Observation of nanometer-spaced modulated structure causing Large Seebeck coefficient of the Tl compound: Angle-resolved photoemission study

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TllnSe_2 with quasi one-dimensional chain structure has attracted much interest because of the structural low-dimensionality, and the interesting electric and thermoelectric properties. Recently, the Seebeck coefficient of TlInSe_2 has then been found to be positive only at temperatures below 473 K, reaching very high values of more than 10^6μV/K below 413 K. It has been pointed out that above record-breaking thermoelectric power may result from the formation of an incommensurate (IC) superlattice phase in TlInSe_2. However, the existence of IC-phase in TlInSe_2 has not been experimentally verified yet, either by the analyses of the electronic structure or by the extended X-ray or neutron scattering examination. In this work, we have studied TlInSe_2 by means of angle-resolved photoemission spectroscopy (ARPES) at 280 and 50K in order to possibly find the imprints of IC-phase in the electronic band structure.The obtained energy bands and the calculated band structure were found to agree well in main futures. The band structure shows quite strong dispersion in the direction perpendicular to the chains and indicates that in fact the one-dimensional electronic features of TlInSe_2 are masked by the strong interchain interactions. The whole spectral features shift rigidly toward lower binding energies with decreasing temperature. The energy gap splitting, furthermore, has been clearly observed in the experimental electronic bands along both directions. A detailed analysis using both experimental and theoretical data was performed and the obtained results were evident of the presence of the IC-phase linked to a certain point on the A-line of the Brillouin zone of TlInSe_2. A self-consistent picture of the phase transition in TlInSe_2 was drawn after discussion using available data for this material. According to this picture, TlInSe_2 at 50K is already a commensurate (C) material while the IC-C phase transition occurs between 280 and 50K.