Phonon anomaly at the charge ordering transition in 1T-TaS2
The infrared reflectance of the transition-metal chalcogenide 1T-TaS2 has been measured at temperatures from 30 K to 360 K over 30-45 000 cm(-1) (4 meV-5.5 eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360 K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudo-gap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1 eV. As the temperature decreases, the low-frequency conductivity also decreases, phonon modes become more pronounced, and a pseudogap develops into a gap at 800 cm(-1) (100 meV). We observe an anomalous frequency dependence of the 208 cm(-1) infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180-K transition. The same mode demonstrates strong hysteresis of the frequency and linewidth changes, similar in its temperature behavior to the hysteresis in the dc resistivity. We discuss a possible relation of the observed softening of the mode to the structural changes and changes in electronic properties associated with the 180-K transition.
Univ N Florida, Dept Chem & Phys, S Jacksonville, FL 32224 USA. Univ Florida, Dept Phys, Gainesville, FL 32611 USA. Ecole Polytech Fed Lausanne, Inst Phys Complex Matter, CH-1015 Lausanne, Switzerland. Gasparov, LV, Univ N Florida, Dept Chem & Phys, 4567 St Johns Bluff Rd, S Jacksonville, FL 32224 USA.
ISI Document Delivery No.: 600GX
Record created on 2006-10-03, modified on 2016-08-08