  We report the direct observation in real space of the charge density wave
(CDW) phase transition in pristine 2H-NbSe2 using atomic-resolution scanning
tunneling microscopy (STM). We find that static CDW order is established in
nanoscale regions in the vicinity of defects at temperatures that are several
times the bulk transition temperature Tcdw. On lowering the temperature, the
correlation length of these patches increases steadily until CDW order is
established in all of space, demonstrating the crucial role played by defects
in the physics of the transition region. The nanoscale CDW order has an energy
and temperature-independent wavelength. Spectroscopic imaging measurements of
the real-space phase of the CDW indicate that an energy gap in NbSe2 occurs at
0.7eV below the Fermi energy in the CDW phase, suggesting that strong
electron-lattice interactions and not Fermi surface physics is the dominant
cause for CDW formation in NbSe2.
