Visualizing the charge density wave transition in 2H - NbSe2 in real space

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. 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 provide indirect evidence that an energy gap in NbSe2 occurs at 0.7 eV below the Fermi energy in the CDW phase, suggesting that strong electron-lattice interactions, and not Fermi surface physics, are the dominant cause for CDW formation in NbSe2.