Lattice-distortion-enhanced electron-phonon coupling and Fermi surface nesting in 1T-TaS 2 F. Clerc,* C. Battaglia, M. Bovet, L. Despont, C. Monney, H. Cercellier, M. G. Garnier, and P. Aebi Institut de Physique, Université de Neuchâtel, Rue A.-L. Breguet 1, CH-2000 Neuchâtel, Switzerland H. Berger and L. Forró Institut de Physique de la Matière Complexe, EPFL, CH-1015 Lausanne, Switzerland The temperature dependence of the electronic structure of the quasi-two-dimensional material 1T-TaS 2 is revisited by considering angle-resolved photoemission spectroscopy ARPES and density functional theory to calculate the imaginary part of the static electronic susceptibility characterizing the nesting strength. While nesting appears to play a role in the high temperature phase, the ARPES line shapes reveal peculiar spectral properties which are not consistent with the standard two-dimensional Peierls scenario for the formation of a charge density wave. The temperature dependence of these anomalous spectral features suggests a lattice- distortion enhanced electron-phonon interaction. I. INTRODUCTION The interplay between lattice and electronic degrees of freedom has received renewed interest in the context of high- temperature superconductivity and colossal magnetoresis- tance materials where electron-phonon coupling and possible polaronic effects are considered. 1,2 A significant contribution to the discussion is given by angle-resolved photoemission experiments via analysis of the spectral function. In this con- text it is important to examine other, more conventional ma- terials with respect to unconventional spectral features. 1T-TaS 2 is a layered transition metal dichalcogenide TMDC with a quasi-two-dimensional character. Reduced dimensionality leads to peculiar electronic properties and an interesting phase diagram. 3 In particular, a charge density wave CDW occurs with three distinct phases as the tem- perature is lowered. It is incommensurate IC phase between 600 and 350 K and commensurate C phase with a   13   13 periodicity below 180 K, resulting in a rotation of 13.9° with respect to the underlying unreconstructed 1  1 lattice. Between 350 and 180 K, a hexagonal array of com- mensurate domains with typical size of 70 Å, the so-called quasicommensurate QC phase, is formed. The domains are separated by domain walls, or discommensurations, where the CDW changes quickly. 4 Moreover, the transition to the C phase at 180 K is accompanied by a jump of the resistivity of more than one order of magnitude. In one dimension 1D the occurrence of a CDW is well explained by the theory of the Peierls instability 5,6 where a metal becomes unstable with respect to a spatially modulated perturbation with wave vector q  CDW equal to two times the Fermi vector 2k  F . This leads to the formation of electron- hole pairs with the same wave vector and finally to the open- ing of a gap which provides a gain in electronic energy in order to compensate the elastic energy paid for the lattice distortion. The driving force for such an instability is given by the topology of the Fermi surface FS which has to present favorable nesting conditions. Namely, large portions of the FS have to be connected or nested by the vector q  CDW . A good indicator of the quality of the nesting is the imagi- nary part of the static electronic susceptibility q   which, in linear response theory, relates the response of the system to the perturbation. The Peierls mechanism is also commonly evoked in order to explain the CDW in 1T-TaS 2 . 3,7,8 The topology of the FS of 1T-TaS 2 with parallel sections spanned by a vector of approximately q  CDW has contributed to the strength of this assumption. In a recent paper, 9 it was shown that in order to confirm this scenario, the knowledge of the gap-momentum dependence is of central importance. Pillo et al., 10 in their FS measurements, have observed a pseudogap over the whole FS in the C phase at a temperature below 180 K as well as in the QC phase at room temperature. The removal of states at the Fermi level E F  is explained by a Mott localization electron localization of a collective nature, 11,12 which also gives rise to the strong resistivity enhancement between the QC to C phase transition. Pillo et al. interpreted the pseudogap observed at room temperature as a possible pre- cursor effect of the Mott transition. Another explanation for the pseudogap is based on the spectral weight change in- duced by the new periodicity due to the CDW lattice distortion. 13 These interpretations of the pseudogap are either based on electron-electron correlations or on one-electron band theory, neglecting possible effects of strong electron-phonon interac- tion in this CDW material where electron phonon interaction necessarily has to play an important role. The only contribu- tion of electron-phonon interaction considered until now is the one allowing momentum transfer between electrons and holes near E F . Therefore it is the aim of this paper to examine the influ- ence of electron-phonon interaction. Indeed, as nested areas of the FS can be removed by the formation of electron hole pairs during the Peierls transition, non-nested FS may also be gapped by the influence of strong enough electron-phonon coupling. 14 Moreover, the elastic energy paid for the Peierls distortion being inversely proportional to the electron- phonon coupling parameter g; strong electron-phonon inter- action can help a 2D Peierls transition. In a CDW material, the electron-lattice interaction leads to a static distortion of Published in Physical Review B 74, issue 155114, 1-7, 2006 which should be used for any reference to this work 1