We report room temperature angle-resolved photoemission experiments on 1T-TaS 2 and 1T-TaSe 2 complemented by density-functional theory calculations. Fermi-surface mapping experiments in the charge-density wave ͑CDW͒ phase are similar for the two compounds and do not show symmetries due to the CDW-induced new Brillouin zones. However, the band structure a few eV below the Fermi level (E F) displays a clear modulation that we relate, in both cases, to the CDW. At E F , the spectral weight distribution reflects the band structure of the normal state, but no clear quasiparticle crossing is located. Near the zone center ⌫ , CDW-split quasilocalized Ta dz 2 subbands are observed in the vicinity of E F. For 1T-TaS 2 and 1T-TaSe 2 , they are thermally populated slightly above E F and close to E F , respectively. The observed behavior can be understood in terms of the CDW reconstructed, spectral function weighted band structure.
Density-functional band-structure calculations including atomic displacements in the charge-density wave phase have been performed for the layered 1T-TaS 2 . This quasi-two-dimensional material exhibits a chargedensity wave extending in all three dimensions. We find that the topmost occupied Ta d band is localized in plane but strongly disperses perpendicular to the layers showing favorable nesting conditions in z direction. The calculations are compared to angle-resolved photoemission experiments and possible consequences on the temperature behavior of the resistivity are discussed.Transition-metal dichalcogenides ͑TMCD's͒ are of strong interest since a long time because of their quasi-twodimensional ͑2D͒ character with unique electronic properties and phase transitions.1 1T-TaS 2 is a prototype for such layered materials with a rich phase diagram of charge-density waves ͑CDW's͒. The phase transitions give rise to strong changes in the resistivity ͓Fig. 1͑a͔͒. It is characterized by a small anomaly at the phase transition at 350 K from an incommensurate CDW phase at high temperatures to a quasicommensurate or nearly-commensurate ͑NC͒ phase at room temperature. Upon further lowering the temperature, a firstorder transition takes place at 180 K into the commensurate ͑C͒ CDW phase together with an increase of the resistivity by an order of magnitude. The CDW formation is widespread in the TMDC family, whose simple form of the Fermi surface is given a high degree of 2D character.1 Surprisingly, in 1T-TaS 2 , the CDW manifests itself also along the third dimension as shown again by a recent x-ray diffraction study.2 Therefore, there has to be interaction between the layers. Furthermore, other unusual properties have been discovered in 1T-TaS 2 , notably a pseudogaped Fermi surface 3 and the absence of backfolding of bands according to the new Brillouin zones ͑BZ's͒ introduced by the periodic lattice distortion of the CDW. 4 -6 The absence of backfolding effects is in apparent contradiction to non-self-consistent tightbinding calculations for a simplified structure describing the CDW phase performed by Smith et al. 7 This calculation shows that the reconstruction introduces three sub-band manifolds separated by gaps. Hence, the influence of the CDW should be manifest.Here, we investigate the electronic properties of 1T-TaS 2 using self-consistent full potential linearized augmented plane wave ͑FLAPW͒ calculations based on the experimental crystal structure using detailed refinement of the atomic positions by analyzing satellites up to high orders.2 Furthermore, we compare our calculation to angle-resolved photoemission ͑ARPES͒ experiments.The calculation shows that the CDW reconstruction induces drastic changes in the electronic structure confirming the three sub-band manifolds. However, we find that the reconstruction induces a localization of Ta-induced states within the plane and a delocalization out of plane. This behavior corresponds to an intriguing signature indicating a possible reason for the three-dimens...
The temperature dependence of the electronic structure of the quasi-two-dimensional material 1T-TaS 2 is revisited by considering angle-resolved photoemission spectroscopy ͑ARPES͒ 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 latticedistortion enhanced electron-phonon interaction.
We report high-resolution angle-resolved photoemission experiments on epitaxial thin films of different rare-earth (RE) dihydrides ͑RE= Gd, La͒ and of YH 2 and ScH 2 . It is found through ab initio calculations and confirmed by Fermi surface mapping that the electronic structure becomes very similar upon hydrogenation, rendering the studied dihydrides isoelectronic. We propose that the dihydride phase acts as a common precursor state for the formation of the insulating trihydride phase. For states with higher binding energies (which exhibit considerable H character) the agreement between calculation and measurement is less convincing. Independent of the difficulties to describe these hydrogen related states, we note in the comparison between experiment and calculation a very convincing description of the Fermi surface for the dihydrides. Therefore we trace the apparent inability of density, functional theory to describe the hygrogenation up to the trihydride phase to an insufficient description of hydrogen states in general and, in particular, involving octahedral sites.
We perform angle-resolved photoemission spectroscopy on 1T-TaS 2 and 1T-TaSe 2 using synchrotron radiation. We observe a characteristic splitting of the chalcogen p-derived valence bands along high symmetry directions. Density functional theory calculation and group theory strongly suggest that this splitting is due to spin-orbit interaction along one direction, and to symmetry along the other direction. We note that, according to the Kramers degeneracy, the spin-orbit interaction leaves every state doubly degenerate. Furthermore, this study allows us to identify a mixing between bands with Ta 5d and Se 4p character, possibly relevant for the different temperature behaviours of the two compounds.
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