Abstract
We present recent results obtained using angle-resolved photoemission spectroscopy performed on 1T-TiSe2. Emphasis is put on the peculiarity of the bandstructure of TiSe2 compared to other transition metal dichalcogenides, which suggests that this system is an excellent candidate for the realization of the excitonic insulator phase. This exotic phase is discussed in relation to the BCS theory, and its spectroscopic signature is computed via a model adapted to the particular bandstructure of 1T-TiSe2. A comparison between photoemission intensity maps calculated with the spectral function derived for this model and experimental results is shown, giving strong support for the exciton condensate phase as the origin of the charge density wave transition observed in 1T-TiSe2. The temperature-dependent order parameter characterizing the exciton condensate phase is discussed, both on a theoretical and an experimental basis, as well as the chemical potential shift occurring in this system. Finally, the transport properties of 1T-TiSe2 are analyzed in the light of the photoemission results.
Highlights
The chalcogen atom being in an ns2 np4 electronic configuration (n is the principal quantum number of the partially filled last shell), four electrons are taken away from the transition metal atom, in a purely ionic picture. This means that, for group IVA transition metals ((n − 1)d2 ns2 electronic configuration), the atom is left in a d0 valence state, while for the group VA transition metals ((n − 1)d3 ns2), it is left in a d1 configuration
Three symmetry equivalent conduction bands having their minima at the border of the Brillouin zone (BZ) (at the L point, see figure 2(a)) are coupled to the valence band having its maximum at the center of the BZ
The good agreement between the photoemission intensity maps calculated within the exciton condensate phase model and the experimental data leads us to give strong support for the realization of this exotic phase in 1T -TiSe2
Summary
Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. 7 Author to whom any correspondence should be addressed. Three symmetry equivalent conduction bands having their minima at the border of the Brillouin zone (BZ) (at the L point, see figure 2(a)) are coupled to the valence band having its maximum at the center of the BZ (the point) For such a bandstructure, the main difference compared to the basic excitonic insulator phase is that the electron–hole coupling does not shift all the conduction bands away from the Fermi energy when opening a gap, leaving . The corresponding phase diagram is reminiscent of the one of the new iron pnictide superconductors In this context, it is worth citing the recent work of Sawatzky et al, emphasizing that the pnictogen- and chalcogen-based materials are of particular interest, since these atoms possess high polarizability, which is, under certain circumstances, favorable to non-phonon-mediated superconductivity [29].
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