The Atomic-Scale Onset of Charge Density Waves in NbSe$_{2}$

Many modern complex materials exhibit spatially ordered electronic states such as charge and spin density waves, and scanning tunneling microscopy (STM) has recently been used successfully to visualize some of these phases. Unfortunately such phases often occur simultaneously in a single sample making the interpretation of experimental data difficult. In order to gain insight into the nature of spatial order in a simple material, we study the transition-metal dichalcogenide NbSe$_{2}$, which displays a 2D charge density wave (CDW) phase at low temperature. How does this CDW phase manifest itself in real space at the atomic scale? In order to answer this question, we have performed variable-temperature STM experiments on NbSe$_{2 }$single crystals at various temperatures around T$_{c}$ (33.5 K). We discover that static, short-range CDW order exists around crystal defects and impurities at temperatures up to 3T$_{c}$. We will describe the temperature evolution as well as the energy dependence of the short-range CDW using spectroscopic mapping. Our experiments provide a high-resolution measurement of the changes in electronic structure caused by the formation of CDW in the dichalcogenides.