High-$Q$-resolution scattering using synchrotron x radiation: $2H$-${\mathrm{TaSe}}_2$ and ${\mathrm{NbSe}}_3$

We report high-resolution synchrotron x-ray-scattering measurements on two compounds with charge-density-wave (CDW) phase transitions, $2H$-${\mathrm{TaSe}}_2$ and ${\mathrm{NbSe}}_3$. In these materials the scattering from the CDW superlattice is typically more than 3 orders of magnitude smaller than that of the host lattice. The increased flux available with a synchrotron source allows one to study weak CDW scattering with a $Q$ resolution on the order of ${10}^{-4\mathrm{}}$ ${\mathrm{\AA }}^{-1\mathrm{}}$, which is 1 to 2 orders of magnitude better than that attainable with a rotating-anode source. In the case of $2H$-${\mathrm{TaSe}}_2$, we were able to measure finite widths of the CDW superlattice peaks and to determine precisely the temperature dependence of the CDW incommensurability near the commensurate-incommensurate transition. We have established a lower bound for the CDW coherence length in ${\mathrm{NbSe}}_3$, and we have shown that the sum of the two CDW wave vectors is not commensurate as previously suspected. We have also looked with higher precision for possible effects of the electric field on the CDW structure in ${\mathrm{NbSe}}_3$.