Semimetal-to-Semimetal Charge Density Wave Transition in $1T\mathrm{\text{−}}{\mathrm{TiSe}}_2$

We report an infrared study on $1T\mathrm{\text{−}}{\mathrm{TiSe}}_2$, the parent compound of the newly discovered superconductor ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$. Previous studies of this compound have not conclusively resolved whether it is a semimetal or a semiconductor—information that is important in determining the origin of its unconventional charge density wave (CDW) transition. Here we present optical spectroscopy results that clearly reveal that the compound is metallic in both the high-temperature normal phase and the low-temperature CDW phase. The carrier scattering rate is dramatically different in the normal and CDW phases and the carrier density is found to change with temperature. We conclude that the observed properties can be explained within the scenario of an Overhauser-type CDW mechanism.

Figure 1

(a) The temperature dependent $R(\omega )$ in the frequency range from $30$ to $6000{\mathrm{cm}}^{-1}$. The inset is the $T$-dependent DC resistivity. (b) The expanded plot of $R(\omega )$ from $30$ to $600{\mathrm{cm}}^{-1}$. The inset shows $R(\omega )$ data up to $50000{\mathrm{cm}}^{-1}$ on a logarithm scale. Dash curves are fitted to a simple Drude-Lorentz model.

Figure 2

The energy loss function spectra at different $T$. Inset: the variation of the plasma frequency and scattering rate for the Drude term in Eq. (1).

Figure 3

Frequency dependence of the optical conductivity at different temperatures.

Figure 4

Schematic picture for the evolution of the band structure with temperature. (a) The Brillouin zone layout for $1T\mathrm{\text{−}}{\mathrm{TiSe}}_2$. The right part shows the new Brillouin zones (dashed lines) for a $2\times 2\times 2$ superlattice in $\Gamma$, $A$, $L$ , and $M$ plane. (b) The semimetal picture in the normal phase. A small number of Se $4p$ holes and Ti $3d$ electrons exist near $\Gamma$ and $L$, respectively. (c) The exciton state in the CDW phase caused by the Coulomb interaction between electrons and holes.