We propose a scanning tunneling spectroscopy experiment to identify the nature and relative strength of collective modes in the high-temperature superconductors (HTSCs). To this end, we show that the pinning of diverse collective modes by impurities leads to qualitatively different fingerprints in the local density of states. These fingerprints directly reflect the modes’ magnetic or nonmagnetic character, as well as their wave vectors and correlation lengths. These results provide an alternative method for identifying collective modes not only in the HTSCs but also in other correlated electron systems.

• Received 21 February 2008

DOI:https://doi.org/10.1103/PhysRevB.78.054504