Impurity-induced spin density wave in the thermoelectric layered cobaltite [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]

TL;DR

The study demonstrates that Sn substitution in the misfit layered cobaltite $Ca_3Co_4O_9$ stabilizes spin-density-wave (SDW) order in the conducting $CoO_2$ layers, as evidenced by rising SDW onset temperatures and enhanced Seebeck responses. Through synthesis, XRD, EPMA, resistivity, Seebeck, and magnetization measurements, the authors show Sn primarily substitutes Co in the conducting layer, causing lattice expansion and a progressive suppression of hole concentration, consistent with pseudo-gap formation associated with SDW. The observed impurity-induced stabilization parallels disorder-driven SDW phenomena in cuprates and underscores the importance of substitution site (conducting vs. block layers) for tuning magnetic and transport properties. These findings illuminate the interplay between impurities, SDW formation, and thermoelectric behavior in layered cobaltites, with potential implications for optimizing thermoelectric performance via controlled disorder.

Abstract

We investigate the Sn-substitution effect on the thermoelectric transport properties of the layered cobaltite [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] single crystals, which exhibit a non-monotonic temperature variation of the electrical resistivity and the Seebeck coefficient owing to the complex electronic and magnetic states. We find that the onset temperature of the short-range spin-density-wave (SDW) formation increases with the substituted Sn content, indicating the impurity-induced stabilization of the SDW order, reminiscent of the disorder/impurity-induced spin order in the cuprate superconductors. The Seebeck coefficient is well related to such impurity effects, as it is slightly enhanced below the onset temperatures, implying a decrease in the carrier concentration due to a pseudo-gap formation associated with the SDW ordering. We discuss the site-dependent substitution effects including earlier studies, and suggest that substitution to the conducting CoO$_2$ layers is essential to increase the onset temperature, consistent with the impurity-induced SDW picture realized in the conducting layers with the cylindrical Fermi surface.

Figures (6)

• Figure 1: Composition images of Ca$_3$Co$_{4-x}$Sn$_x$O$_9$ single crystals [(a) $x$ = 0, (b) $x$ = 0.1, (c) $x$ = 0.2, and (d) $x$ = 0.3] measured by the electron probe micro analyzer (EPMA) experiments. The measured points are marked with filled circles. (e,f) Comparison of nominal and measured atomic contents in Ca$_3$Co$_{4-x}$Sn$_x$O$_9$ single crystals ($0 \le x \le 0.3$) via EPMA measurements.
• Figure 2: (a) X-ray diffraction (XRD) patterns of the as-grown single crystals of Ca$_3$Co$_{4-x}$Sn$_x$O$_9$ ($0 \le x \le 0.3$) measured at room temperature. (b) XRD patterns of the polycrystalline samples of Ca$_3$Co$_{4-x}$Sn$_x$O$_9$ ($0 \le x \le 0.4$) measured at room temperature. Inset compares these data around the (004) reflection. (c,d) Lattice parameters as a function of nominal Sn composition. $b_1$ and $b_2$ correspond to the $b$ parameters of [Ca$_2$CoO$_3$] and [CoO$_2$] layers, respectively. The inset of (d) shows the ratio $b_2/b_1$ of the parameter $b$ of the CoO$_2$ layer ($b_2$) to the parameter $b$ of the Ca$_2$CoO$_3$ layer ($b_1$).
• Figure 3: (a) Temperature dependence of the resistivity $\rho$ for Ca$_3$Co$_{4-x}$Sn$_x$O$_9$ ($0\le x \le 0.3$) single crystals in a semi-logarithmic plot. (b) The resistivity as a function of $T^2$. Dashed lines show the Fermi-liquid behavior of $\rho(T) = \rho_0+AT^2$, which is realized in the intermediate temperature range between the onset temperature $T^{\rm on}_{\rm SDW}$ (solid arrows) and the crossover temperature $T^*$ (dashed arrows). A characteristic temperature $T_{\rm min}$, where the resistivity shows a minimum value, is also depicted as the dotted arrows.
• Figure 4: The characteristic temperatures as a function of the substitution content $x$ in Ca$_3$Co$_{4-x}$Sn$_x$O$_9$. $T_{\rm min}$, $T_{\rm SDW}^{\rm on}$, $T^*$, and $T_{\rm SDW}$ are plotted as the solid circles, triangles, squares, and diamonds respectively. For comparison, $T_{\rm min}$ data in Ca$_3$Co$_{4-x}$Ti$_x$O$_9$Zhao2006 and Ca$_{3-x}$Bi$_x$Co$_4$O$_9$Heieh2014 are also plotted as the open circles.
• Figure 5: (a) Temperature dependence of the magnetic susceptibility $\chi$ for Ca$_3$Co$_{4-x}$Sn$_x$O$_9$ single crystals. (b) Temperature dependence of $d\chi^{-1}/dT$ at low temperatures. The characteristic temperatures below which the high-temperature linear variations deviate are indicated by solid arrows.