Bulk-Sensitive Angle-Resolved Photoemission Spectroscopy on TTF-TCNQ

Abstract

TTF-TCNQ is a quasi-1D molecular conductor exhibiting metal-insulator transition at 54K accompanied by chargedensity-wave (CDW) formation. Its electronic properties in relation to the Peierls instability and CDW transitions are well investigated through the several decades. Past ARPES studies indicate the band dispersion consisting of TCNQ molecules (namely the TCNQ-band) shows the signature of spin–charge separation, together with the suppression of the quasiparticle excitation at the Fermi level. These observations have been discussed in terms of the 1D character of TTF-TCNQ, nevertheless, there are several mysteries remained. One is the band dispersions obtained by ARPES commonly indicating the band-width (or the band group velocity) larger by a factor of 2 with respect to band theory. To account for this discrepancy, the possibility of relaxed tilting of the topmost molecules and/or strong correlation effect have been raised until now. The second is the anomalous suppression of the spectral weight near the Fermi level (EF), which gives negligible amount of spectral intensity at EF even in the highly-conductive state above >60K. The electronic structure of the metallic quasi-1D state has been thus remained to be elucidated. To investigate the precise electronic structure of TTFTCNQ, here we utilized the laser-excited ARPES with h 1⁄4 6:994 eV. Owing to the relatively low photon-energy as compared with past ARPES studies (h 1⁄4 20{40 eV), the bulk-sensitive measurement becomes available. The effect of photoirradiation-induced damage peculiar to molecular compounds is also considered to be weaker compared to the measurements using higher-energy photons. The high cross section of s and p electrons for this photon-energy provides some advantage in studying the light-element based organic conductors. ARPES measurements were performed using a system constructed with VG-Scienta R4000 electron analyzer and an ultraviolet (h 1⁄4 6:994 eV) laser. He II (h 1⁄4 40:8 eV) light source was also used to check the correspondence with past reports. Single crystals of TTF-TCNQ grown by the diffusion method were cleaved in-situ to obtain fresh surfaces. The pressure was below 5 10 11 Torr throughout all the measurements. The energy resolution was E 1⁄4 4:0meV. The Fermi level EF of the sample was referred to that of a Au film evaporated on the sample substrate, with an accuracy of 0:1meV. Figure 1(a) shows the mapping of the ARPES intensity (h 1⁄4 6:994 eV) at the Fermi level, obtained by integrating the ARPES intensity in the energy window of 50meV. The shape of the Fermi surface thus observed makes a straight line at kF 1⁄4 0:24 0:01 A 1 1⁄4 ð0:29 0:01Þb , indicative of 1D electronic structure. The image of band dispersions along –Z direction obtained by h 1⁄4 6:994 and 40.8 eV are shown in Fig. 1(b,c). The peak positions of the ARPES intensity estimated from momentum distribution curve (MDC) and energy distribution curve (EDC) are plotted by circle and cross markers, respectively. The red and blue curves, on the other hand, represent the band dispersions consisting of TTF and TCNQ molecules, obtained by the first-principles calculation. By comparing with the band calculation, we can notice that the h 1⁄4 6:994 eV data is dominated by the signal from TTF bands, whereas the h 1⁄4 40:8 eV data shows both TTF and TCNQ bands that are well in accord with past reports. This h -dependence, which should be due to the photoionization cross section and/or matrix element effect, also explains the slight difference of the present ‘‘Fermi-surface’’ shape from that previously reported. The Fermi momentum obtained by ARPES, kF 1⁄4 0:24 0:01 A , is somewhat smaller compared with the band calculation, reflecting the overestimation of the charge transfer between TTF and TCNQ inevitably arising in the calculated data. Here, let us focus on the gradient of the TTF band dispersion. In the h 1⁄4 40:8 eV ARPES spectrum, the gradient of the TTF band dispersion is about 2 times larger as compared to the calculation. It shows that the observed High