Failed excitonic quantum phase transition in Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$

Abstract

We study the electronic phase diagram of the excitonic insulator candidates Ta2Ni(Se1−xSx)5 [x=0, ... ,1] using Raman spectroscopy. Critical excitonic fluctuations are observed, that diminish with x and ultimately shift to high energies, characteristic of a quantum phase transition. Nonetheless, a symmetry-breaking transition at finite temperatures is detected for all x, exposing a cooperating lattice instability that takes over for large x. Our study reveals a failed excitonic quantum phase transition, masked by a preemptive structural order.

Excitonic insulator states in molecular functionalized atomically-thin semiconductors

“…There are a few materials, which are suspected to possess EI ground states in a solid state, namely 1T-TiSe 2 , Ta 2 NiSe 5 or TmSe 0.45 Te 0.55 [12][13][14][15][16][17][18][19] . However, it has been difficult to establish whether the EI state has been realized, because the expected electronic phase transition is accompanied by a structural phase transition, which makes it difficult to distinguish between EI and normal insulator or Peierls transition [20][21][22][23] . Just recently, the discussion started about EIs in TMDC heterostructures [24][25][26][27] .…”

Third-harmonic generation in excitonic insulators

“…Recently, several transition-metal compounds, including TiSe 2 [7,[32][33][34], Ta 2 NiSe 5 [6,[35][36][37][38][39][40], and WTe 2 [8,41,42], are considered as candidates for the EIs. In particular, the origin of the ordered state in Ta 2 NiSe 5 are actively debated by the Raman and nonequilibrium pump-probe spectroscopies [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].…”