Collective modes of the electron-hole condensate in the (putative) excitonic insulator 1T-TiSe₂
15:30 - 16:30
Department of Physics, University of Illinois at Urbana-Champaign
CFEL (Bldg. 99)
Seminar Room IV, O1.111
TiSe₂ is one of the simplest charge density wave (CDW) materials, forming a 2×2 superlattice below a transition temperature T_C = 200 K, but the origin of this phase is controversial. Its nearly inverted band structure led early authors to identify TiSe₂ as an “excitonic insulator,” which is an electronic instability involving spontaneous proliferation of excitons. The problem is that the CDW also exhibits a sizeable lattice distortion, leading later authors to identify it as a conventional Peierls phase. That said, an excitonic phase would also create an incidental lattice distortion, since the interaction with phonons can’t be switched off. The arguments on the matter have gone in circles for decades.
In this talk I will describe a study of TiSe₂ using momentum-resolved, inelastic electron scattering (M-EELS) with an energy resolution of 4.5 meV. We observed the appearance, at low temperature, of an exciton-like, electronic mode that disperses to ω~0 at the wave vector of the CDW. We identify this mode as the long-sought—but until now never observed—Jerome-Rice-Kohn (JRK) collective mode of an excitonic insulator. This excitation can be thought of as the Goldstone mode or, equivalently, the phason mode of the electron-hole condensate. Unlike an ideal excitonic insulator, this mode mixes strongly with the lattice degrees of freedom on an energy scale below 10 meV. I will argue that phonons contribute to this stability of this phase, which should be thought of as a phonon-stabilized excitonic insulator, which is the necessary manifestation of an excitonic state in a real material.