= Reciprocal effective mass tensor
{title2=$(M^{-1})_{ij}=\hbar^{-2}\partial_{k_i}\partial_{k_j}E$}
The curvature tensor of a nondegenerate <energy band>, determining the linear relation between force and acceleration in semiclassical band dynamics. From <group velocity> $v_i=\hbar^{-1}\partial_{k_i}E$ and $\hbar\dot k_j=F_j$, one obtains $\dot v_i=(M^{-1})_{ij}F_j$. Near a parabolic minimum, $E-E_0=\hbar^2\delta\mathbf k^TM^{-1}\delta\mathbf k/2$. The principal values are reciprocal curvature <effective masses>; a cyclotron experiment measures a direction-dependent combination rather than every tensor entry separately.
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