Optical studies of cuprate superconductors: The nature of low-energy excitations

Other than charge carriers, the only low-energy excitations in HTSC materials are phonons and plasmons, but each of these shows some unusual features. Certain phonons are quite polar, strongly coupled to electrons and anharmonic. An unusual Im(-1/∊) E ² law is attributed to plasmon spectrum being similar to that of the LEG model. Change carriers are described as highly concentrated intermediate polarons, all of which pair in real space at or close to T _c.