltzmann equation (Ref. 19).