The growing charge-density-wave order in CuTe lightens and speeds up electrons

Charge density waves (CDWs) are pervasive orders in solids that usually enhance the effective mass (m*) and reduce the Fermi velocity ( $$ ) of carriers. Here, we report on the inverse - a reduced m* and an enhanced $$ correlated with the growth of the CDW order in CuTe with gapped, practically linearly dispersing bands - reminiscent of emergent CDW-gapped topological semimetals. Using momentum-dependent electron energy-loss spectroscopy (q-EELS), we simultaneously capture m* and $$ of the CDW-related, practically linearly dispersing electrons by plasmon dispersions across the transition (335 K, T_CDW), with m* of 0.28 m₀ (m₀, the electron rest mass) and $$ of ~ 0.005c (c, the speed of light) at 300 K. With the growth of the CDW order-parameter strength toward 100 K, the electrons become lighter and move faster by ~ 20%. Thorough inspection below T_CDW unveils the essential role of the increasing opening of the CDW gap. CuTe is a rich platform for the exploration of CDW/correlation physics with q-EELS established as a useful probe for this type of physics.