  High-resolution angle-resolved photoemission spectroscopy (ARPES) data reveal
evidence of a crossover from one-dimensional (1D) to three-dimensional (3D)
behavior in the prototypical charge density wave (CDW) material NbSe3. In the
low-temperature 3D regime, gaps in the electronic structure are observed due to
two incommensurate CDWs, in agreement with x-ray diffraction and
electronic-structure calculations. At higher temperatures we observe a spectral
weight depletion that approaches the power-law behavior expected in 1D. From
the warping of the quasi-1D Fermi surface at low temperatures, we extract the
energy scale of the dimensional crossover. This is corroborated by a detailed
analysis of the density of states, which reveals a change in dimensional
behavior dependent on binding energy. Our results offer an important insight
into the dimensionality of excitations in quasi-1D materials.
