We report a comprehensive study of the monoclinic ${\text{Nb}}_{12}{\text{O}}_{29}$ that includes material synthesis, chemical and structural characterization, and measurements of physical properties as a function of temperature, magnetic field, and high pressure. The high-pressure structural study has been carried out up to 71 kbar. Critical behavior analysis of $\rho \left(T\right)$ and $C_{\text{p}}\left(T\right)$ as $T_{\text{N}}$ is approached from $T_{\text{N}}^{+}$ is consistent with the picture of localized spins in this metallic antiferromagnet. These results also indicate that thermally driven critical fluctuations are confined to a temperature range in the vicinity of $T_{\text{N}}\approx 12\text{K}$. The thermoelectric power, however, is strongly enhanced at $T<70\text{K}$. The peculiar temperature dependence of the thermoelectric-power enhancement at low temperatures is consistent with predictions based on quantum critical fluctuations. We have also shown that the enhancement is suppressed under either a magnetic field or hydrostatic pressure.

• Received 7 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.134428