Exciton Condensation and Superfluidity in TmSe<sub>0.45</sub>Te<sub>0.55</sub>

Abstract

In this publication details of the calculation of heat conductivity and thermal diffusivity, compressibility, sound velocity and exciton-polaron dispersion of TmSe 0.45 Te 0.55 will be shown. The properties of excitons, coupling to phonons, producing thus polarons, but also blocking the phonons as running waves lead to an exciton condensation or exciton liquid. Surprisingly this exciton liquid is contained in a macroscopic crystal, a solid nevertheless, which becomes extremely hard due to the exciton liquid and finally exhibits a strange type of superfluid in a two fluid model, where the superfluid phase increases more and more below about 20 K until the whole exciton liquid becomes a superfluid at zero temperature. Never else a superfluid phase has been observed at such high temperatures.

Excitonic condensation state in the assistance of the adiabatic and anti-adiabatic phonons

“…In that manner, we show in figure 4 the ground-state phase diagrams of the excitonic condensate in the (U, ε f ) plane for different phonon frequencies ω 0 at a given electron-phonon coupling g = 0.6. Due to the choice with ε c = 0, the onsite energy ε f represents the overlap between the f -and the cbands, measuring the possible coupling between the electrons and the holes to form the excitons caused by the external pressure [9,33]. At a given phonon frequency, we always find the excitonic condensation state for small |ε f |.…”

“…In general, the excitonic condensation state is foremost investigated in purely electronic models, i.e., only the Coulomb interaction between the electron and hole is considered [2][3][4][5][6][7][8]. However, recent several experiments have indicated that the lattice distortions also play important roles to anticipate the excitonic condensation state [9][10][11]. For instance, in the pressure-sensitive mixed valence material TmSe 0.45 Te 0.55 , the excitonic condensation state is found in accompanying with an abrupt change of lattice displacement [12].…”

Thermal transport signatures of the excitonic transition and associated phonon softening in the layered chalcogenide Ta$_2$NiSe$_5$

“…The realization in bulk materials has been discussed, for example, in TmSe 0.45 Te 0.55 [2] and 1T -TiSe 2 [3], but the evidence for excitonic condensation in these compounds has been far from convincing. The former is a 4f 13 Mott-like system, not a band semiconductor/semimetal [4], and the latter remains metallic and may suffer from strong screening of electron-hole interactions even below the proposed excitonic transition temperature [5].…”