Electron tunneling measurements on TSeF-TCNQ and TTF-TCNQ

Using a point contact geometry we realized tunnel junctions between single crystals of TTF-TCNQ, TSeF-TCNQ and an oxidized aluminum plate; we studied the dynamic conductance of these junctions as a function of temperature between 4.2 K and room temperature.Above the transition temperature T_c, i.e., in the metallic regime, dI/dV(V) is remarkably well fitted by a parabolic function, in agreement with theoretical considerations. Below the critical temperature, both with TTF-TCNQ and TSeF-TCNQ we observed a flattering of the dI/dV curves, and thus a clear deviation from the parabola.From the low-temperature results, a more sophisticated fit of the curves enabkes an upper limit value of the Peierls gap in these compounds to be calculated which is in good agreement with known values.