We present a new high-resolution angle-resolved photoemission study of 1T -TiSe2 in both, its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalisations at highsymmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A theoretical calculation of the spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favour of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T -TiSe2. PACS numbers:Transition-metal dichalcogenides (TMDC's) are layered compounds exhibiting a variety of interesting physical properties, mainly due to their reduced dimensionality [1]. One of the most frequent characteristics is a ground state exhibiting a charge-density wave (CDW), with its origin arising from a particular topology of the Fermi surface and/or a strong electron-phonon coupling [2]. Among the TMDC's 1T -TiSe 2 shows a commensurate 2×2×2 structural distortion below 202 K, accompanied by the softening of a zone boundary phonon and with changes in the transport properties [3,4]. In spite of many experimental and theoretical studies, the driving force for the transition remains controversial. Several angle-resolved photoelectron spectroscopy (ARPES) studies suggested either the onset of an excitonic insulator phase [5,6] or a band Jahn-Teller effect [7]. Furthermore, TiSe 2 has recently attracted strong interest due to the observation of superconductivity when intercalated with Cu [8]. In systems showing exotic properties, such as Kondo systems for example [9], the calculation of the spectral function has often been a necessary and decisive step for the interpretation of the ARPES data and the determination of the ground state of the systems. In the case of 1T -TiSe 2 , such a calculation for an excitonic insulator phase lacked so far.In this letter we present a high-resolution ARPES study of 1T -TiSe 2 , together with theoretical calculations of the excitonic insulator phase spectral function for this compound. We find that the experimental ARPES spectra show strong band renormalisations with a very large transfer of spectral weight into backfolded bands in the low-temperature phase. The spectral function calculated for the excitonic insulator phase is in strikingly good * Electronic address: herve.cercellier@unine.ch agreement with the experiments, giving strong evidence for the excitonic origin of the transition.The excitonic insulator model was first introduced in the sixties, for a semi-conductor or a semi-metal with a very small indirect gap E G [10,11,12,13]. Thermal excitations lead to the formation of holes in the valence band and electrons in the conduction band. For low free carrier densities, the weak screening of the electronhole Coulomb interaction leads to the formation of stable electron-hole bound states, called excito...

Recently strong evidence has been found in favor of a BCS-like condensation of excitons in 1T -TiSe2. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the excitonic condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. Here, the calculations in the framework of this model are presented in detail. They represent an extension of the original excitonic insulator phase model of Jérome et al. [Phys. Rev. 158, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and further comparison with experiment are also discussed.

We investigate band bending, electron affinity and work function of differently terminated, doped and oriented diamond surfaces by X-ray and ultraviolet photoelectron spectroscopy ( XPS and UPS ). The diamond surfaces were polished by a hydrogen plasma treatment and present a mean roughness below 10 Å . The hydrogen-terminated diamond surfaces have negative electron affinity (NEA), whereas the hydrogen-free surfaces present positive electron affinity (PEA). The NEA peak is only observed for the borondoped diamond (100)-(2×1):H surface, whereas it is not visible for the nitrogen-doped diamond (100)-(2×1):H surface due to strong upward band bending. For the boron-doped diamond (111)-(1×1):H surface, the NEA peak is also absent due to the conservation of the parallel wavevector component (k d ) in photoemission. Electron emission from energy levels below the conduction band minimum (CBM ) up to the vacuum level E vac allowed the electron affinity to be measured quantitatively for PEA as well as for NEA. The emission from populated surface states forms a shoulder or a peak at lower kinetic energies, depending on the NEA behavior and additionally shows a dispersion behavior. The low boron-doped diamond (100)-(2×1):H surface presents a highintensity NEA peak with a FWHM of 250 meV. Its cut-off is situated at a kinetic energy of 4.9 eV, whereas the upper limit of the vacuum level is situated at 3.9 eV, resulting in a NEA of at least −1.0 eV and a maximum work function of 3.9 eV. The high-borondoped diamond (100) surface behaves similarly, showing that the NEA peak is present due to the downward band bending independent of the boron concentration. The nitrogen-doped (100)-(2×1):H surface shows a low NEA of −0.2 eV but no NEA peak due to the strong upward band bending. The (111)-(1×1):H surface does not show a NEA peak due to the k d conservation in photoemission; E vac is situated at 4.2 eV or below, resulting in a NEA of at least −0.9 eV and a maximum work function of 4.2 eV. The high-intensity NEA peak of boron-doped diamond seems to be due to the downward band bending together with the reduced work function because of hydrogen termination. Upon hydrogen desorption at higher annealing temperatures, the work function increases, and NEA disappears. For the nitrogen-doped diamond (100) surface, the work function behaves similarly, but the observation of a NEA peak is absent because of the surface barrier formed by the high upward band bending.

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