Giant Nernst Effect and Bipolarity in the Quasi-One-Dimensional Metal ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathbf{O}}_{17}$

The Nernst coefficient for the quasi-one-dimensional metal, ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$, is found to be among the largest known for metals ($\nu \simeq 500\mu \mathrm{V}/\mathrm{KT}$ at $T\sim 20\mathrm{K}$), and is enhanced in a broad range of temperature by orders of magnitude over the value expected from Boltzmann theory for carrier diffusion. A comparatively small Seebeck coefficient implies that ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ is bipolar with large, partial Seebeck coefficients of opposite sign. A very large thermomagnetic figure of merit, $ZT\sim 0.5$, is found at high field in the range $T\approx 35–50\mathrm{K}$.

Figure 1

(a) Nernst coefficient, $\nu (T)$, for crystals $A2$ (dashed lines), $C$ (circles and squares), and $D$ (triangles). For crystal $C$: ${\nu }_{cb}$ and ${\nu }_{bc}$ determined with primary heat current along $b$ (solid circles) and $c$ (solid squares), respectively. The specimen, originally with thickness $110\mu \mathrm{m}$ along $a$, was mechanically thinned to $55\mu \mathrm{m}$ and remeasured with primary heat current along $b$ (open circles) [18]. (b) $\nu /T$ vs $T$ for crystal $C$, compared to bismuth [Refs. 19 (solid curve) and 20 (dashed curve)] and graphite (Ref. 21). The dash-dot-dotted curve labeled “$283\mu /T_F$” represents the carrier-diffusion contribution from Boltzmann theory, determined from the carrier mobilities and estimated Fermi temperature [18]. (c) Figure of merit as a function of magnetic field for crystal $C$ at several temperatures. (d) zero-field thermal conductivities along the $b$ and $c$ axes, inset: same data plotted as $\kappa T$ vs $T$ to emphasize the large constant term in ${\kappa }_b(T)$. Main Inset: Perspective view of the LMO monoclinic unit cell viewed along the $b$ axis; Q1D Mo1-O-Mo4 zigzag double chains composed of ${\mathrm{MoO}}_6$ octahedra (highlighted bonds) are part of planar Mo-O networks whose two dimensionality is broken by the Li ions.

Figure 2

Seebeck coefficient ($S$), resistivity, and Hall coefficient for specimen $C$. Inset: Fermi surface for ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ (adapted from Ref. 9).

Figure 3

Zero-field $b$-axis Seebeck coefficient for four ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ specimens. Combined results of x-ray, compositional, and Hall measurements indicate that specimens with large peaks ($A1$, $A2$) are more oxygenated and particle-hole asymmetric ($R^h\gg R^e$ and ${\sigma }^h<{\sigma }^e$) compared to those with linear-$T$ thermopowers ($C$, $D$, $E$) [18]. Inset: Hole and electron partial thermopowers computed using the equation [25], the measured Nernst coefficient (${\nu }_{cb}$), and two-band model parameters; error bars (25%) are propagated from those of the Hall data [18].