Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3

The charge response in the barium vanadium sulfide (BaVS3) single crystals is characterized by dc resistivity and low frequency dielectric spectroscopy. A broad relaxation mode in MHz range with huge dielectric constant ~= 10^6 emerges at the metal-to-insulator phase transition TMI ~= 67 K, weakens with lowering temperature and eventually levels off below the magnetic transition Tchi ~= 30 K. The mean relaxation time is thermally activated in a manner similar to the dc resistivity. These features are interpreted as signatures of the collective charge excitations characteristic for the orbital ordering that gradually develops below TMI and stabilizes at long-range scale below Tchi.