Study of C–I–C CDW phase transition in TTF–TCNQ with STM

TTF–TCNQ is a one-dimensional charge transfer compound. Charge density wave (CDW) transition occurs at 54 K in TTF–TCNQ, and CDW presents three phases: high temperature commensurate phase (T>49 K; 2a×3.39b); inter-mediated incommensurate phase (T=38–49 K; μa×3.39b) and low temperature commensurate phase (T<38 K; 4a×3.39b). In this report, we present our experimental result on cleaved (0 0 1) surfaces of TTF–TCNQ with UHV–LT–STM (Omicron LTSTM). For a broad temperature range (33–100 K), molecular resolution of TTF–TCNQ has been obtained. Below 54 K an additional two-dimensional superstructure of CDW appears in our images. The ordering of CDW modulation both along a and b direction are first revealed by our STM measurement. Our result shows that, on (0 0 1) surface of TTF–TCNQ, C–I–C CDW phase transition persists in nanometer scale and C–I–C transition is homogeneous. The determination of local CDW phase shift will be presented in this report too. At high temperature (60–280 K), we observed a v-shape pseudo-gap. The pseudo-gap and Peierls-gap co-exist at low temperature.