Orthorhombic-to-monoclinic phase transition of Ta2NiSe5induced by the Bose-Einstein condensation of excitons

Abstract

Using the band structure calculation and mean-field analysis of the derived three-chain Hubbard model with phonon degrees of freedom, we discuss the origin of the orthorhombic-to-monoclinic phase transition of the layered chalcogenide Ta2NiSe5. We show that the Bose-Einstein condensation of excitonic electron-hole pairs cooperatively induces the instability of the phonon mode at momentum q → 0 in the quasi-one-dimensional Ta-NiSe-Ta chain, resulting in the structural phase transition of the system. The calculated single-particle spectra reproduce the deformation of the band structure observed in the angle-resolved photoemission spectroscopy experiment. PACS numbers: 71.10.Fd, 71.30.+h, The electron-hole pair condensation in thermal equilibrium into the excitonic insulator (EI) state has attracted renewed interest in recent years because of the discoveries of a number of new materials. The idea of EI was proposed about half a century ago [1,2], where the EI state was predicted to be realized either in a semiconductor with a small band gap or in a semimetal with a small band overlap. The formation of excitons is driven by poorly screened Coulomb interaction between conduction-band electrons and valence-band holes under condition of a low carrier concentration. If the binding energy of excitons is larger than the band gap, they may spontaneously condense at low temperatures and drive the system into a new ground state with exotic properties. This new state, which is a condensed state of a macroscopic number of excitons acquiring quantum phase coherence, is called EI [3,4]. It has been pointed out that the semimetal-EI transition may be described in analogy with the BCS theory of superconductivity and the semiconductor-EI transition is discussed in terms of a Bose-Einstein condensation (BEC) of preformed excitons [5][6][7][8][9].An example of materials for possible realization of EI is Tm(Se,Te), where it has been claimed that a transition into EI occurs by applying pressure [10][11][12] and that the superfluidity of condensed excitons is responsible for the observed anomalous properties [13]. Another promising candidate for EI is CaB 6 [14], where the observed weak ferromagnetism with an unexpectedly high Curie temperature was interpreted in terms of a doped EI [15,16]. Also known as a candidate for EI is TiSe 2 , where the observed charge-density-wave state was interpreted to be of an excitonic type [17][18][19]. The spin-density-wave state of the iron-pnictide superconductors has also been argued to be of the excitonic type [20][21][22]. Semiconductor bilayer systems have attracted attention as well in relation to the BEC of excitons [23].Recently, a transition-metal chalcogenide Ta 2 NiSe 5 has been studied in this respect [24,25]. This mate-rial has a layered structure stacked loosely by a weak van der Waals interaction, and in each layer, Ni single chains and Ta double chains are running along the aaxis of the lattice to form a quasi-one-dimensional (1D) chain structure [26]. The observed resistivity shows a se...

Fate of the excitonic insulator in the presence of phonons

“…Finally, at the transition to the suggested EI phase in Ta 2 NiSe 5 the lattice structure changes from orthorhombic to monoclinic, although the charge does not modulate. 18,19,27 Therefore, the electronphonon interaction seems non-negligible in this material as well.…”

“…First attempts to include a coupling to the lattice degrees of freedom have been made quite recently, motivated by several experiments indicating that the lattice is involved at the phase transition to the anticipated EI phase. [17][18][19][20][21] For example, in the TmSe 0. 45 Te 0.55 compound a drop of the specific heat and an increase of the lattice constant have been interpreted as a strong coupling between excitons and phonons.…”

Photoinduced Enhancement of Excitonic Order

“…So far, the theoretical works on EIs have mainly focused on the equilibrium properties such as the BEC-BCS crossover [22,23], the coupling of the EI to phonons [24][25][26], linear susceptibilities [27][28][29], and the effect of strong interactions and new emergent phases [30][31][32]. In contrast, the nonequilibrium investigation of EIs has just begun [10].…”

“…We denote the phonon creation operator at site i byb † i , the e-ph coupling by g, and the phonon frequency by ω 0 , and define the effective e-ph interaction by λ ≡ 2g 2 =ω 0 . This phonon is associated with a lattice distortion in TNS, which hybridizes the electronic bands [24]. The hopping parameter J is our unit of energy.…”

Low-Energy Excitation Spectra in the Excitonic Phase of Cobalt Oxides

“…After Mott's prediction of the EP half a century ago, 5) a number of candidate materials for this phase have come to our attention. Among them are the transition-metal chalcogenides 1T -TiSe 2 6-8) and Ta 2 NiSe 5 , [9][10][11] where the electrons and holes on different atoms are considered to form spin-singlet pairs to condense into the EP, which is accompanied by lattice distortion.…”