Evidence for an Excitonic Insulator Phase in1T−TiSe2

Abstract

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...

Time-domain classification of charge-density-wave insulators

“…3a-d display qualitative electronic structure changes happening within 65-210 fs after excitation, which is much faster than the 0.35-4 ps it takes to transfer the excess energy in the electron system to the phonon bath 16,17,19,[39][40][41] . These changes therefore do not reflect transitions to quasi-equilibrium states at elevated effective temperatures, although the band maps look fairly similar to temperature-dependent equilibrium ARPES data 9,[12][13][14] .…”

“…On the one hand, quantitative microscopic explanations 26 are difficult and deviate too much from experiment [12][13][14] . On the other hand, qualitative explanations in terms of an electron-lattice instability (variously called Peierls instability, band-type/indirect Jahn-Teller effect or antiferroelectric transition) 12,25,[27][28][29] , a purely electronic phenomenon (an excitonic insulator instability) 14,20,30 , or a combination of the two scenarios 13,31 are ambiguous as they all seem consistent with subsets of the extensive body of experimental and theoretical results. All proposed models explicitly involve the interaction between the Se 4p valence band maximum at Γ and the conduction band minimum at M, the band extrema that are 'nested' by the wavevector of the CDW and mapped onto each other in the CDW phase (Fig.…”

“…Two prominent examples are 1T-TaS 2 and 1T-TiSe 2 . In these two compounds, the electronic structures [8][9][10][11][12][13][14][15] and ultrafast spectral responses [16][17][18][19] indicate the presence of significant electron correlation effects. In both compounds, electron-electron interactions may in fact have an important role in CDW formation [12][13][14][18][19][20] and in the emergence of superconductivity from CDW states upon intercalation 21 or under pressure 22,23 .…”

Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

“…In these systems, the valence and conduction bands are formed by orbitals located on different atoms. For example, in 1T -TiSe 2 , the 4p orbitals of Se ions account for the valence bands and the 3d orbitals of Ti ions account for the conduction bands [7][8][9][10][11][12][13][14] , and in Ta 2 NiSe 5 , the 3d orbitals of Ni ions form the valence bands and the 5d orbitals of Ta ions form the conduction bands [21][22][23][24] . Hund's rule coupling, acting between electrons on different orbitals of a single ion and favoring the spin-triplet excitons, is therefore negligible.…”

“…In this regard, Tm(Se,Te) was argued to exhibit a pressureinduced excitonic instability, related to an anomalous increase in the electrical resistivity and thermal diffusivity 5,6 . The charge-density-wave (CDW) state observed in 1T -TiSe 2 was claimed to be of excitonic origin [7][8][9][10][11][12][13][14] . In Ca 1−x La x B 6 , the weak ferromagnetism was interpreted in terms of doped spin-triplet excitons [15][16][17] .…”