63Cu, 65Cu, and 125Te NMR measurements are reported for Cu2-xTe and Cu1.98Ag0.2Te.
The results demonstrate an onset of Cu-ion hopping below room temperature, including
an activation behavior consistent with high-temperature transport measurements but
with a significant enhancement of the hopping barriers with Ag substitution. We also
separated the Korringa behavior by combining NMR line shape and relaxation
measurements, thereby identifying large negative chemical shifts for both nuclei,
as well as large Cu and Te s-state contributions in the valence band. Further
comparison was obtained through heat capacity measurements and chemical shifts
computed by density functional methods. The large diamagnetic chemical shifts coincide
with behavior previously identified for materials with topologically nontrivial band
inversion, and in addition, the large metallic shifts point to analogous features in
the valence band density of states, suggesting that Cu2Te may have similar inverted features.